Paper: Skin Analysis Program Boosts Matrix Stimulation Results

Matrix stimulation treatments (most commonly using acid as a solvent) are aimed at overcoming the effect of near wellbore damage by dissolving solids and, hence, increasing permeability. The skin factor, a parameter to account for altered flow conditions in the near wellbore vicinity, is often used as a quantitative indicator of the level of damage. In the absence of other effects, such as partial completion or poor perforations, a high positive skin factor indicates severe damage; a zero skin factor results for an undamaged well, and a negative skin factor is due to higher effective permeability in the near-wellbore region'. Thus, determining the evolution of the skin factor during a matrix stimulation treatment is a means of measuring the effectiveness of the acid or other solvent in overcoming formation damage. Based on the theory of pressure behavior in transient flow, the skin factor can be calculated from the bottomhole injection pressure and the injection rate during a treatment. However, the bottomhole pressure is seldom measured during an acidizing treatment and the injection rate is usually variable, complicating the determination of the skin factor. In a typical matrix stimulation treatment, only the surface pressure and injection rate are measured and multiple rate changes occur during the course of the treatment. These effects must be considered to properly calculate the evolving skin factor. We have developed a computer program, UTRTM (University of Texas Real-Time Monitoring), that calculates and displays the skin factor during a matrix stimulation treatment from the measured surface injection conditions. The program can be used in real-time to monitor the progress of a stimulation treatment or after completion of a treatment to analyze its effectiveness. We have used this program for a wide variety of acid treatments and found it to be extremely valuable. We present examples in this paper that illustrate how the skin analysis program can be used to optimize a particular treatment on the fly, to evaluate the effectiveness of acidizing methods applied in an area, and to quantify secondary effects, such as diversion.