Leak-off control in acid stimulation using dissolvable material

Abstract

Reducing leak-off during acid stimulation with dissolvable material sized to preferentially block crevices and wormholes, thus allowing the acid to travel further down the fractures, etching deeper into the reservoir than a similar process not using such dissolvable materials. After stimulation, the materials dissolve and production can proceed and will be improved over what is possible without such dissolvable materials.

Claims

1. A method of acid fracturing a carbonate reservoir, said method comprising: a) pumping a viscous pad fluid into a carbonate reservoir at a pressure exceeding a fracture pressure of said carbonate reservoir to fracture said carbonate reservoir, fractures being of a width ≤D; b) pumping a slurry containing a gel plus a water-dissolvable fiber and/or water-dissolvable particulate material of a size ≥1.2D into said carbonate reservoir at a pressure exceeding said fracture pressure, said slurry preferentially restricting entry of a fluid into crevices that are larger than D, but not said fractures; c) pumping an acid downhole at a pressure exceeding said fracture pressure to preferentially etch said fractures; and d) repeating steps b) and c) as needed; e) wherein said carbonate reservoir has higher conductivity than a similar reservoir similarly fractured, but without said slurry.

2. The method of claim 1, wherein said acid is a slick acid comprising a friction reducer plus an acid.

3. The method of claim 1, wherein said viscous pad fluid and said slurry are pumped into said carbonate reservoir at the same time.

4. The method of claim 1, wherein said viscous pad fluid is pumped into said carbonate reservoir before said slurry.

5. The method of claim 1, wherein said slurry is pumped into said carbonate reservoir at the same time as said acid.

6. The method of claim 1, wherein i) increasing concentrations of dissolvable fiber or particulate material in said slurry are used in each subsequent repetition, or ii) increasing volumes of said slurry are used in each subsequent repetition, or both i) and ii).

7. The method of claim 6, wherein said water-dissolvable fiber and/or said water-dissolvable particulate material comprises polylactic acid (PLA) or poly-glycolic acid (PGA) or derivatives thereof.

8. The method of claim 6, wherein said water-dissolvable fiber and/or said water-dissolvable particulate material comprises polylactic acid (PLA) or poly-glycolic acid (PGA) or derivatives thereof in the form of fibers or particles of greater than 20 mesh and less than 100 mesh.

9. The method of claim 8, wherein said water-dissolvable fiber and/or said water-dissolvable particulate material is used at about 25 to 75 lbm/Mgal or at about 30 to 40 lbm/Mgal.

10. A method of producing hydrocarbon from a carbonate reservoir, said method comprising: a) pumping a viscous pad fluid downhole into a carbonate reservoir at a pressure exceeding a fracture pressure of said carbonate reservoir to produce fractures; b) pumping a slurry of gel plus a dissolvable fiber or a dissolvable particulate material downhole at a pressure exceeding said fracture pressure, said slurry preferentially blocking crevices that are larger than said fractures, wherein said dissolvable fiber or said dissolvable particulate material will dissolve in water under reservoir conditions in less than 48 hours; c) pumping an acid plus a friction reducer downhole at a pressure exceeding said fracture pressure to preferentially etch said fractures, wherein said acid penetrates deeper into said carbonate reservoir than it would in a similar method without said slurry; and d) repeating one or more of these steps as needed to acid fracture said carbonate reservoir; e) dissolving said dissolvable fiber and said dissolvable particulate material and flushing said carbonate reservoir; and f) producing hydrocarbon from said carbonate reservoir.

11. The method of claim 10, further comprising a step of pumping an acid plus a friction reducer downhole at a pressure exceeding said fracture pressure before step b).

12. The method of claim 10, wherein step a) and b) are partially at the same time.

13. The method of claim 10, wherein step a) and b) are at the same time.

14. The method of claim 10, wherein step b) and c) are partially at the same time.

15. The method of claim 10, wherein step b) and c) are at the same time.

16. The method of claim 10, further comprising a step of pumping a cooling fluid downhole before step a) to reduce a temperature of said carbonate reservoir.

17. The method of claim 16, wherein increasing concentrations of dissolvable fiber or particulate material in said slurry are used in each subsequent repetition.

18. The method of claim 16, wherein said dissolvable fiber and said dissolvable particulate material comprises polylactic acid (PLA) or poly-glycolic acid (PGA) or derivatives thereof.

19. The method of claim 16, wherein said dissolvable fiber or dissolvable particulate material comprises polylactic acid (PLA) or poly-glycolic acid (PGA) or derivatives thereof in the form of fibers or particles of greater than 20 mesh and less than 100 mesh.

20. The method of claim 17, wherein said dissolvable fiber or said dissolvable particulate material is used at total concentration of about 25 to 75 lbm/Mgal.

21. The method of claim 17, wherein said dissolvable fiber or dissolvable particulate material is used at a total concentration of about 30 to 40 lbm/Mgal.

22. An improved method of acid stimulating a reservoir wherein a carbonate reservoir is fractured and said fractures are etched with an acid pad, the improvement comprising alternating or combining slurry pads with acid pads, said slurry pads comprising a slurry of gel plus a dissolvable material of a size larger than a fracture size, so as to preferentially block wormholes and crevices and preferentially etch fractures, said dissolvable material dissolving in water or acidic water in less than 48 hours under reservoir conditions.

Description

DESCRIPTION OF FIGURES

(1) FIG. 1A. Various acid treatments.

(2) FIG. 1B Acid-flow behavior in wide (a) and narrow (b) fractures.

(3) FIG. 2. Effect of fracture width on acid-penetration distance.

(4) FIG. 3. Effect of temperature, lithology, and acid concentration on acid-penetration distance.

(5) FIG. 4. Typical Acid Fracturing Treatment. A common pump schedule includes the pumping of an initial, highly viscous pad stage to create the fracture geometry. A stage of acid follows the pad stage, creating the etched width and fracture conductivity. The acid, however, is reactive and less viscous. It will leak-off quickly and the fracture may close. Therefore, acid fracturing treatments include alternating pad and acid stages with a final flush stage after the last pumped acid stage, protecting the wellbore from long term acid exposure and minimizing the potential for corrosion in the tubulars.

(6) FIG. 5. Plot showing typical treatments that are pumped without leak-control materials that result in continuous reduction of bottom hole injection pressures in successive pad cycles, because the formation leak-off is not readily controlled by pad alone.

(7) FIG. 6. Plot showing the benefits of using D-FLAC where there is a clear indication of gain of bottomhole pressures after introduction of D-FLAC, which when compared to previous pad cycle points to improved leak-off control.

DETAILED DESCRIPTION

(8) Subterranean hydrocarbon bearing formations are routinely fracture stimulated to enhance well productivity and improve well performance. In acid fracturing, as described above, a non-reactive fluid often termed as “pad” is pumped under high pressure down the well tubular into the hydrocarbon-bearing formation to generate a hydraulic fracture.

(9) After the creation of fracture, acid is introduced in the formation to react on the walls of fracture and generate an uneven spending pattern that will result in a non-uniform fracture face. The acid stages are often smaller in volume than pad stages, and apart from reacting with fracture walls, may also “wormhole” into the formation via fracture face in a plane orthogonal to fracture. This phenomenon leads to increased opportunity for fluid to leak away from the fracture face into the adjacent rock instead of traversing down the fracture, which will not benefit productivity in low permeability limestone or chalk formations.

(10) To control this, another pad stage is introduced with a purpose of first controlling some of the leak-off and then extending the fracture even further. The pad/acid alternating sequence are pumped until the treatment objectives are met. Eventually, when such a “differentially” etched fracture face closes after the pumping pressures are removed, it leaves behind conductive pathway that enables flow of fluids (oil/gas/condensate, etc.) into the well with relative ease, during the production phase. FIG. 4 shows one such typical plot of acid fracturing in multiple stages.

(11) In FIG. 4, the enhancement of near well region leak-off resulting from acid-mineral reaction is often detrimental to etched fracture geometry as shorter penetration lengths and smaller etched fracture heights may result because of acid's inability to travel further away from the point of injection. The “thieving” of acid in the near well region must be prevented.

(12) FIG. 5 shows the pressure response analysis from treatment showed in FIG. 4. The continuously diminishing formation face pressure is a strong indicator of enhanced leak-off after every acid cycle reacts with the formation.

(13) We have addressed the leak-off problem by developing technology that utilizes available hydrolyzable materials (materials that will decompose in water after a certain exposure time) to plug leak-off of acid into the formation. This limits overspending of the acid in the near wellbore region in all acid cycles and promotes acid placement to the farthest (deepest) points in the fracture network.

(14) We generally begin by carrying out injectivity tests to determine the extent of leak-off by measuring the pressure decline. Depending on the degree of leak-off determined from the pressure fall-off or if estimation of leak-off is already done from previous injections in the area, the treatment design will be based on any of the following methods.

(15) Technique A:

(16) Recommended to be used if the leak-off coefficient is more than 0.0045 ft/min.sup.0.5 (high).

(17) Start the treatment by pumping a “cooldown” stage to lower reservoir temperature, as needed. Around 300 bbls of cool friction reducer laden water is sufficient for the purpose of cool down, depending of course on well length and downhole temperatures. We prefer to call this a “cooling-down” stage as the primary purpose of this batch of fluid is to lower the near-well temperature in order to slow down acid spending speed.

(18) This is followed by linear or cross-linked gel pad of pre-determined volume, either polymeric or visco-elastic fluid, anything that provides sufficient viscosity and enables the creation and propagation of the desired fractures. It is known in the art how to select pad fluid, volumes and pump pressures to influence fracture size, depth and propagation.

(19) The pad is then followed with a slick acid stage (generally 15 or 28% HCl or any of the organic acids, individual or in mixture) consisting of acid (un-inhibited or inhibited—with all relevant additives) along with a friction reducer. Friction reducers are commercially available and concentrations may range from 0.75 to 3.0 gal/Mgal [0.75 to 3.0 L/m.sup.3] depending on optimality of performance. The acid fracturing step occurs at volumes of about 100 to 500 gal [1.24 to 6.2 m.sup.3] of acid per foot [meter] of producing formation, and typically is done above the fracture pressure of the formation.

(20) The slick acid stage is followed with a D-FLAC stage, comprising a slurry constructed out of dissolvable fiber and/or dissolvable particulates in a carrier fluid of linear gel made from dry polymer or gel concentrate, to effectively yield concentrations such as 30 to 40 lbm/Mgal (amount of polymer by weight in pounds in 1000 US gallons of water).

(21) The D-FLAC stage inhibits formation fluid leak-off in following steps: 1. Efficiently transported downhole and remaining fully suspended during the process because of the viscosity of carrier gel. 2. Enter the wormholes/bridge at the throat of the wormholes and other cracks/crevices in the formation. 3. Prevent further entry of the subsequent fluids (acid) into the blocked wormholes and other cracks/crevices. 4. Enabling the subsequent fluids to traverse down and finger through the fractures, thus etching and deepening the fractures.

(22) The invention differs from fluid loss control through formation of filter-cake. Filter cake occurs in low to medium permeability formation as the filtrate leak-offs off into the formation leaving behind a wall comprising dehydrated polymeric fluid solids that are of the order of microns in size. Higher fluid velocities such as the ones that occur in near the wellbore region during pumping, as the fluid enters the formation, can also erode filter cake and or prevent its formation up to a distance away from wellbore, till favorable conditions are achieved, leading to ineffective fluid loss control. Furthermore, filter-cake formation in heavily fractured, fissured or wormholed formations is difficult because the fluid tends to be “lost” in these larger features in its entirety.

(23) In this application of the use of D-FLAC with acid, no substantial solids other than D-FLAC are used, which because of their larger size, tend to physically “block” the entry of fluid at all plausible sites such as described in sentence above. Thus, the larger leakage sites are blocked. However, the dissolvable material size is selected so it cannot “block” fractures as it will be too large to do so (the hydraulic fracture width is typically in the order of 0.15 to 0.5 inch when being pumped). Preferential blocking of larger wormholes and crevices allows the fluid to travel down and further etch fractures farther away from the wellbore than would be possible without blocking the larger crevices.

(24) To date we have used D-FLAC only in the PAD, and prior to starting the acid etching or after the first etch and before subsequent etches. In this manner, the dissolvable material is included with at least 2 of the 3 gel pads (2nd and 3rd gel pads), for example, up to 50% of each gel pad volume. These trials have already been conducted successfully.

(25) The next step is to start using the dissolvable material in the acid itself, with the help of a carrier fluid such as concentrated gel generally used for frictional reduction purposes. We anticipate that more dissolvable material may be needed in the long run, as the acid is expected to initiate dissolution. Dissolution of D-FLAC is by hydrolysis, which will speed up in presence of temperature and even extreme low pH environment, such as during acid treatment. To mitigate this, however, we may coat the fiber and particulates in a coating that can delay dissolution for a suitable length of time. Such coatings are available or can be designed.

(26) Particulate matter (solids) will also be suspended in the gel and it will be run along with acid such that the effective concentration of the D-FLAC material will be anywhere from 25 to 35 lbm/Mgal downhole. The volumes and rates will be metered accordingly—the D-FLAC material is pumped using a separate pump at about 5-10 or about 7.0 bbl/min max, and acid will be commingled with it downstream at a rate of about 53-60 bbl/min, prior to entering the wellhead.

(27) The process is not diversion because it does not plug all entry points to “divert” the fluid, but only makes the acid more “efficient,” allowing it to propagate as any normal fracture would propagate, but making sure it does not leak-off via the larger wormholes and crevices as rapidly during the process. The additional fluid being pumped in the fracture does not create “new” fractures, as would be done in a diversion process, but only to continue to “extend” the already created fractures.

(28) The following variations are also possible:

(29) D-FLAC stage can be introduced in tandem with the second PAD stage: If a cross-linked gel pad is used, then run the D-FLAC slurry in the first 35% of the second PAD cycle. However, if a linear-gel pad is used, then run D-FLAC slurry for the entire duration of the second PAD Cycle.

(30) Additionally:

(31) Start the D-FLAC stage with 35 lbm/Mgal concentration of total solids inclusive of dissolvable fiber and dissolvable particles.

(32) If using a linear gel PAD, increase the D-FLAC stage concentration by 5 lbm/Mgal every linear gel pad cycle going forward.

(33) Follow the D-FLAC stage with an acid stage of twice or more the volume of preceding D-FLAC stage.

(34) If using cross-linked gel in the third PAD stage, increase D-FLAC up to 50% of designed volume of the third PAD. If using a linear-gel PAD, continue running D-FLAC for entire duration of the third PAD.

(35) Repeat the cycles until all designed acid is pumped.

(36) Increase D-FLAC stage volume as necessary.

(37) Technique B:

(38) Recommended to be used if the leak-off is less than 0.0045 ft/min.sup.0.5 (high) and only a linear gel pad is used, because the leak-off is not high and linear gel with leak-off control will be sufficient to generate and propagate fractures.

(39) Technique B is like Technique A, the difference being that the D-FLAC material is used throughout the treatment, both in PAD and in the slick acid stages. Note that Acid-Linear Gel compatibility must be pre-determined to provide fluid stability (retain linear gel viscosity despite presence of surrounding acid medium).

(40) Increase D-FLAC material concentration and volume as treatment progresses.

(41) Technique C:

(42) Similar to Technique B with following differences:

(43) Alternate D-FLAC/Acid stages more frequently e.g. every 50 to 75 bbls with equal amounts of the volumes with the leak-off material (fiber+solids) running throughout the treatment. We may need specialized equipment to carry this out, which we may be able to influence during design phase. To administer D-FLAC material so frequently, a gate controller and/or automated controller to the upstream (suction side) of fracturing pumps designated to pump D-FLAC may be installed to obviate the manual controls.

(44) The method in any of its variations can provide any one or more of the following benefits: Improve Fracture penetration Enhanced rock stimulation Better production ($$) Better long-term fracture conductivity as more etching throughout the fracture face instead of near wellbore Mitigate wellbore integrity issues ($$) Simplify operations

(45) The method can eliminate any one or more of the following: Intermediate brine cushion X-linked pad Need to control the pumping rates to the extent it is done today

(46) The data in FIG. 6 was obtained as follows: A polymeric gel or friction reducer laden water fluid was pumped at higher pressures to (a) fracture the rock, and (b) propagate the fracture to generate a desired geometry in two pad cycles. In the third cycle, D-FLAC was pumped in a dedicated “stage” or “pill” using 25 pounds of D-FLAC in 1000 gallons of gel carrier fluid (2.96 kg/m3).

(47) We used a 20/40 U.S. Mesh Size of a PLA polymer indicating a particle size distribution of 0.033 to 0.017 inch [0.838 to 0.432 mm] in the mix. Pump rate was at 60 bbl/min, equivalent to 0.16 cubic meter per second [0.158987 m.sup.3/s]. FIG. 6 shows the benefits of using D-FLAC where there is a clear indication of gain of bottomhole pressures after introduction of D-FLAC, which when compared to previous pad cycle, points to leak-off control.

(48) Each of the following is incorporated by reference in its entirety for all purposes. Williams, B. B., et al., 1979. Acidizing Fundamentals, 55. New York: SPE/AIME. US2004152601 Generating Acid Downhole in Acid Fracturing US2015041132 Method of Using Diverter and Proppant Mixture U.S. Pat. No. 7,219,731 Degradable Additive for Viscoelastic Surfactant Based Fluid Systems