Experimental and theoretical study of water and solute transport mechanisms in organic-rich carbonate mudrocks

Abstract

Physicochemical processes behind transport mechanisms of water and solute in shales have been extensively studied for over fifty years. In hydraulic fracturing or low-salinity waterflooding operations, fresh water with a relatively small concentration of ions is injected into the formation through the use of large hydraulic heads from the surface. For both techniques, the fluid travels through the various scales of artificial and natural fractures until it reaches the matrix of the rock. At that point, the fluid interacts with the rock surfaces amongst the micro-scale fractures and the matrix. Water-to-rock surface interaction has therefore become a critical matter for the economic development of source rocks. The objective of this research study is to investigate the fundamental physics that dominate physicochemical-dependent water and solute transport phenomena in organic-rich source rocks and the interactions between completion/EOR fluids and the rock surfaces. The microscopic composition and fabric of the rock and the existing relationships between mineralogy, organic matter, porosity and strength are studied, along with the implication of these relationships for fluid-rock rheology. In addition, the stress dependence of permeability for organic-rich mudrocks using various experimental methods is measured at reservoir conditions. In order to address the fundamental questions with regard to physicochemical transport phenomena of water and solute, a set of long-term osmosis tests (up to 120 days) were also completed, showing the transmission of pressure through the matrix due to osmosis. In addition to these experiments, a novel conceptual approach to transport phenomena is presented. This model pays attention to the fundamentals of water and solute transport. Empirical interpretations were compared against the existing theory on the transport mechanisms on clay-rich, low-permeability rocks, revealing that pressure behavior could not be explained accurately with the current theory. The approach is based on two fundamental modifications of prior work: previously, the rock was assumed to be a semi-permeable membrane, while its thickness is much larger than a membrane. In addition, osmotic pressure was considered the imbibing force, even though it is not, because osmotic pressure is the consequence of the selective molecular transport by the solvent. The model captures these modifications and treats solute and solvent flow entirely as diffusive transport.