Method for sealing perforation tunnels with swelling elastomer material

A method for sealing perforation tunnels in a well includes steps of pumping an initial volume of fluid into the well, mixing expandable material into a carrier fluid to form an expandable fluid mixture, pumping the expandable fluid mixture into the well to force the expandable material into the perforation tunnels in the well, and holding the expandable material under a hold pressure in the perforation tunnels for a cure period to form perforation plugs.

FIELD OF THE INVENTION

The present invention generally relates to the field of the production of oil and gas from subterranean reservoirs and more specifically, but not by way of limitation, to a method for sealing perforations in a wellbore.

BACKGROUND OF THE INVENTION

During the completion stage of an oil well drilling process, perforations are made in the casing or liner of the well to place the interior of the wellbore in communication with the surrounding geologic formation. The perforations may be made through a variety of methods including using shape charges and perforation guns. Once the casing has been perforated in the desired locations, the remaining steps in well completion process can take place. In certain situations, this may involve a hydraulic fracturing process (“fracking”) that will improve the permeability of the formation and increase the rate of recovering hydrocarbons through the well.

In some formations, the production of hydrocarbons from the well quickly decreases following the initial completion. In those wells, the producer may choose to close the well if the decreased production is not economically viable. However, due to the significant costs associated with drilling and completing new wells, there is renewed interest in increasing the production from older wells by conducting a subsequent perforation and hydraulic fracturing operation on a new zone within the producing formation.

To perform a subsequent hydraulic fracturing operation, sometimes referred to as a “refrac,” the open perforations must be sealed to prevent the high pressure frac fluid from entering the older perforations. In the past, operators have used bridge plugs to isolate zones within the wellbore during the hydraulic fracturing operation. Although widely accepted, the bridge plugs are expensive and are often removed following the hydraulic fracturing operation. This adds cost and complexity to the process of recompleting a well. In other situations, operators have pressed cement into each perforation. This is also expensive and difficult to control. In yet other situations, operators have pumped thixotropic gels into the perforations to temporarily block fluid from entering those zones. The use of thixotropic gels is expensive and may not provide a suitable solution under elevated fracturing pressures.

In light of the deficiencies in the prior art, there remains a need for an improved system and method for preparing a well for sealing perforations in a wellbore in preparation for a subsequent hydraulic fracturing operation.

SUMMARY OF THE INVENTION

In one embodiment, the present invention includes a method for sealing perforation tunnels in a well includes steps of pumping an initial volume of fluid into the well, mixing expandable material into a carrier fluid to form an expandable fluid mixture, pumping the expandable fluid mixture into the well to force the expandable material into the perforation tunnels in the well, and holding the expandable material under a hold pressure in the perforation tunnels for a cure period to form perforation plugs.

In another embodiment, a method for sealing perforation tunnels in a well includes the steps pumping an initial volume of fluid into the well, mixing expandable material into a carrier fluid to form an expandable fluid mixture, pumping the expandable fluid mixture into the well to force the expandable material into the perforation tunnels in the well, holding the expandable material under a hold pressure in the perforation tunnels for a cure period to form perforation plugs, and performing a static pressure test after the step of holding the expandable material under a hold pressure.

In yet another embodiment, the present invention includes a method for sealing perforation tunnels in a well comprising the steps of pumping an initial volume of fluid into the well, and mixing expandable material into a carrier fluid to form an expandable fluid mixture, wherein the step of mixing expandable material into a carrier fluid further comprises mixing a combination of hydrophilic granules and swellable fibers into the water-based carrier fluid. The method continues with the step of pumping the expandable fluid mixture into the well to force the expandable material into the perforation tunnels in the well, and holding the expandable material under a hold pressure in the perforation tunnels for a cure period to form perforation plugs.

WRITTEN DESCRIPTION

Referring first toFIG. 1, shown therein is a cross-sectional depiction of a well100that has been drilled in a geologic formation102that contains hydrocarbons. As used in this disclosure, the term “hydrocarbons” will include all liquid and gaseous petroleum products. The well100includes a wellbore104and a casing106. The well100includes cement108around the exterior of the casing106. Although the well100is shown as entirely cased and cemented, it will be appreciated that the well100may also be an “open-hole” or only partially cased and cemented. Additionally, the well100inFIG. 1is depicted as a horizontal well that includes a vertical section that leads to a lateral section. It will be further appreciated that the system and methods disclosed in this application will find utility in wells that are vertical, deviated or that take other profiles.

As depicted inFIG. 1, the well100has been perforated and hydraulically fractured. Perforations110have been punched through the casing106at desired locations using known perforating techniques. Perforation tunnels112extend from the perforations110through the cement108and into the formation102. Fractures114resulting from the application of high pressure hydraulic fracturing fluid extend into the formation102from the perforation tunnels112. The production of perforations110, perforation tunnels112and hydraulic fractures114is well known in the art and the systems and methods disclosed in this application are not limited to the form or configuration of the perforations110, perforation tunnels112and hydraulic fracturing114depicted inFIG. 1.

The well100further includes a wellhead116disposed on top of the casing106. A perforation sealing system118is assembled on the surface and connected to the wellhead106. The perforation sealing system118includes a high pressure pump120, a blender122, a fluid source124and an expandable material hopper126. The pump120may be a conventional triplex pump that is commonly used in hydraulic fracturing operations. The fluid source124is used to supply water-based or oil-based carrier fluids to the blender122. The blender122combines the carrier fluid with the expandable material128from the expandable material hopper126. The blended carrier fluid and expandable material is then provided to the pump120, where it can be pressurized and delivered into the casing106through the wellhead116. Other configurations of the perforation sealing system118are contemplated as falling within the scope of exemplary embodiments. The perforation sealing system118is well suited for delivering a slurry or mixture of carrier fluid and expandable material128into the well100.

The expandable material128is configured to absorb fluid and swell when exposed to the carrier fluid or fluids in the wellbore. In some embodiments, the expandable material128includes small hydrophilic granules130that swell when exposed to water-based carrier fluids. The granules130may range from about 1 to about 2,000 microns in diameter. In other embodiments, the granules130are configured to swell in the presence of hydrocarbons or acidic fluids. In other embodiments, the expandable material128includes a combination of granules130and swellable fibers132. In exemplary embodiments, the expandable material128swells to many times its original size under well-established, predictable rates of expansion.

The expandable material128may be manufactured from suitable compounds, including those listed in the table below:

Additional swellable materials suitable for use in connection with the present invention are disclosed in U.S. Pat. Nos. 3,385,367, 7,059,415, 8,051,914 and 9,464,500, the entire disclosures of which are herein incorporated by reference. It will be generally appreciated that any swellable material that swells when placed in contact with any type of fluid may find utility within the scope of exemplary embodiments of the present invention. In certain applications, it may be desirable to manufacture the expandable material128such that it degrades in the presence of an acid. This permits the removal of the expandable material128through an acidizing operation.

Turning toFIGS. 2 and 3, shown therein are cross-sectional, close-up views of a portion of the well100. These figures illustrate the plugging effect achieved by placing the expandable material128into the perforations110, perforation tunnels112and fractures114. As shown inFIG. 2, the perforation sealing system118is used to push the mixture of carrier fluid and expandable material into the casing106, where the mixture enters the formation102through the perforation tunnels112and fractures114. As the mixture enters the perforation tunnels112and fractures114, the expandable material128becomes trapped and accumulates in these relatively small voids. As shown inFIG. 4, the expandable material128then absorbs the surrounding carrier fluid or fluids in the well100and expands to fill the voids within the perforation tunnels112and larger fractures114. The expandable material128forms a substantially impenetrable plug134that prevents the flow of fluid through the perforations110and perforation tunnels112. As used herein, the term “mixture” includes, but is not limited to, slurries of expandable material in carrier fluid.

Turning toFIG. 4, shown therein is a process flow diagram for a perforation sealing method200that provides for emplacing the expandable material128into the perforation tunnels112to seal the perforations110. The method200begins at step202as the perforation sealing system118is “rigged-up” on the well100. Service crews arrive to the wellsite of a previously producing oil and gas well100that contains existing perforations in the wellbore104. The crews rig up and prepare the necessary equipment, which may include some combination of the pump120, the blender122, the fluid source124, and the expandable material hopper126. High pressure pumping iron places the pump120in fluid communication with the wellhead116. The expandable material128can be blended into the carrier fluid with the blender122or added using an injector or eductor into the high pressure fluid discharged from the pump120.

At step204, crews begin pumping the water or hydrocarbon-based carrier fluid into the well casing106to establish an injection rate and pressure. This is a commonly referred to process of “bullheading.” A recommended pump rate of 4 to 8 barrels per minute (BPM) may be necessary for casing sizes ranging from 4.500″ to 9.625″ diameter. The step of bullheading the well100will continue until a desired volume of fluid has been pumped into the well100. In some embodiments, the amount of fluid pumped into the well may range from about half to about twice the total casing volume.

Once a sufficient injection rate and pressure has been established, the expandable material128is added at step206to the output from the perforation sealing system118. The expandable material may be added to the carrier fluid in ratios ranging from about 0.25 to about 1.0 pounds of expandable material per gallon of carrier fluid. Each perforation110may take between about 0.25 pounds and about 0.50 pounds of expandable material128before the perforation110is sealed. For example, if the well100has 100 perforations in the 5.500″ diameter casing106, the well100will need a minimum quantity of about 25 to about 50 pounds of expandable material128to occupy and seal all of the 100 perforation tunnels112.

Next, at step208, the perforation sealing system118continues to pump the slurry of carrier fluid and expandable material128into the well100while the pressures in the well100are closely monitored. As each perforation tunnel112begins to fill up with the expandable material128, there will be slight increase in the pumping pressure. The pumping pressures will continue to rise as additional perforation tunnels112become occupied with the expandable material128.

At step210, the operator determines if the desired maximum pumping pressure has been reached. The maximum pressure may be indicated by the operator or when the rate of increase in the pumping pressure decreases to a threshold value. Once all the perforation tunnels112have been packed off with the expandable material128fluid losses through the well100will decrease and the pump rate will gradually be reduced at step212to a “hold” pressure that is maintained in the wellbore104. A trapped hold pressure of between about 1,000 to about 3,000 pounds-per-square inch (psi) is recommended. As the expandable material128begins to swell and occupy the perforation tunnels112, the pressures in the well100will begin to stabilize as fluid losses continue to decline. In exemplary embodiments, the pump120remains connected to the well100to maintain the hold pressure while this swelling process takes place.

It will be noted that the expandable material128is at first differentially held in place by applying positive pump pressure to the material. The hold pressure is held for a desired cure time at step214. In some cases, a suitable pressurized cure time is between about 8 and about 12 hours. Once the expandable material has fully expanded, the hold pressure can be bled off the well100. At step216, the pump pressure is removed following the pressurized cure time and the perforation sealing system118can be disconnected from the wellhead116. During this time, the well100is kept under pressure, but the pump120is no longer needed to maintain the pressure in the well100.

Once the pump pressure has been removed, a static pressure test can optionally be performed on the well100at step218. In exemplary embodiments, the static pressure test is run for from about 50 to about 100 hours, with a preferred test time of about 72 hours to allow the expandable material sufficient time to swell out and fully seal off the occupied perforation tunnels112. During the static pressure test, the expandable material128fully expands and forms the plugs134to seal the perforations110and perforation tunnels112. The perforation plugs134will create an effective seal against the formation fracture matrix as well as seal off any existing fractures114that have been created in the cement108behind the production casing106.

Following the static pressure test, the operator can slowly bleed off the trapped static pressure within the well100. The well100is now in a state to allow the producer to re-perforate and stimulate this well100. The target of the new perforations will be the space and distance between the existing perforations110that have been effectively plugged off with the expandable material128.

Although the inventive systems and methods are well suited for preparing the well100for a subsequent hydraulic fracturing operation, it will be appreciated that the expandable material128and method200will find utility in other applications. For example, the embodiments disclosed herein are also helpful in limiting the migration of hydraulic fracturing fluid along the outside of the casing106caused by compromised cement108around the casing106. By sealing the perforation tunnels112, hydraulic fracturing fluid is prevented from migrating through cracks or fissures in the cement108.

In other embodiments, the expandable material128is pumped into the well100in preparation for abandoning the well100. In many wells designated for “plugging” or abandonment, there is insufficient bottom-hole pressure to allow circulating fluids and cement to plug the well. The expandable material128can be pushed into the well100using the perforation sealing method200to provide a cost effective method of plugging the well100for abandonment.

In yet other embodiments, the expandable material128can be used to provide zonal control for wells in which the casing106has been parted. In a modification to the perforation sealing method200, the expandable material128can be pumped behind the parted casing to create an effective barrier for well control and zonal isolation purposes.