Modelling temperature-influenced acidizing process in fractured carbonate rocks

Abstract

A three-dimensional thermal-hydro-chemical (T-H-C) coupled unified pipe-network method (UPM) is developed to simulate the process of acidizing treatment in fractured carbonate rock. The concept of the traditional two-scale continuum model is extended by considering thermal effects and incorporating multiphysical governing equations for explicitly represented fractures in the current method. An effective conforming mesh method is employed in the element discretizing procedure for the complex fracture network system. Both rock matrix and fractures are modeled by three-dimensional equivalent pipe networks to simulate the H-T-C coupling process in fractured carbonate rock. The fluid transfer between rock matrix and fractures is solved by a superposition principle, without introducing the interchange terms. This straightforward method simplifies the programming process in simulating rock dissolution by acid. The accuracy of the thermal-hydro coupled process is verified against commercial software (COMSOL Multiphysics) result. The reliability of modelling the acidizing process at different temperatures is demonstrated through comparison with previous experimental results. Numerical experiments indicate that the presence of a fracture network, varying reservoir or inlet acid temperature, and formation heterogeneity all influence the acidizing efficiency. The fracture network reduces the volume of acid being injected but has no influence on the dissolution pattern corresponding with specific injection rates. The optimal injection rate increases to from a dominant wormhole in the case with high formation temperature. The acidizing efficiency is reduced when acid with a high temperature is injected. And both optimal injection rate and injection volume increase. For the influence of formation heterogeneity on the dissolving patterns, conical dissolution is observed in comparatively more homogeneous formations with lower porosity heterogeneity. However, ramified dissolution occurs in the extremely heterogeneous formation, and the existence of fracture networks have less influence on the development of wormhole branches. These aforementioned factors should be considered in practical field design to approach optimal acidizing results.