It has been well-understood that there exists an optimal condition in matrix acidizing in carbonate formations. When a treatment is not pumped at the optimal conditions, the efficiency of the stimulation suffers. A cost-effective procedure for maximizing matrix acidizing treatment results in carbonates is proposed. This procedure includes a systematic approach to altering the acid schedule, pump schedule and the placement technique. The procedure is particularly important for high temperature or long treatment intervals where achieving optimal injection rate is difficult.
Injecting acid below the optimal injection rate in matrix acidizing of carbonates can negatively impact productivity/injectivity by significantly limiting the final skin reduction. To design a treatment that creates dominant wormholes to improve well performance, the optimal pumping rate must first be determined based on the field conditions which can be scaled up from laboratory core flood data. For cases where the optimal injection rate cannot be achieved because of the limitation of pumping equipment, this paper proposes a counter-intuitive procedure for altering the treatment design. There are means to reduce optimal injection rate for a treatment, such as changing acid concentration, acid type or additives. Reducing acid strength can lead to a slower reaction rate and deeper wormhole penetration. There is however a limit since reducing the acid strength will not only reduce the optimal interstitial velocity but also increase the acid volumes required for breakthrough because of lower acid dissolving power. If the optimal pumping rate cannot be sufficiently reduced, then the placement techniques must be modified to facilitate higher injection rates. Furthermore, the pump schedule must also consider heterogeneity and this may require the use of multiple stages. Using published core flood data along with hypothetical well designs, various acid treatments have been simulated, and the results are presented in terms of skin factor along the well length for each injection strategy. The study demonstrates the application of the proposed procedure.
The results of the study showed that in cases where the pumping rate is less than optimal, simulation results show less than optimal skin factors are predicted. When pumping at low rates, using the highest concentration of acid does not necessarily result in the most efficient wormhole creation and propagation. There is a clear limit to which reducing acid strength can benefit the degree of penetration for a given injection rate. The proposed procedure is a practical means of creating a cost effective acidizing design that not only removes formation damage but creates deep dominant wormholes that minimize the skin factor.