Several techniques for the real-time evaluation of matrix acidizing have been presented over the years. All rely on the calculation of the skin factor of the formation during the course of the treatment as the indicator for treatment effectiveness.
Two general approaches are currently used for calculating the skin factor of a well during a matrix acidizing treatment. The first assumes that a steady-state flow regime is in effect during injection while the second uses a pressure transient solution to calculate the skin factor during the treatment and assumes the well is in an infinite-acting transient flow regime. Comparisons of both calculation methods will be presented in this paper as will the advantages and limitations common to both methods. Also presented are two field case histories in which a pressure transient solution was used to continuously monitor the skin factor during the treatments. These case histories show in detail the effect of the different fluid stages injected during the treatments, including the effect of diversion stages. The concept of using the derivative of the skin factor with respect to time to track the rate of damage removal during acid injection or the rate of skin factor increase during diversion stage injection is also introduced.
Matrix acidizing provides an effective method for removing or bypassing near well bore damage. The method has been proven by many years of successful application around the world. One of the primary advantages of matrix acidizing is the comparatively low cost of performing a treatment in relation to alternative stimulation methods such as hydraulic fracturing. It is because of the low cost of matrix acidizing treatments that there has been little incentive to provide sophisticated on-site treatment interpretation and post-job analysis such as that which currently exists for hydraulic fracturing treatments.
The advent of compact, PC based data acquisition systems that can record real-time values of well head and annulus pressure, injection rate, and fluid density now allows the most important parameter in interpreting the effects of matrix acidizing treatments, the bottom hole treating pressure, to be calculated continuously as the treatment progresses.