| There are fractures,pores and beddings at micro-nano scale developed in shale gas formation.The shale instability occurs frequently in horizontal well drilling,which seriously hinders the pace of shale gas drilling.Oil-based drilling fluid is the preferred system for drilling shale gas formation,but its application is limited due to its poor environmental protection and high cost.Exploring water-based anti-sloughing drilling fluids that can replace oil-based drilling fluids has become a key technology for efficient development of shale gas resources,and is also a worldwide engineering and technical problems.Shale stability is closely related to the micro-fabric and physicochemical characteristics of shale minerals.Aiming at the shale gas strata of Longmaxi formation in southern Sichuan and Jingmen exploration areas,the micro-fabrics and physicochemical properties of shale gas strata were deeply analyzed.The key factors leading to wellbore instability were fractures,pores and beddings at micro-nano scale development,strong local hydration and capillary effect.The high content of organic matter in shale also had an important impact on shale instability.In addition,based on the theory of fracture mechanics,the effects of pore pressure,drilling fluid column pressure,hydration stress and capillary force on the propagation of micro-fractures in shale were comprehensively analyzed.The micro-fractures propagation behavior and damage characteristics of shale samples treated with different testing fluids were studied.Strengthening dense plugging of micro-fractures,inhibiting shale hydration and weakening shale capillary effect could effectively inhibit the propagation of micro-fractures and mitigate shale microstructure damage.Adopting emulsion polymerization technology,nano polymer microspheres SD-PM as a novel shale stabilizer was prepared.SD-PM had excellent dispersion stability in high temperature,high salinity solutions and water-based drilling fluids,and SD-PM had good compatibility with water-based drilling fluids.The plugging effect was evaluated by pressure transmission and permeability test,and the weakening capillary effect was tested by spontaneous imbibition experiment.SD-PM could greatly improve shale stability due to the synergistic effect of micro-nano scale dense plugging and shale capillary effect weakness.The synergistic mechanism of SD-PM for improving shale stability was further revealed by means of scanning electron microscopy,nuclear magnetic resonance,wettability experiment and surface tension test.Adopting free radical polymerization technology,amphoteric polymer/nano-silica SD-NFC with core-shell structure as a novel shale stabilizer was prepared.SD-NFC had excellent dispersion stability in high temperature,high salinity solutions and water-based drilling fluids,and SD-NFC had good compatibility with water-based drilling fluids.Pressure transmission and permeability test were used to evaluate the plugging effect of SD-NFC.Na-bentonite plate immersion and interval transit time test were applied to evaluate the hydration inhibition effect of SD-NFC.The elasticity of polymer structure and the rigidity of nano-silica combined to form a dense plugging layer on shale surface,and the plugging performance was significantly better than that of SD-PM.SD-NFC could greatly improve shale stability due to the synergistic effect of micro-nano scale dense plugging and shale hydration inhibition.The synergistic mechanism of SD-NFC for improving shale stability was further revealed by means of scanning electron microscopy,nuclear magnetic resonance,adsorption characteristics test,X-ray diffraction analysis and wettability test.Based on the experimental research of micro-fabric characteristics,physicochemical properties and micro-fractures propagation behavior of shale in the target area,the shale instability mechanism was determined.The technical measures of anti-sloughing drilling fluids for shale gas formation were put forward.Micro-nano scale shale stabilizers SD-PM,SD-NFC and strong inhibitor OMB were used as key additives to optimize water-based drilling fluids SDSF.SDSF could effectively plug the fractures and pores in shale,inhibit shale hydration and weaken shale capillary effect.The comprehensive performance of SDSF was close to that of oil-based drilling fluids. |
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