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Substantial research has been conducted on the mechanisms responsible for the wellbore collapse in shale formations. For chemically active soft shale formations, pore pressure and stress change due to osmotic pressure and hydration swelling are usually recognized as the primary mechanisms, while for chemically inactive hard shale formation, wellbore instability is generally attributed to weak structural planes like bedding planes and microcracks. In this paper, experimental testing and analytical and numerical analyses are performed to reveal the dominant mechanism of the frequently encountered severe wellbore instability events in the middle-deep shale formation in the Bohai oil field of China. It is evidenced from the physical and chemical experimental testing that the middle-deep shale features both medium chemical activity and abundant bedding planes and microcracks, indicating that the middle-deep shale is in the transition process from chemically actively soft shale to the chemically inactive but laminated and fractured hard shale. Mechanical testing also shows considerable strength degradation of the middle-deep transition shale due to drilling fluid-shale interaction. Analysis through a hydro-chemo-mechanical coupling theory shows that the extent of the damage zone around the wellbore is limited if only pore pressure change and hydration swelling caused by the chemical difference between the drilling fluid and the formation fluid are considered, which cannot explain the severe wellbore collapse in the drilling process. In contrast, when pore pressure increase and strength degradation of the shale due to drilling fluid penetration along the bedding planes and microcracks are taken into account, a damage zone of 3~4 times of the wellbore diameter can be generated, implying that the dominant mechanism of the wellbore instability in the middle-deep transition shale formation should be the pore pressure change and strength degradation resulted from drilling fluid penetration along the bedding planes and microcracks. [ABSTRACT FROM AUTHOR]

The shale of the Lower Silurian Longmaxi Formation is an important gas-producing layer for shale gas development in southern China. This set of shale reservoir characteristics and shale gas development potential provide an important foundation for shale gas development. This study takes wellblock XN111 in the Sichuan Basin, China, as an example and uses X-ray diffraction (XRD), scanning electron microscopy (SEM), isothermal adsorption, and other techniques to analyze the shale reservoir characteristics of the Lower Silurian Longmaxi Formation. The results show that the Lower Silurian Longmaxi Formation was deposited in a deep-water shelf environment. During this period, carbonaceous shale and siliceous shale characterized by a high brittle mineral content (quartz > 40 wt. % , carbonate mineral > 10 wt. %) and a low clay mineral content (<30 wt.%, mainly illite) were widely deposited throughout the region. The total organic carbon (TOC) content reaches up to 6.07 wt.%, with an average of 2.66 wt.%. The vitrinite reflectance is 1.6–2.28%, with an average of 2.05%. The methane adsorption capacity is 0.84–4.69 m3/t, with an average of 2.92 m3/t. Pores and fractures are developed in the shale reservoirs. The main reservoir space is composed of connected mesopores with an average porosity of 4.78%. The characteristics and development potential of the shale reservoirs in the Lower Silurian Longmaxi Formation are controlled by the following factors: (1) the widespread deep-water shelf deposition in wellblock XN111 was a favorable environment for the development of high-quality shale reservoirs with a cumulative thickness of up to 50 m; (2) the high TOC content enabled the shale reservoir to have a high free gas content and a high adsorptive gas storage capacity; and (3) the shale's high maturity or over maturity is conducive to the development of pores and fractures in the organic matter, which effectively improves the storage capacity of the shale reservoirs. The reservoir characteristic index was constructed using the high-quality shale's thickness, gas content, TOC, fracture density, and clay content. Using production data from shale gas wells in adjacent blocks, a mathematical relationship was established between the Estimated Ultimate Recovery (EUR) of a single well and the Reservoir Characteristics Index (Rci). The EUR of a single well in wellblock XN111 was estimated. [ABSTRACT FROM AUTHOR]

The lower Silurian Longmaxi Formation hosts a highly productive shale gas play in the Zhaotong region of southern China. According to core observation, X-ray diffraction analyses, and scanning electron microscopy (SEM) observations, the shale comprises primarily quartz, carbonate minerals, and clay minerals, with minor amounts of plagioclase, K-feldspar, and pyrite. The clay mineral content ranges from 15.0% to 46.1%, with an average of 29.3% in the Zhaotong region. Organic geochemical analyses show that the Longmaxi Formation has good potential for shale gas resources by calculating total organic carbon, vitrinite reflectance, and gas content. Scanning electron microscope images demonstrate that reservoir pore types in the Longmaxi shale include organic pores, interparticle pores, intercrystalline pores, intraparticle pores, and fractures. Reservoir distribution is controlled by lithofacies, mineral composition, and geochemical factors. In addition, we investigated the relationships between reservoir parameters and production from 15 individual wells in the Zhaotong region by correlation coefficients. As a result, the brittleness index, total organic carbon (TOC), porosity, and gas content were used to define high-quality reservoirs in the Longmaxi shale. Based on these criteria, we mapped the thickness and distribution of high-quality reservoirs in the Longmaxi Formation and selected highlighted several key sites for future exploration and development. [ABSTRACT FROM AUTHOR]