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

Abstract

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.