Model Captures Carbonate Matrix Acidizing in Horizontal Well Completions
Authors: Carpenter Chris 
Published: June 2024

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 23178, “A Comprehensive Model for Carbonate Matrix Acidizing in Complex Horizontal Well Completions,” by Mahmoud T. Ali, Ahmed Zakaria, SPE, and Jiliang Wang, Baker Hughes, et al. The paper has not been peer reviewed. Copyright 2024 International Petroleum Technology Conference.

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The complete paper presents an extensively validated model to simulate acid flow from the wellhead to the wormhole tip in carbonate formations. The model accounts for upper and lower completion wellbore hydraulics, pressure drop across valves, annular flow, and wormhole growth. The comprehensive model introduced in this work provides stimulation engineers with a reliable tool to design successful acid stimulation jobs in complex horizontal well completions.

Introduction
Prediction and analysis of wormhole growth and the corresponding skin relies on the ability to predict the reservoir-face pressure and the understanding of physics and chemistry of acid flow in porous media. In many situations, reservoir-face pressure must be predicted from surface treating pressure. In some situations, a downhole gauge may be set at the bottom of the upper completion. In certain horizontal wells, however, the reservoir face can be far from the downhole gauges and detailed mathematical models still are required for accurate reservoir-face pressure calculations.
The travel of the treatment fluids from the well surface through tubulars is accompanied with pressure losses caused by friction with the walls and pressure gain caused by the change in vertical depth. The flow through the horizontal section of the well is controlled by the friction calculations. In the lower completion, the fluid needs to travel radially, usually through limited entries to the reservoir. The simplest completion is the open hole, where fluid travels radially to the reservoir face with no mechanical constraints. In advanced completions, the fluid must go through orifices or more complicated pathways such as an inflow control device (ICD) before hitting the reservoir face. Mathematical models are needed to account for the frictional losses through those mechanical constraints. In many cases, one ICD can be used to stimulate more than 200 ft of the reservoir, which requires the implementation of advanced algorithms to account for the flow behind the ICD and distribute the fluid precisely.
Once the fluid reaches the reservoir face with the accurate pressure, then, using the classical production-engineering equations, the injection rate can be calculated for each zone. Acids usually are injected in carbonate formations to create thin tunnels, called wormholes, to bypass damage and improve well productivity or injectivity. An experimentally validated model was implemented to predict the wormhole growth as a function of rate, acid type, concentration, temperature, rock type, and mineralogy. The generated wormholes were then translated into reduction in skin and increase in well productivity or injectivity.
In the complete paper, a wellbore/reservoir coupled-flow model was developed to predict and analyze acid flow accurately through advanced horizontal lateral completions in carbonate reservoirs. The model accounts for the friction losses through the tubulars and lower completions. Also, the distribution of the fluids behind the liners is handled through mathematical algorithms. A field-validated carbonate acidizing model was used.