Enhanced Water Recovery Improves Stimulation Results

Abstract

Summary. The effects of capillary pressure, wettability, and relative permeability in controlling load water recovery following hydraulic-fracturing treatments have been examined. Laboratory studies have indicated that the alteration of wettability to control capillary pressure and/or relative permeability can promote a rapid, thorough cleanup of injected water. Field applications of these concepts have resulted in enhanced load water recoveries and higher production because of longer effective fracture lengths and/or higher effective fracture conductivities after treatment cleanup. Introduction The impact of water retention on hydrocarbon production has been discussed by several investigators. In a conventional water-wet treatment, water strongly associates with sandstone and limestone surfaces. During cleanup in a water-wet condition, the hydrocarbon tends to break through the water, leaving high water saturation and low relative permeability to hydrocarbon. On the surface, one may see only 10 to 15 % of the treating fluid recovered when the hydrocarbon breaks through; the remaining fluid is held in place by high capillary pressures. If this hydrocarbon break-through is near the wellbore, cleanup of the remainder of the treated area may be slowed or stopped. Posttreatment analyses have indicated effective fracture lengths of less than 100 ft [less than 30 m] when the jobs were designed for 1,000 ft [305 m]. Application of enhanced-load-recovery additives is meant to leave contacted surfaces nonwet. The nonwet surface exhibits a water/hydrocarbon/solid contact angle of nearly 90 degrees [1.6 rad], meaning that neither water nor hydrocarbon strongly associates with the surface. During cleanup in a nonwet state, the hydrocarbon displaces the treating fluid in a piston-like fashion. At hydrocarbon breakthrough, a greater percentage of fluid is recovered, leaving a lower water saturation and a higher relative permeability to hydrocarbon. During fracture cleanup, the nonwet condition minimizes rapid hydrocarbon breakthrough near the wellbore, and load water is displaced by the hydrocarbon. This delay in hydrocarbon breakthrough enhances the chance of the fracture to clean up from the tip. The result is that a greater percentage of water is recovered and the effective fracture length following cleanup is greater. This work examines the effects of capillary pressure, wettability, and relative permeability in controlling load water recovery. Results have shown that alteration of wettability to control capillary pressure and/or relative permeability can promote a rapid, thorough cleanup of injected aqueous fracturing fluids. Field results, where these concepts have been used, have shown enhanced load water recoveries and higher production because of longer initial effective fracture lengths and higher effective fracture conductivities after cleanup. Experimental Flow-Column Preparation. The column was made from stainless- steel tubing of 3/4 -in. [ 1.91-cm] OD. Column length, including the end fittings, was 10 in. [30.48 cm]. Outer female pipe-thread fittings on each end of the column contained a 1/8-in. [0.32-cm] -diameter stainless-steel tube centered in the fitting and cemented into place with epoxy resin. Epoxy resin was also used to fill the remaining void inside the fitting, eliminating dead volume. This resin was poured smooth to the same level as the 1/8-in. [0.32-cm] tube. The tube entrance was then countersunk with a drill bit of diameter larger than that of the tube. Flow channels away from the entrance of the tube were made in a wagon-spoke pattern. This ensured that the flow would enter the sand column in a more uniform pattern, not in a channel-like flow. A screen of approximately 200 mesh was made to place inside each fitting to prevent sand from entering and plugging the small tubing. The sand was a sieved 100/200-mesh Oklahoma No. 1. About 123 g of this sand was used to fill the column on each test. One end of the column was fitted together and the column suspended vertically. The sand was added slowly while the tube was vibrated to cause the sand to contact the sides of the column and to achieve a more uniform packing. Testing Procedure With Oil. Test equipment used to evaluate enhanced-water-recovery compounds in low-permeability sand columns with oil is illustrated in Fig. 1. The complete system contains a digital balance, fluid reservoir, fluid-injection pump, sand column, 0- to 100-psi [0- to 690-kPa] pressure transducer with digital readout, oil-injection pump, and oil reservoir. API standard brine was flowed into the column at 1.0 cm3 /min until the column was saturated. PV was determined by measuring fluid intake into the column. Isopar M TM, a refined oil with a viscosity of about 2.46 cp [2.46 mPa.s], was then flowed in the reverse direction at 1.0 cm3/min to a residual water saturation. Enhanced-water-recovery treating fluid was flowed into the column in the same direction as the API standard brine at 1.0 cm3/min. Injection ceased when the first drop of fluid was eluted. Isopar M was then flowed in the opposite direction of the enhanced-water-recovery treating fluid at rates of 0.5, 1.0, 5.0, and 10.0 cm3/min for 2 hours at each rate or until pressure leveled out. Volume of water recovered and pressure at each rate were recorded. Equilibrium water saturation within the column at each rate and the effective oil permeability were then calculated. It was generally found that 90 to 95 % of the equilibrium water saturation and pressure were reached at each rate within 2 hours. Testing Procedure With Gas. Test equipment used to evaluate enhanced-water-recovery compounds in low-permeability sand columns with gas is illustrated in Fig. 2. The complete system contains a digital balance, fluid reservoir, fluid-injection pump, sand column, visual flow cell, gas flowmeter, pressure gauge, manometer, and regulated nitrogen supply. Nitrogen was flowed through the sand column to determine maximum flow rate and the permeability of the column to nitrogen at a pressure differential of 10 psi [69 kPa]. API standard brine was flowed into the column in the reverse direction at 1.0 cm3/min until the column was saturated. PV was determined by measuring fluid intake into the column. Nitrogen was then flowed in the reverse direction at a pressure differential of 5 psi [34.5 kPa] to 50% water saturation. SPEPE P. 515^