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Porosity Creation in Carbonate Reservoirs by Pre-Oil Charging With Source Rock-Derived Corrosive Fluids: One-Dimensional Reactive Mass Transport Modeling


The origin of locally high porosity in carbonate reservoirs is still a matter of debate. One hypothetical concept is the late diagenetic calcite dissolution caused by acidic corrosive fluids (ACF) which migrated through the reservoir immediately before the actual oil phase. Our hydrogeochemical 1D-reactive mass transport (1DRT) modeling aims to test this concept by providing quantitative evidence based on the principles of chemical equilibrium thermodynamics and reactive mass transport. Such 1DRT calculations link 1D-purely advective pore water flow with the equilibrium thermodynamics of water-rock-gas interactions. Our motivation comes from the work of Ehrenberg et al. (2012) about dissolution of micritic calcite in Middle East oil fields and from their conclusions that (1) “diagenetic reactions must be constrained geochemically”, and (2) that “the theory about dissolution by acid pore water has produced significant net increases in bulk porosity has not been supported by models of mineral solubility and fluid flow”. Our modeling results reveal that the calcite solubility in the system “ACF/calcite/CO2(g)” increases along with decreasing pressure/temperature. During their migration, ACF may reach saturation with respect to calcite (e.g., at 305 atm and 85°C) before they enter the carbonate reservoir. Although saturated with respect to calcite, further migration of such ACF through the reservoir into areas of lower pressure/temperature (e.g., 295 atm and 82°C) causes undersaturation of ACF with respect to calcite. Consequently, such migrating ACF dissolve calcite until the saturation is re-established again (e.g., 295 atm and 82°C). Thus, thermodynamically controlled and repeated chemical re-equilibration among ACF, calcite, and CO2(g) during ACF migration from higher to lower pressure/temperature conditions is the driving force for continuous calcite dissolution. This continuous calcite dissolution along the flow path of ACF is the inevitable consequence of the chemical-thermodynamic behavior of the system “ACF/calcite/CO2(g)” under decreasing pressure/temperature –at least in the modeled p/T range from 295 atm/82°C to 205 atm/55°C. This holds also true for dolomitic reservoir rocks. Such self-propagating calcite dissolution before or during migration of the oil phase can increase reservoir porosity and permeability to a certain extent. In contrast, intense ACF-induced calcite dissolution may also lead to a local structural breakdown of the calcite matrix.