Development of a New Method for Estimation of Permeability and Saturation-Dependent Relative Permeability in Irregularly Shaped Tight Rock Samples

Summary

Conventional laboratory-based rock measurement techniques, including saturation-dependent relative permeability measurements, require perfectly-cut cylindrical core plugs. Saturating tight rock samples for saturation-dependent measurements can also be time-consuming in tight rocks, making these measurements expensive and unreliable. Moreover, achieving a perfectly-cut cylindrical core sample is difficult in the case of brittle rock types or naturally fractured rocks. In this work, we introduce a new method for the assessment of permeability and saturation-dependent relative permeability measurements using any regularly and irregularly shaped rock samples. The permeability values of the rock samples are estimated using a pressure decay experimental setup, which records the decay of pressure with respect to time using helium as the penetrating gas. We solve the pressure diffusion equation numerically using the finite volume method (FVM) (cylindrically and irregularly shaped sample) and the finite difference method (FDM) (cylindrically shaped sample). We obtain the geometric profile of the samples from computed tomography (CT)-scan imaging. The next step involves partially saturating the sample to a desired water saturation level using the recently developed enhanced saturation setup and workflow, which expedites saturation of tight rocks. We then estimate the permeability of the partially-saturated sample through integration of the numerical solver and the experimental measurements, and repeat the process to obtain the saturation-dependent relative permeability values of the samples.

We tested the introduced workflow on cylindrically shaped and irregularly shaped samples belonging to the Lower Eagle Ford, the Wolfcamp, the Scioto Sandstone, and the Nugget Sandstone formations and compared the results with saturation-dependent relative permeability estimates using the pulse-decay measurements. The estimated gas permeability values of the samples were in the range of 10–55 nd at dry conditions for organic-rich mudrock samples. We also estimated the gas permeability values of these samples at different partial water saturation levels to estimate the relative permeability values. The estimated gas permeability of the samples reduced to less than 1 nd at a water saturation of ~40%. The estimated gas permeability values ranged from 33 µd to 83 µd at dry conditions for clay-free tight samples, and these values reduced to 0.3–0.7 µd at maximum water saturation levels for the respective samples. The total time duration of the experiment for each sample was reduced from a minimum of 25 days for conventional techniques, such as the steady-state method, to a maximum of 7 days using the introduced method. The developed numerical forward simulator and the inversion algorithm enable, for the first time, relative permeability and absolute permeability measurements in irregularly shaped rock samples such as cuttings, broken core samples, and outcrop samples, and integration of numerical modeling and experimental methods to estimate permeability values in tight formations.