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Poroviscoelasticity of The Eagleford Shale Depending on the Drilling Mud and Fracturing Fluid Chemistry

Abstract

It is well-known that chemically active clay minerals make shale mechanical properties highly sensitive when it is in contact with fluids. From the drilling process to hydraulic fracturing, shale formation is exposed to fracturing fluid and drilling mud with various chemical compositions. Therefore, a key point of success in shale gas/oil drilling, stimulation and production is to estimate the effects of various fluids on the geomechanical properties of shales. This study focuses on the time-dependent mechanical properties of the Eagleford Shale and fluid effects on its properties under three subjects: anisotropic elastic properties, stress-dependent poroelastic properties and time-dependent creep deformation. Shales display acoustic anisotropy that reflects their intrinsic properties due to their depositional processes. Ultrasonic pulse velocity tests were performed to measure acoustic anisotropy to quantify the elastic moduli in isotropic and transverse directions. Inclined Direct Shear Testing Device (IDSTD) setup allows taking acoustic velocities during the loading stages to estimate the stress-dependent geomechanical properties and fluid effects as well. In addition, creep tests were performed to quantify the poroviscoelastic properties. Dry, decane and water exposed shale samples were tested under several loading steps. The magnitude of anisotropy reflected by the Thomsen's coefficients is reaching up to 21% in ε (P wave anisotropy) and 14% in γ (S wave anisotropy). Anisotropy of the corresponding engineering moduli reached up to 28% in E, 19% in G, 14% in v and 20% in α. The results showed some degree of sensitivity to fluid contact. The IDSTD tests exhibited that water circulated tests on shale samples have less strength than the decane circulated ones. The difference is reaching up to 3.7%. The low difference is related to low amount of chemically active clay minerals, illite and smectite. The Eagleford Shale samples displayed creep behavior for all conditions. Water saturated sample had the largest creep deformation. Decane exposed and dry tested samples followed subsequently. Strain and time relation was best characterized by power-law function of time, ε = Btn, where n determines the contribution of the time dependent deformation to the total strain, a measure of creep tendency of the rock. The outcomes of this study are essential inputs for well planning, hydraulic fracturing design as well as wellbore seismic measurements.