Study On The Distribution Of Fracturing Fluid In Shale And Its Influence On Pore Structure

Hydraulic fracturing is a key technology for shale gas reservoir reconstruction.A large amount of fracturing fluid is injected into the shale layer at a high pressure to generate fractures to increase the permeability and connectivity.However,the fracturing fluid is always retained in shale,and causing changes in the pore structure of shale,which has a great impact on the storage and flow of shale gas.Therefore,studying the distribution of fracturing fluid in shale and its influence on the pore structure is important for evaluating the hydraulic fracturing results.In this study,the shale samples were collected from the Pengshui area of southeast Chongqing,and the distribution of fracturing fluid in the shale pores and the influence of the fracturing fluid on the pore structure are analyzed by several experimental methods,including low pressure nitrogen adsorption experiment,fracturing fluid immersion experiment,nuclear magnetic resonance(NMR),centrifugal experiments and X-ray diffraction(XRD).We focused on the influence of acidic and neutral slick water fracturing fluid on shale pore structure,basic physical properties,wettability,and the distribution characteristics of fracturing fluid in the shale pore structure.Based on low pressure nitrogen adsorption experiment results,the fractal characteristics of pore structure before and after fracturing fluid intrusion were further analyzed.The research of this study has achieved the following results:(1)The mineral composition of shale samples in Pengshui District is mainly brittle minerals,followed by clay minerals with a small amount of pyrite.The porosity is basically in the range of 0.64%to 1.12%,The permeability is mostly between 0.005-0.007m D.The contact angle is generally between 20°and 55°,and the average contanct angle is 35.54°.The pore structure of the samples in this study is mostly open pores,with strong heterogeneity and complex structure.(2)Low-temperature liquid nitrogen experiment and fractal analysis are carried out.After the action of acid fracturing fluid,the specific surface area and pore volume of reservoir decrease,and the average pore diameter increases.The results show that the pore structure before and after immersion in the neutral fracturing fluids tend to be smaller.The relationship between the fractal dimension D₁ and the immersion time before and after the two kinds of fracturing fluid immersion is better than D₂.After soaking in acidic fracturing fluid,pores larger than 4 nm have greater influence on the change of basic parameters.After the action of neutral fracturing fluid,the pores less than 4 nm have greater influence on the surface,and the pores greater than 4 nm have greater influence on the pore volume and average pore diameter.(3)The wettability experiments,nuclear magnetic resonance,and centrifugal experiments were carried out.We found that the samples after saturated by fracturing fluids,the corresponding wettability will be less hydrophilic.In addition,we also found the effect of acidic fracturing fluid in the early stage of shale is affected by the expansion of clay minerals and the composition of the fracturing fluid,which may reduce the pore volume and pore connectivity.In the late interaction stage,the acidic fracturing liquid may induce solute pores due to the chemical reactions,so that enhance pore connectivity.The effect of neutral fracturing fluid is generally affected by clay mineral swelling and high pressure.The study found that the average pore size had the greatest effect on contact angle.Combined with NMR and centrifugal experiments,it was found that fracturing fluid was more likely to be trapped in reservoirs than formation water.