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% HC1/1.5% 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.

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