In a matrix acidizing treatment, it is extremely important that the appropriate acid formulation be used, that no damage be left in the reservoir, and that no excess acid be injected. Nowadays, this can be easily accomplished with the aid of computerized real-time monitoring techniques, which provide on-site evaluation of the stimulation results. We applied a real-time monitoring technique to a series of acidizing treatments to evaluate stimulation performance and to optimize subsequent treatments in the same area. From the measured surface pressure and injection rate, a computer program calculated the bottomhole pressure and estimated the skin factor as the treatments progressed. In this way, the responses of the wells to stimulation were evaluated. Real-time monitoring of the evolving skin factor showed when further acid injection resulted in no further stimulation, leading to reduced acid volumes in subsequent treatments.
By applying this real-time monitoring method, we have improved acidizing practices in two fields, located in the northern part of Brazil. In a series of injection well treatments, acid volumes were reduced by 25% from standard practice, with no loss in acidizing efficiency. We were also able to change one of the acid compositions from regular mud acid to a more compatible 6% HCl/l.5% HF.
In more sensitive production well treatments, the real-time monitoring of skin factor may be even more useful by ensuring total damage removal or in identifying deleterious effects of overtreatment. More importantly, this technique will also indicate when design changes such as increased acid volume, different acid formulation, or different injection rate schedule are needed to reach the treatment objectives.
For many years, the design of acid volumes and concentrations for a matrix acidizing treatment could not be evaluated in the field. The only indication of success was the increase in rate, because well tests were seldom performed and no one could tell whether or not the damage was completely removed.
Nowadays, there are several methods to monitor damage evolution during acid injection, aimed at stopping it as soon as damage remove has been completed; i.e., when the damage ratio equals one or skin factor equals zero.
The model used in this work was developed by Hill and Zhu. It consists of plotting the inverse injectivity versus the superposition time function, from which the skin factor may be derived. To apply this technique, a simple program, called UTRTM (University of Texas Real-Time Monitoring) was developed. It monitors the skin factor in real-time, using input data such as formation permeability and initial skin. This method allows the operator to take control of the treatment and to decide when to stop acid injection or if it is necessary to increase the volume initially designed. Besides, the results of one stimulation help improve the design of subsequent treatments in the same field, in terms of volumes and concentrations.
This way, the introduction of this technology ensures the fulfillment of matrix acidizing objectives and fosters the selection of the most adequate formulations for the various types of formations that require acidizing
Bottomhole Pressure Calculation. The method relies on the accurate calculation of bottomhole pressures from surface pressure. Friction losses and the hydrostatic pressures of each phase inside the tubing are accounted for, using the values of density and viscosity supplied by the user. To do so, the position of each injected fluid inside the tubing is tracked by the program, as illustrated in Fig. 1. The bottomhole pressure is given by
where hj and fj are the hydrostatic and frictional pressure drops of sequence j, ps is surface injection pressure, and pwf is bottomhole pressure.