Effect of Rock Dissolution on Two-Phase Relative Permeabilities: Pore-Scale Simulations Based on Experimental Data

Abstract

Relative phase permeability is an important characteristic of multiphase flow in porous media. Its assessment is an urgent issue when the pore structure changes due to rock dissolution. This article examines the effect of carbonate rock dissolution on two-phase flow based on images obtained by X-ray microcomputed tomography with a spatial resolution of ~18 µm. The characteristics of the two-phase flow were calculated through pore network modeling. The studies were conducted on 20 sub-volumes, which were extracted from cylindrical samples A and B with permeabilities of 0.72 and 0.29 D. HCl solutions (12% and 18%) were injected into samples A and B at a rate of 8 and 2 mL/min, respectively. Due to rock dissolution, the porosity and absolute permeability of the sub-volumes increased by 1.1–33% and 44–368%, respectively. Due to dissolution, the residual oil and water saturations decreased by 20–46% and 25–60%, respectively. These results showed that an increase in absolute permeability led to a significant reduction in residual oil and water saturations. These results also demonstrated that rock dissolution resulted in a change in the spatial heterogeneity of the relative phase permeabilities. The spatial heterogeneity increased in sample A after rock dissolution, while in sample B, it decreased.

Experimental and Numerical Study of the Effect of Rock Dissolution on Absolute Permeability of Limestone Sub-Samples

“…The results demonstrate that the relationship between connected total porosity is described well by the parabolic equation ϕ con = aϕ 2 + bϕ + c both before and after rock dissolution, giving high correlation coefficients R 2 ≈ 0.96 − 0.99. The percolation threshold porosity values found in this article align well with those reported in [31], and are approximately three times higher than the values for sandstones. It has been shown that, as a result of rock dissolution, the connectivity between pores increases for heterogeneous samples, as evident from the decrease in the percolation threshold porosity.…”

“…The experimental procedure involved cleaning each sample from oil, bitumen, and water-soluble salts, saturating it with a 2% NaCl solution, and maintaining it for 12 h at a temperature of 20 • C and a pressure of 3 MPa (reservoir conditions) [31]. Then, each sample was scanned using an X-ray microcomputed tomography General Electric V|tome|X S240 (Baker Hughes, Houston, TX, USA) at a resolution of about 18 µm before injecting HCl solution.…”

“…The diameter and length of the samples are about 3 and 5 cm, respectively. The study of the mineral composition using a Bruker D2 Phaser X-ray diffractometer (Bruker, Billerica, MA, USA) [31] showed that all samples were limestones almost completely composed of calcite, and the quartz content did not exceed 1% (see Table 1). This allowed us to exclude the influence of the mineral heterogeneity of the samples on rock dissolution.…”