Achieving 100 Percent Success in Acid Stimulation of Sandstone Reservoirs

Abstract

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. Introduction 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(2). However, it was not until the 1930's that attempts were made to improve production from sandstone reservoirs by injecting mixtures of hydrochloric and hydrofluoric acids. These early treatments were not particularly successful, however, and this relegated these HCl/HF mixtures to only occasional use in those wells that were deemed to have suffered drilling mud damage. It was not until the 1960's that treatments containing hydrofluoric acid again saw widespread use in well remediation. This was largely due to the publication of studies on the chemical interactions of HF with typical sandstone formation minerals, along with guidelines for treatment optimisation. This work eliminated much of the mystery surrounding the use of HF and put its use in practical terms that petroleum engineers could understand(3). However, in spite of this, acidising sandstone formations remained a hit-or-miss enterprise(4). It was fabulously successful in some areas, totally disastrous in others. Indeed, throughout the 1970's and 80's, most production engineers and service company personnel could recite details of some catastrophic acid job that they had either heard about, or actually been involved with.

Rotating Jet Technology Improves Stimulation Fluid Placement to Effectively Treat Horizontal and Gravel Packed Wells - Lab Model Tests and Global Results are Reviewed

“…The Aida A-9 well is a 500 ft horizontal open-hole GP (5 ½ " premium screens with 20/40 GP sand) completed in 11/98. As part of the completion practices in the field, Aida A-9 was initially stimulated using 2000 gals of a polymer specific enzyme solution to remove drill-in fluid (DIF) damage from the drilling operations, a standard operation in the area to remove and dissolve polymer and calcium carbonate particles 3,14 . CT and nitrogen foam were utilized for enzyme placement across the 500 ft interval.…”

Deviation From Standard Sandstone Acidizing Practices B/V Chronicle of a Forgiving Acid System

“…Their job designs were similar to those of Martin et al and they compared stimulation gains with wells stimulated previously with conventional mud acid blends. Examples of such include publications by 7 McLeod et al, 6 Phil Rae and Gino Di Lullo. They also made contributions in the area of gas well stimulation and job success definition.…”