Method For Cleaning Well Perforations

Abstract

Sand control operations in cased and perforated wells completed into unconsolidated formations are improved by first cleaning the perforations by packing off the perforated interval of the well, rapidly opening the packed-off interval of the well into fluid communication with a low-pressure chamber to surge fluid from surrounding earth formations through the perforations and into the well, allowing debris carried by the surging fluid to settle, and circulating debris from the well.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of completing wells, and more particularly, to a method for cleaning perforations in a cased and perforated well.

2. Description of the Prior Art

In completing wells, such as oil wells, it is common practice to drill the well bore hole into an oil containing formation, cement a tubular casing throughout the length of the bore hole, and thereafter perforate the casing and cement adjacent the oil containing formation to open the formation into fluid communication with the interior of the casing. If the oil containing formation behind the perforated casing is unconsolidated, it may be necessary to provide means for excluding sand from the well bore before the well can be put on production.

It has been found that the presence of unconsolidated sand even in small quantities can seriously effect the operation of both the producing and injection wells. Besides high rate of wear on subsurface and surface equipment due to sand abrasion, partial or complete plugging of a well can occur. If no sand control means is provided, expensive well cleanout is frequently required. In extreme instances the cleanout may not be possible and the well must be abandoned.

Two widely used sand exclusion methods are the installation of gravel or sand packs in the well bore adjacent the production interval and the consolidation of the formation sand in and/or around the well bore. Consolidation may, for example, be achieved by injecting plastics or resins into the formation or by metal plating the formation. Such consolidation techniques are described in U.S. Pat. Nos. 3,294,166 and 3,393,737.

In some oil producing regions where unconsolidated sands are common and where the above-mentioned sand control methods are frequently used, a high incidence of sand control failure with associated casing damage has been observed. An important factor causing this failure is injectivity impairment into the unconsolidated sands adjacent the perforated interval of the well bore. It is believed that injectivity impairment may be caused by a number of factors. Among these are pulvarized material force from the cement sheath, pulvarized and compacted formation material, and perforation change debris. When using sand consolidating techniques to control sand, the impairment can prevent injection of consolidating fluids into the formation through some of the perforations. This prevents complete consolidation of the interval adjacent the well bore. Additionally, treatment of impaired perforations can cause the impairment to become "locked in place" by the consolidating material.

It has also been found that permeability impairment in unconsolidated sands can inhibit deposition of gravel in perforation tunnels where sand control is by gravel packing. Tunnels are best filled with gravel if fluid flows out of the wellbore through the perforation tunnels thus carrying packing material into these tunnels. The absence of gravel in perforation tunnels allows fine formation sand to fill in the tunnels when the well is placed on production. This fine formation sand severely restricts fluid flow, and thereby decreases well productivity.

A number of techniques have been proposed for improving injectivity into the unconsolidated interval prior to the installation of said control means. Among these techniques are acid stimulation and fracturing. It has been found that each of these methods has the disadvantage that in many cases acid or fracturing fluid moves into the formation through only some of the perforations. Since all perforations are not opened, uniform placement of said control means throughout the interval is not assured.

It is known that flow impairing materials can be removed from perforations which extend into consolidated formations by perforating the well with the pressure in the unperforated well casing lower than the formation pressure. This causes formations fluids to surge into the casing carrying debris from the formation. The method is referred to as "underbalanced perforating". A second technique which has been successfully used in consolidated formations is to backflow material into the well bore after the well is perforated by reducing well bore pressure. This can be done by swabbing fluid from the well bore to reduce the fluid column in the well.

Unfortunately, these techniques have often not been successful in unconsolidated formations which are pressure depleted (i.e. formations in which the pressure of reservoir fluids is significantly less than hydrostatic pressure). A major disadvantage of these methods is that it is very difficult to control the amount of fluid which flows into the well bore. If the well bore fluid column is drawn down too much, then an excessively large volume of fluid may surge into the well carrying a large volume of formation sand. When this occurs, it is necessary to remove the sand before sand control means are installed. This, of course, results in additional expenditures of time and increases the cost of sand control installation. Additionally, if an excessively large volume of fluid flows into the well bore, formation sand around the entire casing can be disturbed. This may result in impairment of formation permeability adjacent the well, thus reducing well productivity.

SUMMARY OF THE INVENTION

We have now found that well perforations in a well cased with a tubular casing which is substantially perforated to open the well into fluid communication with an unconsolidated, fluid containing sub-surface earth formation can be cleared of perforation debris without sanding up the well bore and without creating permeability impairment of the adjacent formation by disturbing sands adjacent well bore by a method which comprises the steps of packing off the perforated interval of the well with at least one packer; positioning adjacent the packed off interval a chamber containing a gas at a pressure substantially lower than the formation pressure, the chamber having selected volume of at least about one-fourth gallon per perforation in the perforated interval of the well but less than 1 gallon per perforation; and rapidly opening the chamber into fluid communication with the packed-off perforated interval whereby fluid surges from the fluid containing formation through the perforations and into the well carrying debris from the perforations into the well. The debris may be permitted to settle to the bottom of the well or, advantageously, be removed by unseating the packer to open the packed-off interval to fluid communication with the surface through the casing and circulating a fluid from the surface to the bottom of the well to remove debris from the well. The method is advantageously used prior to the installation of sand exclusion means.

In a preferred embodiment the chamber of lower pressure is provided by disposing upper and lower valve means in a string of tubular pipe. A chamber-portion is formed within the string of pipe between the valve means when they are closed. The chamber is operatively positioned by extending the string of pipe into the well. Opening the lower valve opens the chamber into fluid communication with the packed-off interval.

BRIEF DESCRIPTION OF THE DRAWING

The drawing shows a cross-sectional view of a sub-surface earth formation traversed by a well suitably equipped for the practice of this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to the FIGURE we see an hydrocarbon bearing, unconsolidated sub-surface earth formation 10 traversed by bore hole of a well 11 in which tubular casing 12 has been cemented with cement 13. The casing 12 and cement 13 adjacent the formation 10 have been perforated by conventional means, such as jet or gun perforating, with a number of perforations 14.

To insure uniform placement of sand exclusion means such as a gravel pack or in situ consolidating materials (for example resin or metal plating solutions) according to the method of this invention, a string of tubing 15 carrying a packer 16 is run into the well 11. The packer 16 is set above the perforating interval 17 of the casing 12 to pack off the annular space 22 between the casing 12 and the tubing 15. In the drawing, the formation 10 is the lowest productive interval in the well 11. It should be understood that this method may be used in treating upper zones in wells which traverse more than one productive interval. When upper zones are treated, lower zones should be isolated by any suitable conventional means such as the positioning of a bridge plug in the casing 12 below the perforated interval 17 adjacent the formation 10 to be treated.

The tubing 15 carries a sealed low pressure chamber 18 containing a gas at a pressure substantially lower than the fluid pressure in the formation 10. In practice the gas in the chamber 18 is advantageously air at substantially atmospheric pressure since this eliminates the need for special equipment to prepare the chamber. However, other gases and pressures may be used with equal effect. It is only necessary that the pressure in the chamber 18 be sufficiently lower than the pressure in the formation 10 to allow an adequate volume of fluid to flow into the chamber 18. In practice operators may prefer that the pressure difference between the fluids in the formation 10 and the low pressure chamber 18 be the maximum available to insure creation of an initial surge of fluid through the perforations 14 at a rate sufficient to dislodge debris tightly packed in the perforations 14 during the perforating process. However, we have found that initial pressure differences in the range of 1,200 psi to 4,000 psi have worked equally well.

The volume of the chamber 18 for best results is at least about one-fourth gallon per perforation in the perforated interval 17 and less than about 1 gallon per perforation. We have found that for chamber volumes less than about one-fourth gallon per perforation, productivity through subsequently placed sand control installations is not greatly improved. For volumes greater than about 1 gallon per perforation, the well tends to become filled with sand washed through the perforations 14, and further productivity improvement is not achieved.

The chamber 18 is connected with the interior of the tubing 15 above and below the chamber 18 by valve means 19 and 20 of a type which may be opened from the surface. For example, the lower valve 20 may be a full opening shock valve which is opened by means such as applying tubing weight or by pressurizing the annular space between the casing 12 and tubing 15. Similarly, the upper valve 19 may be of a type opened by dropping a bar, application of additional tubing weight, or pressurizing the tubing 15. To clean the perforations 14 the lower valve 20 is rapidly opened after the packer 16 is set. This allows a controlled volume of fluids from the formation 10 to surge through the perforations 14 into the well 11 as fluid in the well moves into the low pressure chamber 18. The fluid flowing through the perforations 14 carries debris such as pulvarized formation material into the well 11.

In some cases, the debris merely can be permitted to settle to the bottom of the well 11 prior to the installation of sand control means. This is achieved by waiting a sufficient time, usually about 1 hour, for settling to take place prior to disturbing fluids in the well 11.

However, in many cases it is advantageous to remove the debris from the well 11. This insures that the debris is not carried back into the perforations 14 as fluid, for example in situ consolidation fluid, is injected into the formation. To remove the debris from the well 11, the packer 16 is released, and the upper valve 19 is opened. Cleaning fluid such as salt water, a liquid hydrocarbon, or other suitable liquid can then be circulated from the surface down the tubing 15 and up the annular space between the tubing 15 and casing 12 to carry debris away from the perforated interval 17. Sand control means is then installed in a conventional manner.

In some cases best results may be obtained by allowing debris to settle in the well 11 for 15 minutes or more before circulating cleaning fluid. Advantageously, sufficient tail pipe 21 is run below the packer to enable lowering the tubing string 15 to the bottom of the well 11 when circulating debris out of the well 11 without running the packer 16 through the perforated interval. This avoids washing additional sand into the well bore as the packer passes up through the perforations 14 after washing to bottom.

It has been found that in some cases when the pressure in the formation 10 is depleted, the circulating cleaning fluid may flow in large amounts back through the perforations 14 and into the formation 10 carrying debris into the perforations 14. Therefore it is advantageous in such cases to inject a temporary plugging material into the perforations 14 prior to circulating debris out of the well. A suitable material is an aqueous suspension of about 25 pounds per barrel of a mixture of lignolsulfonate and 22-40 micron calcium carbonate particles. The suspended material screens out on the formation to plug the perforations. After debris has been circulated from the well, the material is dissolved by pumping an acid, such as hydrochloric acid, down the tubing 16.

The method of this invention is most advantageously practiced by using one or more sections or joints of the tubing 15 and full opening valves 19 and 20 to form the low pressure chamber 18. This provides flexibility in choosing chamber size without requiring equipment additional to that normally used in sand consolidation other than the two valves 19 and 20. For example, if the length of the perforated interval 17 is 60 feet and if the casing 12 is provided with four perforations per foot, then if the tubing 15 is 27/8 inch O.D. tubing (which has a capacity of about 20 gallons per hundred feed of length) the low pressure chamber 18 can comprise 300 to 1,200 feet of tubing (i.e., a capacity between about 60 and 240 gallons which is one-fourth to 1 gallon per perforation) between an upper valve 19 and a lower valve 20. If other chamber volumes are desired, it is only necessary to change the length of the tubing section.

The use of full opening valves 19 and 20 allows fluid to be circulated through the tubing string 21 after the perforations have been cleaned without obstruction from the back surge apparatus. Therefore, if the sand control means to be installed around the casing 12 is installed by injecting fluids into the formation, as in resin or metal plating type consolidation, sand control means placement can continue without withdrawing the tubing string 15 from the well 11. After debris carried into the well 11 has been circulated out of the well, or permitted to settle, the necessary resin or metal plating fluids may be injected down the tubing string 15 and into the formation 17. In most cases, it will be advantageous to reset the packer 16 before injecting these materials. In some cases, it may be advantageous to leave the valve 19 and 20 in the well as a permanent part of the tuning string 15 which is then used as a production tubing string. Thus perforations 14 may be cleaned, the formation 10 consolidated in situ, and the well 11 put on production without withdrawing the tubing 15 from the well.

Claims

I claim as my invention:

1. In a method of completing a well wherein the well bore hole is cased with a tubular casing that is subsequently perforated with a number of perforations to open the bore hole into fluid communication with a fluid-containing subsurface earth formation, the improvement which comprises cleaning the perforations to remove debris therefrom by:

packing off a perforated interval of the casing with at least one packer;

positioning adjacent the packed-off interval a chamber containing a gas at a pressure substantially lower than the formation pressure, the chamber having a selected volume of at least about one-fourth gallon times the number of perforations in the packed-off perforated interval of the well;

rapidly opening the chamber into fluid communication with the packed-off perforated interval whereby fluid surges from the fluid-containing formation through the perforations and into the casing carrying debris from the perforations into the well; and

permitting debris to settle in the well.

2. The method of claim 1 wherein the step of positioning a chamber adjacent the packed-off interval comprises the steps of:

disposing first and second valve means at longitudinally displaced points in a tubular string of pipe to form a chamber portion within the string of pipe between the two valve means; and

extending the string of pipe, including the chamber portion, into the well.

3. The method of claim 1 wherein the chamber positioned adjacent the packed-off interval contains air at substantially atmospheric pressure.

4. In a method of completing a well wherein the well bore hole is cased with a tubular casing that is subsequently perforated with a number of perforations to open the borehole into fluid communication with a fluid-containing subsurface earth formation, the improvement which comprises cleaning the perforations to remove debris therefrom by:

packing off a perforated interval of the casing with at least one packer;

positioning adjacent the packed-off interval a chamber containing a gas at a pressure substantially lower than the formation pressure, the chamber having a selected volume of at least about one-fourth gallon times the number of perforations in the packed-off perforated interval of the well;

rapidly opening the chamber into fluid communication with the packed-off perforated interval whereby a controlled volume of fluid substantially equal to the selected volume of the chamber surges from the fluid-containing formation through the perforations and into the casing carrying debris from the perforations into the well;

unseating the packer to open the packed-off interval to fluid communication with the surface through the casing; and

circulating a fluid from the surface into the well to remove debris from the well.

5. The method of claim 4 wherein the step of positioning a chamber adjacent the packed-off interval comprises the steps of:

disposing first and second valve means at longitudinally displaced points in a tubular string of pipe to form a chamber portion within the string of pipe between the two valve means; and

extending the string of pipe, including the chamber portion, into the well.

6. The method of claim 4 including the step of allowing debris carried by the surging fluid into the packed-off interval to settle for at least 15 minutes before circulating fluid to remove debris from the well.

7. In a well having a cemented casing perforated with a plurality of perforations which open the interior of the casing into fluid communication with an unconsolidated sand formation containing a fluid at a pressure, a method of excluding sand from the casing without locking perforation-plugging debris in the perforations comprising the steps of:

installing upper and lower valve means in a string of tubular pipe to form a closed chamber between the valve means having a volume of at least about one-fourth gallon times the number of perforations in the plurality of perforations but less than 1 gallon times that number;

lowering the string of tubular pipe into the well through the casing with the pressure in the closed chamber substantially less than the pressure in the sand formation;

packing-off the annular space between the tubular pipe and the casing above the perforations by setting a packer;

opening the lower valve to connect the chamber in fluid communication with the perforated portion of the casing whereby fluid surges from the formation, through the perforations and into the casing carrying debris into the casing; and

installing sand exclusion means to exclude sand from the casing.

8. The method of claim 7 including the steps of after opening the lower valve:

releasing the packer to open the well to fluid communication between the earth surface and the perforated portion of the casing through the annular space between the tubular pipe and the casing;

opening the upper valve; and

circulating a fluid from the surface through the tubular pipe and annular space between the tubular pipe and the casing to remove debris from the casing.

9. The method of claim 8 including the steps of:

temporarily plugging the perforations by injecting an acid soluble plugging material into the perforations before circulating the debris-removing fluid; and

injecting an acid into the perforations after circulating debris from the casing to open the temporarily plugged perforations.

10. The method of claim 7 wherein the sand exclusion means is a gravel pack.

11. The method of claim 7 wherein the sand exclusion means is a resin coating applied to sand adjacent the casing.

12. The method of claim 7 wherein the sand exclusion means is a metal coating applied to sand adjacent the casing by electroless metal plating.

13. The method of claim 7 wherein the sand exclusion means is installed without withdrawing the tubular string of pipe from the well and including the step of

producing fluid from the formation through the tubular string of pipe including the open upper and lower valves.