Experimental Study On Imbibition And Flowback Of Organic-rich Shale Fracturing Fluids

Shale gas reservoirs are usually developed by horizontal Wells plus staged fracturing.After fracturing,the gas well return rate is usually less than 30%,and the field statistics show that Wells with low return rate tend to have higher production.Water-shale interaction induces fracture initiation and propagation and continuous micro-modification of reservoir is considered as one of the main reasons,which also becomes the theoretical basis for the application of post-pressure soaking in the mine.However,the process of imbibition of fracturing fluid during shut in,the effect of imbibition of fracturing fluid on shale gas Wells,and the effect of imbibition on shale gas Wells’ output are still not clear,and relevant theories still need to be further studied.There is almost no research on the effect of high salinity flowback fluid on gas reservoir fracture in the flowback process of gas Wells after water-shale interaction.Therefore,it is of great significance to carry out experimental research on the imbibition and backflow of fracturing fluid,and optimize the gas well soaking and backflow system through experimental results and relevant theories.This paper takes the Longmaxi formation shale in Changning block as the research object,The composition and pore structure of the shale in the study area are analyzed,Experiments were carried out to evaluate the effects of fracturing fluid on reservoir permeability under confining pressure,with permeability as the evaluation index.According to the experimental results and literature investigation,the damage control countermeasures of fracturing fluid were proposed,and Suggestions were put forward for the optimization of soaking and flowback system.The specific research results are as follows:The improvement mechanism and improvement scope of the permeability of shale gas reservoir by the retention fracturing fluid in the process of gas well shut-in are revealed.The permeability of microfractures after shale imbibition fracturing fluid is about 20%higher than that of developed rock samples and bedding base blocks.The interaction between fracturing fluid and shale dissolves soluble salts in pores and fractures,reduces the mechanical strength of shale,and promotes the initiation and expansion of micro fractures along the direction of stratification and original fractures.The initiation and expansion of micro fractures also reduces the water saturation of the main fractures,leading to the self-release of water facies traps,and the overall improvement of gas reservoir permeability through the comprehensive effect.When the imbibition time reached 60d,the imbibition depth of the base block was 25 cm,and the imbibition depth of the crack was 230cm.The characteristics of shale ion dissolution behavior and solid phase distribution in the process of fracturing fluid imbibition were clarified.During the shut-in of shale gas Wells,the imbibition of fracturing fluid will dissolve the soluble salt filled in shale pores/fractures or mix with reservoir high-salinity fluid,which will continuously increase the salinity of the fracturing fluid in the retained reservoir,and the increase of the salinity of fracturing fluid will inhibit shale imbibition and micro-fracture expansion.With the increase of gas well flowback time,the solid particle size in flowback fluid first increases and then decreases.The main solid particle size is about 10 μm,which is in the same level as the reservoir micro-fracture size.The damage to shale fracture permeability cannot be ignored.The damage mechanism of fracturing fluid to gas reservoir in different flowback stages is revealed.At the initial stage of gas well flowback,that is,the liquid-solid two-phase flow stage,the phase trapping effect of fracturing fluid on fractures and the plugging effect of solid phase residue on fractures reduced the fracture permeability by more than 50%.In the middle and late stage of gas well fracturing fluid flowback,that is,the gas phase flow stage,the water phase trap is alleviated and the damage rate of fracture permeability is reduced by more than 10%.However,particle migration occurs in the solid phase residue of fracturing fluid under the action of airflow erosion.In the later stage of gas well flowback,the soluble salt in the supersaturated fracturing fluid precipitates out under the action of gas phase fluid carrying,which reduces the effective volume of fractures and blocks the gas seepage channel.The comprehensive effect causes serious damage to shale permeability in the flowback process of fracturing fluid.The idea and method of fracturing fluid damage control without backflow in shale gas reservoir are put forward.Based on the effect of fracturing fluid on gas reservoir reconstruction and the damage effect of fracturing fluid flowback.The idea and method of non-flowback control of fracturing fluid damage were put forward,which is adding strong oxidant into fracturing fluid and synergetic hydration to expand the reform scope of fracturing fluid infiltration and absorption,and finally achieving the goal of non-flowback fracturing fluid.