Acid jetting, as a well stimulation method for carbonate reservoirs, has shown optimistic results in the production enhancement of some extended reach horizontal wells. It was used initially to promote damage removal along a wellbore via multiple strategically-located injection nozzles. It has the potential to place the injecting fluid at the locations that need stimulation. Jetting may also enhance wormhole propagation compared with conventional matrix acidizing. These hypotheses are currently being investigated to improve the design for more efficient stimulation treatments. We have conducted an experimental study to investigate the effect of jetting on wormhole efficiency. The jetting experiments are conducted at a constant pressure with linear core-flood tests, where a stand-off distance is maintained between the injection nozzle tip and the core. At low-velocity acid injection, jetting effectively removes mud filter-cake by mechanical actions. Jetting also creates wormholes in carbonate cores. The combination of mechanical and chemical reactions stimulates carbonate rocks better than matrix acidizing without the jetting nozzle. When the jetting velocity increases, the dissolution pattern changes. An isolated locally compact dissolution results in a cavity at the entries of core samples by jetting, followed by a wormhole structure. With the known dissolution pattern, sensitivity studies are carried out to investigate the effect of various parameters on the experimental outcome. We used Indiana and Winterset limestone rocks in the experiments. 15% HCl (by weight) at ambient temperature was used and the core dimensions were 4 inches in diameter and 16 inches in length. Various combinations of acid jetting velocities and acid fluxes were considered. The Winterset limestone cores are more heterogeneous, with higher porosity and lower permeability than the Indiana limestone cores. From the experimental results, the results of acid jetting from the two different rock samples are compared. The experimental results indicate that acid jetting follows the same trend as matrix acidizing regarding wormhole propagation once cavities are created. Jetting velocity and acid flux are the critical parameters in jetting design for optimal stimulation results. Acid jetting tends to create different dissolution patterns for Indiana limestone and Winterset limestone cores. The observations from this work highlight the importance of understanding the dynamic chemical process of jetting in the design of successful acid stimulation jobs.
Summary Acid jetting, as a well-stimulation method for carbonate reservoirs, has shown optimistic results in the production enhancement of some extended-reach horizontal wells. It was used initially to promote damage removal along a wellbore by means of multiple strategically located injection nozzles. It has the potential to place the injecting fluid at the locations that need stimulation. Jetting may also enhance wormhole propagation compared with conventional matrix acidizing. The hypotheses for the design of more-efficient stimulation treatments are currently being investigated. We have conducted an experimental study to investigate the effect of jetting on wormhole efficiency. Each jetting experiment was conducted as a constant-pressure (equivalent to a desired initial flux through the core) linear coreflood test, in which a standoff distance is maintained between the injection nozzle tip and the core. At low-velocity acid injection, jetting effectively removes mud filter cake by mechanical actions. Jetting also creates wormholes in limestone cores. The combination of mechanical and chemical reaction stimulates limestone rocks better than matrix acidizing without the jetting nozzle. When the jetting velocity increased, the dissolution pattern changed. An isolated local compact dissolution results in a cavity at the entries of core samples by jetting, followed by a wormhole structure. With the known dissolution pattern, sensitivity studies are carried out to investigate the effect of various parameters on the experimental outcome. We used Indiana and Winterset limestone rocks in the experiments. A 15% hydrochloric acid (HCl) (by weight) at ambient temperature was used, and the core dimensions were 4 in. in diameter and 16 in. in length. Various combinations of acid jetting velocities and acid fluxes were considered. The Winterset limestone cores are more heterogeneous, with higher porosity and lower permeability than the Indiana limestone cores. The experimental results from the two different rock samples are compared. Overall, the experimental results indicate that acid jetting follows the same trend as matrix acidizing, regarding wormhole propagation after cavities are created. Jetting velocity and acid flux are the critical parameters in jetting design for optimal stimulation results. Acid jetting tends to create different dissolution patterns for the cores from Indiana limestone and Winterset limestone. The observations from this work highlight the importance of understanding the dynamic physical and chemical process of jetting in the design of successful acid-stimulation jobs.
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.
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