Computational Fluid Dynamics Modeling and Simulation of Acid Jetting on Limestone Carbonates

Abstract

Acid jetting is a well stimulation method for carbonate reservoirs, with promising outcomes for the production enhancement in horizontal wells. It is a process where an acid solution is injected at a high rate via relatively smaller localized nozzles. The flow out of the nozzles is designed to be a fully turbulent jet which impinges on the porous surface of the rock, leading to a dissolution structure. That structure is of great interest as it determines the quality of the well stimulation job, and correlates directly to the well productivity. Preliminary experimental acid jetting studies, aiming to understand the acid jetting mechanism on carbonate cores and its key parameters, revealed the recurring creation of a large dissolution structure at the impingement location in the shape of a cavity and, depending on injection conditions, the propagation of wormholes through the core. The objective became to model/describe acid jetting from a mathematical standpoint. A computational fluid dynamics model was thus developed to simulate acid jetting. A core-scale model was developed to simulate cavity and wormhole growth during acid jetting. It is a three-dimensional model which alternates between the two fundamental aspects of the overall acid jetting process. Firstly, it models the fluid mechanics of the turbulent jet exiting the nozzle and continuously impinging on the porous media transient surface. The jet fluid dynamics are implemented using a transient finite volume numerical solver using Large Eddy Simulations with the Dynamic Smagorinsky-Lilly sub-grid model to solve the Navier-Stokes and continuity equations. The results of this simulation include velocity and pressure distributions at the porous media surface. Secondly, it models an irreversible chemical reaction with dissolution and transport at the impingement location between the fluid and the rock matrix. This two-step model successfully replicates experimental results and observations. When coupled with a wormhole growth model, it can represent the entire experimental acid jetting outcome. The tool developed in this study builds the understanding for the upscaling and integrated dynamic modeling of acid jetting in the field and can therefore lead to the establishment of a standard for predicting and improving field applications of acid jetting.