For shale gas reservoirs with the characteristics of bedding planes developed, hydrated easily and strong anisotropy of the mechanical properties, this paper couples conventional logging, image logging and real-time drilling data with laboratory rock mechanical and in-site stress tests, and establishes a wellbore stability analysis model for shale reservoirs. This analysis covers weak bedding planes, mechanical anisotropy and time effects. Besides, the effect of these above factors on the wellbore stability for horizontal wells in shale reservoirs is studied, and wellbore collapse mechanism for the shale reservoirs in Sichuan Basin is also analyzed.
Based on the established wellbore stability analysis model, this paper analyses the well W-H1, the analysis results are in good agreement with real drilling data.
Study and field application results indicate that weak bedding plane effects mainly affect wellbore collapse of horizontal wells for shale reservoirs, which differs from conventional sand reservoirs and carbonate reservoirs, even for the vertical wells in the shale reservoirs.
The study provides a theoretical basis for W-H1 well drilling and completion design, such as drilling fluid design, wellbore structure optimization and completion method optimization, etc.
The Lower Cambrian Longwangmiao Fm carbonatite gas reservoirs in the Leshan-Longnüsi Paleouplift in the Sichuan Basin feature strong heterogeneity, well-developed fractures and caverns, and a high content of H 2 S, so these reservoirs are prone to reservoir damages caused by the invasion of drilling fluid or the improper well completion, so to minimize the reservoir skin factor is key to achieving high yield of oil and gas in this study area. Therefore, based on the geological characteristics of the Longwangmiao reservoirs, the binomial productivity equation was applied to demonstrate the possibility and scientificity of minimizing the skin factor. According to the current status of reservoir stimulation, the overall skin factors of reservoir damage caused by drilling fluid invasion, improper drilling and completion modes etc were analyzed, which shows there is still potential for skin factor reduction. Analysis of reservoir damage factors indicates that the main skin factor of Longwangmiao Fm reservoirs consists of that caused by drilling fluid and by improper completion modes. Along with the minimization of skin factor caused by drilling and improper completion, a fracture-network acidizing process to achieve "non-radial & network-fracture" plug-removal by making good use of natural fractures was proposed according to the characteristics of Longwangmiao Fm carbonatite reservoirs.
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