Insights into the Process of Effectively Acidizing Naturally Fractured Reservoirs

Abstract

This paper presents the simulation results of acidizing process in naturally fractured reservoir (NFR) by application of advanced numerical technique. Accurately predicting fracture and matrix flow is often critical to assessing well productivity in naturally fractured reservoirs.Fracturing with acid (usually hydrochloric acid [HCl]) is an alternative to propped fractures in acid-soluble formations such as dolomites and limestones. Computational Fluid Dynamics (CFD) is a computational technology that enables study of the dynamics of materials that flow. The CFD code is used to simulate the fluid flow through the fracture and matrix. The model is based on coupled multiphysics phenomena such as Darcy's law in porous media, reaction flow equation for the fracture and fracture growth by acid dissolution. Reaction flow in the fracture is controlled by diffusion and convection terms. The model simulates the impact of fracture geometry, acid properties, fracture width, matrix permeability on the acidizing process.The results show that diffusion and convection terms will control the transport of acid through the fracture (as there are limits to this process). when the mass transfer coefficient (K g ) is higher than 10e-5 m 2 /sec, the mechansim of acid trasnport will be controlled by convection term on the fracture surface. Physically ,this means that acid transport to wall by diffussion term is negligilble. When the fracture width is higher than 200 micron (0.00002 m), the acid will react with the most of the surface of the fracture and it will be dissolved by acid considerably. The mass transfer coefficient will also play an important role during acidizing process as the results show. The results of this study were used as guidelines to design a more effective acid job by predicting the acid penetration and acid volume for matrix acidizing in naturally fractured reservoirs. Furthermore, the contrasts in job design for lithology of the carbonate formation are also presented.