Experimental Investigation of Fracture Orientation Effect on Carbonate Matrix Acidizing Using a Visual Micromodel Setup

Abstract

Many oil reservoirs face production decline, so using enhanced oil recovery methods and well stimulation techniques to increase well productivity has been beneficial and essential. Acid injection into the rock matrix is one of the best stimulation methods to improve oil production. The type of acid used in acid injection operations varies based on reservoir conditions, such as temperature, lithology, and acid‐rock contact time. Reservoir heterogeneity, such as the density and orientation of fractures, affects the performance of acidizing operations. Fracture orientation affects the hydraulic conductivity of post‐acid formation in the near‐wellbore area. In this study, the effect of fracture orientation on acid flow in porous media was visually observed using a transparent cell (micromodel). Image processing techniques were employed to study acid distribution in the carbonate‐fractured rock sample. The results showed that the effectiveness of acid treatment is higher in intersecting fractures compared to parallel fractures, and in parallel fractures compared to fractures perpendicular to the flow direction. At the end of the acid injection, the cumulative dimensionless index (DI), defined as the ratio of the aperture surface area to the surface area in contact with the acid, was 0.57, 0.38, and −0.11 for the crossed, parallel, and vertical fractures, respectively. The aperture surface area refers to the total area of the openings within the fracture network, while the surface area in contact with the acid denotes the portion of the fracture surface directly exposed to the acid during the acidizing process. This introduced index quantitatively measures acidizing effectiveness in widening fracture networks in carbonate reservoirs providing insight into alterations in fracture size and permeability due to acid treatment. After three stages of acid injection, the DI for the fracture aperture in the parallel section of intersecting fractures is 33.3% higher than that for the fracture aperture in the parallel fracture. Considering the additional fracture aperture in the vertical sections of the intersecting fractures, compared to parallel fractures, the superior effectiveness of intersecting fractures becomes more evident. Nevertheless, vertical fractures demonstrated minimal and sometimes negative impact. Fracture orientation impacts acidizing operations, guiding strategies for better production, recovery rates, and reservoir characterization in the oil industry.