Petrophysical interpretation of laboratory pressure-step-decay measurements on ultra-tight rock samples. Part 2 – In the presence of gas slippage, transitional flow, and diffusion mechanisms

摘要: Abstract Ultra-tight formations generally exhibit heterogeneous, anisotropic, and pressure-dependent petrophysical properties. Conventional core analyses tend to generate inconsistent estimates when the physical measurements are performed on different samples extracted from ultra-tight formations. The discrepancies in further escalated due pressure- pore-size-dependent fluid flow mechanisms nanopores of rocks. In first part this two-paper series publication, a method is proposed simultaneously estimate four properties by inverting laboratory-based pressure-step-decay measurement single rock sample; thereby, circumventing inconsistencies heterogeneity anisotropy. second part, an inversion algorithm developed six parameters processing laboratory measurements. Similar step-decay involves nitrogen gas injection into sample at multiple stepwise pressure increments high-resolution pressure-decay outlet, which followed deterministic measured downstream data based numerical finite-difference modeling sample. Unlike forward model through nanoscale pores accounts for not only slippage but also transitional Knudsen diffusion. This work improves previously obtained modeled Klinkenberg-type as part. A transport implemented account separate simultaneous occurrence diffusion across during pore range 5 psi 500 room temperature. interpretation was applied nine 2-cm-long, 2.5-cm-diameter plugs 1-ft 3 pyrophyllite block. We estimated intrinsic permeability, effective porosity, pore-volume compressibility, throat diameter, two slippage-diffusion coefficients each Estimation accuracy relies error minimization scheme. For samples, average, diameter 86 nd, 0.036, 2.6E-3 −1 , 195 nm, respectively. Notably, indicate that mechanism completely dominated slip without any or flow, despite numbers entire duration were determined be 0.01–1. observation contradicts widely accepted qualitative classification mandates inversion-based approach identify appropriate pores.