TREATING SUBTERRANEAN FORMATIONS WITH DELAYED CROSSLINKING GELS

Abstract: A subterranean formation is treated with a delayed crosslinking gel fluid by first preparing an aqueous gel by mixing an organic carboxylated gelling agent with an aqueous liquid, combining a buffer with the aqueous gel in an amount to give a pH of 5 or above at ambient temperature, adding a zirconium IV-containing crosslinking compound to the gel, and then introducing the mixture into a subterranean formation whereby the mixture is heated and caused to crosslink. Carbon dioxide may be included in the gel.

Description: The present invention relates generally to the treatment of subterranean formations with delayed crosslinking gel fluids. A variety of subterranean formation treatments utilizing crosslinked aqueous gels have been developed heretofore. For example, a well known technique for stimulating the production of oil and/or gas from a subterranean formation using a high viscosity crosslinked aqueous gel involves fracturing the formation. That is, the aqueous gel is injected down the wellbore penetrating the formation at a flow rate and pressure sufficient to develop hydraulic forces which fracture the formation. Continued pumping of the fluid containing propping agent, e.g. sand, extends the fractures and deposits the propping agent in the fractures to hold them open. Other treatments using high viscosity crosslinked aqueous gels include the temporary plugging or blocking of portions of a formation, enhancing the recovery of oil or gas from subterranean formations by the injection of high viscosity flooding fluids therein, etc. In order to avoid the necessity of pumping crosslinked gel fluids from the surface to a subterranean formation to be treated, delayed crosslinking gel fluids have been developed. That is, instead of incurring the high surface pressures and power requirements involved in pumping highly viscous crosslinked gel fluids, the crosslinking of the fluids is delayed until after the fluids have been pumped into the formation. Once in the formation, crosslinking occurs and useful high viscosity is developed. In some applications, and particularly in carrying out subterranean formation fracturing treatments, it is highly advantageous to combine carbon dioxide with the aqueous gel fluid used. The purpose of the carbon dioxide is to reduce the water content of the treating fluid and to energize the fluid whereby upon the termination of the treatment, it readily flows back from the formation and is recovered therefrom. The particular quantity of carbon dioxide utilized with the aqueous gel fluid can range from commingled fluids containing from about 5% to about 50% by volume carbon dioxide to foamed fluids containing up to in excess of 96% by volume. A fracturing method for stimulating subterranean formations utilizing foams comprised of aqueous gels and carbon dioxide is described in United States patent no. RE.32,302. United States patent no. 4,488,975 is directed to a fracturing method which utilizes an aqueous gel containing a crosslinking agent and carbon dioxide. The crosslinking agent is a zirconium or aluminum chelate, and a sufficient quantity of carbon dioxide is used to reduce the pH of the aqueous gel to a level below about 5.5 whereupon crosslinking occurs. United States patent no. 4,657,081 is directed to the delayed crosslinking of a fracturing fluid comprised of an aqueous gel and carbon dioxide. A crosslinking agent is combined with a chelating agent resulting in a chelate solution having a specific pH. When combined with an aqueous gel fluid, the chelate solution causes a delayed crosslinking to take place. U.S. patent specification no. 4505826 describes a composition for treating a subterranean formation which comprises a carboxylated gelling agent, a Zr (IV) crosslinking agent and a buffer to provide a preferred pH of between about 5 and about 6. However, these compositions crosslink quickly at ambient temperature. We have now found that such compositions can be made which have delayed crosslinking at ambient temperature but which will crosslink when heated by the subterranean formation into which they are injected. The compositions comprise zirconium lactate as the crosslinking compound. According to the present invention, there is provided a method of treating a subterranean formation comprising: (a) preparing an aqueous gel by admixing an organic carboxylated gelling agent with an aqueous liquid; (b) combining a buffer with said aqueous gel in an amount sufficient to result in a pH of about 5 or above at ambient temperature; (c) combining a zirconium IV-containing crosslinking compound with said aqueous gel in an amount sufficient to result in crosslinking of said gel; and (d) introducing the aqueous gel mixture formed in step (c) into said subterranean formation; characterised in that the crosslinking compound is zirconium lactate and the crosslinking of the gel is delayed until it is heated by said formation and thereby caused to crosslink therein. By the present invention, an improved method of treating a subterranean formation with a delayed crosslinking gel is provided which can be utilized with or without carbon dioxide, and which results in the formation of crosslinked gels having a good stability at high temperature. The term "ambient temperature" is used herein to mean the above freezing temperatures normally encountered on the surface. Because most subterranean formations are relatively hot, i.e. have temperatures in the range of from about 110°F to 275°F (43 to 135°C), the aqueous gel fluid is heated during passage through the wellbore and in the formation. For example, upon being heated to a temperature of about 110°F (43°C) and higher, the aqueous gel rapidly crosslinks to form a highly viscous treating fluid in the wellbore or formation. Carbon dioxide can be included in the gel fluid. The presence of carbon dioxide in the delayed crosslinking gel fluid does not appreciably change its delayed crosslinking characteristics. With carbon dioxide included, the delayed crosslinking gel fluid is particularly suitable for use in carrying out subterranean formation fracturing procedures, both wherein the carbon dioxide is commingled with the gel fluid and where it is foamed therewith. A commingled fluid generally contains carbon dioxide in an amount of from about 5% to 50% by volume of the fluid, and a foam contains high concentrations of carbon dioxide as the internal phase, i.e. up to in excess of 96% by volume of the fluid. In both forms of fluids, the crosslinking of the aqueous gel portions of the fluids is delayed until the fluids are heated by the formation. The aqueous liquid can comprise substantially any aqueous liquid which does not adversely react with the components of the delayed cross linking gel fluid. The aqueous liquid can comprise, for example, fresh water or salt solutions. Preferably, the aqueous liquid is a salt solution containing about 2% potassium chloride or other salt which functions to stabilize the formation to be treated. The gelling agent can be any of a variety of hydratable organic carboxylated compounds. Preferably, the gelling agent is a derivatized natural material such as guar gum or cellulose containing carboxymethyl groups, e.g., carboxymethylguar, carboxymethylhydroxyethylguar, carboxymethylhydroxypropylguar, carboxymethylhydroxyethylcelluose, carboxymethylhydroxypropylcellulose, and so on. Of these, carboxymethylhydroxypropylguar and carboxymethylhydroxyethylcellulose are the most preferred. The above-described carboxylated gelling agents are hydratable whereby they form a gel in an aqueous liquid, and when at a pH of about 5 and above, the gel is delayedly crosslinkable by a zirconium IV-containing compound to produce a crosslinked gel having good viscosity at high temperatures. More specifically, a buffer is combined with the aqueous gel formed in an amount sufficient to result in and maintain the pH of the aqueous gel at a level of about 5 or above at ambient temperature. When the crosslinking agent is combined with the aqueous gel, the crosslinking reaction that takes place is very slow at ambient temperature and therefore delayed. As indicated above, when the aqueous gel is heated by the formation, the crosslinking reaction takes place at a relatively rapid rate. While various buffers can be utilized, particularly suitable buffers are sodium diacetate and a mixture of a weak acid such as acetic acid and sodium carbonate. The crosslinking compound which is useful in accordance with this invention is zirconium lactate. When carbon dioxide is combined with the aqueous gel mixture prior to the introduction of the mixture into the subterranean formation, the presence of the carbon dioxide lowers the pH of the aqueous gel, but does not adversely affect the delay in crosslinking of the aqueous gel or the stability of the crosslinked gel formed. In carrying out the method of the present invention, the organic carboxylated gelling agent used, for example, carboxymethylhydroxypropylguar, is admixed with an aqueous liquid such as fresh water containing 2% by weight potassium chloride whereby the gelling agent is hydrated and an aqueous gel is formed. Generally, the gelling agent is admixed with the aqueous liquid in an amount in the range of from about 0.15% to about 1.0% by weight of the aqueous liquid, preferably in an amount of about 0.36% to about 0.60% by weight of the aqueous liquid. A buffer, e.g. sodium di-acetate, is combined with the aqueous gel in an amount sufficient to result in and maintain the pH of the aqueous gel at a level of about 5 or above at ambient temperature. Generally, a quantity of buffer in the range of from about 0.01% to about 0.20% by weight of aqueous liquid used will be required. The zirconium IV-containing crosslinking compound, zirconium lactate, is combined with the buffered aqueous gel in an amount whereby the zirconium in the plus 4 oxidation state is present in an amount in the range of from about 0.5% to about 7% by weight of gelling agent utilized. When the subterranean formation to be treated has a temperature below about 200°F (93°C), the pH of the aqueous gel is preferably adjusted to a level in the range of from about 5 to about 6, and most preferably from about 5 to about 5.5 using sodium diacetate or an equivalent buffer. The crosslinking agent, zirconium lactate, is then combined with the buffered aqueous gel in an amount in the range of from about 0.5% to about 7% as zirconium (plus 4) by weight of gelling agent used. The resulting gel mixture is pumped into the formation to be treated, and upon being heated and crosslinked, the resulting crosslinked gel maintains good viscosity at temperatures up to about 200°F (93°C). When the subterranean formation has a temperature above 200°F (93°C), a buffer comprised of a mixture of acetic acid present in an amount of about 0.015% by weight of aqueous liquid used and sodium carbonate present in an amount of about 0.12% by weight of aqueous liquid used, or the equivalent is preferably combined with the aqueous gel. The above amounts of the acetic acid-sodium carbonate buffer generally are sufficient to adjust the pH to a level in the range of from about 8 to about 10. The most preferred pH level is from about 9.5 to about 10. Zirconium lactate crosslinking agent is then combined with the buffered gel in an amount in the range of from about 2.0% to about 25% as zirconium (plus 4) by weight of gelling agent used. The crosslinked aqueous gel formed in the subterranean formation after being heated maintains good viscosity at temperatures up to about 275°F (135°C). Carbon dioxide can be combined with the delayed crosslinking gel fluid in amounts in the range of from about 5% or that amount of carbon dioxide required to effect saturation at the existing temperature and pressure of the fluid, to in excess of 96% by volume of the aqueous gel fluid and carbon dioxide. Preferably, the carbon dioxide is combined in the liquid state with the aqueous gel on-the-fly just prior to the introduction of the resulting mixture into the well bore and subterranean formation to be treated. In the following description, reference is made to the accompanying drawings, wherein: FIGURE 1 is a graph of temperature versus apparent viscosity for aqueous gel fluids of the present invention containing varying amounts of crosslinking compound after the simulated introduction of the fluids into a subterranean formation. FIGURE 2 is a graph similar to Figure 1 for aqueous gel fluids of this invention containing varying amounts of a particular buffer. FIGURE 3 is a graph similar to Figure 1 for aqueous gel fluids of the invention containing various buffers. FIGURE 4 is a graph similar to Figure 1 for aqueous gel fluids of the invention containing varying amounts of gelling agent and crosslinking compound. FIGURE 5 is a graph similar to Figure 1 for additional aqueous gel fluids of the invention containing varying amounts of crosslinking compound. FIGURE 6 is a graph similar to Figure 1 for yet additional aqueous gel fluids of the invention containing varying amounts of crosslinking compound. In order to further illustrate the method of the present invention the following Examples are given by way of illustration only. Example 1 Tests were carried out using a recirculation flow loop viscometer. The viscometer is constructed of 0.305 inch (7.7mm) ID stainless steel tubing. Fluid pressure loss is measured across a 10.0 foot (3.05m) section of the tubing by a Viatran 0-50 psi (0-345 kPa) differential pressure transducer. Fluid is circulated with a Zenith precision metering pump. The pump allows circulation of fluid at a shear rate of 80-2000 sec⁻¹ in the pipe. Fluid properties may be visually observed through a lighted Penberthy sight gauge. A 0-2000 psi (0-13.8 MPa) pressure gauge allows monitoring of system pressure. The entire length of tubing is wrapped with electrical heating tape. Power is supplied through two Honeywell temperature controllers. Thermocouples sense fluid temperature at two points in the flow loop and connect to the Honeywell controllers. System fluid pressure is controlled by a Grove backpressure regulator. The flow loop is capable of temperatures from 70 to 300°F (21 to 149°C) at pressures between ambient and 2000 psi (13.8 MPa). Three test mixtures of delayed crosslinking gel fluids were prepared as follows. Carboxymethylhydroxyethylcellulose is admixed with fresh water having 2.7% potassium nitrate dissolved therein in a Waring Blender in an amount equivalent to 40 pounds of gelling agent per 1000 gallons of water (4.8 kg/m³; 0.48% by weight of water). Sodium diacetate buffer in an amount equivalent to 5 pounds per 1000 gallons of water (0.6 kg/m³; 0.04% by weight of water), and various quantities of a zirconium lactate solution containing 26% by weight zirconium lactate were combined with the aqueous gel in the blender. The pH of the three aqueous gel mixtures formed ranged from 5.18 to 5.21. The three test mixtures were individually injected into the recirculating flow loop viscometer along with a small quantity of a foaming surfactant, and while circulating, carbon dioxide was injected into the flow loop at a gauge pressure of 1000 psi (6.89 MPa) in a total quantity of 20% by volume of the circulating aqueous gel mixture and carbon dioxide. The circulating fluid was heated during each test over the temperature range of 70°F to 250°F (21 to 121°C). Fluids were circulated at a high shear rate (500 to 1100 sec⁻¹) to simulate blending operations and tubing transit time (4 to 5 minutes). The shear rate was then reduced to 170 sec⁻¹ for the remainder of the test to simulate flow in a subterranean formation. The apparent viscosity of the circulating fluid was calculated at various temperatures over the above mentioned range. The results of the three tests are shown graphically in Figure 1. In Figure 1, where 1,2 and 3 are, respectively, for zirconium lactate concentrations of 2.5%, 3.8% and 5.0% by weight of gelling agent used. As can be seen, delayed crosslinking of the fluids occurred after heating, and the particular quantity of crosslinking compound used affected the viscosity and stability of the crosslinked gel formed. Example 2 Four additional tests were carried out as described in Example 1 except that the zirconium lactate solution used contained 26% by weight zirconium lactate and was combined with each of the aqueous gels formed in an amount of 0.8 vols. per 1000 vols. of water (5.0% by weight of gelling agent used). In addition, various quantities of sodium diacetate buffer were used. The results of these tests are shown in Figure 2 where the curves are as follows: curve Sodium diacetate buffer used (% by weight of aqueous liquid) Initial pH 1 0.0% 5.11 (adjusted with acid) 2 0.25% 5.40 3 0.5% 5.26 4 1.0% 5.13 Example 3 Three additional tests were carried out identical to those of Example 2 except that the kind and quantity of buffer used were changed. The results of these tests are shown in Figure 3, where the curves are as follows: curve buffer amount (% by wt aqueous liquid) initial pH 1 sodium diacetate 0.5% 5.21 2 1:1 by weight fumaric acid and sodium bicarbonate 0.5% 5.09 3 1:1 by weight ammonium acetate and acetic acid 0.5% 5.32 Example 4 Three additional tests were carried out identical to the tests of Example 2 except that the sodium diacetate buffer was used in the quantity given in Example 1 and various quantities of gelling agent and crosslinking compound were used. The results of these tests are shown in Figure 4, where the curves are as follows: curve amount of gelling agent (% by wt of aqueous fluid) amount of zirconium lactate (% by weight of gelling agent) 1 0.48 2.5 2 0.36 2.5 3 0.30 2.5 Example 5 The procedure of Example 1 was repeated except that the gelling agent used was carboxymethylhydroxypropylguar in an amount of 50 pounds of gelling agent per 1000 gallons of water (6.0 kg/m³; 0.60% by weight of water). The buffer used was a mixture of 0.015% by weight of water acetic acid and 0.12% by weight of water sodium carbonate which resulted in the test mixture having pH levels ranging from 8.54 to 9.24 before addition of CO₂. Also, various amounts of crosslinking compound were used. The results of these tests are shown in Figure 5, where the curves are as follows: curve amount of zirconium lactate (% by wt. gelling agent) 1 17.5% 2 10% 3 5% As illustrated in Figure 5, the use of greater amounts of gelling agent and crosslinker compound coupled with a higher initial pH results in a delayed crosslinking gel having good viscosity up to about 275°F (135°C). Example 6 The procedure of Example 5 is repeated except that carboxymethylhydroxyethylcellulose is substituted for carboxymethylhydroxypropylguar. The results of these tests are shown in Figure 6, where the curves are as follows: curve amount of zirconium lactate (% by wt. of gelling agent) 1 10 2 7.5 3 5 While particular embodiments of the invention have been described for purposes of this disclosure, it is to be understood that the invention is not limited to such embodiments only, and that reasonable variations and modifications which will be apparent to those skilled in the art are possible.

Claims: A method of treating a subterranean formation comprising: (a) preparing an aqueous gel by admixing an organic carboxylated gelling agent with an aqueous liquid; (b) combining a buffer with said aqueous gel in an amount sufficient to result in a pH of about 5 or above at ambient temperature; (c) combining a zirconium IV-containing crosslinking compound with said aqueous gel in an amount sufficient to result in crosslinking of said gel; and (d) introducing the aqueous gel mixture formed in step (c) into said subterranean formation; characterised in that the crosslinking compound is zirconium lactate and the crosslinking of the gel is delayed until it is heated by said formation and thereby caused to crosslink therein. A method according to claim 1, wherein said organic carboxylated crosslinkable gelling agent is carboxymethylhydroxypropylguar or carboxymethylhydroxyethylcellulose. A method according to claim 1 or 2, wherein the gelling agent is mixed with the aqueous liquid in an amount of from 0.15% to 1.0% by weight of the aqueous liquid. A method according to claim 1,2 or 3, wherein the buffer is sodium diacetate or a mixture of acetic acid and sodium carbonate. A method according to any of claims 1 to 4, wherein the zirconium lactate is combined with the aqueous gel in an amount to provide zirconium (in the plus 4 valence state) in an amount of from 0.5, to 25% by weight of the gelling agent used. A method according to any of claims 1 to 5, which further includes the step of combining carbon dioxide with the aqueous gel mixture prior to introducing the mixture into the formation. A method according to claim 6, wherein said carbon dioxide is combined with said aqueous gel mixture in an amount of from 5% (or that quantity required to effect saturation at the temperature and pressure of said mixture) to in excess of 96% by volume of the mixture of aqueous gel and carbon dioxide.

Inventors: HARRIS, PHILLIP C., SANDY, JOE M.
Applicants: HALLIBURTON COMPANY
Application year: 1988
Main classification: E21B43
IPC codes: C09K8