GB2175942A - Fracturing compositions and their use - Google Patents

Abstract

A fracturing composition for treating a subterranean formation containing hydrocarbon deposits which comprises a mixture of a major proportion of liquid carbon dioxide and a minor proportion of an immiscible organic liquid which does not adversely react with the carbon dioxide, the subterranean formation or the hydrocarbon therein, and a 'surfactant.' A method of fracturing a hydrocarbon-containing, subterranean formation penetrated by a well bore comprising the steps of: (a) admixing a major portion of liquid carbon dioxide with a minor portion of an immiscible organic liquid which does not adversely react with the carbon dioxide, the subterranean formation or the hydrocarbon therein and a surfactant; (b) introducing the admixture into the well bore and formation at a temperature below the critical temperature of the carbon dioxide and at a pressure to maintain the admixture as a liquid; (c) maintaining the admixture within the formation sufficiently long to permit the admixture to be volatilized; and (d) fracturing the formation with the volatilized admixture. n

Description

SPECIFICATION Fracturing compositions and their use The present invention relates to two-phase systems suitable for use in the hydraulicfracturing of hydrocar- bon reservoirs. More particularly, it relates to liquid carbon dioxide-containing combinations or mixtures useful as fracturing fluids for deep gas wells and oil wells which mixtures demonstrate increased viscosity and provide improved lubricity over conventional carbon dioxide fracturing fluids.

The treatment of subterranean formations penetrated by a well bore to stimulate the production of hydrocarbons therefrom has long been known in the art. One ofthe most common methods of increasing productivity of a hydrocarbon-bearing formation is to subjectthe formation to a fracturing treatment. This treatment is effected by injecting a liquid, gas or two-phase fluid which generally is referred to as a fracturing fluid down the well bore at sufficient pressure and flow rate to fracture the subterranean formation. A propant material such as sand, fine gravel, sintered bauxite, glass beads orthe like can be introduced into thefractures to keep them open. The propped fracture provides largerflow channels through which an increased quantity of hydrocarbon can flow, thereby increasing the productive capability of a well.

Carbon dioxide (CO2) has been used for over twenty years as a component offluids used for hydraulic fracturing and numerous fracturing methods using this compound have been proposed. Thus, U.S.

Patent No. 3,368,627 describes the use of a mixture of liquefied CO2 and a liquefied hydrocarbon such as propane asafracturing fluid. in U.S. Patent No.

3,310,112, the use of a gelled hydrocarbon slurry or emulsion in admixture with liquid CO2 is proposed for the same purpose. U.S. Patent No. 3,396,107 describes a fracturing composition consisting of a liquid CO2 and water mixture. U.S. Patent No. 3,623,552 describes a well displacement process wherein liquid CO2 is injected into a formation in three phases or stages, each phase having a different density. In U.S. Patent No.3,664,422, the use of liquid CO2 in combination with a gelled alcohol is proposed for use in a well treating system. U.S. Patent No. 3,765,488 discloses the use of a fracturing fluid comprising liquid CO2 mixed with a gelled alcohol, using a specific hyd roxyalkyl cellulose gelling agent. In U.S. Patent No.

3,842,910, a particularwell-treating method making use of liquid CO2 is described. U.S. Patent No.

3,954,626 provides a well-treating composition comprising liquid CO2, alcohol and a hydroxy-propyl methylcellulose gelling agent.

While all ofthe above noted inventions are meritorious, none has completely aileviated the problems associated with the use of liquid CO2 as a wellfracturing fluid. Although liquid CO2 is a near ideal fracturing fluid, since it vaporizes in situ, it nevertheless has a very low viscosity and, hence, must be combined with,forexample, a gelled alcohol or similar material in order to support a propping agent and to provide a suitable fracture width in all but shallow gas wells. The presence of, for example, hydroxyalkyl cellulose thickening agents, however, often results in the deposit of an undesirable residue in the rockfissures. Furthermore, hydroxyalkyl cellu lose thickeners are cross-linked only with difficulty and manyspeciesare notcompatiblewith liquid CO2.

Foaming ofthe liquid mixtures during pumping may also present problems when such thickeners are present The description "thickened liquid CO2,, found in the prior patent literaturetendsto be misleading since it is the alcohol which is gelle'd orthickened and which is then diluted by the presence ofthe liquid CO2.

What is required in the industry is a well-fracturing fluid which is sufficiently viscous to be delivered under pressure into a subterranean formation art a high pumping rate, which will not boil orfoam during pumping, which carries a suspended propping agent without difficulty and which will completely degrade in the underground location without leaving any interfering residue in the formation.

The present invention provides high phase volume liquid CO2 fluids having a higherthan expected viscosity and lu bricity suita ble for use for the hydraulic facturing of hydrocarbon reservoirs and placing of proppanttherein. Generallyspeaking,thefluidsofthe present invention are combinations or mixtures of liquid CO2, and an organic liquid which is not miscible with liquid CO2forexample, crude oil or diesel oil, or non-aqueous organic liquids,for example, glyceryl triacetate (triacetin) alcohols, and the like.

According to the present invention there is provided a fracturing composition for treating a subterranean formation containing hydrocarbon deposits which comprises a mixture of a major proportion ofliquid carbon dioxide and a minor proportion of an immiscible organic liquid which does not adversely react witch the carbon dioxide,the subterranean formation orthe hydrocarbon therein, and a surfactant.

The present invention also provides a method of fracturing a hydrocarbon-containing, subterranean formation penetrated by a well bore comprising the steps of: (a) admixing a major portion of liquid carbon dioxide with a minor portion of an immiscible organic liquid which does not adversely react with the carbon dioxide, the subterranean formation or the hydrocarbon therein and a surfactant; (b) introducing the admixture intothewell bore and formation at a temperature below the critical temperature ofthe carbon dioxide and ata pressure to maintain the admixture as a liquid; (c) maintaining the admixture within the formation sufficiently long to permit the admixture to be volatilized; and (d) fracturing the formation with the volatilized admixture.

More particularly, compositions are mixtures con taining from 75% to 99.5% byweightofliquidCO2and from 0.5% to 25% by weight of a second phase which comprises an oil or other non-miscible liquid organic containing from 0.1% to 10% by weight ofsurfactant.

The surfactant chosen will be appropriate to the nature ofthe organic liquid employed, and will be compatibletherewith,the organic liquid/surfactant imparting to the liquid carbon dioxide improved properties of viscosity and lubricity.

The immiscible, organic liquid may be selected from (a) crude oil, (b) diesel oil, (c) a solution oftriacetin and ethylene glycol with or without isopropyl alcohol, (d) a solution oftriolein andaikoxy-terminated polyglycol, (e) a solution of dibutyl phthalate and ethylene or propylene glycol, (f) a solution oftriolein and a glycol ether, (g) a solution of alcohols having up to 30 carbon atoms and diesel oil, and (h) a solution of alcohols having up to 30 carbon atoms and diesel oil wherein part ofthe alcohol is replaced with an organic acid having up to 30 carbon atoms, or mixtures thereof.

Suitable surfactants include ethoxylated resin acids, ammonium linear alcohol ethoxysulfate and sorbitan oleate.

The composition may also contain a solid propping agent.

Mixtures or combinations that have been found particularly useful from the standpoint of desired viscosity arethose containing from 75% to 99.5% by weight of liquid andfrom 0.5% to 25% byweight of a second phase comprising (a) crude or diesel oil containingfrom O.f%to 10% byweightofan ethoxylated resin acid surfactant or (b) a solution of triacetin and ethylene glycol with our without isopropyl alcohol and containing from 0.1% to byweightof an ammonium linear alcohol ethoxysulfate surfactant or (c) solutions oftriolein and a mixture of a homologous series of alkoxy-terminated polyglycols (Selexol) containing from 0.1% to 10% ofanammo- niumlinear alcohol ethoxysulfate surfactant (Fenopon CD-1 28), or (d) solutions ofconventional plasticizers such as, for example, dibutyl phthalate, a glycol such as,forexample, ethylene or propylene glycol and Fenopbn, or (e) solutions oftriolein, glycol ethers such as,-forexample,triglyme and Fenopon, or (f) solutions of alcohols having up to 30 carbon atoms such as, for example,oleyl alcohol, diesel oil and a selection of sorbitan oleate surfactants (Span) or (g) solutions such as in (f) above wherein partofthe alcohol is replaced byan organic acid having upto 30 carbon atoms such as,forexample, oleicacid. "Selaxol", "Fenopon" and "Span" are Trade Marks.

The fracturing fluid composition ofthe present invention can be introduced into the subterranean formation in the conventional manner used in carbon dioxidefracturing. Thefiuid can be prepared in a suitable closed mixing apparatus and delivered by means of a high pressure pump into the well bore.

After introduction ofthe celculated volume of fluid, the well bore can be shut in for a period adequateto stabilize the fractured formation. After stabilization, the well bore can be opened to allow escape ofthe carbon dioxide gas.

The hydraulicfracturing fluid compositions ofthe present invention, being non-aqueous, are non- damaging to water-sensitive zones within the formation. All ofthe inherent advantages of a liquid CO2 fracture fluid are maintained The second phase additive is compatible with hydrocarbons and no solid residue is left behind in the underground formation.

The viscosityofthefluid isincreased oversimple CO2 systems to provide improved proppant-carrying capacity as well as lowerfrictional pumping pressure.

The present invention will now befurther illustrated by, but is in no manner limited to the following Examples, wherein some ofthe compositions may be in the form of emulsions.

EXAMPLE I The following procedure was used to prepare mixtures of liquid CO2 and diesel or crude oil brother non-aqueous solution.

Two Jerguson gauges were set up with a Koch motionless mixer placed in between them ("Koch" is a Trade Mark). The required amounts of oil and surfactantwere put into the gaugesfirst and then the required volume of liquid CO2 was added The ingredients were pumped through the motionless mixer a number of timestoform the emulsion-like mixtures. Once formed, the mixtures were introduced directly into the couette of a Rheometric Pressure Rheometerforviscosity measurements. During the addition, the couette was kept spinning at at least 1000 ;ec-7 to maintain the emulsified state. When addition Nas complete, a pre-programmed shear rate scan was run and viscosityversus shear rate and/ortime rate turves were generated.Results are illustrated in TABLES land IA where concentrations of ingredients are given as parts by volume.

TABLE I

Ethoxylated resin acid 1 1 1 surfactant Liquid C02 90 85 80 Beaverhill Lake Crude 10 15 20 Oil (parts by volume) Shear rate (Sec-l) Viscosity (cup) 250 2.1 2.9 4.0 500 1.4 2.6 2.9 750 1.6 2.6 2.9 1000 1.5 2.2 - 2.2 1250 1.4 2.2 1.7 1500 0.6 1.7 0.7 1750 0.8 1.1 0.6 2000 0.9 1.5 1.0 TABLE IA

Ethoxylated resin acid 1 1 1 surfactant Liquid CO2 90 69 60 Diesel Oil 10 15 20 (parts by volume) Shear rate (Sec-l) Viscosity (cp) 500 - 1l - 750 - 0.79 - 1000 - 0.53 0.61 1250 - 0.48 0.62 1500 - 0.56 0. 87 1750 0.45 0.69 1.04 2000 0.42 0.58 1.00 In all cesses, the viscosity ofthe mixtures was found to be higherthan would be expected from simple dilution behaviour.

EXAMPLE II The following procedure was used to prepare mixtures of liquid CO2 and triacetin.

5 ml of ammonium linearalcohol ethoxysulfate surfactant (sold underthe trade mark "Fenopon CD-128") were mixed with 100 ml of ethylene glycol (M.W. 218.21; M.P. 3"C; density 1.155 g/ml) and 1.5 kg/m3 guar (hydroxypropyl) were then mixed in on a stirrer plate. 8 ml of the resultant mixture were placed in a CO2cell and8mloftriacetin added thereto together with 5 ml of isopropyl alcohol. Liquid CO2 was then added to bring the volumeto 100 ml and the mixturewas pumped through an in-line mixerfour times. Viscosity measurements were then made as in Example 1 and repeated at 15, 60 and 90 minute intervals. Results are given in TABLE II.

TABLE II

TAe (minutes) 0 15 60 90 Shear rate (Sec1) Viscosity (cp) 250 5.187 5.550 4.498 2.860 500 4.442 4.986 3.837 2.324 750 4.652 5.027 4.252 2.677 1000 5.364 4.880 4.311 2.769 1250 5.016 4.293 3.894 2.873 1500 4.460 3.910 3.533 2.934 1750 4.123 3.581 3.309 2.942 2000 3.914 3.484 3.208 2.909 EXAMPLE 111 The procedure of Example II was repeated using 5 ml of ammonium linear alcohol ethoxysulfatesurfac- tant, 40 ml of ethylene glycol, 40 ml of triacetin and 25 ml of isopropyl alcohol but no guar, 25 ml of the mixture being placed in the CO2 cell and liquid CO2 added to a final CO2/second phase weight ratio of 75/25. Viscosity results are given in TABLE Ill.

TABLE III

Time (minutes) 0 15 60 90 Shear rate (Sec-1) Viscosity (cp) 250 5.578 7.617 6.533 6.313 500 4.700 6.696 5.529 5.250 750 1.972 6.343 5.144 5.536 1000 2.373 6.784 5.791 5.762 1250 4.099 6 795 6.795 5.972 5.157 1500 6.402 6.152 5.933 4.477 1750 5.876 5.553 4.424 3.601 2000 5.834 5.441 4.769 3.821 EXAMPLE IV 15 ml ofthe surfactant/ethylene glycol/triacetin/ isopropyl alcohol mixture of Example Ill were used to prepare a mixture as in Example Ill having a CO2/second phase weight ratio fo 85/15. Viscosity measurements are tabulated in TABLE IV.

TABLE IV

Time (minutes) 0 15 60 1 9O Shear rate (Sec-1) Viscosity (cp) 290 5.468 5.167 3.582 2.972 500 3.750 4.629 3.320 2.632 750 3.036 4.405 3.172 2.543 1000 3.311 4.635 3.353 2.783 1250 4.251 5.104 3.673 2.095 1500 4.614 4.581 3.778 3.194 1750 3.944 4.226 3.846 3.326 2000 4.177 4.022 3.749 3.461 EXAMPLE V A solution of 47.7% of ethylene glycol,47.7% of triacetin and 4.6% Fenopon was used to prepare, as in Example 1,a mixture having a liquid CO2/second phase ratio of 85/15. The viscosity was determined to be 10 cp at -10 C and 1000 sec-1 of shear.

EXAMPLE VI The mixture of Example V was made in a modified mixing apparatus wherein the Koch motionless mixer was replaced by either of a 2 or 5 micron in-line filter element. One pass was then required to obtain complete mixing. A viscosity of 14 cp was achieved at -3"C.

EXAMPLE VII Afield trial of a mixture of liquid CO2/diesel oil of 85/15, stabilized by addition of part of an 8 mole ethoxylate resin acid was successfully run. 78 m3 of the mixture was used to hydraulicallyfracture a 1330 metergaswell. 15,000kg of or proppantwas placed at proppantaddition rates of up to 600 kg/m3. Pumping pressure due to friction was 50% of that usually encountered when using unadulterated liquid CO2.

EXAMPLE VIII Asolution of 47% triolein,47% of a mixtureofa homologous series of alkoxy-terminated polyglycols (trade name Selexol) and 6% Fenopon was used as in Example VI to make a mixture having a liquid CO2/second phase ratio of 95/5. The viscosity of this mixture was 24 cp at - 1 55C at 500 sec- of shear.

Other glycol ethers were tested (diglyme, triglyme, etc) but the viscosities were significantly lower.

EXAMPLE IX A solution of 47% dibutyl phthalate,47% of either ethylene or propylene glycol and 6% Fenopon was used as in Example Vlli (95/5 liquid CO2/second phase) to give viscosities of 8 and 6 cp respectively.

EXAMPLE X Solutions of 47% oleyl alcohol, 47% diesel oil and 6% sorbitan oleate surfactants (Spans) were used as in Example Vlil to make mixtures having a liquid CO2/second phase ratio of98/2. Viscosities ranged from 4to 13 cp depending on the qualityofthe oleyl alcohol used. The use ofdecanol or oleic acid to replace the oleyl alcohol gave lower viscosities (1 -5 cp), still at a concentration of 98% liquid CO2.

Replacementof part ofthe oleyl alcohol by oleic acid results in a controllable viscosity change from 7 to 20 cp,while maintaining the liquid CO2 at 98%, (e.g.

replacement of 10% ofoleyl alcohol gave 8cp, 20% gave 20 cp,30 and 40% gave7cpeach).

The viscosities ofthe mixtures prepared in Exam ples 11,111 and IV were found to be higher than would be suggested by simple dilution theory.

Claims (14)

1. Afracturing composition for treating a subterranean formation containing hydrocarbon de posits which comprises a mixture of a major propor tion of liquid carbon dioxide and a minor proportion of an immiscible organic liquid which does not adversely react with the carbon dioxide, the subterra nean formation orthe hydrocarbon therein, and a surfactant.

2. A composition as claimed in claim 1, in which the surfactant is compatible with the organic liquid, the organic liquid/surfactant combination imparting to the liquid carbon dioxide improved properties of viscosity and lubricity.

3. Afracturing composition as claimed in claim 1 or 2, in which the immiscible, organic liquid is selected from (a) crude oil, (b) diesel oil, (c) a solution oftriacetin and ethylene glycol with orwithout isopropyl alcohol, (d) a solution oftriolein and alkoxy-terminated polyglycol, (e) a solution of dibutyl phthalate and ethylene or propylene glycol, (f) a solution oftriolein and a glycol ether, (g) a solution of alcohols having upto 30 carbon atoms and diesel oil, and (h) a solution of alcohols having up to 30 carbon atoms and diesel oil wherein partofthealcohol is replaced with an organic acid having up to 30 carbon atoms, or mixtures thereof.

4. Afracturing composition as claimed in any of claims 1 to 3, which comprises from 75% to 99.5% by weightofliquid carbon dioxide, from 90.5% to 25% byweight ofthe immiscible organic liquid and upto 10% byweight of the surfactant

5. Afracturing composition as claimed in any of claims 1 to 4, in which the surfactant is selected from ethoxylated resin acids, ammonium linear alcohol ethoxysulfate and sorbitan oleate.

6. Afracturing composition as claimed in any of claims 1 to 5, which also contains a solid propping agent.

7. Afracturing composition according to claim 1, substantially as hereinbefore described with particular reference to anyoftheforegoing Examples.

8. Amethodoffracturing a hydrocarbon-containing, subterranean formation penetrated by a well bore comprising the steps of: (a) admixing a major portion of liquid carbon dioxide with a minor portion of an immiscible organic liquid which does not adversely reactwith the carbon dioxide, the subterranean formation orthe hydrocarbon therein and a surfactant; (br introducing the admixture into the well bore and formation at a temperature below the critical temperatureofthe carbon dioxide and ata pressure to maintain the admixture as a liquid; (c) maintaining the admixture within the formation sufficiently long to permitthe admixture to be volatilized; and (d) fracturing theformationwith the volatilized admixture.

9. A method as claimed in claim 8, in which the surfactantis compatible with the organic liquid, the organic liquid/surfactant combination imparting to the liquid carbon dioxide improved properties of viscosity and lubricity.

10. A method as claimed in claim 8 or9,in which the surfactant is selected from (a) crude oil, (b) diesel oil, (c) a solution oftriacetin and ethylene glycol with orwithoutisopropyl alcohol,(d) a solution oftriolein and alkoxy-termianted polyglycol, (e) a solution of dibutyl phthalate and ethylene or propylene glycol, (f) a solution oftriolein and a glycol ether, (g) asolution ofalcohols having up to 30 carbon atoms and diesel oil, and (h) a solution of alcohols having upto 30 carbon atoms and diesel oil wherein part ofthe alcohol is replaced with an organic acid having up to 30 carbon atoms, or mixtures thereof.

11. A method as claimed in any of claims 8to 10, in which there is admixed from 75% to 99.5% by weight of liquid carbon dioxide, from 0.5% to 25% by weightofthe immiscible organic liquid and upto 10% byweightofthe surfactant.

12. A method as claimed in any of claims 8 to 11, in which the surfactant is selected from ethoxylated resin acids, ammonium linear alcohol ethoxysulfate and sorbitan oleate.

13. A method as claimed in any of claims 8 to 12, in which the admixture also contains a solid propping agent.

14. A method according to claim 8, substantially as hereinbefore described with particular reference to anyoftheforegoing Examples.