Methods of fracturing subterranean zones to produce maximum productivity

[0001] 1. Field of the Invention

[0002] The present invention relates to methods of fracturing subterranean zones whereby maximum hydrocarbon productivity therefrom results. 2. Description of the Prior Art

[0003] Hydrocarbon producing wells are often stimulated by hydraulic fracturing treatments. In such treatments, a viscous fracturing fluid which also functions as a carrier fluid is pumped into a producing zone to be fractured at a rate and pressure such that one or more fractures are formed in the zone. Proppant particles, e.g., graded sand, for propping the fractures are suspended in at least a portion of the fracturing fluid so that the proppant particles are deposited in the fractures when the fracturing fluid is broken. That is, a viscosity breaker is included in the fracturing fluid whereby the fracturing fluid reverts to a thin fluid which is returned to the surface. The proppant particles deposited in the fractures when the fracturing fluid looses its viscosity function to prevent the fractures from closing so that conductive channels are formed through which produced hydrocarbons can readily flow.

[0004] The fracturing fluids utilized heretofore have damaged the proppant particle packs formed in the fractures whereby the production of hydrocarbons through the proppant particle packs is less than the maximum possible. The damage has been the result of gelled fracturing fluid that forms a filter cake on the fracture faces subsequently flowing into the proppant particle packs. That is, after the fracturing fluid is recovered from the subterranean zone in which the fractures are formed and formation fluids are produced from the subterranean zone through the fractures and through the proppant packs therein, the gelled fracturing fluid filter cake on the fracture faces breaks up and flows into the proppant particle packs. A major portion of the gelled fracturing fluid remains in the proppant particle packs and reduces their conductivities to produced hydrocarbon fluids. In addition, the fracturing fluids utilized heretofore have included high loadings of viscosity increasing gelling agents. When the fracturing fluids have been broken, i.e., reduced in viscosity, a significant amount of polymer residue attaches to the proppant particle packs thereby further reducing the produced hydrocarbon conductivity of the packs.

[0005] Thus, there are continuing needs for improved methods of fracturing subterranean zones which produce maximum hydrocarbon productivity from the zones.

[0006] The present invention provides improved methods of fracturing subterranean zones whereby maximum hydrocarbon productivity results therefrom which meet the needs described above and overcome the deficiencies of the prior art. The methods of the present invention utilize a fracturing fluid which is comprised of fresh water or salt water having a low salt content, a gelling agent and an inorganic salt for preventing clay swelling. The gelling agent and the inorganic salt are included in the fracturing fluid in very low amounts so that the fracturing fluid produces very little damage to the conductivities of the proppant particle packs placed in the fractures. Stated another way, the fracturing fluids utilized in accordance with the methods of this invention produce little or no gelled fracturing fluid, filter cake or residue in the permeable proppant particle packs thereby allowing maximum production of hydrocarbons from a fractured subterranean producing zone.

[0007] The methods of the present invention for fracturing a subterranean zone whereby maximum hydrocarbon production therefrom results is comprised of the following steps. A fracturing fluid is provided comprised of water, a gelling agent present in an amount in the range of from about 0.06% to about 0.3% by weight of the water and an inorganic salt for preventing clay swelling present in an amount in the range of from about 0.01% to about 1% by weight of the water. The fracturing fluid is introduced into a subterranean zone at a rate and pressure sufficient to form one or more fractures in the zone. Thereafter, the fracturing fluid is recovered from the zone.

[0008] As a result of the very low quantities of gelling agent and clay swelling preventing salt in the fracturing fluid, the fracturing fluid produces very little filter cake and/or residue in the formed fractures whereby the conductivities of the proppant particle packs in the fractures are not significantly impaired.

[0009] The objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.

[0010] The present invention provides improved methods of fracturing subterranean zones whereby maximum hydrocarbon production from the zone is achieved. The methods are basically comprised of the following steps. A fracturing fluid comprised of water, a gelling agent present in an amount in the range of from about 0.06% to about 0.3% by weight of the water and an inorganic salt for preventing clay swelling present in an amount in the range of from about 0.01% to about 1% by weight of the water. The fracturing fluid is introduced into a subterranean zone at a rate and pressure sufficient to form one or more fractures in the zone. Thereafter, the fracturing fluid is recovered from the zone.

[0011] The fracturing fluid generally also includes a cross-linking agent for the gelling agent which increases the viscosity of the fracturing fluid as well as suspended proppant particles and a delayed breaker for reducing the viscosity of the fracturing fluid. When the viscosity of the fracturing fluid is reduced, the suspended proppant particles are deposited in the formed fractures and the low viscosity fracturing fluid is recovered from the subterranean zone.

[0012] The water in the fracturing fluid is preferably fresh water, but it can also be salt water containing dissolved salts in no greater a concentration than about 3.5% by weight of the salt water.

[0013] A variety of gelling agents can be utilized in the fracturing fluid to provide viscosity thereto. Examples of gelling agents which can be utilized include, but are not limited to, galactomannans gums, modified or derivatized galactomannan gums, cellulose derivatives, xanthan biopolymer, succinoglycon biopolymer, polyacrylamides and polyacrylates. Of the foregoing, galactomannan gums, modified or derivatized galactomannan gums and cellulose derivatives are generally preferred. Examples of suitable cellulose derivatives and galactomannan gums and derivatives include, but are not limited to, hydroxyethylcellulose, hydroxypropylguar, guar and anionically charged guar gelling agents such as carboxymethylguar, carboxymethylhydroxypropylguar, carboxymethylguar and carboxymethylhydroxyethylcellulose. Of these, the anionically charged guar gelling agents are preferred with carboxymethylhydroxypropylguar being the most preferred.

[0014] The gelling agent is included in the fracturing fluid in a minimum amount to prevent damage to the proppant packs in the formed fractures as described above. The gelling agent is generally present in the fracturing fluid in an amount in the range of from about 0.06% to about 0.3% by weight of the water in the fracturing fluid. More preferably, the gelling agent is present in the fracturing fluid in an amount in the range of from about 0.12% to about 0.3% by weight of water therein, most preferably from about 0.18% to about 0.24%.

[0015] Various inorganic salts for preventing clay swelling can also be utilized. Examples of such inorganic salts include, but are not limited to, potassium chloride, sodium chloride, potassium nitrate and ammonium chloride. Of these, potassium chloride is preferred.

[0016] The inorganic salt utilized is included in the fracturing fluid in an amount in the range of from about 0.01% to about 1% by weight of the water in the fracturing fluid, more preferably in an amount in the range of from about 0.05% to about 0.5% by weight of the water and most preferably from about 0. 1% to about 0.2%.

[0017] The fracturing fluid can also and generally does include a cross-linking agent for cross-linking the gelling agent and increasing the viscosity of the fracturing fluid. A variety of cross-linking agents can be utilized, including, but not limited to, borate releasing compounds, a source of titanium ions, a source of zirconium ions, a source of antimony ions and a source of aluminum ions. Of these, a source of zirconium ions and a borate releasing compound such as boric acid, borax, ulexite and colemanite are preferred with a source of zirconium ions being the most preferred.

[0018] The cross-linking agent utilized is generally present in the fracturing fluid in an amount in the range of from about 0.02% to about 0.6% by weight of water therein and when a source of zirconium ions or a borate releasing compound is utilized from about 0.04% to about 0.3%.

[0019] As mentioned above, the fracturing fluid generally also includes a delayed breaker for reducing the viscosity of the fracturing fluid whereby the fracturing fluid can be recovered from the subterranean zone. Examples of delayed breakers which can be used include, but are not limited to, alkali metal and ammonium persulfates which are delayed by being encapsulated in a material which slowly releases the breaker, alkali metal chlorites, alkali metal hypochlorites and calcium hypochlorite. Of these, sodium chlorite is preferred. The delayed breaker is generally included in the fracturing fluid in an amount in the range of from about 0.001% to about 0.25% by weight of water therein, more preferably from about 0.012% to about 0.06%.

[0020] As also mentioned above, the fracturing fluid includes proppant particles suspended therein which are deposited in the fractures formed when the viscosity of the fracturing fluid is reduced by the delayed breaker. Various proppant particles can be used such as graded sand, bauxite particles, ceramic particles, glass particles, walnut hull particles, polymer particles and the like. Generally, the proppant particles have a size in the range of from about 2 to about 400 mesh, U.S. Sieve Series. The preferred proppant is graded sand having a particle size in the range of from about 10 to about 70 mesh, U.S. Sieve Series. Preferred sand particle size distribution ranges are one or more of 10-20 mesh, 20-40 mesh, 40-60 mesh or 50-70 mesh, depending on the particular size and distribution of formation solids to be screened out by the consolidated proppant particles.

[0021] A preferred method of this invention for fracturing a subterranean zone whereby maximum hydrocarbon productivity therefrom results is comprised of the steps of: (a) providing a fracturing fluid comprised of water, a gelling agent present in an amount in the range of from about 0.06% to about 0.3% by weight of the water and an inorganic salt for preventing clay swelling present in an amount in the range of from about 0.01% to about 1% by weight of the water; (b) introducing the fracturing fluid into the subterranean zone at a rate and pressure sufficient to form one or more fractures in the zone; and (c) recovering the fracturing fluid from the zone.

[0022] Another preferred method of fracturing a subterranean zone whereby maximum hydrocarbon production therefrom results is comprised of the steps of: (a) providing a fracturing fluid comprised of water, a gelling agent selected from the group consisting of anionically charged guar gelling agents present in an amount in the range of from about 0.06% to about 0.3% by weight of the water, potassium chloride for preventing clay swelling present in an amount in the range of from about 0.1% to about 1% by weight of water, a cross-linking agent for cross-linking the gelling agent selected from the group consisting of a source of zirconium ions and a borate releasing compound present in an amount in the range of from about 0.04% to about 0.3% by weight of water and a sodium chlorite breaker present in an amount in the range of from about 0.001% to about 0.25% by weight of water; (b) introducing the fracturing fluid into the subterranean zone at a rate and pressure sufficient to form one or more fractures in the zone; (c) allowing the fracturing fluid to break into a low viscosity fluid; and (d) recovering the fracturing fluid from the zone.

[0023] In order to further illustrate the methods of this invention, the following example is given.

[0024] Four gelled fracturing fluids were prepared in the laboratory comprised of tap water, carboxymethylhydroxypropylguar gelling agent in an amount of 0.26% by weight of the water, various salts, i.e., potassium chloride, sodium chloride, ammonium chloride and potassium nitrate in amounts of 0.1% by weight of the water and a zirconium complex cross-linking agent in an amount of 0.04% by weight of the water.

[0025] A 65 milliliter sample of each of the fracturing fluids was placed in a Fann Model 50 viscometer equipped with a 420 spring, a 316 stainless steel cup and a B2 bob. The bath was preheated to 250 F. and the fracturing fluid samples were each transferred into the viscometer cup at 75 F. and placed on the viscometer. The cup was rotated at 106 rpm (40 sec¹) and recovery was set at 5 F. per minute. The fracturing fluid sample viscosities and temperatures versus time were recorded. The results of the tests are given in the Table below.

TABLE
Fracturing Fluid Viscosities And Temperatures Versus Time
Time,	Potassium Chloride	Sodium Chloride	Ammonium Chloride	Potassium Nitrate
min.	Temp., F.	Viscosity, cp	Temp., F.	Viscosity, cp	Temp., F.	Viscosity, cp	Temp., F.	Viscosity, cp
2	128	190	124		127	17	127	153
5	177	1213	178	1131	175	628	176	1140
10	230	922	232	773	229	733	231	717
20	254	925	255	717	253	763	255	559
30	254	948	254	823	253	902	253	724
40	254	988	253	879	253	816	253	790
50	253	1015	253	929	253	813	253	872
60	253	1008	253	935	254	733	253	945
70			253	948	254	664	253	1005
80			253	945

[0026] From the above Table, it can be seen that fracturing fluids containing low concentrations of carboxymethylhydroxypropylguar and various inorganic salts for preventing clay swelling have sufficient viscosity to function successfully as fracturing fluids while producing very low filter cake and/or residue in the formed fractures.

[0027] Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While numerous changes can be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims.