A General-Purpose, Geochemical Reservoir Simulator
Authors: Liu X. , Ortoleva P. 
Published: December 2007

Abstract
A geochemical simulator for the analysis of coupled reaction and transport processes is presented. The simulator is based on the numerical solution of the equations of coupled multiphase fluid flow, species transport, energy balance and rock/fluid reactions. It also accounts for the effects of grain growth/dissolution and the alteration of porosity and permeability due to mineral reactions. The simulator can be used to analyze core floods, single-well scenarios and multiple production/injection well systems on the reservoir scale. Additionally, the simulator provides two flow options: the Darcy law for fluid flow in porous media and the Brinkman law that subsumes both free and porous medium flows.
The simulator was tested using core acidizing data and results were in good agreement with laboratory observations. The simulator was applied to analyze matrix acidizing treatments for a horizontal well. The evolution of the skin factor was predicted and the optimal volume of acid required to remove the near-wellbore damage was determined. Reactive fluid infiltration was shown to lead to reaction-front fingering under certain conditions. Viscosity contrast in multiphase flow could also result in viscous fingering. Examples in this study also address these nonlinear fingering phenomena. A waterflood on the reservoir scale was analyzed and simulation results show that scale formation during waterfloods can occur far beyond injection wells. Two cases of waste disposal by deep well injection were evaluated and our simulation results were consistent with field measured data.

Introduction
Many processes including acid stimulation, waterflooding and waste storage may involve the injection of reactive fluids into formation rocks. The injection of reactive fluids into a rock can activate a host of complex reaction and transport phenomena. A quantitative model can be used to analyze them. To this end, a number of geochemical models and simulators have been developed for matrix acidizing and other geochemical processes. A common feature of these geochemical models is the coupling between reaction and transport. But in most models, both the reaction chemistry and transport processes have been simplified.
In this paper, a geochemical simulator - CIRF. A (Chemical Interactions of Rocks and Fluids) is presented. The CIRF. A simulator was designed to analyze the effects of the injection of a fluid into a rock that arise out of the reaction and transport processes. Our simulator has been used to analyze a variety of systems. It simulates the fluid/fluid and fluid/rock reactions and single or multi-phase transport phenomena that take place when a fluid of given chemistry is injected into a formation. The simulator contains large built-in kinetic and thermodynamic data bases and has great chemical generality. The CIRF. A simulator accounts for the following processes:finite rate mineral dissolution/precipitation reactions,aqueous pore fluid equilibrium reactions,fluid flow with a permeability that reflects changes in grain size and shape due to precipitation/dissolution reactions and relative permeability and capillarity effects for two fluid phase flow,conservation of pore fluid solute mass due to flow, dispersion/diffusion and aqueous phase and mineral reactions,energy conservation,the effects of temperature, pressure and ionic strength on solute species activities.
A key feature of CIRF. A is its capacity to account for the strong coupling between the operating processes. For example, the infiltrating fluid can dissolve certain minerals and thereby affect rock permeability. The latter affects the flow of the infilling reacting fluid. One cannot, therefore, analyze fluid flow and mineral reactions separately. These processes are strongly coupled and must be co-evolved. The co-evolution of all processes is achieved in CIRF. A via a finite difference approximation to the partial differential equations of mass, momentum and energy conservation and grain growth/dissolution. At each time step, all these equations are solved iteratively before advancing the system to next time step as shown in Fig. 1.
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