Simulation and Experimental Study on the Effect of Acid-fracturing in Carbonaceous Shale

Abstract

Unconventional oil and gas reservoirs have been given a lot of attention due to oil prices in the market. Production costs of these formations are very high due to directional drilling completion and hydraulic fracturing operations. However, the primary recovery factor of these reservoirs is as low as 2% to 8%. Therefore, recent studies have focused on recovery factor improvement. Some of these methods in conventional reserves are also applicable for unconventional layers like waterflooding, miscible gas injection, and chemical material utilization. Acidizing can also be used to extend the productivity of the reservoir as an improved recovery method. It can be in the form of acid washing, matrix acidizing and acid fracturing. However, acid application in the already propped fracture is a new area that has not been fully investigated. This study performed a detailed analysis of HCl acidizing in Eagle Ford shale as a typical unconventional oil reservoir. As a first step, a simulation model was created to explore the effect of HCl on fractured medium numerically. The model was firstly validated by an experimental result and then the effects of acid concentration, acid injection rate, and temperature were fully investigated. It was revealed that for each of these parameters there is an optimum condition that makes required water breakthrough into the minimum. Since non-carbonate content is also present within carbonaceous shale formations, the acid front movement within the formation is not always reactive. Instead, there is a two-phase fluid inside the medium, and saturation changes because of injected fluid. Due to the saturation gradient between the injected fluid and in-situ irreducible wetting phase, there is a spontaneous imbibition within the porous medium when there is no injection. An exact analytical solution was proposed for co-current and counter-current imbibition of this movement and validated by experimental results. The suggested equational form can be used to present the distribution of injected fluid within the porous medium. Finally, experimental tests were conducted to monitor the short-term effect of high concentration acid and the long-term impact of low concentration acid. During the short-term contact of acid with carbonaceous shale (Eagle Ford shale sample), the rock samples were confined by in-situ condition (pressure and temperature) and the rock surface was prone to the acid by keeping it open through utilization of proppant, as the permeability of the rock sample was very low. Again, different effects including acid concentration, proppant type and size and acid injection rate were investigated. It was found that 5% HCl acid concentration, injection with 8 ml/min and with propant size and concentration of 600-700 μm and 0.3 lb/ft2, respectively. For long-term effect of acid on the rock surface, mathematical model as well as experimental setup were constructed. A semi-analytical solution achieved with very good matching with experimental results. Diffusion coefficient of acid into the rock matrix was also measured at different acid concentration and with various brines. Altogether, analytical, experimental and simulation results of this study are the tools for different production strategies for carbonaceous shale formations as a major unconventional oil and gas resources.