Experimental and Theoretical Study of Water and Solute Transport in Organic-Rich Carbonate Mudrocks

Abstract

Summary The objective of this research is to determine the physicochemical processes underlying water and solute transport in organic-rich source rocks. To achieve this goal, a custom-designed experimental apparatus was constructed to conduct flow tests, founded on a high-pressure triaxial assembly. The apparatus is capable of maintaining core samples at reservoir pressure, temperature, and confining stress. We conducted several 120-day low-salinity osmotic tests in low-clay, organic-rich Eagle Ford carbonate-shale samples. Test results showed gradual, slow increase of pressure within the samples. Because this pressure behavior could not be explained properly with classical models, we formulated a mass-transport mathematical model that relies on fundamental chemical osmosis principles driving low-salinity brine into high-salinity core samples. Our mathematical model was articulated to simulate flow into the core as a 3D porous medium rather than transport across a thin, molecule-selective membrane. The model is dependent on the following principles: The low-salinity brine selectively enters the pores by diffusion mass transport, and the pre-existing, ionized dissolved salt molecules within the core are restrained by internal electrostatic forces to counterdiffuse in the direction opposite to that of the low-salinity-brine molecules entering the pore network. Critical model input data, such as permeability, porosity, and rock compressibility, were obtained from flow experiments on twin cores, and the diffusion coefficient was chosen by history matching. The strengths of the numerical simulation include reliance on mass-transport fundamental principles; not requiring the use of an ambiguously defined membrane-efficiency term; and relying on chemical-potential gradient as the driving force for the low-salinity brine to enter the high-salinity core, generating osmotic pressure within the pore network. The latter implies that osmotic pressure is the consequence of water entering the cores, not the cause. Results of this research have provided a more plausible explanation of pore-scale mass transport in organic-rich shales, and provide useful insights for design of effective enhanced-oil-recovery (EOR) processes.