Porosity and permeability development in compacting chalks during flooding of nonequilibrium brines: Insights from long‐term experiment

Abstract

We report the complete chemical alteration of a Liège outcrop chalk core resulting from a 1072 flow-through experiment performed during mechanical compaction at 130°C. Chemical rock-fluid interactions alter the volumetric strain, porosity, and permeability in a nontrivial way. The porosity reduced only from 41.32% to 40.14%, even though the plug compacted more than 25%. We present a novel analysis of the experimental data, which demonstrates that the geochemical alteration does not conserve the volume of the solids, and therefore, the strain is partitioned additively into a pore volume and solid volume component. At stresses beyond yield, the observed deformation can be explained by grain reorganization reducing the pore space between grains and solid volume changes from the rock-fluid interactions. The mechanical and chemical effects are discussed in relation to the observed permeability development.

How Stress and Temperature Conditions Affect Rock-Fluid Chemistry and Mechanical Deformation

“…These predictions have been supported by direct XRD and SEM observations that were published recently by Zimmermann et al [21]. As such, calculations and direct rock-observations have shown that chalk is prone to chemical rock-fluid reactions at high temperatures [17].…”

“…The porosity estimate from mass difference measurements was performed both before and after the experiment. Based on previous experience in related geochemistry, pycnometry, saturation and dry mass estimates, the porosity of connected pores is equal to the overall porosity of the chalk sample [17]. Helium gas pycnometer measurements, using the Micromeritics Gas Pycnometer model AccuPyc II 1340, were also employed to estimate the solid density ρ s of tested and un-tested material.…”

“…For example adsorption processes occur at short time-intervals since it may occur within 1-2 pore volumes injected, while dissolution/precipitation is time-dependent occurring during injection for hundreds of pore volumes [17]. Rock-fluid interactions estimated from equilibrium calculations become increasingly important with temperatures above 100 • C, reported in e.g., [18][19][20].…”

“…The rate of deformation with time is linked to mineral alteration (see e.g., [17]). Any dissolution related to solidvolume changes may affect the bulk volume directly.…”

“…Since these mineral reactions, expected when MgCl 2 are flooded through chalks, are associated with density increase, the total volumetric deformation can be partitioned into a solid volume and a pore volume component. Depending on how the solid volume and pore volume changes, the porosity may change in a non-intuitive way [17].…”