Study On Shale Water - Based Drilling Fluid In Longxasi Group Of Changning

During the development of shale gas, oil-based drilling fluid is dominant in shale gas well drilling. However, with the continuous improvement of people’s awareness of environmental protection and in order to increase economic efficiency, water-based drilling fluid for shale gradually attracts more attention. Nevertheless, using water-based drilling fluid has a few disadvantages and seriously restricts the application in shale gas well drilling. For example, the lubricating property and cleaning ability of water-based drilling fluid are poor, which will lead to a high friction and high torque phenomenon prone segment in kickoff section and long horizontal section. At the same time, the aqueous phase invasion of water-based drilling fluid will cause hydration, structure damage and well collapse. Therefore, it is very important to research and resolve these issues of water-based drilling fluid for the development of shale gas.This article analyzed fabric characteristics, drilled geological data and engineering data of Changning Longmaxi shale. Based on these analysis, the assessments and optimizations of sealing, fluid loss, inhibitor and lubricant have been studied. Through a lot of the compatibility tests, a water-based drilling fluid system is initially developed which can be used in horizontal shale well. And this system has been carried on the field application test. Following understandings and the results are obtained:(1) The studies showed that Changning Longmaxi shale mainly contains quartz and clay. The clay minerals are mainly illite, with a little illite/smectite formation and chlorite. According to SEM analysis, there are well developed micron-sized pore throats and micro-fractures in the shale. Therefore, effective seal and hydration inhibition are very important for designing and optimizing the water-based drilling fluid.(2) The experiment results of evaluation and optimization showed that the main treatment agents of shale water-based drilling fluid are NR-1, PAV-LV, RSTF, SMP-II,PEG and interior emulsion preparation. Besides, through the distribution experiments of these additives best adding amounts were found.(3) The developed water-based drilling fluid has good performance in sealing, inhibiting and lubricating. Its borehole stability ability is stronger than polysulfide drilling fluid and polymer drilling fluid, which are used in Changning block, but weaker than oil-based drilling fluid. Two shale wells using water-based drilling fluid system in Changning block showed excellent performance. This system not only can greatly reduce the cost of drilling fluid but also reduce the risk of environmental pollution.(4) The field test application showed that, in order to drilling safety and smoothly in shale horizontal well, the system properties, such as sealing ability and the lubrication performance still need further improvements. In the continuing research, this system need to select or develope stronger inhibitor.