Research And Performance Evaluation Of N₂ Foam Fracturing System For Sandstone Reservoirs In Sulige Gas Field

The technology,theory and method of the exploration and development of oil and gas reservoirs have been improved in China.A large number of low pressure and low permeability oil and gas reservoirs have been identified.The reserves of low pressure and low permeability oil and gas fields continue to increase in China.The development of low pressure and low permeability oil and gas fields has been paid more and more attention.The Sulige gas field is a typical low pressure,low permeability and low pore formation,and natural productivity can not reach the demand.Therefore,it is imperative to improve the productivity by fracturing,and it is of great significance to exploit the fracturing fluid technology with strong increase in productivity and good performance to the exploitation of oil and gas reservoirs in China.Compared with conventional water base fracturing fluid,foam fracturing fluid has many advantages such as high viscosity,low filtration rate,strong sand carrying capacity,low residue content,easy return and little damage to formation,and it is suitable for low pressure,low permeability and water sensitive formations.However,the application of foam fracturing fluid is limited because of its stability.In the foam fracturing fluid system,the volume fraction of gas is much larger than the volume fraction of liquid,which greatly increases the surface energy of the liquid,but the material tends to toward the direction of decrease energy.So the stability of foam fluid is poor.Therefore,it is of great significance to prepare a stable foam fracturing fluid.This article studied the properties of three kinds of anionic foaming agent(SDBS,SDS,AOS)and five kinds of amphoteric ionic foaming agent(CHSB,CAB,OA-12,BS-12,CAO)and two kinds of negative nonionic foaming agent(NPES,AES),and through the synergistic effect of compound foaming agent has excellent foam properties,determined the foam fracturing fluid foaming system;To improve the stability of foam fracturing fluid,using acrylamide(AM),acrylic acid(AA)and methacryloyloxyethyl trimethyl ammonium chloride(DMC),2-propylene amide-2-methyl propane sulfonic acid(AMPS)by four elements polymerize a series of polymers,and the polymer with good foam stability was compared with the crosslinking instant guanidine gum,and the foam stabilization system of foam fracturing fluid was determined.The formulation of foam fracturing fluid:0.3%instant guanidine gum+0.2%synthesis polymer+2%KCl+0.2%organic clay anti swelling agent+0.1%bactericide+0.2%AOS+0.05%AES+0.05%OA-12+60%N2+0.1%pH regulator+0.1%ammonium persulfate+0.4%organic boron crosslinking agent.We evaluate the comprehensive performance of foam fracturing fluid system.The viscosity remains above 120 mPa·s,when the system is shearing for 1 hour at 90? and 170 s-1,the system has good resistance to high temperature and shear rate;Sand concentration can reach more than 1000 kg/m3,the sedimentation rate of proppant in the foam fracturing fluid is very small,almost no settlement in 24 hours when the concentration of sand is 1000 kg/m3,a slight settlement at 96 hours,while the conventional fracturing fluid carrying sand concentration of 500 kg/m3 has been completely settlement in 64 hours;After 120min,the gel breaking is complete,and the average surface tension and interfacial tension of gel breaking fluid reach 27.5 mN/m and 0.3004 mN/m respectively,which is favorable for the fracturing fluid reflow;The residue is very low,which is 66.67 mg/L,only about 20%of the standard;The static filtration coefficient is 2.73×10-4 m/min1/2,which is 36%of the conventional fracturing fluid;The core damage rate is only 19.34%.The mechanism of foam stabilization and the decay process were analyzed by electronic Leica microscope.By observing the size of the foam,film thickness with time and the bubble of the two-dimensional structure,explore the foam decay mechanism;According to the structure of the foaming agent and the foam stabilizer in the foam fracturing fluid,the schematic diagram of the possible foaming and foam stabilization mechanism of foam fracturing fluid is drawn.