With the recent development on several shale gas plays in the Sichuan Basin, shale gas has emerged as one of the viable and economical energy options in China. The drilling and completion practices are mostly echoed the success of shale gas developments in US, and hydraulic fracturing treatments are primarily dominated by slickwater owing to its low cost and benefit in maximizing the stimulated reservoir volume (SRV). However, these shale plays are usually in high tectonic stress environment, complex in geological structures and usually with rich natural fractures, which are quite different from most of the plays in US with relatively simple structure and more relaxed stress environment. Thus, critical design parameters for hydraulic fracturing that drive the production performance still remain unclear with the lack of integrated production evaluation.
Although microseismic mapping results were widely collected and used for the SRV estimation and post treatment evaluations, it cannot provide the insight of effective SRV or underlying production mechanisms because of the uncertainty in correlating micro-seismicity with hydraulic fracture propagation and proppant placement. A direct production comparison with different treatment types and design strategy would often lead to inappropriate conclusions due to too many variables involved in reservoir quality, stresses, completion design and flowback practices. Thus it is necessary to apply an integrated approach to obtain a thorough understanding of production performance in relation to fracturing design and flowback management.
This paper discusses an integrated workflow that incorporates all the valuable data into production evaluation, and presents the detailed analysis on the horizontal wells in Longmaxi shale gas reservoirs in Huangjinba block. The workflow starts with the use of 3D geological models to build numerical reservoir models that honor key features of reservoir heterogeneity, and advanced fracture modeling to integrate 3D geomechanics and microseismic results into fracture simulations and provide inputs to reservoir models for production history matching.
The study revealed the stress sensitivity in all the wells in which the fracture system starts to deteriorate when the effective stress reaches the critical value, resulted in many perforation clusters not contributing to production due to the loss of connectivity to wellbore. This leads to an important conclusion that fracture conductivity is equally important as the SRV for such a high stress shale gas play, and the treatment strategy shall be recalibrated from current slickwater treatments that do not provide sufficient fracture conductivity due to low proppant concentration as well as challenges of proppant placement experienced in many stages. This study has provided a much better understanding of production mechanism and effectiveness of stimulation treatments, as well as a list of design changes and actions on the flowback management to overcome current production challenges.