Relative Permeability Modification in Gas/Liquid Systems Through Wettability Alteration to Intermediate Gas Wetting

Abstract

Summary The wettability of Berea and chalk samples for gas/oil and gas/ water fluids were altered from strong liquid wetting to intermediate gas wetting. Two polymers, FC-722 and FC-759, were used to alter the wettability. FC-759 is soluble in water and 1/20 as expensive as FC-722. Gas and liquid relative permeabilities were measured before and after wettability alteration. The results demonstrated a significant increase in liquid-phase relative permeability. Gas-phase relative permeability for a fixed saturation may increase or decrease; however, because of the very high liquid mobility and reduced liquid saturation, the gas mobility also increased for a fixed pressure drop. A number of liquid injectivity tests were also carried out. The results revealed that the liquid-phase mobility could increase significantly when the wettability of rocks was altered from strong liquid wetting to intermediate gas wetting. All the results show clearly that the application of wettability alteration to intermediate gas wetting may significantly increase deliverability in gas condensate reservoirs. Introduction In some gas condensate reservoirs, the well deliverability drops severely owing to liquid dropout around the wellbore.^1-4 Wettability plays an important role in condensate accumulation around the wellbore. The effect of wettability on condensate accumulation in porous media can be explained with the Young-Laplace equation. The capillary pressure, P _c, is proportional to interfacial tension, s, and the cosine of the pseudocontact angle, cos?, and is inversely proportional to pore size, r. For a gas/liquid system, strong liquidwet porous media can hold high liquid saturation because of the low mobility of the wetting phase. Experimental and numerical studies show that the condensate saturation around the wellbore could be reduced by (1) an increase in viscous forces (also gravity forces),^5-6 (2) a decrease in interfacial tension,⁷ (3) gas injection,⁸ or (4) a decrease in liquid wetting.⁹ In 1995, Cowney et al.¹⁰ studied the feasibility of improving gas and brine relative permeabilities in Blue Greek coal and Ohio sandstone for a gas/brine/ rock system by adding surface-tension-reducing agents. They found that it might not be possible to increase gas relative permeability through a lowering of the interfacial tension. Penny et al.¹¹ studied removal of the load water from gas and oil wells through wettability alteration from strong water wetting to intermediate wetting. They showed that the load water could be removed easily from either gas or oil wells when the rock was neither oil wetting nor water wetting. After wettability alteration, the productivity following cleanup increased two to three times. Penny et al.¹¹ and Cowney et al.¹⁰ did not study the wettability alteration for a gas/ oil/rock system. Recently, Li and Firoozabadi^6,9 have proposed the enhancement of gas-well deliverability by wettability alteration from strong liquid wetting to preferential gas wetting in gas condensate reservoirs. The laboratory study by Li and Firoozabadi⁹ showed that a permanent intermediate gas wetting could be established in Berea and chalk through chemical treatment. The major goal of this work is to study the mobility of the gas and liquid phases (both water and hydrocarbon liquids) before and after wettability alteration from strong liquid wetting to intermediate gas wetting. For this purpose, in addition to relative permeability measurements, we also conducted various other tests to demonstrate that liquid mobility can be improved significantly because of wettability alteration. Li and Firoozabadi⁹ used the polymer FC-722 in their wettability alteration. This polymer does not dissolve in water, and the solvent is expensive (20 times the cost of FC-759). In this work, in addition to FC-722, we used the polymer FC-759, which is soluble in water and less expensive than FC-722. This chemical has a specific application in porous media.¹² In this paper, we first present the experimental procedures and the apparatus. Then we discuss the results of (1) the spontaneous imbibition tests without and with initial water saturation; (2) the effect of chemical adsorption on permeability; (3) the gas/oil and gas/water relative permeabilities; (4) the effect of wettability alteration on oil accumulation; and (5) the effect of wettability on oil injectivity. Experimental Fluids and Rocks. Normal-decane (n-C ₁₀) with a specific gravity of 0.73 and a viscosity of 0.92 cp at T=24°C was used as the oil phase. Distilled water was used to prepare 0.2% (wt) NaCl brine as the water phase. The specific gravity and viscosity of 0.2% NaCl brine at 24°C were 1.012 and 1.0 cp, respectively. Air was used as the gas phase. The surface tension is 23.4 dynes/cm for air/n-C10 and approximately 70 dynes/cm for air/water. Berea sandstone and Kansas chalk were the rock samples. For Berea sandstone, the samples were divided into two groups. The first group of Berea samples had an air permeability of 500 md and a porosity of 21 to 22%. The second group of Berea samples had an air permeability of 340 md and a porosity of 20%. For Kansas chalk, the air permeability was 1.3 to 1.5 md, and the porosity was approximately 30 to 32%. Table 1 provides permeability, porosity, dimensions, chemical treatment, and other relevant data for Berea and chalk samples used in our work. In this table, B represents Berea and C represents chalk. Polymer FC-722 and FC-759, manufactured by the 3M corporation, were used to alter the wettability of Berea sandstone and Kansas chalk from strong liquid wet to intermediate gas wet. These two chemicals are the fluoropolymer type with some specific functional groups. Fig. 1 shows the chemical structure of...