Challenges in Deep Shale Gas Drilling: A Case Study in Sichuan Basin

摘要

As an unconventional gas resource, shale gas has been developed mostly in shallow reservoirs which are mainly buried in depth form 76 m to 2439 m in the world. However, the deep shale gas is drawing more attention as time goes by. In this paper, deep shale gas in Sichuan Basin is studied as an example where the shale gas reservoirs are mostly buried deep in depth from 2300 m to 4500 m. Because the shale gas in Sichuan Basin is buried deep with high formation pressure and the formation conditions are complex, there are many problems that will be encountered during drilling, such as wellbore instability, serious formation damage by high drilling fluid pressure, and low drilling efficiency, etc. In this paper, shale gas reservoir in Long maxi Group of Sichuan Basin is studied as a case for deep shale gas drilling. Basic characteristics of strata profile, lithological characteristics, formation fluids and formation pressure are analyzed combined with previous drilling data. Based on this, wellbore stability and formation damage are evaluated. The study indicates that in a relatively high borehole stress condition with a Mechanic-Chemical coupling, shale with lamellar, brittle and high clay content has a high collapse stress, and borehole trajectory design and drilling fluid performance are crucial to wellbore stabilization. Meanwhile, in deep shale drilling of this area, liquid phase will invade into bedding fissures and tectonic fractures to form a liquid phase trap between the fractures and matrix, so that the shale formation in near bore area will be isolated by the aqueous phase trapping and unable to penetrate, and it is the main formation damage. This will be helpful in improving wellbore stability in basin deep shale gas drilling, reducing formation damage, and promoting drilling and exploration efficiency.