Modeling the Closure of Thief Zones in Carbonate Reservoirs Using Acids

Carbonate oil reservoirs can contain fractures and dissolution-vug pore space that may cause short-circuit flows between injection anal production well. As a result, injected water bypasses the larger part of the reservoir, leaving much oil unproduced. This paper investigates the concept of reducing the permeability in the fracture that is causing the short-circuit flow by injecting a mixture of sulfuric acid and hydrochloric acid. During the process, reservoir rock dissolves and gypsum subsequently precipitates. We formulate a mathematical and numerical model that simulates the coinjection of sulfuric acid (H2SO4) with hydrochloric acid (HCl) into carbonate rock for both one- and two-dimensional settings. The carbonate rock contains a single fracture. A one-dimensional simulation of the process shows that upstream dissolution is the dominant process, whereas precipitation of gypsum occurs downstream. The two-dimensional simulation shows that downstream fractures are clogged, whereas continuous paths are created upstream. We find that the injection flow rate and acid ratio are the two main factors that influence precipitation and dissolution patterns within a fracture. The results show qualitative agreement with experimental results obtained in the Dietz De Josseling laboratory. We conclude that injection of an acid mixture can be used to improve water-drive recovery performance from oil reservoirs that contain fractures between injection and production well.