Formation Damage Induced by Oil-Based Drilling Fluid in a Longmaxi Shale Gas Reservoir: A Comprehensive View of the Drilling, Stimulation, and Production Processes

Abstract

Formation damage during drilling, completion, and stimulation stages is an important reason inducing the obviously different initial production rates and rapid declines in shale gas wells. How the formation damage at all of the exploration and development stages is reduced is a major engineering problem to be solved urgently in shale gas reservoirs. Oil-based drilling fluid (OBDF) is the first choice for drilling horizontal long sections for its good inhibition and lubrication. However, formation damage induced by OBDF during the entire exploration and development cycle of the shale gas reservoir is still not well understood. In this study, formation damage degree during the drill-in stage was evaluated simulating OBDF circulation and shearing flow on the wellbore surface. A working fluid sequential contact test was designed to evaluate the impact of the OBDF on the fracturing fluid damage. A permeability stress sensitivity test on shale samples before and after OBDF invasion was carried out to analyze the formation damage during the depletion exploitation stage. Results indicate that the damage rate of the OBDF in the drill-in stage to induced fractures is 72.0%. Compared with a single fracturing fluid, the sequential contact between the OBDF and the fracturing fluid further exacerbates damage degree with a permeability damage rate increase of 27.3%. Matrix strength weakening, fracture surface asperity dissolution, and oil phase trapping induced by OBDF invasion strengthen the permeability stress sensitivity of the fractured samples. The formation damage caused by OBDF becomes permanent, multicycle superimposed damage, which makes the formation damage more severe and difficult to be eliminated. The invasion of OBDF into shale is the premise of inducing shale gas formation damage. Improving the plugging efficiency and pressure-bearing capacity for the multiscale intrusion channels is the primary factor in controlling formation damage in shale gas reservoirs. This study provides guidance for the development of formation protection drilling fluids in shale gas reservoirs.