Research On The Performance Optimization Of Water-based Drilling Fluid In Longmaxi Formation Shale In Zhaotong

The shale gas in the Zhaotong block has large geological reserves with high-quality reservoir thickness,high organic carbon and siliceous content,which is favorable for shale gas development.Drilling technology of horizontal well is one of the key technologies for shale gas development.Nowadays,horizontal wells in shale sections are often drilled with oil-based drilling fluid,which effectively solve engineering problems such as well-bore instability,high horizontal friction and large torque.However,with the continuous improvement of environmental protection requirements and the pursuit of economic benefits,shale gas water-based drilling fluids have received more and more concerns.Nevertheless,conventional water-based drilling fluids cannot fully meet the construction requirements of horizontal well drilling in shale sections.Domestic and foreign research institutes have carried out a series of research and field application on high-performance water-based drilling fluids suitable for horizontal wells in shale sections.At present,the high-performance water-based drilling fluid has been applied in the Zhaotong shale gas demonstration area and achieved satisfying results.It has made great progress in stabilizing the borehole wall and reducing the friction resistance,but it has also revealed the problems that the drilling fluid is vulnerable to geological conditions and has insufficient regional adaptability,which needs further optimization and improvement.This paper takes the Longmaxi formation shale in Zhaotong as the research object,and compares the mineral composition,hydration characteristics and surface wettability of rock samples in different well areas.This paper also optimizes the water-based drilling fluids from the aspects of hydration inhibition,surface wettability and high efficiency lubrication.Following understandings and the results are obtained:The mineral composition of the Longmaxi Formation shale in Zhaotong is mainly composed of quartz and clay minerals which are dominated by illite,but the mineral composition of different wells is different.In some well areas,the content of active clay components such as Iraqi/Mongolian mixed layer increased,and the hydration ability increased as well.Therefore,in the design and optimization of drilling fluid system,targeted research should be carried out in combination with the mineral composition characteristics of different regions.Through the bentonite pulping and linear expansion experiments,the treatment agents QBZ and MK with good inhibition ability are preferred,and the inhibition performance is further improved by synergistic compounding;the surface modifier G1230 is chosen by the test of surface contact angle,which can effectively increase the water contact angle on the surface of the rock sample and improve the surface hydrophobicity;based on the test of extreme pressure lubrication,two kinds of lubricants are selected to reduce the lubrication coefficient of extreme pressure to less than 0.1,which further improves the lubrication performance.Based on the preferred key agent,the optimal addition amount of key agent in water-based drilling fluid is analyzed through a large number of drilling fluid compatibility experiments,and the formula of shale water-based drilling fluid is obtained as follows:3%soil slurry+2%JS-2a+2%NBG-2+3%QBZ+5%MK+1%G1230+1%SKS-1+1%SKS-2+7%KC1+barite.Meanwhile,the performance evaluation of the optimized formulation shows that the system has good inhibition,lubricity and hydrophobicity.The extreme pressure lubrication coefficient is reduced from 0.112 to 0.082,the reduction rate is 36.6%;the contact angle is increased from 26°to 93.95°.The temperature capability reaches 120? and the density can reach 1.8 g/cm3,which has a good prospect of popularization and application.