Acid jetting is a well stimulation method for carbonate reservoirs, with observed positive production enhancement in some extended-reach horizontal wells. It is a process in which a reactive chemical solution is injected at a high rate at specific entry points via relatively smaller 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 dissolution structure is of great interest as it determines the quality of the well stimulation job, which correlates directly to the well productivity. This work is the second step in the overall project about a comprehensive study of acid jetting as a successful stimulation method for carbonate formations. The first step was an experimental study performed using a linear core-flood setup including a jetting nozzle. The objective was to understand the mechanism of acid jetting on carbonate cores and identify the important parameters in the experimental outcome. The current study aims at describing acid jetting from a mathematical standpoint, while using experimental results as model validation and improvement tools. Previously published acid jetting laboratory experiments results 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.
A core-scale computational fluid dynamics model has been developed to simulate cavity and wormhole growth in 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 3D transient finite volume numerical solver using Large Eddy Simulations (LES) with the Smagorinsky-Lilly sub-grid model to solve the Navier-Stokes and continuity equations. The results of this simulation include a velocity and pressure distribution 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. The reactive transport is modeled using the conventional kinetics of the dissolution of calcite by hydrochloric acid. This two-step model successfully replicates experimental results and observations for the cavity growth. It can then be coupled with a wormhole growth model to represent the entire experimental acid jetting outcome.
The modeling and computational tool for acid jetting developed in this paper will build the understanding for the upscaling and integrated dynamic modeling of an acid jetting stimulation job in the field. It will thus lead to the establishment of a standard for predicting and improving field applications of acid jetting.
