Modeling Wormhole Propagation During Closed-Fracture-Acidizing Stimulation in Tight-Carbonate Formations

Abstract

Summary Closed-fracture acidizing (CFA) is a well-stimulation technique that can be applied to stimulate carbonate reservoirs at the end of acid-fracturing treatments. In CFA, acid is injected into the closed fracture at rates lower than the fracturing pressure to enhance the fracture conductivity. The objective of this study is to develop a robust model that can capture the dissolution process and wormhole-propagation phenomena that occur during CFA. This work develops a CFA model using computational-fluid-dynamics (CFD) techniques coupled with a two-scale continuum model that can predict accurately the reactive-flow mechanisms of hydrochloric acid (HCl) in carbonate formations. The developed CFA model is constructed and populated with the actual porosity-distribution profiles of tight carbonates. The model was tested against the experimental work performed on a fracture-conductivity apparatus. Sensitivity analysis is performed for several parameters that affect the performance of CFA in tight-carbonate formations. The developed model has successfully captured the dissolution patterns and wormhole-propagation phenomena that occur during CFA. In calcite formations, high temperatures promote acid leakoff into the formation, resulting in inefficient fracture stimulation. On the contrary, low temperatures reduce the overall reaction kinetics and attenuate the HCl reaction with calcite. Also, simulation results show that high acid concentration is favorable in treating low-conductivity fractures. In dolomite formations, it is essential to adapt a strong-acid system with an extended treatment duration to ensure efficient acid stimulation to the closed fractures. This paper provides a simulation study of the CFA process in a carbonates formation by establishing a 3D CFD model using the two-scale continuum approach. Fracture-surface etching and the associated acid-wormhole behavior during CFA are experimentally validated. This study optimizes the acid volumes and injection rates that can be used in conducting CFA.