Matrix Acid Stimulation - A Review of the State-Of-The-Art

Abstract

Abstract Acid stimulation of oil and gas reservoirs, with a view to increasing well productivity, has been applied since the late- 19th century. Initially applied in carbonate reservoirs, the technique was extended to more complex mineralogies, over a number of years. However, it's fair to say that acid stimulation of wells is the exception rather than the rule. This probably stems from the complex, heterogeneous nature of formation minerals and the unpredictability of their response to conventional oilfield acid formulations. With inappropriate acid designs, or poor job procedures, even the best candidate wells can be damaged, sometimes irreversibly. This paper discusses the current state-of-the art in matrix acidising and makes the case for the wider implementation of acidising, as a cost-effective method for production enhancement. It reviews the many rules used today in the design of acid treatments and how these rules have evolved with improvements in our understanding of the interactions between acids, formation constituents and well tubulars. The paper also reviews the rationale behind the use of additives such as corrosion inhibitors, iron control agents, clay control additives, surfactants, solvents, anti-sludges and diverting agents, etc. and makes general recommendations on appropriate loadings, where applicable. Finally, the latest developments in acidising are considered, including the use of novel acid systems, to overcome many of the problems inherent in earlier formulations. Innovative equipment design, coupled with real-time monitoring capabilities, improved placement techniques and environmentally-friendly materials, are helping to transform acidising into a valuable asset in the quest for optimum performance from every oil and gas well. The paper references many key publications and provides the engineer with an upto- date overview of the state-of-the-art in this very important discipline. Introduction Matrix acid stimulation is viewed by many as a risky enterprise and one that should be undertaken only as a last resort. Yet, this relatively simple technique certainly represents one of the most cost-effective methods to enhance well productivity and improve hydrocarbon recovery. When properly applied, it is also an effective way to reduce the environmental impact of our industry, ensuring that reservoir drainage is efficient by optimising productive capacity from previously damaged wells. The science of acidising has its origins over 100 years ago when Herman Frasch of Standard Oil patented the use of hydrochloric acid to stimulate carbonate formations.(1) Simultaneously, one of his colleagues patented the use of sulphuric acid for the same purpose(2). Apart from a brief flurry of activity, resulting from the original idea, neither technique was applied on a widespread basis, during the next thirty years. Then, in the early 1930's several serendipitous events occurred. The Dow Chemical Company developed an effective acid corrosion inhibitor for mineral acids and was asked to provide it for an acid treatment being performed by the Pure Oil Company on one of their wells in Michigan. The effect of the treatment on production was positive, if not spectacular, but it provided the impetus to perform further treatments. Some of these later treatments produced excellent results and news of the technique quickly spread, spawning a whole host of small companies, eager to participate in this new business.(3)