Liquid CO2/hydrocarbon oil emulsion fracturing system

[0002] The present invention relates to a novel formulation for a fracturing fluid for use in fracturing subterranean formations such as oil and gas wells.

[0003] It is known to utilize an emulsion consisting of water or water with a polymer therein as the continuous phase, and from about 50% to about 80% oil (crude or refined) as the discontinuous phase, with a surfactant such as sodium tallate or a quaternary amine in the aqueous phase. This is shown in Polymer Emulsion Fracturing (Sinclair et al), a 1973 publication of the Society of Petroleum Engineers. It is also known, as shown in U.S. Pat. No. 4,233,165, to utilize a water-in-oil emulsion as a fracturing fluid. In that case, a surfactant soluble in oil is utilized to maintain an aqueous internal phase in a continuous diesel phase.

[0004] Moreover, stable foams have been developed where the external phase is water and the internal phase is CO₂and the foam is stabilized by selective chemical foamers. It has been found that the internal phase (CO₂) in such foam has to be a minimum of 53%. The Western Company of North America has developed two foams, the first one where a combination of CO₂and Nitrogen is used to create a stable foam with the addition of chemical foamers. The combined internal phase of gas is still over 52% by volume. The second foam involves stabilizing the CO₂foam with a crosslinker without the use of foamer. By using less CO₂the produced gas from a well can be put on a pipe-line faster without flaring for a long time to meet pipeline CO₂content regulations. Moreover, all the above systems use gelling agents from guar to CMHPG to stabilize the foams. There have also been attempts made to foam CO₂in diesel or crudes with limited success because of the natural antifoam ability of these oils.

[0005] Conventionally, instead oil gels have been used. However, with the potential for downstream problems of the phosphate esters used in gelling oils, there is a need for alternatives. Attempts have been made to pump CO₂with gelled oils, mainly to use the energy of the phase change from liquid to CO₂gas to clean up wells efficiently. However, most phosphate ester based oil gels are not compatible with CO₂; when these systems are made to survive under CO₂conditions, breaking with conventional breakers for these systems, which tend to be high pH buffers tend to be neutralized by the CO₂or in the presence of any water, will chelate the aluminum or iron crosslinkers (complexers) and cause precipitation resulting in formation change.

[0006] The approach of the present invention is to provide an emulsion of oil in liquid CO₂. This is possible with the selection of an appropriate surfactant, in particular methoxy-or ethoxy nonafluorobutane. This approach provides an emulsion of high sustained permeability, without the need or gelling and breaking, and the costs associated therewith. The amount of CO₂required is less than 48%, which results in cost effectiveness, and the inherent ability of CO₂simply to gasify and escape to the atmosphere indicates simplified clean-up after fracturing.

[0007] The object of the present invention, is to provide a fracturing fluid consisting of an emulsion of hydrocarbon in a liquified gas such as liquid CO₂.

[0008] A further object of the present invention is to provide an emulsion of liquid CO₂in which is dissolved a surfactant, and in which are suspended droplets of a hydrocarbon fluid, for use in fracturing oil and/or gas well formations.

[0009] In a broad aspect, then, the present invention relates to a fracturing fluid consisting of an emulsion having a continuous phase of a liquified gas, a discontinuous phase of a hydrocarbon, and a surfactant soluble in the two phases.

[0010] The present invention will be described by way of example below, and in connection with the description the following drawings are provided:

[0011] FIG. 1 is a graph of Diesel/CO₂solubility;

[0012] FIG. 2 is a graph of Frac Oil 200/CO₂solubility.

[0013] In order to determine the solubility of CO₂with two candidate hydrocarbons, labstock diesel and Frac Oil 200™ (a hydrocarbon based fracturing oil from Amoco Canada Petroleum Company Ltd., Calgary, Alberta) the following test was conducted.

[0014] The tests were conducted with labstock diesel and Frac Oil 200. The volumes were visual within a sight glass of a known volume. Due to the small volume in the sight glass (75 mL) weight %, was not used. 25 mL of hydrocarbon was added to the cell at −20° C., CO₂was added until total volume was 75 mL (50 mL of CO₂). Pressure in the cell was 500 psi for the diesel and 700 psi for the Frac Oil 200. The cell was then observed at various temperatures to see the volume changes in hydrocarbon and CO₂.

[0015] The observed results are shown in FIGS. 1 and 2.

[0016] The volume change could be caused by a number of factors: CO₂volume increases with lower temperature and the solubility of CO₂in hydrocarbon decreases with lower temperature. The volume of the hydrocarbons both increased about the same, 25 mL to 36-38 mL, indicating both the fluids had similar CO₂solubility. Both fluids match up well in solubility and volume changes at the various temperatures. The pressure of the cell in both tests did not increase until the temperature was 10° C. and vapor was created. When the vapor is created then the fluid changes from a polyemulsion into a CO₂foam. However, since the volume of CO₂may not be above 53 quality, there will be little viscosity to this foam. If the pressure exceeds 500 psi at −20° C. the emulsion will be formed until the temperature increases to 5-10° C. or enough pressure is applied to keep the CO₂in a liquid phase.

[0017] The emulsions of the present invention were then created, using labstock diesel, liquid CO₂and hydrofluoroether surfactant HFE 7200 and HFE 7100 from 3M Chemicals. HFE 7200 (C₄F₉OC₂H₅) consists of two inseparable isomers with essentially identical properties. These are (CF₃)₂CFCF₂OC₂H₅(CAS No. 163702-06-5) and CF₃CF₂CF₂CF₂H₅(CAS No. 163702-05-4). HFE-7100 (C₄F₉OCH₃) consists of two inseparable isomers with essentially identical properties. These are (CF₃)₂CFCF₂OCH₃(CAS No. 163702-08-7) and CF₃CF₂CF₂CF₂OCH₃(CAS No. 163702-07-6). Both surfactant are soluble in liquid CO₂and diesel, and both produced similar results.

[0018] The surfactant is added at a rate of from about 1 to 30, preferably about 4 L/m³to diesel. The diesel/surfactant solution is then mixed with liquid CO₂at a rate of about 5%-48% CO₂, preferably ;about 30-40% CO₂. The mixture is then vigorously agitated or subject to shear, resulting in a stable emulsion (stability increasing with shear), with appropriate viscosity for use as a fracturing fluid.