Application of gas wettability alteration to improve methane drainage performance:A case study

Hydraulic fracturing technique is widely used for methane drainage and has achieved good effects in numerous coal mines,but negative effects may occur as the fracturing fluids are absorbed into the coal seam.Gas wettability alteration(GWA)technology can be used as it can enhance the gas and water mobility during dewatering process as a result of capillary pressure change.However,there have been few reported field tests in coal mines using GWA technology.This paper describes a pilot-scale field test in Xinjing coal mine,Yangquan,China.The fluorocarbon surfactants perfluorooctyl methacrylate monomer-containing polymethacrylate(PMP)was used to alter the wettability of coal seam to strong gas-wetness during the hydraulic fracturing process.The study focuses on the comparison of two boreholes(Boreholes#9 and#10)and one other borehole(Borehole#8)with and without using GWA approach.A well-defined monitoring program was established by measuring the dewatering volume of the fracturing fluid and the drainage volume of methane as well as the concentration.The field test results showed that the average methane drainage rates of Boreholes#9(39.28 m^(3)/d)and#10(51.04 m^(3)/d)with GWA treatment exceeded that of Borehole#8(21.09 m^(3)/d)without GWA treatment,with an increase of 86.3%and 142.1%,respectively.The average methane concentrations of Boreholes#9(4.05%)and#10(6.18%)were 64.6%and 151.2%higher than that of Borehole#8(2.46%),respectively.On the other hand,the dewatering ratio of Boreholes#9(4.36%)and#10(3.11%)was almost 19 times and 13 times greater than that of Borehole#8(0.22%).These field test results were in agreement with the experimental data.The significant increase in both methane concentration and dewatering ratio demonstrated that GWA technology could be applied for enhanced methane drainage in coal mines.Important lessons learned at Xinjing coal mine might be applied to other coal mines in China and elsewhere.

Effect of a synthesized anionic fluorinated surfactant on wettability alteration for chemical treatment of near-wellbore zone in carbonate gas condensate reservoirs

The pressure drop during production in the near-wellbore zone of gas condensate reservoirs causes condensate formation in this area.Condensate blockage in this area causes an additional pressure drop that weakens the effective parameters of production,such as permeability.Reservoir rock wettability alteration to gas-wet through chemical treatment is one of the solutions to produce these condensates and eliminate condensate blockage in the area.In this study,an anionic fluorinated surfactant was synthesized and used for chemical treatment and carbonate rock wettability alteration.The synthesized surfactant was characterized by Fourier transform infrared spectroscopy and thermogravimetric analysis.Then,using surface tension tests,its critical micelle concentration(CMC)was determined.Contact angle experiments on chemically treated sections with surfactant solutions and spontaneous imbibition were performed to investigate the wettability alteration.Surfactant adsorption on porous media was calculated using flooding.Finally,the surfactant foamability was investigated using a Ross-Miles foam generator.According to the results,the synthesized surfactant has suitable thermal stability for use in gas condensate reservoirs.A CMC of 3500 ppm was obtained for the surfactant based on the surface tension experiments.Contact angle experiments show the ability of the surfactant to chemical treatment and wettability alteration of carbonate rocks to gas-wet so that at the constant concentration of CMC and at 373 K,the contact angles at treatment times of 30,60,120 and 240 min were obtained 87.94°,93.50°,99.79°and 106.03°,respectively.However,this ability varies at different surfactant concentrations and temperatures.The foamability test also shows the suitable stability of the foam generated by the surfactant,and a foam half-life time of 13 min was obtained for the surfactant at CMC.

Evaluation of gas wettability and its effects on fluid distribution and fluid flow in porous media

The special gas wettability phenomenon of reservoir rocks has been recognized by more and more researchers.It has a significant effect on efficient development of unconventional reservoirs.First,based on the preferentially gas-covered ability and surface free energy changes,definition and evaluation methods have been established.Second,a method for altering rock wettability and its mechanisms have been studied,surface oriented phenomena of functional groups with low surface energy are the fundamental reason for gas wettability alteration of rock.Third,the effect of gas wettability on the surface energy,electrical properties and dilatability are investigated.Last,the effects of gas wettability on capillary pressure,oil/gas/water distribution and flow are investigated with capillary tubes and etchedglass network models.The gas wettability theory of reservoir rocks has been initially established,which provides theoretical support for the efficient production of unconventional reservoirs and has great significance.