Acid stimulation of sandstone reservoirs is viewed by many as a risky enterprise and one that should be undertaken only as a last resort. Much of this reluctance stems from the complex, heterogeneous nature of sandstones(1) and the unpredictability of their response to conventional oilfield acid formulations. Indeed, with an inappropriate acid design, or poor job procedures, even the best candidate well can be damaged, sometimes irreversibly. With the loss of experience and expertise due to industry downsizing, there is some concern that the risk of such failures has increased.
This paper discusses the implementation of an acidising methodology with the goal of attaining 100 percent success in acid stimulation treatments. A cornerstone of the strategy involved the development of a personal acidising mentor for every engineer to help them avoid the pitfalls of inappropriate design. The only practical way to achieve this was by the use of computers, programmed with the many rules used by human experts in the design of acid treatments. These rules take into account such variables as mineralogy, temperature, formation fluids, permeability, porosity and tubular metallurgy, amongst others. Integration of these rules, based on an array of algorithms coupled with heuristic principles, outputs a preferred acid design along with recommended acid volumes, pre- and post-flushes, nitrogen ratios for flowback and a customized message board with hints, tips or warnings about the potential treatment. The design advisor also incorporates recommended loadings of corrosion inhibitors, iron control agents, clay control additives, surfactants, solvents, anti-sludges and diverting agents, etc. as and when required.
Application of this approach has helped fill the gap caused by the widespread loss of experienced industry personnel and has accelerated the learning process among younger engineers. It has also helped ensure that only appropriate acid treatments are recommended and pumped in any location, worldwide. The paper reviews the results of over 100 acid jobs designed using this methodology.