Abstract
Shale gas production after drill-in, completion, and hydraulic fracturing is strongly affected by formation damage. In order to determine the damage mechanisms for nonmarine shale reservoir, a series of assessments of sensitivity damage, water block damage, water-based drill-in fluids damage, and water damage to gas diffusion on 20 shale samples obtained from Chang 7 Formation were conducted and analyzed. Results indicate that, in the Chang 7 Formation shale, there is extremely strong stress sensitivity and moderately weak water sensitivity damage. Although the liquid phase invasion depth is shallow and the water block damage is limited, the liquid phase and solid particles would enter the microfractures in the reservoir.The P-1 water-based drill-in fluid is compatible with the Chang 7 Formation shale reservoir which can meet the requirement of Chang 7 Formation shale damage controlling, the effect of water-based drill-in fluid on wellbore stability should be paid more attention. The diffusion coefficient of the shale decreases with the presence of water.A systematic damage evaluation method of working fluid considering the multi-mechanism and multi-scale mass transfer process of shale gas is needed to establish.
Highlights
Shale gas is popular in the exploration and development of unconventional oil and gas in the world due to advances in drilling and fracturing techniques (Chareonsuppanimit et al, 2012).In recent years, a significant breakthrough has been made in the southern marine shale gas field represented by the Fluling, Changning-Weiyuan, and Shaotong shale gas in Sichuan Basin, they are the mainly commercial shale gas fields in China, the cumulative developed reserves reached 10,450  m3 by the end of 2018, yearly production  109 m3 in 2018 (Jiping, 2019)
The instruments of fluid and stress sensitivity damage assessment, water block damage assessment, and water-based drill-in fluids damage assessment are at the Research Center of Development and Management for Length Diameter Depth
The filter loss of the P-1 water-based drill-in fluid is 3.5 ml at room temperature and is 6.5 ml at a high temperature of 85C, which meets the requirement of the industrial standard, and the formed mud cake is thin and tough, and has good plugging performance.The rheological characteristics of water-based drill-in fluid were measured at 25C, 50C, and 80C, the results show that its plastic viscosity is low and it has a good shearing force
Summary
Shale gas is popular in the exploration and development of unconventional oil and gas in the world due to advances in drilling and fracturing techniques (Chareonsuppanimit et al, 2012).In recent years, a significant breakthrough has been made in the southern marine shale gas field represented by the Fluling, Changning-Weiyuan, and Shaotong shale gas in Sichuan Basin, they are the mainly commercial shale gas fields in China, the cumulative developed reserves reached 10,450  m3 by the end of 2018, yearly production  109 m3 in 2018 (Jiping, 2019). In addition to marine shale gas, nonmarine shale gas is an important development direction in the future. Ltd took the lead in theoretical research and technical practice of nonmarine shale gas exploration and development (Xiangzeng, 2018). With the exploitation of shale reservoirs, the formation damage and protection which affect the shale gas well productivity has become more and more significant and urgent (Xu et al, 2016)
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