Abstract

Acidizing is often applied in horizontal wells to improve well performance. Acidizing in horizontal wells differs significantly from the acidizing of vertical wells. The length of acid contact is much longer, the formation properties over the contact length likely vary in a larger range, and the cost is generally higher. To enhance the effectiveness of matrix stimulation, methods to evaluate and optimize acid treatments similar to those used for vertical wells can be adopted for horizontal wells.

This paper introduces a simple approach to evaluate horizontal well response to acid treatments. Skin factor was calculated explicitly in the method from pressure and flow rate, which can be directly measured during a treatment. Skin factor is then used to identify the well response to acid injection, and therefore to understand and control the stimulation process. There have been several mathematical models published to simulate the pressure/flow rate behavior of horizontal wells. Most of them have complicated solutions and have to be solved numerically. We selected and modified analytical solutions for horizontal well performance that can be implemented for field evaluation of acid treatments.

Field examples presented in the paper show how to apply the evaluation method for horizontal wells. The method can assess the effectiveness of acid treatments and can be used to optimize subsequent treatments.

Introduction

Acidizing in horizontal wells is significantly different than in vertical well. In a horizontal well, formation contact area is much longer, and acid distribution inside the wellbore is a greater issue1. The acid injection rate per length of formation treated is also generally much lower for horizontal wells than for vertical wells. This reduced injection rate can reduce the efficiency of the acidizing process, particularly in carbonate reservoirs, where the creation of wormholes is desired2. Simulating the acidizing process in horizontal wells involves more uncertainty, which directly affects the reliability of acidizing treatment design. In addition, acid treatment costs for horizontal wells are generally higher than for vertical wells because of higher injection volumes, the common use of coiled tubing, and more complicated injection procedures. For these reasons, it is important, but also very hard, to understand what has happened in horizontal well acidizing, and to identify the reasons resulting in success or failure. To enhance the effectiveness of acidizing treatments in horizontal wells, we introduce a skin analysis method similar to that used in vertical well acidizing. The method can be applied to matrix acidizing either during or after the treatment, to identify the well response to the acidizing treatment3,4. The method is based on transient flow theory for pressure behavior in horizontal wells. It calculates skin factor from pressure, flow rate, and other information about the well. In order to use the method during an acid treatment in real time, the model has to be simple and analytically solvable. This is a key criterion when selecting the mathematical models for acidizing evaluation.

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