Effective Pressure Law for the Intrinsic Formation Factor in Low Permeability Sandstones

Abstract

The effective pressure law for the intrinsic formation factor of low permeability sandstones from the Ordos Basin (northwest of China) was studied experimentally by measuring the electrical resistivity of brine saturated rock samples while cycling the pore and confining pressures, pp and pc. We used a 100,000 ppm NaCl solution so that the results could be directly interpreted in terms of the intrinsic formation factor F. The Response Surface method was used to construct a quadratic function of pp and pc fitting the experimental data, from which the coefficient α of the effective pressure law, peff = pc – αpp, was determined. We found that the coefficient α generally had low values, mainly from 0.2 to 0.4, thus contradicting the theoretical prediction that α should be equal to 1 for scale‐invariant properties, such as F, in linear elastic materials having a homogeneous solid matrix. Since the two underlying assumptions, linear elasticity and homogeneity of the solid matrix, are likely violated in reservoir rocks, we tried to assess their effects on α using simple analytical models, based on idealized geometries of the pore and solid phases. Analysis of these models suggests that non‐linear elasticity associated with the formation of solid–solid contacts in the compressed rocks (e.g., during closure of rough cracks or crack‐like pores along the grain boundaries), may be more effective in producing the low α values observed than inhomogeneity of the solid matrix.

Laboratory Research on Gas Transport in Shale Nanopores Considering the Stress Effect and Slippage Effect

“…Meanwhile, in other studies, the effective stress is corrected by the introduction of a constant n value, which translates into a better estimation of the stress effect of permeability. The improved effective stress law is frequently used but the approach for obtaining n varies (Chen et al, 2017; Heller et al, 2014; Zhao et al, 2011). The study by Letham and Bustin (2016a) shows that the n value is affected by the gas slippage effect.…”

Laboratory Observation of Fluid Flow in Depleted Shale Gas Reservoir: Coupling Effect of Sorbing and Non-sorbing Gas on Stress, Slippage, Flow Regime in Anisotropic and Fractured Shales

“…However, to accurately measure apparent permeability ( k app ), it is essential to undergo a prestressing (referred to as “seasoning”) process in all the core samples. This process is crucial for eliminating the hysteresis effect, which stems from the impact of stress loading history on permeability test results. − This hysteresis phenomenon is typically linked to the relaxation of internal stress and frictional sliding between grain boundaries . In our investigation, each gas was utilized before conducting apparent permeability measurements, serving as part of the prestressing procedure.…”

“…The average pore width (w a ) conduits for the bedding perpendicular sample (KG−C3-H) had pore widths ranging from 993. 53 8a). The pore width (w a ) for all gases decreases as the effective stress (σ eff ) increases.…”

Acoustothermoelasticity for Joint Effects of Stress and Thermal Fields on Wave Dispersion and Attenuation

“…The stress and thermal fields are coupled in the Earth's subsurface (Anderson, 1988; Qi et al., 2020; Schubnel et al., 2013). Considerable theoretical researches and experimental measurements demonstrate that the stress and thermal effects are related to the rock properties such as microscopic permeability and pore structure and distribution (Bernabé, 1987; Chen et al., 2017; David & Zimmerman, 2012; Sun & Gurevich, 2020) and macroscopic elasticity and wave velocities (Carcione et al., 2019; Chen et al., 2021; Pao et al., 1984; Thurston & Brugger, 1964; Winkler & McGowan, 2004). Generally, compressive stress and decreasing temperature can compress the rock pore space and bulk volume, leading to the increasing wave propagation velocities in rocks (Liu et al., 2021; Wang et al., 2021).…”