Unraveling the mechanisms underlying permeability control in unconventional reservoirs (shale, coal, and tight sandstones) is essential for accurate production forecasting, although it poses unique challenges. The complex interactions among various processes within the matrix, encompassing pore structures and fractures, require a more comprehensive understanding. This investigation specifically focuses on examining the impacts of poro-elastic and fluid-dynamic effects unconventional reservoir rocks. As the reservoir undergoes depletion, the reduction in pore pressure induces changes in the apparent permeability coefficients (kgas) due to the influences of poro-elasticity and fluid dynamics. The combination of these opposing effects theoretically results in an identifiable minimum. However, the experimental exploration of the pressure ranges wherein fluid-dynamic and poro-elastic effects coexist, and one dominates the evolution of matrix permeability remains unexplored. To effectively disentangle these effects, conducting comprehensive laboratory measurements encompassing a wide range of pore- (Pm) and confining pressures (Pc) is imperative. The primary objective is to experimentally verify and validate the differentiation of fluid-dynamic and poro-elastic effects and ascertain the specific pore pressures at which each effect predominates in three different types of unconventional reservoir rocks: Krihna-Godavari (KG) shale, Janjhra coal, and Gwalior tight sandstones.
We created iso-permeable contour lines by plotting 3D surface contours (Pc, Pm, and kgas) and projecting them onto Pc and Pm planes. In shale and coal, the iso-permeable line initially decreases and then increases as Pm increases, while in tight sandstone, it only decreases with increasing Pm. At lower Pm, gas molecules with a mean free path equivalent to pore conduits collide more with pore walls, resulting in increased permeability (the slippage effect or fluid dynamic effect). However, beyond a certain Pm, the poro-elastic effect governs flow, causing apparent permeability to increase with Pm. Unlike sandstone, shale, and coal exhibit fluid dynamics and poro-elastic effects, which decouple at Pm values of 6-7 MPa and 2-3 MPa, respectively. The effective stress coefficient (χ) in the effective stress law (σeff = Pc - χPm) governs the variation of intrinsic permeability in the poro-elastic region. In pore-elastic region, the iso-permeable line becomes parallel, the slope of these lines providing the χ value. For shale and coal, χ is determined to be 0.64 and 0.58, respectively. We also investigated the apparent permeability evolution during reservoir depletion; experimental results were extrapolated to represent in-situ reservoir conditions.
