Study on the hydration of hard and brittle mudstone and its influence on the wellbore stability

Abstract

The instability of hard and brittle mudstone wellbore in the Xujiahe Formation in the Western Sichuan Depression is widespread, which has led to accidents such as sticking and collapse. In this paper, the Xujiahe Formation in the Western Sichuan Depression is taken as the research object, and the method of indoor experiment and theoretical simulation is used to study the instability phenomenon of the wellbore. The following points of knowledge have been obtained: The clay minerals in the mudstones of the Xujiahe Formation in the Western Sichuan Depression are mainly illite and chlorite (content>90%), with a small amount of illite/montmorillonite mixed layer, and the pore throat radius is between 14.56nm-24.47nm, belonging to the mesoporous type; After the mudstone hydrates, its compressive strength, cohesion, elastic modulus, and tensile strength will continue to decrease with the increase of water, while the Poisson’s ratio will continue to rise; The relationship between the dry core and the wet core can be used to correct the acoustic velocity of the collapsed mudstone section; the nature of the wellbore instability is that the liquid pressure in the wellbore is less than the formation pressure, and the formation pressure exceeds the rock strength, which leads to instability. We use the borehole expansion rate to realize the evaluation of wellbore stability. The research results have guiding significance for the evaluation of the stability of the hard and brittle mudstone wellbore of the Xujiahe Formation in the Western Sichuan Depression, and provide technical support for the next step of drilling safety.

Study of wellbore instability in shale formation considering the effect of hydration on strength weakening

“…Based on the in situ stress coordinates, the distribution of in situ stress around the well in the Cartesian coordinate system can be obtained through a series of coordinate transformations, which can be expressed as follows (Huang and Zhang, 1998;Tang, 2011;Li et al, 2012;Wen, 2015;Ma, 2015;Huang et al, 2023;Qiu et al, 2023a),…”

“…The micro-scale strength of force chains depends on the key performance parameters of plugging materials such as microscale permeability, particle size distribution, fiber aspect ratio, friction coefficient, compressive strength, high-temperature resistance, and solubility (Liu and Li, 2012;Lin et al, 2013;Teng et al, 2017;Xu et al, 2020;Tang et al, 2021;Ma et al, 2022;Zhou et al, 2022). Tang (2011) proposed that the essence of inhibiting shale hydration is to control hydration stress, hinder fluid invasion, and pressure transmission, which can prevent/control shale hydration and promote stable wellbore in complex shale formations (Huang and Zhang, 1998;Lu et al, 1999;Tang, 2011;Hang et al, 2014).…”