Insights of Wormhole Propagation During Carbonate Acidizing: Constant Pressure vs. Constant Rate

Abstract

Acidizing of carbonate reservoirs is a common technique used to restore and enhance production by dissolving a small fraction of the rock to create highly conductive channels. Literature review reveals that most acidizing studies are focused on acid injection at a constant volumetric rate (CVR) instead of at a constant injection pressure (CIP). Therefore, the primary objective of the present work is to investigate the benefits and recommended applications of each technique. The study analyzes dissolution patterns, and wormhole propagation rate. A coreflood study was conducted using different Indiana limestone cores to assess both techniques. Additionally, a 2-D wormhole model was used to mathematically describe the acidizing phenomena. This model describes the reactive transport of acid as a coupling between Darcy scale flows. The algorithm captures the essential physics and chemistry of the acid reaction in a carbonate porous medium. The study confirmed that all types of acid dissolution patterns (i.e. face, conical, wormhole, and branched) exist for both techniques (CVR or CIP). Unlike in the CVR technique, dissolution patterns during the CIP technique can change, tending toward a branched dissolution regime. The CIP technique required a lower acid volume to achieve breakthrough in the conical dissolution regime and a higher acid volume to achieve breakthrough the branched dissolution regime compared to the CVR technique. In a dominant wormhole pattern, both techniques required nearly the same acid volume for breakthrough. A CT scan confirmed that the CIP technique develops a uniform wormhole at a low initial injection rate. For the CIP technique, the acid injection rate increased exponentially with the volume of the acid injected.

Interaction Law between Natural Fractures-Vugs and Acid-Etched Fracture during Steering Acid Fracturing in Carbonate Reservoirs

“…Self-generating acid (Figure 4(c)) is a fracturing fluid used in ordinary acid fracturing. For slick water, its special properties such as particularly low viscosity and ability to generate complex fracture network in the shale reservoir have 2 Geofluids been extensively studied [10]. Although vugs and natural fractures in carbonate reservoirs might lead to the loss of slick water, slick water still can be used in the acid fracturing treatment as ahead fluid.…”

“…The Gaussian curvature of each point on the area was calculated, as shown in Figure 17. In order to display the relative size of Gaussian curvature, different colors were used to mark, red 10 Geofluids for high Gaussian curvature and rougher surface and blue for low Gaussian curvature and smoother surface. The result is shown in Figure 17.…”