U.S. patent application number 10/963436 was filed with the patent office on 2006-04-13 for well stimulation.
This patent application is currently assigned to MAERSK OLIE OG GAS A/S. Invention is credited to Jens Thorlak.
Application Number | 20060076134 10/963436 |
Document ID | / |
Family ID | 35445872 |
Filed Date | 2006-04-13 |
United States Patent
Application |
20060076134 |
Kind Code |
A1 |
Thorlak; Jens |
April 13, 2006 |
Well stimulation
Abstract
Methods of conducting matrix acidizing in a well in a formation
by simultaneously or alternatingly injecting an acid and a diverter
into the formation through tubing. The acid is injected into the
formation through a first tubing and the diverter is injected into
the formation through a second tubing. The second tubing may extend
within said first tubing and the formation may be a chalk
formation.
Inventors: |
Thorlak; Jens; (Hedehusene,
DK) |
Correspondence
Address: |
LADAS & PARRY
26 WEST 61ST STREET
NEW YORK
NY
10023
US
|
Assignee: |
MAERSK OLIE OG GAS A/S
|
Family ID: |
35445872 |
Appl. No.: |
10/963436 |
Filed: |
October 12, 2004 |
Current U.S.
Class: |
166/282 ;
166/294; 166/307; 166/313; 507/263; 507/267 |
Current CPC
Class: |
E21B 43/26 20130101 |
Class at
Publication: |
166/282 ;
166/307; 166/313; 166/294; 507/263; 507/267 |
International
Class: |
E21B 43/27 20060101
E21B043/27; E21B 33/138 20060101 E21B033/138 |
Claims
1. A method of conducting matrix acidizing in a well in a formation
by simultaneously or alternatingly injecting an acid and a diverter
into the formation through tubing, said acid being injected into
the formation through a first tubing and said diverter being
injected into the formation through a second tubing.
2. A method according to claim 1, wherein before said acid and said
diverter are injected alternatingly through the first and the
second tubings respectively, the formation is pre-flushed by acid
that is injected into the formation through said second tubing.
3. A method of conducting matrix acidizing as defined in claim 1,
wherein said diverter comprises benzoic acid diluted in
methanol.
4. A method of conducting matrix acidizing as defined in claim 1,
wherein said formation is a chalk formation.
5. A method of conducting matrix acidizing in a well in a formation
by simultaneously or alternatingly injecting an acid and a diverter
into the formation through tubing, said acid being injected into
the formation through a first tubing and said diverter being
injected into the formation through a second tubing, said second
tubing extending within said first tubing.
6. A method according to claim 5, wherein before said acid and said
diverter are injected alternatingly through the first and the
second tubings respectively, the formation is pre-flushed by acid
that is injected into the formation through said second tubing.
7. A method of conducting matrix acidizing as defined in claim 5,
wherein said diverter comprises benzoic acid diluted in
methanol.
8. A method of conducting matrix acidizing as defined in claim 4,
wherein said formation is a chalk formation.
Description
[0001] A well for the production of hydrocarbons may be stimulated
by one or more procedures. The purpose of stimulation is to
increase the permeability to the flow of hydrocarbons. These
procedures may increase the permeability to values not only equal
to the original natural permeability prior to drilling. If a
hydrocarbon reservoir has acceptable saturation, but insufficient
permeability for economic development, stimulation procedures may
also increase the permeability to a value justifying the cost for
development of the reservoir.
[0002] Matrix acidizing is a common way to stimulate a well by
pumping acid into the near wellbore region to dissolve formation
damage and create pathways for the hydrocarbons.
[0003] Various acids may be used to Improve production by
dissolving formation damage or creating new pathways around the
borehole depending on the nature of the formation, rock, or the
types of damage which may exist. In acidizing a well, an acid
solution Is pumped under pressure down the well and into the
perforations around the well, chemically removing contaminants as
well as rock structure itself. However, if the pressure exceeds the
fracture closure pressure (FCP), pumping acid into the matrix of
the formation may cause a hydraulic fracture of the formation and
create an undesirable flow path between wells. It Is therefore
essential to keep the pressure and rates of the fluid below
fracture closure pressure (FCP). The FCP is a function of formation
parameters and pressure.
[0004] Low acid pressure injection rates often cause other
problems, because when acid is injected at low rates the most
reactive parts of the formation in a group of perforations or zones
may take up all the acid. With continued reaction at low injection
rates it is even more likely that only these more reactive
perforations or zones will receive acid.
[0005] As acid is pumped it preferably flows along the most
permeable path into the formation. The acid opens up these paths
even more, and the process becomes self-perpetuating; and less
permeable, damaged zones are almost guaranteed not to receive
adequate treatment. Some technique to divert the treatment fluid is
therefore essential. Such technique is called diversion.
[0006] There exist a variety of diversion techniques. Treatment
fluid can be directed exclusively towards a low-permeable zone
using mechanical aids or the flow itself can be blocked at
individual perforations using e.g. ball sealers that seal the
perforation. In order to cover all zones as completely as possible,
a frequent practice is to use diverting materials in combination
with stages of acid.
[0007] The most common materials used to divert the acid are
particulates that are poorly soluble in acid, but soluble in
hydrocarbons for effective cleanup so as not to obstruct the
production.
[0008] Because of both Its solubility in hydrocarbons and its
capability to build up filter cakes within the holes created by the
acid, and thereby forcing the acid to divert, stimulation is often
performed with benzoic acid flakes, emulsified acids, oil soluble
resins, foam and self-viscosifying acids as a diverting
material.
[0009] One method is to use chemical diversion during the matrix
stimulation treatment and this can be accomplished by adding
diversion material in stages at the surface such that the diversion
material is separated by stages of acid and pumping the stages of
acid and diversion material commingled through the tubing without
mixing. These stimulation treatments are normally pumped at high
rates and at high treatment pressures and therefore they are called
High Rate Matrix stimulation methods. The diversion volumes are
large compared to the total treatment volumes and the diverting
materials are physically very coarse. Therefore, the High Rate
Matrix stimulation methods often cause severe operational problems,
such as stuck pipes or tubing, or the FCP is in many cases even
exceeded during the treatment causing the formation to
fracture.
[0010] These problems can be partly avoided by using a coiled
tubing which is basically a second smaller tubing disposed within
the production tubing (first tubing) running from the well to the
surface. Coiled tubing is widely used in the oil and gas industry
for completion, production and work-over operations. Some of the
oilfield operations in which such a secondary tubing is used are
completion operations in which the second tubing can be used to
transport fluids from the surface down into the well. The treatment
fluids in the prior art technique involving coiled tubing are
pumped through the interior of the second tubing and into the well
to perform the particular required operation which can take place
at a predetermined location or in a predetermined depth. The second
tubing may remain suspended in the well for continuous use, and it
may extend from the surface to the bottom of the well. The second
tubing may also extend to an intermediate point between the surface
and the well bottom. The second tubing may possibly also be
temporarily suspended into the well for the duration of a
particular operation during which the tubing may be raised and/or
lowered to various levels in the well. Upon completion of the
operation, the tubing may be retracted so as to be used in another
operation or be transported to another well.
[0011] As will be familiar to those having operating experience
with well stimulation by matrix acidizing, the prior art technique
often poses a potential hazard to the environment and pollution is
often the unsuccessful outcome in situations where the formation is
blocked with diversion materials and the treatment pressure at the
same time approaches or even exceeds the FCP. The tubing is then
filled with a mixture of acid and diversion material, and further
pumping is not possible without unavoidable fracturing of the
formation. The tubing content of acid and diversion material
(possibly solid) then inevitably have to be circulated out back to
surface and discharged to the sea where both the acid and the
diversion material present a high degree of hazard to the
environment.
[0012] It is an object of the invention to provide a method for
diverting acid into all intervals of a formation via a diverting
agent, which method further minimizes chemical consumption and
prevents discharge of acid and diverting material to the sea.
[0013] This is achieved by the present invention as defined by the
claims relating to methods of conducting matrix acidizing in a well
in a formation by simultaneously or alternatingly injecting an acid
and a diverter into the formation through tubing, said acid being
injected into the formation through a first tubing and said
diverter being injected into the formation through a second tubing.
The second tubing may extend within said first tubing and the
formation may be a chalk formation.
[0014] Having acid and diverter in both the first and the second
tubing respectively makes it possible to inject one of these fluids
at the precise moment where the particular treatment fluid is
needed in the matrix acidizing process thus improving the
utilization of well treatment fluids.
[0015] It is a further object of this invention to increase the
pressure during acidizing and thereby to improve permeability of a
the formation and stimulate said formation to produce increased
volumes of hydrocarbons.
[0016] Increasing the pressure at which acid is injected into the
formation improves the matrix stimulation of a well since the
higher pressure forces the acid further into the formation.
Accordingly an even better exploitation of especially the acid can
be accomplished by monitoring of the pressure e.g. at the wellbore
or formation to facilitate an almost instantaneous injection of
acid whenever the pressure is approaching FCP pressure or/and
injection of diverter whenever the pressure decreases to a level
inadequate for the matrix acidizing of the well.
[0017] In a preferred embodiment the formation is pre-flushed by
acid that is injected into the formation through-said second
tubing.
[0018] Preferably, the acid treatment is initiated through the
second tubing, whilst taking fluids back to surface via the first
tubing inlet until the acid reaches the end of the second tubing.
The initial step of Injecting a relatively small amount of acid
into the formation through the second tubing causes the formation
pressure to decrease. This will improve the injectivity into the
formation so the annular volume (between the first and the second
tubing) of non-reactive fluid can be injected to the formation,
without exceeding the FCP. Advantageously the injection occurs by
displacement of the annular volume by well treatment fluid. However
the substitution of said liquid could also, in a less preferred
embodiment, be accomplished by it being displaced through the top
from the bottom end (at the formation) with the aid of the inner
(second) tubing.
[0019] The use of benzoic flakes dissolved in methanol achieves a
highly flowable diverter that can flow into all parts of a
formation and provide good diversion without exhibiting defects due
to segregation in the tubing delivering it.
[0020] The formation being a chalk formation.
[0021] The invention will be described in detail in the following
with reference to the drawing in which
[0022] FIG. 1 shows a sectional view of a well showing the initial
step of displacing fluid in second tubing by acid whilst taking
fluid back to surface through first tubing inlet,
[0023] FIG. 2 shows a well being pre-flushed with acid through the
second tubing,
[0024] FIG. 3 shows a well with diverter in the second tubing,
[0025] FIG. 4 shows a sectional view of a well showing the step of
injecting the annular volume of non-reactive fluid (between first
and second tubing) into formation by displacement with acid,
[0026] FIG. 5 shows a sectional view of well showing displacement
of fluid in the second tubing by diverter,
[0027] FIG. 6 shows a sectional view of a well having diverter
present in the second tubing and acid present in the first tubing
at the formation,
[0028] FIG. 7 shows a sectional view of a well where the acid is
being displaced by a non-reactive fluid,
[0029] FIG. 8 shows a sectional view of a well where the acid is
being displaced by a non reacting fluid, and
[0030] FIG. 9 shows a sectional view of a prior art well with a
single tubing having acid and diverter commingled through the
tubing.
[0031] Referring more particularly to the drawings, FIG. 1
discloses a well having a casing 3 extending throughout the
wellbore, which is usually secured in place by e.g cement. The
casing 3 is perforated 5 adjacent to the production formation 20 to
provide flow passages for fluids from the formation 20 into the
casing 3. A first tubing or production tubing I has a sliding door
6 and a tubing Inlet 12 extends into the casing 3 and has a packer
19 (already known in the prior art) for isolating the production
interval of the well-bore. A second tubing 2 extends down within
the production tubing 1 into the well-bore and has an inlet 11. The
second tubing, which is commonly called coiled tubing, may be wound
around a reel or wheel and Is passed into the well through
an--already known--injector head.
[0032] The second tubing and production tubing further comprises a
system of valves (not shown) having means for individually and
independently regulating and turning off the fluid flow through
first and second tubings at both inlet and welbore end. The second
tubing usually passes through a blowout-preventer (not shown
either) to facilitate regulation of the well-bore pressure. The
coiled tubing usually consists of a singular tubing but may consist
of two or more tubings.
[0033] The outcome of matrix acidizing depends highly on the
pressure at which diverter and acid are injected and the aim of the
current matrix stimulation method is to deploy acid at the highest
pressure possible without exceeding of the pressure at which the
formation breaks down and a fracture is generated. The pressure at
which the formation breaks down is called the fracture closure
pressure (FCP). To facilitate a more precise pressure in the
wellbore, the pressure may be monitored. Monitoring of the wellbore
pressure facilitates injection of acid whenever the pressure is
adequate for acidizing and diverter whenever the pressure drops to
a level inadequate for acidizing thus optimizing utilization of the
stimulation fluids.
[0034] When initiating the stimulation of a well as shown in FIG.
1, the non-reactive liquid, present in the second 2 (coiled) tubing
is displaced by acid (illustrated by being a little darker in FIG.
1). The acid 8, which e.g. can be a 15% HCl, is then pumped in from
the top of the second tubing 2 whilst taking fluid back via the
first tubing 1, thus essentially maintaining formation pressure at
the wellbore.
[0035] The tubing 1 is then closed and the acid treatment is
initiated with a pre-flush of acid (bull-heading) to the formation
via the second tubing as shown in FIG. 2. The purpose of initially
injecting of a pre-flush of acid 8 through the second tubing is to
open up the formation so as to increase its injectivity
sufficiently to absorb the volume of non-reactive fluid present in
the first tubing 1 (production tubing) without exceeding the
fracture propagating pressure of the formation. Field testing shows
that this is advantageously done by injecting approximately 50 bbl
(barrel) of acid. The acid pre-flush is advantageously finished by
displacement of acid in the second tubing by diverter 9 as shown in
FIG. 3.
[0036] Preferably, the main acid treatment is then started by
injection of the (annular) volume of non-reactive liquid present in
the first tubing 1 into the formation by displacement with acid 8
as shown in FIG. 4. The flow is kept at rate such that the pressure
does not exceed the FCP pressure. The acid injection preferably
continues until the pressure starts to decrease. However, a
pressure insufficient for acidizing may at any time during the well
stimulation procedure cause injection of diverter 9.
[0037] When the main treatment acid 8 starts to react with the
formation, FIG. 5 shown by a pressure drop, a batch of diverter is
pumped into the main treatment fluid and further to the formation
via the second tubing (coiled tubing), as shown in FIG. 6. The
diverter will follow the main stream, which is also the path of
least restriction, into the formation and preferably block off the
coming acid accession to recently stimulated areas in the
formation.
[0038] When the path into the formation is blocked by diverter 9,
the treatment pressure will increase thus indicating that acid 8
would be diverted to other parts of the formation. When the
pressure has increased to a level adequate for acid treatment but
preferably still below FCP, acid 8 is injected via the first
tubing. This treatment is then continued until pressure has again
dropped to a level sufficiently below FCP-pressure. The main
treatment acid 8 is displaced to perforations, as shown in FIG. 7
and 8.
[0039] The worst operational scenario during prior art methods of
matrix stimulation with diverter 9 is illustrated in FIG. 9. The
FCP is exceeded and further pumping into the formation will
invariably create fracture of the formation. Further, the string
content has to be circulated back to surface and disposed off.
[0040] The acid utilized may be any of the aqueous solutions of
acid commonly employed for acidizing formations. The solution of
acid may be an aqueous solution of hydrochloric acid and
hydroflouric acid, which is employed for acidizing formations.
Certain organic acids can be utilized alone or in combination with
inorganic acids. Organic acids Include formic, acetic and oxalic
acids. When hydrochloric acid Is utilized, it should be in a
concentration from about 5 to about 30 percent. When combined with
hydrochloric acid, the concentration of hydroflouric acid will
range from about 0.3 to about 8 percent.
[0041] Although the method has been described as comprising
diverter in the second tubing and acid in the first tubing it is
clear that the opposite condition may as well be the case without
departing from the scope of the invention.
* * * * *