Permeability Evolution in Carbonates: Competing Roles of Stress and Fluid Reactivity

Thomas P. McGuire¹, Takuya Ishibashi², Derek Elsworth¹, Zvi Karcz³, and Noriyoshi Tsuchiya²
¹Energy and Mineral Engineering, EMS Energy Institute and G3 Center, Pennsylvania State University, University Park, PA, USA
²Graduate School, Tohoku University, Sendai, Japan
³ExxonMobil Upstream Research Company: Stratigraphic and Reservoir Systems, Houston TX, USA, Now at Delek Group, Israel

We explore the evolution of fracture permeability in tight carbonates due to the combined action of stress and reactive fluids. We independently control stress and fluid reactivity (pH) in flow-through experiments on core plugs of Capitan Massive Limestone containing artificial fractures of repeatable surface roughness. Mass balance of both fluid and dissolved mineral species independently constrain the redistribution of mineral mass under the influence of stress. By measuring in regimes of both increasing and decreasing permeability we quantitatively constrain the transition between fracture-gaping and fracture-closing modes of behavior. For minimally reactive systems (pH~7), permeability decreases at a rate that increases with confining stress. Conversely for highly reactive systems (pH<6) permeability increases at a rate that is independent of confining stress. We parameterize this transition in permeability evolution by the ratio of mechanically-controlled to chemically-controlled dissolved mass fluxes and compare this mass flux ratio to the inferred rate of hydraulic aperture change. The transition from regimes of fracture closing to regimes of fracture gaping occurs when stress and chemically driven mass fluxes are equal. These transformations result in an increase in permeability for the transmission of acidic fluids and reduction in permeability due to the influence of follower stresses during flow of near neutral fluids.

We represent this behavior through lumped parameter modeling of key processes: free-face dissolution and diffusion-controlled stress-enhanced dissolution of fracture contacts. This accommodates the roles of confining stress, fracture contact area, composition and reactivity of the permeating fluid together with identification of stagnant and diffusion-dominated zones of mass transfer within the fracture. Fracture contact area is defined by the digital mating of adjacent fracture surfaces with topography defined by white light interferometry. Confining stress is applied to the fracture and the two fracture halves mate until the bridging contact stress is no longer exceeded. Finally chemical kinetics and Fick’s law are applied to solve for the mass transport away from the free-face of the fracture and the contacting asperities, respectively. Dissolved mineral mass is transported from the fracture contacts by diffusion into the fracture void where advective flux is insufficient to remove it from the system. The dissolved mineral mass diffuses across the near-asperity region where diffusive transport dominates over advective transport to reach the advection-dominated region that surrounds this diffusive halo. Initial aperture distributions for both smooth and rough fracture surfaces replicate distributions measured in experiments. Fractures permeated by fluid with little free-face reactivity (pH~7) are predicted to have an average fracture aperture that decreases with time at rates similar to those measured in experiments. Increasing the free-face chemical dissolution coefficient (decreasing the pH of the permeating fluid) changes the evolution of the fracture aperture from closing to gaping as also observed in experiments. These rates of fracture gaping during flow of acidic solutions (pH~5 and pH~6) are also consistent with model predictions.

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012