U.S. patent application number 09/772039 was filed with the patent office on 2001-12-20 for method and apparatus for monitoring the advance of seawater into fresh water aquifers near coastal cities.
Invention is credited to Babour, Kamal, Delhomme, Jean-Pierre R., Howard, Peter V., Rossi, David J., Wijnberg, Willem A..
Application Number | 20010052774 09/772039 |
Document ID | / |
Family ID | 26876446 |
Filed Date | 2001-12-20 |
United States Patent
Application |
20010052774 |
Kind Code |
A1 |
Rossi, David J. ; et
al. |
December 20, 2001 |
Method and apparatus for monitoring the advance of seawater into
fresh water aquifers near coastal cities
Abstract
Sensors are permanently placed in the ground near observation
and injection wells in order to passively and continuously monitor
the status of seawater advance toward fresh water aquifers near
coastal cities as well as the status of fresh water injected into
the injection wells. Such sensor devices are installed in the
ground and electrically connected to surface acquisition equipment
that would, without human intervention, transmit acquired data to a
centralized facility for processing and interpretation. Various
types of sensors can be used: the sensors used for general
reservoir monitoring and/or the sensors used for leak detection,
soil heating, and temperature mapping. Alternatively, a special
type of sensor can be designed and provided for the purpose of
monitoring the status of seawater advance toward fresh water
aquifers.
Inventors: |
Rossi, David J.; (Katy,
TX) ; Wijnberg, Willem A.; (Houston, TX) ;
Howard, Peter V.; (Belleville, TX) ; Delhomme,
Jean-Pierre R.; (Boulogne-Billancourt, FR) ; Babour,
Kamal; (Bures sur Yvette, FR) |
Correspondence
Address: |
GeoQuest, a division of Schlumberger Technology Co
ATTN: J. H. Bouchard, Patent Counsel
Suite 1700
5599 San Felipe
Houston
TX
77056-2722
US
|
Family ID: |
26876446 |
Appl. No.: |
09/772039 |
Filed: |
January 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60180572 |
Feb 4, 2000 |
|
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60180981 |
Feb 8, 2000 |
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Current U.S.
Class: |
324/357 ;
324/713 |
Current CPC
Class: |
E21B 47/113 20200501;
G01V 9/02 20130101; E03B 1/00 20130101 |
Class at
Publication: |
324/357 ;
324/713 |
International
Class: |
G01V 003/02 |
Claims
We claim:
1. A method of passively and continuously monitoring a status of
seawater advance toward fresh water aquifers which are located
adjacent coastal cities, one or more observation wells being
located in the fresh water aquifers adjacent said coastal cities,
comprising the steps of: installing sensors in or around said
observation wells, using said sensors, monitoring the advance of
said seawater toward said fresh water aquifers.
2. The method of claim 1, wherein said one or more observation
wells are located adjacent one or more injection wells, and wherein
said sensors associated with said observation wells detect said
advance of said seawater toward said fresh water aquifers and
further detect a pressure in a mound of fresh water that has been
injected into said injection wells, said pressure in said mound
being above a pressure in said seawater for mitigating a landward
advance of said seawater toward said fresh water aquifers.
3. The method of claim 1, wherein said sensors associated with said
observation wells comprise a resistivity array located within or
around a casing of said observation wells adapted for measuring a
conductivity and a resistivity of an earth formation within or
around said observation wells, said conductivity and resistivity
being representative of a presence or absence of either seawater or
fresh water within or around said observation wells and being
further representative of a location of a seawater/fresh water
boundary within or around said observation wells, said boundary
being further representative of an advance of seawater toward fresh
water aquifers located adjacent coastal cities.
4. The method of claim 3, wherein each said resistivity array is
located around said casing of said observation wells and adjacent a
water sand unit of said earth formation, said resistivity array
comprising a top subarray portion, a middle subarray portion, and a
bottom subarray portion for measuring and determining the
conductivity and resistivity of a top part, a middle part, and a
bottom part of said water sand unit thereby determining said
location of said seawater/fresh water boundary.
5. The method of claim 4, wherein said top subarray portion, said
middle subarray portion, and said bottom subarray portion of said
resistivity array each comprise a plurality of electrodes, the
plurality of electrodes of the top subarray portion, said middle
subarray portion, and said bottom subarray portion being located
adjacent, respectively, the top part, the middle part, and the
bottom part of the water sand unit.
6. The method of claim 5, wherein each said subarray portion
comprises a plurality of electrodes, a first pair of the electrodes
generating and receiving current, a second pair of the electrodes
generating a potential difference, the current and the potential
difference being used to determine a resistivity of said water sand
unit, said resistivity being indicative of a presence or absence of
seawater in the water sand unit adjacent said observation well.
7. The method of claim 6, wherein each of the electrodes, which
comprise said each said subarray portion of said resistivity array,
are spaced apart from an adjacent electrode by a distance, said
distance being chosen such that a particular resolution is
achieved, a distance "d" between electrodes achieving one
resolution, a distance "3d" achieving another resolution, and
distances "6d" and "9d" achieving still other resolutions.
8. The method of claim 6, wherein said each said subarray portion
comprises a distinct unit comprised of an insulating material
having interleaved integral electrodes.
9. The method of claim 6, wherein said each said subarray portion
comprises a set of solid plated metal electrodes wrapped around a
cable.
10. The method of claim 3, further comprising the step of: further
installing a plurality of isolated additional electrodes within or
around the observation well, in combination with said resistivity
array located within or around said observation well, for
monitoring a free water level within said observation well.
11. The method of claim 4, wherein each said subarray portion, that
is located adjacent said top part and said middle part and said
bottom part of said water sand unit, comprises a quadrupole
subarray electrode set, said electrode set further including a
plurality of quadrupole subarray electrodes, one quadrupole
subarray electrode being located adjacent said top part of said
water sand unit, another said quadrupole subarray electrode being
located adjacent said middle part of said water sand unit, and
still another said quadrupole subarray electrode being located
adjacent said bottom part of said water sand unit.
12. A method of monitoring an advance of seawater toward a fresh
water aquifer, an observation well penetrating said fresh water
aquifer, said observation well having a casing, comprising the
steps of locating a resistivity array within or around said casing
of said observation well, an aquifer being located around said
resistivity array; using said resistivity array, measuring a
resistivity value or a conductivity value of a water based
substance in said aquifer around said resistivity array, the values
of said resistivity or conductivity being representative of a
presence or an absence of either seawater or fresh water in said
water based substance in said aquifer.
13. The method of claim 12, wherein said water based substance in
said aquifer around said resistivity array includes a top part, a
middle part, and a bottom part, said resistivity array includes a
top subarray, a middle subarray, and a bottom subarray, and wherein
the measuring step further comprises the steps of: using the top
subarray of the resistivity array, taking measurements to obtain a
first resistivity value or a first conductivity value of the top
part of said water based substance in said aquifer around said
resistivity array, using the middle subarray of the resistivity
array, taking measurements to obtain a second resistivity value or
a second conductivity value of the middle part of said water based
substance in said aquifer around said resistivity array, and using
the bottom subarray of the resistivity array, taking measurements
to obtain a third resistivity value or a third conductivity value
of the bottom part of said water based substance in said aquifer
around said resistivity array.
14. The method of claim 13, wherein a seawater/fresh water boundary
is located in said top part, said middle part, and said bottom part
of said water based substance in said aquifer, the first
resistivity value or conductivity value, the second resistivity
value or conductivity value, and the third resistivity value or
conductivity value being used to determine a location of said
seawater/fresh water boundary in said water based substance in said
aquifer.
15. The method of claim 14, wherein each subarray includes a
quadrupole electrode set, said quadrupole electrode set including a
plurality of electrodes, the step of taking measurements by each
subarray to obtain a resistivity value or a conductivity value of
the top, middle, and bottom part of said water based substance in
said aquifer comprising the steps of: injecting current into said
water based substance in said aquifer from a first electrode of the
plurality of electrodes, receiving current from said aquifer into a
second electrode of the plurality of electrodes, and measuring a
potential difference using a third and fourth electrode of the
plurality of electrodes, said potential difference being
representative of said resistivity value or said conductivity value
of said water based substance in said aquifer, said resistivity
value or conductivity value of said water based substance in said
aquifer being further representative of a presence or absence of
seawater in said water based substance in said aquifer.
16. The method of claim 15, wherein the electrodes of said
quadrupole electrode set are spaced apart by a distance, said
distance being chosen such that a particular resolution is
obtained.
17. A method of using an observation well which penetrates a water
aquifer in an earth formation that is located near coastal cities
to measure and obtain data representative of a characteristic of
the water in said aquifer, said observation well having water
disposed therein, comprising the steps of: (a) locating an
electrode inside said observation well, a depth of said electrode
in said observation well being known; and (b) using said electrode,
determining when a level of said water in said observation well
traverses said electrode thereby determining a free head level of
said water in said observation well through knowledge of said depth
of said electrode in said observation well.
18. The method of claim 17, further comprising the steps of: (c)
locating a resistivity array within or around a casing of said
observation well, said casing being disposed within said water
aquifer in said earth formation; (d) using said resistivity array,
determining a resistivity of a water based substance disposed
around said casing within said aquifer, said resistivity being
representative of a presence or absence of seawater in said aquifer
and being determinative of any advance of said seawater into said
aquifer.
19. The method of claim 18, wherein said resistivity array
comprises a plurality of subarrays including a top subarray, a
middle subarray, and a bottom subarray, said water based substance
around said casing within said aquifer including a top part
disposed adjacent said top subarray, a middle part disposed
adjacent said middle subarray, and a bottom part disposed adjacent
said bottom subarray.
20. The method of claim 19, wherein the determining step (d) for
determining said resistivity of said water based substance disposed
around said casing within said aquifer comprises the steps of: (d1)
using said top subarray, determining a resistivity of the top part
of said water based substance around said casing within said
aquifer; (d2) using said middle subarray, determining a resistivity
of the middle part of said water based substance around said casing
within said aquifer; and (d3) using said bottom subarray,
determining a resistivity of the bottom part of said water based
substance around said casing within said aquifer.
21. The method of claim 20, wherein said top subarray, said middle
subarray, and said bottom subarray each comprise an electrode set
including a plurality of electrodes, the determining steps (d1),
(d2), and (d3) each comprising the steps of: using a first
electrode of said plurality of electrodes, injecting a current into
said water based substance around said casing within said aquifer;
using a second electrode of said plurality of electrodes, receiving
said current from said aquifer; and using a third and fourth
electrode of said plurality of electrodes, measuring and
determining a potential difference which exists between the third
and fourth electrodes, said potential difference being
representative of said resistivity of the top, middle, and bottom
parts of the water based substance around said casing within said
aquifer, said resistivity in the top, middle, and bottom parts of
the water based substance within said aquifer being further
representative of the presence or absence of said seawater in said
aquifer and any advance of said seawater into said aquifer.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a utility application of prior pending provisional
application serial No. 60/180,572 filed Feb. 4, 2000 entitled
"Seawater Barrier Monitoring", and of prior pending provisional
application serial No. 60/180,981 filed Feb. 8, 2000 entitled
"Water Aquifer Electrical Monitoring Electrode Cable System".
BACKGROUND OF THE INVENTION
[0002] The subject matter of the present invention relates to a
method and apparatus for passively and continuously monitoring the
status of seawater advance toward water acquifers near coastal
cities by placing sensors, such as a particular type of electrode
cable system, in the ground near observation and injection wells,
transmitting acquired data back to centralized processing
facilities, and, responsive thereto, subsequently mitigating the
advance of the seawater into potable water acquifers which are
situated near the coastal cities.
[0003] Coastal cities rely on groundwater from subsurface aquifers
to meet all or part of their municipal water needs. In cases of
historical overdraft, water is withdrawn from the subsurface
aquifers at a rate exceeding the rate of natural aquifer recharge.
As illustrated in FIG. 1, such overdraft results in a lowering of
the water table in the aquifers and is accompanied by possible
intrusion of seawater into the aquifer. The breakthrough of
seawater at wells supplying the drinking water has severe long term
consequences on municipal potable water deliverability. For
example, Los Angeles (LA) experienced such overdraft in the first
half of the twentieth century (the 1900s). As a result, LA
subsequently created the Water Replenishment District (WRD) agency
to define and enforce reduced aquifer pumping rates and mitigate
the effects of seawater intrusion.
[0004] Various means exist to mitigate seawater advance. One of
these methods is to recharge the aquifers from surface spreading
grounds. Another method involves injecting inert gas or fresh water
into the ground using special injection wells, as illustrated in
FIG. 2. The water injection method consists of injecting fresh
water into the aquifer, forming a fresh water `mound` in the local
area around the well, to create a zone with pressure above the
pressure in the seawater, thereby mitigating the landward advance
of the seawater. As an example, since the 1950s, Los Angeles has
constructed approximately 250 seawater barrier injection wells of
this type thereby resulting in three lines of injectors. These
three different lines of injectors are illustrated in FIG. 3, where
the position of the water injection wells is shown by the
adjacently connected circles.
[0005] In addition to the wells constructed for water injection
(i.e., injection wells) as shown in FIGS. 2 and 3, a number of
observation or monitoring wells (i.e., observation wells) are
typically constructed in the vicinity of the injection wells. These
observation wells are used to periodically measure the pressure
(i.e., the hydraulic head) of the aquifer in the neighborhood of
the injection wells, and for occasional sampling of water chloride
(i.e., salinity) levels. This gives information about how
efficiently the injection wells are limiting the seawater advance.
In Los Angeles, for example, over 700 observation wells have been
constructed along the three lines of injection wells (shown in FIG.
3) in order to monitor the position of the seawater wedge. In a
typical municipal setting with seawater advance into the aquifers,
the water authorities sample the chloride concentrations at three
positions (top, middle, and bottom) of each water sand unit, as
illustrated in FIG. 7. This provides information about how
efficiently the injection wells are limiting the seawater advance
in each sand unit.
[0006] However, a need exists to passively and continuously
monitor, in the observation wells, the status of the seawater wedge
and the resultant seawater advance toward coastal city water
aquifers as well as the status of the injected fresh water in the
injection wells.
[0007] Depending on the application, various types of in-situ
sensors have been employed in the oil industry for general
reservoir monitoring. See the following "first reference" which
discloses general reservoir monitoring: Babour, K. A., Belani and
J. Pilla, `Method and Apparatus for Surveying and Monitoring a
Reservoir Penetrated by a Well Including Fixing Electrodes
Hydraulically Isolated within a Well`, U.S. Pat. No. 5,642,051, the
disclosure of which is incorporated by reference into the
specification of this application. In addition, such sensors have
been proposed for leak detection, soil heating and temperature
mapping. See the following three "second set of references" which
disclose leak detection, soil heating, and temperature mapping: (1)
Berryman, James G., Daily, William D., `Optimal joule heating of
the subsurface, U.S. Pat. No. 5,325,918, the disclosure of which is
incorporated by reference into the specification of this
application, (2) Daily, William D., Laine, Daren L., Laine, Edwin
F., `Methods for Detecting and Locating Leaks in Containment
Facilities using Electrical Potential Data and Electrical
Resistance Tomographic Imaging Techniques`, U.S. Pat. No.
5,661,406, the disclosure of which is incorporated by reference
into the specification of this application, and (3) Ramirez,
Abelardo L.; Dwayne A.; Daily, William D., `Using Electrical
Resistance Tomography to Map Subsurface Temperatures`, U.S. Pat.
No. 5,346,307, the disclosure of which is incorporated by reference
into the specification of this application.
[0008] Consequently, in connection with the aforementioned need to
passively and continuously monitor, in the injection wells and the
observation wells, the status of the seawater wedge and the
resultant seawater advance in addition to the status of the
injected fresh water, there is a further need to utilize `special
sensors` in the injection wells and in the observation wells to
perform the step of monitoring the seawater advance and the status
of the injected fresh water. These `special sensors` can be the
sensors disclosed above in connection with the "first reference" or
in connection with the "second set of references". Alternatively,
these `special sensors` can be new sensors which are adapted for
the above stated purpose of monitoring the seawater advance and the
status of the injected fresh water.
SUMMARY OF THE INVENTION
[0009] Accordingly, it is a primary object of the present invention
to passively and continuously monitor, in the observation wells,
the status of the seawater wedge and the resultant seawater advance
toward fresh water aquifers located near coastal cities in addition
to the status of any fresh water injected into the injection
wells.
[0010] Accordingly, it is a primary aspect of the present invention
to permanently install sensors in the ground in or around an
observation well located near the injection wells in order to
passively and continuously monitor the status of the seawater
advance toward fresh water aquifers near coastal cities in addition
to the status of the fresh water injected into the injection wells
(hereinafter, the `monitoring step`).
[0011] It is a further aspect of the present invention to implement
the aforementioned `monitoring step` by utilizing sensors which
have been used in the oil industry, such as the sensors used for
general reservoir monitoring, and/or the sensors used for leak
detection, soil heating, and temperature mapping.
[0012] It is a further aspect of the present invention to implement
the aforementioned `monitoring step` by utilizing sensors that are
specially designed for use during the steps of monitoring the
seawater advance toward fresh water aquifers near coastal cities
and monitoring the status of fresh water injected into the
injection wells.
[0013] It is a further aspect of the present invention to use
oilfield related techniques/methods [that are currently being used
in the oil industry to detect and record the existance of
underground deposits of hydrocarbon (such as oil)] for the purpose
of: (1) detecting the advance of seawater toward fresh water
aquifers near coastal cities, and (2) detecting the pressure in the
`mound` of fresh water that has been injected into injection wells,
the purpose of which is to create a zone of pressure above the
pressure in the seawater for mitigating (i.e., slowing) the
landward advance of the seawater toward the fresh water aquifers
that are situated near the coastal cities.
[0014] It is a further aspect of the present invention to install a
resistivity array around a casing of an observation well, or inside
the casing of the observation well, which is located near an
injection well, for the purpose of measuring the conductivity and
resistivity of the earth formation within or around the observation
well, the conductivity and resistivity values being representative
of the presence or absence of either seawater or fresh water within
or near the observation well, the conductivity and resistivity
values being further representative of the location of a
seawater/fresh water boundary within or near the observation well,
the boundary being further representative of the advance of the
seawater toward fresh water aquifers near coastal cities.
[0015] It is a further aspect of the present invention to install a
resistivity array within or around an observation well for the
purpose of measuring for and determining the presence or absence of
a seawater/fresh water boundary in a water sand unit located
adjacent to the observation well, the resistivity array having a
top subarray portion, a middle subarray portion, and a bottom
subarray portion for measuring and determining the conductivity and
resistivity of a top part, a middle part, and a bottom part of the
water sand unit thereby determining the location of said
seawater/fresh water boundary.
[0016] It is a further aspect of the present invention to install a
resistivity array around an observation well for the purpose of
measuring for and determining the presence or absence of a
seawater/fresh water boundary in a water sand unit located adjacent
to the observation well, the resistivity array including a
plurality of subarrays located adjacent the top part and the middle
part and the bottom part of the water sand unit, each subarray of
the resistivity array including a plurality of electrodes, a first
pair of the electrodes generating and receiving current, a second
pair of the electrodes used to measure the resulting potential
difference, the current and the potential difference being used to
calculate the resistivity of the water sand unit, the resistivity
being indicative of the presence or absence of seawater in the
water sand unit near the observation well.
[0017] It is a further aspect of the present invention to install a
resistivity array around an observation well for the purpose of
measuring for and determining the presence or absence of a
seawater/fresh water boundary in a water sand unit located adjacent
to the observation well, wherein the electrodes which comprise each
subarray of the resistivity array are spaced apart from each other
by a distance, said distance being chosen such that a particular
resolution is achieved, a distance of "d" between electrodes
achieving one resolution, the distance "3d" achieving still another
resolution, and the distances "6d" and "9d" achieving still another
resolution. The larger the distance between the electrodes, the
deeper into the earth formation the electrical current will flow,
and the deeper the depth of investigation on sensitivity.
Electrodes spaced at distances "6d" and "9d" are sensitive to a
seawater/freshwater boundary situated away from the well containing
the permanently installed electrode array. This allows the presence
of seawater to be determined before it arrives in physical contact
with the monitoring electrode array.
[0018] It is a further aspect of the present invention to install a
resistivity array around an observation well for the purpose of
measuring for and determining the presence or absence of a
seawater/fresh water boundary in a water sand unit located adjacent
to the observation well, wherein each resistivity subarray could
comprise a distinct unit comprised of an insulating material (such
as plastic or ceramic) having interleaved integral electrodes, or
the resistivity subarray could comprise a set of solid plated metal
electrodes wrapped around a cable.
[0019] It is a further aspect of the present invention to install a
resistivity array around an observation well for the purpose of
measuring for and determining the presence or absence of a
seawater/fresh water boundary in a water sand unit located adjacent
to the observation well, wherein a plurality of isolated additional
electrodes are used in combination with the resistivity array in
order to monitor a free water level inside the observation
well.
[0020] It is a further aspect of the present invention to install a
resistivity array around an observation well for the purpose of
measuring for and determining the presence or absence of a
seawater/fresh water boundary in a water sand unit located adjacent
to the observation well, where each subarray of the resistivity
array that is located adjacent the top part and the middle part and
the bottom part of the water sand unit comprises a quadrapole
electrode set, one quadrapole electrode set being located adjacent
the top part, another quadrapole electrode set being located
adjacent the middle part, and another quadrapole electrode set
being located adjacent the bottom part of the water sand unit.
[0021] In accordance with the above object and aspects of the
present invention, sensors are permanently placed in the ground
near observation and injection wells in order to passively and
continuously monitor the status of seawater advance toward fresh
water aquifers near coastal cities as well as the status of fresh
water injected into the injection wells. Such sensor devices are
installed in the ground and electrically connected to surface
acquisition equipment that would, without human intervention,
transmit acquired data to a centralized facility for processing and
interpretation. This would avoid the process of manual data
collection and provide more frequent and timely data for better
control of water injection. Various types of sensors can be used:
the sensors used for general reservoir monitoring that are
disclosed in the `first reference` cited above, and/or the sensors
used for leak detection, soil heating, and temperature mapping that
are disclosed in the `second set of reference` cited above.
Alternatively, a special type of sensor can be designed and
provided for the purpose of monitoring the status of seawater
advance toward fresh water aquifers. An array of such sensors
(hereinafter called a "resistivity array") is installed in the
Earth along the exterior of the injection or observation well
casings, or such sensors can be placed inside the well. Then,
time-lapse in-situ electrical resistivity measurements are carried
out. A basic quadrapole measurement consists of injecting and
withdrawing current (I) between two outer electrodes of a group of
four electrodes, and measuring the voltage potential (V) between
the two inner electrodes. The resistivity of the Earth in the
vicinity of the quadrapole sensors is computed from the current "I"
and the potential "V". Thus, the electrode quadrapoles may be used
to obtain resistivity measurements at one or more depths in the
water bearing sands in the earth. Consequently, each single
formation or water resistivity measurement may be used to
discriminate between fresh water and saline (salt) water along the
length of the (injection or observation) well.
[0022] Further scope of applicability of the present invention will
become apparent from the detailed description presented
hereinafter. It should be understood, however, that the detailed
description and the specific examples, while representing a
preferred embodiment of the present invention, are given by way of
illustration only, since various changes and modifications within
the spirit and scope of the invention will become obvious to one
skilled in the art from a reading of the following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] A full understanding of the present invention will be
obtained from the detailed description of the preferred embodiment
presented hereinbelow, and the accompanying drawings, which are
given by way of illustration only and are not intended to be
limitative of the present invention, and wherein:
[0024] FIG. 1 illustrates seawater intrusion into a potable
groundwater aquifer;
[0025] FIG. 2 illustrates water injection to mitigate seawater
advance into a potable aquifer;
[0026] FIG. 3 illustrates three lines of water injectors in the Los
Angeles basin to inject fresh water and for monitoring the
resulting a seawater barrier to restrict the landward advance of
subsurface seawater into potable aquifers;
[0027] FIG. 4 illustrates monitoring the seawater position with an
electrical resistivity array buried in the subsurface;
[0028] FIG. 5 illustrates an electrical conductivity (curve at
left) measured with an in-situ resistivity array indicating the
vertical position of the interface between the seawater and potable
water;
[0029] FIG. 6, which is identical to FIG. 2, also illustrates water
injection to mitigate seawater advance into a potable aquifer;
[0030] FIG. 7 illustrates how municipal authorities measure
chloride concentrations in observation wells at the top, middle,
and bottom of each water sand;
[0031] FIG. 7a illustrates a resistivity array including a
plurality of subarrays located outside the casing of an observation
well;
[0032] FIG. 8 illustrates a resistivity array flatpack cable with
an integrally constructed passive electrode formed as a coil of
wire and overmolded on its ends;
[0033] FIG. 9 illustrates how a basic resistivity 4-electrode
(quadrapole) measurement flows electrical current I between the
outer two electrodes and measures the voltage potential V between
the inner pair of electrodes, where the resistivity in the vicinity
of the four electrodes is computed from I and V;
[0034] FIG. 10 illustrates three quadrapole subarrays for measuring
the resistivity at the top, middle, and bottom of water bearing
sand;
[0035] FIGS. 11a and 11b illustrate modular quadrapole subarrays
for measuring resistivity, where (a) on the left shows a modular
assembly prefabricated and attached to the cable with one manual
intervention, and (b) on the right shows solid metal electrodes
wrapped around the cable and attached into the cable with four
manual interventions;
[0036] FIGS. 12a, 12b, 12c, and 12d illustrate multiple-scale
resistivity measurements using sets of non-uniformly spaced
electrodes; and
[0037] FIG. 13 illustrates isolated electrodes for monitoring the
`head` or free water level inside the well casing to within the
separation of two electrodes.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0038] This specification is divided into two parts: (1) a first
part which discloses a first concept called "Seawater Barrier
Monitoring", and (2) a second part which discloses a second related
concept called "Water Aquifer Electrical Monitoring Electrode cable
System".
[0039] Seawater Barrier Monitoring
[0040] Referring to FIG. 1, coastal cities rely on groundwater from
subsurface aquifers to meet all or part of their municipal water
needs. In cases of historical overdraft, water is withdrawn from
the subsurface aquifer(s) at a rate exceeding the rate of natural
aquifer recharge. As illustrated in FIG. 1, such overdraft results
in a lowering of the water table in the aquifer(s), and is
accompanied by possible intrusion of seawater 12 into the aquifer.
The breakthrough of seawater 12 at wells 10 supplying drinking
water has severe long-term consequences on municipal potable water
deliverability. For example, Los Angeles (LA) experienced such
overdraft in the first half of the 1900's. As a result, LA
subsequently created the Water Replenishment District (WRD) agency
to define and enforce reduced aquifer pumping rates and mitigate
the effects of seawater intrusion. Referring to FIG. 2, various
means exist to mitigate seawater advance. One of these methods is
to recharge the aquifers from surface spreading grounds. Another is
to inject inert gas or fresh water with special injection wells 14,
as illustrated in FIG. 2. The water injection method consists of
injecting fresh water 16 into the aquifer, forming a fresh water
"mound" 18 in a local area around the well, thereby creating a zone
having a pressure which is above the pressure in the seawater and
therefore mitigating the landward advance of the seawater.
[0041] Referring to FIG. 3, as an example, since the 1950s, Los
Angeles has constructed approximately 250 seawater barrier
injection wells 14 of this type in three lines of injectors. These
three lines of injection wells are illustrated in FIG. 3 by element
numerals 20, 22, and 24, where the position of the water injection
wells along each line of injectors 20, 22, and 24 is shown in FIG.
3 by the interconnected circles.
[0042] In addition to the wells constructed for water injection, a
number of observation or monitoring wells (hereinafter,
`observation/monitoring wells`) are typically constructed in the
vicinity of the injection wells. These `observation/monitoring
wells` are used to periodically measure the pressure (hydraulic
head) of the aquifer in the neighborhood of the injection wells,
and for occasional sampling of water chloride (salinity) levels.
This gives information about how efficiently the injectors (i.e.,
the injection wells) are limiting the seawater advance. In Los
Angeles, over 700 `observation/monitoring wells` have been
constructed along the three lines of injection wells (20, 22, and
24 in FIG. 3) to monitor the position of the seawater wedge.
[0043] Referring to FIGS. 2, 4 and 9, in accordance with a first
aspect of the present invention, a plurality of `permanently
installed aquifer sensors` are permanently placed in the ground
near both the `observation/monitoring wells` and the injection
wells in order to passively and continuously monitor: (1) the
status of the advance of the seawater toward the fresh water
aquifers located near coastal cities, and (2) the status of the
injected fresh water when the fresh water is injected into the
ground in the manner illustrated in FIG. 2. Such `permanently
installed aquifer sensors` would be installed in the ground (i.e.,
in the earth formation). Such sensors would be electrically
connected to surface acquisition equipment, in the manner
illustrated in FIGS. 4 and 9, that would, without human
intervention, transmit the acquired data to a centralized facility
for processing and interpretation. FIG. 9 shows the cable to the
wellsite acquisition electronics box 15. This box 15 is, in turn,
connected to a data telemetry system (not shown) which transmits
the data to a centralized facility. This would avoid the process of
manual data collection and provide more frequent and timely data
for better control of water injection.
[0044] Depending on the application, in connection with the
`permanently installed aquifer sensors`, various types of in-situ
sensors may be employed. One appropriate technology is electrical
resistivity arrays that would be used to monitor the subsurface
electrical resistivity in the vicinity of the monitoring array.
Such devices have been proposed for general reservoir monitoring
[refer to: Babour, K. A., Belani and J. Pilla, `Method and
Apparatus for Surveying and Monitoring a Reservoir Penetrated by a
Well Including Fixing Electrodes Hydraulically Isolated within a
Well`, U.S. Pat. No. 5,642,051, the disclosure of which has been
incorporated by reference into this specification] and for leak
detection, soil heating and temperature mapping [refer to: (1)
Berryman, James G., Daily, William D., `Optimal joule heating of
the subsurface, U.S. Pat. No. 5,325,918, (2) Daily, William D.,
Laine, Daren L., Laine, Edwin F., `Methods for Detecting and
Locating Leaks in Containment Facilities using Electrical Potential
Data and Electrical Resistance Tomographic Imaging Techniques`,
U.S. Pat. No. 5,661,406, and (3) Ramirez, Abelardo L.; Dwayne A.;
Daily, William D., `Using Electrical Resistance Tomography to Map
Subsurface Temperatures`, U.S. Pat. No. 5,346,307, the disclosures
of which have been incorporated by reference into this
specification].
[0045] Referring to FIG. 4, an observation/monitoring well 26 is
illustrated, the observation/monitoring well 26 being equipped with
a permanently installed resistivity array 28. The resistivity array
28 is described in the "Babour" reference cited above, the Babour
reference being disclosed in U.S. Pat. No. 5,642,051. The
resistivity array 28 in FIG. 4 includes the `permanently installed
aquifer sensors` mentioned above, such `permanently installed
aquifer sensors` comprising a plurality of `electrically conductive
electrodes` (i.e., a plurality of the `permanently installed
aquifer sensors`). In operation, during the measurement process,
electrical current is injected into the earth using one or more of
the `electrically conductive electrodes` (which comprise the
resistivity array 28 shown in FIG. 4) and electrical voltage
potentials are measured using the other remaining `electrically
conductive electrodes`. A plurality of `measured data` is collected
during the measurement. The set of all such `measured data` then
undergoes computer processing in order to estimate and produce a
`plurality of values` which represent values of electrical
conductivity (or its inverse, resistivity) which exist at several
positions or locations in the earth formation along the length of
the resistivity array 28 in FIG. 4.
[0046] Referring to FIG. 5, refer now to the curve 30 located at
the left side of FIG. 5. The curve 30 represents the aforementioned
`plurality of values` which further represent values of `electrical
conductivity` (or `resistivity`) which exists at the several
locations in the formation along the length of the resistivity
array 28 in FIGS. 4 and 5. The value of `electrical conductivity`
at each position along the resistivity array 28 may then be related
to the earth rock and fluid properties at each location along the
length of the array 28. In particular, the aforementioned
`electrical conductivity` information may be used to distinguish
between: (1) briny, electrically conductive seawater zones, from
(2) potable, electrically more resistive freshwater zones. With the
resistivity array 28 in FIG. 5, the position of the boundary or
contact between the seawater zone and the freshwater zone may be
estimated by determining the vertical position of the transition
between high and low conductivity values and monitored in time
thereby providing: (1) improved knowledge of the state of the
seawater advance toward fresh water aquifers near coastal cities,
and (2) better mitigation of the advance of such seawater into the
potable water (i.e., fresh water) aquifers.
[0047] In FIG. 5, in accordance with another implementation of the
present invention, the resistivity array 28 may be lowered down
into the inside of a dedicated observation/monitoring well 26. The
interface between the seawater and potable water shown in FIG. 5 is
also present on the interior of the `observation/monitoring well`
26. A resistivity array 28 installed on the inside of the
observation well may similarly be used to infer the presence and
position of the seawater wedge from electrical voltage-current
measurements. If configured with a long interval of coverage, a
resistivity array 28 may also be used to detect the presence of (or
the absence of) water at each depth along the array 28, providing
an indictor of hydraulic head (i.e., free water level) in the well
and its variation with time.
[0048] Water Aquifer Electrical Monitoring Electrode cable
System
[0049] Referring to FIGS. 2 and 6, recall again that coastal cities
rely on groundwater from subsurface aquifers to meet all or part of
their municipal water needs. In cases of historical overdraft,
water is withdrawn from the subsurface aquifer(s) at a rate
exceeding the rate of natural aquifer recharge. Such overdraft
results in a lowering of the water table in the aquifer(s), and is
accompanied by possible intrusion of seawater into the aquifer. The
breakthrough of seawater at wells supplying drinking water has
severe long term consequences on municipal potable water
deliverability. Various means exist to mitigate seawater advance.
One of these methods is to inject fresh water into the aquifer
using special-purpose injection wells 32, as illustrated in FIG. 6
(or injection well 14 in FIG. 2). The water injection method
consists of injecting fresh water into the aquifer, forming a
region 34 in FIG. 6 (region 16 in FIG. 2) of high fresh water
pressure around the well, mitigating the landward advance of the
seawater.
[0050] Referring to FIG. 7, in the prior art, in addition to the
injection wells 32 constructed for water injection, a number of
observation/monitoring wells 30 are typically constructed in the
vicinity of the injection wells 32. These observation wells 30 are
used to periodically measure the pressure (hydraulic head) of the
aquifer in the neighborhood of the injectors 32, and for occasional
sampling of water chloride (salinity) levels. In FIG. 7, assume now
that one or more `water sand units` 36 exist along and adjacent to
each observation/monitoring well 30. In FIG. 7, in a typical
municipal (city) setting, when seawater is advancing and moving
into the fresh water aquifers, the city water authorities perform
the following `technique`: (1) lowering a wireline conductivity
cell into the center of an observation/monitoring well 30, and (2)
taking measurements at three (3) depths by sampling the chloride
concentrations at three positions along each `water sand unit` 36
disposed adjacent to each observation well 30. That is, in FIG. 7,
the measurements are taken at a top position 30a, a middle position
30b, and a bottom position 30c. The aforementioned `technique`
provides information about how efficiently the injectors (i.e., the
injection wells 32) are limiting the seawater advance into each
`water sand unit` 36. However, in FIG. 7, in the prior art, there
were no permanent arrays (e.g., a resistivity arrays) disposed
inside or outside the casing of the `observation/monitoring well`
30.
[0051] In the "Seawater Barrier Monitoring" section of this
specification set forth above, in accordance with one aspect of the
present invention, certain types of sensors (such as resistivity
array 28 in FIG. 4) are permanently placed in the ground around or
inside observation/monitoring wells 14 in order to passively and
continuously monitor the status of: (1) the seawater advance into
fresh water aquifers near coastal cities, and (2) fresh drinking
water. Such sensors would be installed in the ground outside the
casing or be appropriately suspended in the interior of the
observation wells 14. The sensor array (resistivity array) 28 would
be electrically connected to surface acquisition equipment that
would, without human intervention, transmit the acquired data to a
centralized facility for processing and interpretation. This would
avoid the process of manual data collection and provide more
frequent and timely data for better control of water injection.
[0052] Referring to FIG. 7a, in accordance with another aspect of
the present invention, an observation well 30 includes a casing,
and a permanent array (e.g., a resistivity array) is disposed
outside the casing of the monitoring/observation well 30. More
particularly, the permanent array comprises a multi-conductor cable
40. The multi-conductor cable 40, which is an example of a
resistivity array, is attached to the exterior of a casing of the
monitoring/observation well 30. The multi-conductor
cable/resistivity array 40 includes a plurality of electrical
conductivity measurement 42a, 42b, and 42c (where each electrical
conductivity measurement 42a, 42b, 42c comprises a subarray of the
full array 28 in FIG. 4; the subarray will be discussed later in
this specification). The cable 40 and the plurality of electrical
conductivity measurements 42a, 42b, and 42c collectively comprise
an example of the resistivity array 28 of FIG. 4 and 5. The
multi-conductor cable/resistivity array 40 in FIG. 7a disposed
along the exterior of the observation well 30 casing would monitor
the subsurface electrical resistivity in the vicinity of the
resistivity array 28 in FIG. 4. Since the electrical resistivity of
the fresh aquifer water is much higher than the resistivity of
saline (salty) seawater, the electrical contrast in resistivity
between seawater and fresh water is large. Therefore, bearing in
mind that a `water sand unit` having a top part 30a, a middle part
30b, and a bottom part 30c in FIG. 7 is disposed adjacent the above
referenced electrical conductivity measurements 42a, 42b, and 42c
of FIG. 7a, the electrical conductivity measurements 42a-42c made
at the top part 42a, the middle part 42b, and the bottom part 42c
of each `water sand unit` should be capable of distinguishing
between saline (salty) seawater and fresh aquifer water at each
level. Further, electrical measurements made (by the resistivity
array 28) along an entire interval located inside an
observation/monitoring well 30 should also be capable of
distinguishing fresh aquifer water (which has a high resistivity)
from air (which has an infinite resistivity) at each level, thereby
providing a continuous remote-reading indication of the free water
level inside the observation well 30 of FIG. 7a.
[0053] Referring to FIG. 9, after the resistivity array cable 40 of
FIG. 7a is installed in the earth along the exterior of the well
casing of observation well 30 or inside the observation well 30,
time-lapse in-situ electrical resistivity measurements are carried
out. Each "electrical conductivity measurement" (that is, each
`subarray`) comprises a `plurality of electrodes`, and that
`plurality of electrodes` further comprises a set of 4 electrodes
called a `quadrupole`. The basic `quadrupole` measurement,
illustrated in FIG. 9, consists of: (1) injecting and withdrawing
electrical current `I` between the outer two electrodes of the
group of four, and measuring the voltage potential `V` between the
two inner electrodes. The resistivity of the earth formation in the
vicinity of the quadrupole is computed from knowledge of `I` and
`V`.
[0054] Referring to FIG. 8, the multi-conductor cable 40 of FIG. 7a
including the electrodes 28 of FIG. 5 is illustrated. When using
the aforementioned technique previously discussed regarding the
installation of electrical resistivity arrays in subsurface
reservoirs, an electrical cable (such as cable 40 of FIG. 7a) is
placed on the exterior of the well casing (of the
observation/monitoring well 30 of FIG. 7a) or tubing to position an
array of resistivity electrodes (such as subarrays 42a, 42b, and
42c of FIG. 7a) in direct contact with the earth. A similar
deployment may be used inside the well. FIG. 8 shows a short
section of the electrical cable 40. It is a multi-conductor cable,
and, at selected points along the cable, `individual conductors` of
the cable are pulled out of a bundle of conductors, and an
`individual conductor` is attached to an `electrode` (such as
electrode 28 in FIG. 5) as shown in FIG. 8. The `electrode` is
formed by manually opening the cable exterior jacket, fishing out
one of the internal conductors, winding a coil of plated wire
around the exterior of the cable, welding the external coil to the
internal conductor, and overmolding the coil ends with rubber to
establish a hydraulic seal. In this approach, an array of "N"
electrodes distributed along the cable requires "N" such manual
interventions.
[0055] Referring to FIG. 10, by deploying a resistivity array (such
as resistivity array 28 in FIG. 5 or subarrays 42a, 42b, and 42c of
FIG. 7a or the resistivity array shown in FIG. 9) along the inside
or outside of a casing of an observation/monitoring well 30
situated in a water aquifer, the electrode quadrupoles (such as the
quadrupoles shown in FIG. 9) may be used to obtain resistivity
measurements at one or more depths in the water bearing sands. As
just described, each quadrupole is capable of a single formation or
water resistivity measurement, and this, in turn, may be used to
discriminate fresh water from saline water along the length of the
well. Thus, in order to obtain three independent samples of
resistivity at the top 30a, middle 30b, and bottom 30c of a `water
sand unit`, for example, as illustrated in FIG. 7, a series of
three electrode quadrupole subarrays 44, 46, and 48 illustrated in
FIG. 10 would be necessary, where each quadrupole subarray includes
4 electrodes and where the three electrode quadrupole subarrays
have a total of 12 electrodes. The fabrication of twelve electrodes
requires twelve manual operations of opening the cable jacket and
manually winding each of the electrode elements. Fabrication of
twelve electrodes will likely raise the total cable array cost to
an unacceptably high level for the water industry, especially if
more than one sand unit is to be monitored in this way.
[0056] Referring to FIGS. 11a and 11b, in accordance with another
aspect of the present invention, in order to reduce the total cost
to fabricate subarrays of quadrupole electrodes on a cable, it is
proposed that each quadrupole subarray of four electrodes, such as
quadrapole subarray 44 or 46 or 48 in FIG. 10, be manufactured as a
single integral assembly, and that each four-electrode quadrupole
subarray be connected into the cable in a single manual operation.
This would allow three quadrupole subarrays (the number necessary
to sample the top 30a, middle 30b, and bottom 30c of a water sand
unit) to be fabricated with only three manual interventions as
opposed to twelve manual interventions, reducing total array
fabrication cost. Each quadrupole subarray assembly 44 or 46 or 48
may be, for example, a distinct unit, as shown in FIG. 11a, made
out of an insulating material like plastic or ceramic with integral
electrodes. Alternatively, each quadrupole subarray assembly 44 or
46 or 48 may be a set of solid plated metal electrodes wrapped
around the cable (of FIG. 8) as shown in FIG. 11b. In the case of
FIG. 11a, the wires attached to the four electrodes comprising each
quadrupole subarray 44 or 46 or 48 enter the cable jacket at one
entry point via a single manual intervention. In the case of FIG.
11b, the four wires attached to the four electrodes enter the cable
jacket at four locations, thereby requiring four manual
interventions.
[0057] Referring to FIGS. 12a, 12b, 12c, and 12d, in accordance
with still another aspect of the present invention, by combining
quadrupole subarrays fabricated in this way (i.e., the quadrupole
subarrays discussed above with reference to FIG. 11b) with single
electrodes along the cable, it would be possible to create a
"Multi-Resolution" electrical array. One example is illustrated in
FIGS. 12a through 12d. As shown in FIG. 12a, the basic quadrupole
is fabricated with an inter-electrode spacing "d". This might be,
for example, in the order of inches for a compact quadrupole
resistivity measurement. Then, as illustrated in FIG. 12b, 4
electrodes at spacing "3d" may be used to make a lower resolution
quadrupole measurement. By virtue of the longer inter-electrode
spacing, this quadrupole measurement would be sensitive to
resistivity variations in a somewhat larger zone around the
quadrupole, as indicated pictorially by the larger diameter circles
in FIG. 12b compared to FIG. 12a. In a similar way, electrodes at
spacings of "6d" and "9d" may be used as shown in FIGS. 12c and 12d
to measure the resistivity in even larger zones around the array,
each at correspondingly lower vertical resolution.
[0058] Referring to FIG. 13, in accordance with still another
aspect of the present invention, in connection with a concept
called "head monitoring/calibration", an additional concept
includes the addition of one or several isolated additional
electrodes in order to monitor the free water level inside an
observation/monitoring well, as illustrated in FIG. 13. Here,
individual electrodes may be used as follows: to determine the
approximate level of `head` in the well by distinguishing whether
each electrode is in a water or air environment. Another way such a
measurement may be used is as follows: to continuously monitor an
electrode as the free water level is varying over a time scale of
days to weeks to months. The moment the water level traverses the
monitored electrode, the free head level is known precisely,
through knowledge of the electrode depth in the well. This
information (precise knowledge of `head` at one time in a well) may
then be used to periodically recalibrate an in-situ permanent
pressure gauge in the same well, correcting the gauge for long-term
drift that is prevalent in low-cost pressure gauges.
[0059] Finally, another implementation is as follows: to use two or
three electrodes 50, 52, and 54, as shown in FIG. 13, as high and
low-level "event detectors". In salt water barrier applications, it
is desired to maintain the `head` or pressure distribution near
some nominal distribution across the aquifer. In any one well, that
corresponds to a "set point" of pressure needed to maintain control
in retarding advance of the sea water wedge. By placing two or
three electrodes in the well at appropriate depths, the electrode
system can function as a continuous alarm system, alerting the user
that the water level has: (1) fallen too low, so that injection is
needed, or (2) has risen too high, so that injection may be
terminated. In this way, a feedback system may be set up to control
water injection and the free water level distribution in the
aquifer.
[0060] The invention being thus described, it will be obvious that
the same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention,
and all such modifications as would be obvious to one skilled in
the art are intended to be included within the scope of the
following claims.
* * * * *