The Research On Borehole Stability Technology Of Hard-brittle Shale In Xinchang Gas Field Xujiahe

The newly proved reserves in Xujiahe section,Xinchang gas field in Western Sichuan is one of the four largest strategic zones determined by SINOPEC.The geological condition is complicated due to the tectonic movement taken place frequently,drilling troubles often occur because of hole sloughing caused by brittle mudstone and shale,resulting in longer drilling time and higher cost,the development schedule has been restricted seriously.The paper describes the mechanism of unstable wellbore caused by brittle mudstone and shale,affecting factors and controlling technique,and provides a basis for stabilizing wellbore and safe drilling in high efficiency.Through the comprehensive analysis of the data,including tectonic background that related to wellbore stability,lithology and lithofacies,pore pressure and temperature etc,combined with geological profile,the relationship among geological environment,wellbore sloughing degree and real stress status has been obtained.By using various testing methods in laboratory,the fabric feature and physical and chemical properties of brittle mudstone and shale in Xu Jiahe section are determined,related to the main composition and contents of mineral,microstructure features,hydration and swelling,as well as dispersing degree etc.According to Mohr-Coulomb shear failure criterion,considering permeability and stress distribution under the status of well inclination,collapse pressure in straight well and directional wellshas been established,and got the pressure curves and distribution for stabilizing wellbore in Xu Jiahe section.Based on the several factors,including the inflow feature of foreign fluids,the effect of it on microstructure and mechanical characteristics,deformation and failure inside the rocks,and introduced CT micro-imaginganalysis,the microstructure changing course of brittle mudstone and shale,the secondary fracture change,damaging ways,unstable chemical effect on mudstone and shale,etc,have been obtained.With the invasion of foreign fluids,self-absorption hydration of capillary,the change regularity with time and space,soaking test of drilling fluid at high pressure and high temperature,the relation of original fracture and its destruction caused by hydration from capillaryhas been found.Through the optimization of prescription,a new anti-self-adsorption agent has been developped,it has oil and water wet characteristics,which provides a scientific basis for studying the unstable mechanism of wellbore in brittle mudstone and shale formation.According to Fabric feature of brittle mudstone and shale,investigation of unstable hydration has been done to the macro-degree,the evaluation of wellbore stability for mudstone and shalecan be made in different ways.Through the rock mechanics experiments with the fluid soaking under the condition of certain temperature and pressure,combined with microstructure analysis and numerical simulation,The strength weakening effect of mudstone and shale,and the effect of different orientation of structure plane on unstable mode have been investigated with different fluids,formed a three-dimensional structure plane mode for analyzing sidewall stability,made up a deficiency of investigation in brittle mudstone and shale performance.Through the analysis of indoor testing data,combined with drilling problems encountered,the unstable mechanism,performance and controllable factors of brittle mudstone and shale are determined,and developed a countermeasure and drilling fluid that called multi-functional polyamine aluminum inhibitor to prevent sloughing of wellbore,wellsite testing and indoor evaluation show that it is suitable for stablizing sidewall in Xujiahe section of Xinchang gasfield,so as to lower drilling troubles,create huge economic and social value in the future.