Acidizing has typically been one of the most popular techniques used for the removal of damage in gas storage wells. Acidizing is often useful for carbonates, but surveys have indicated that as many as 30% of the sandstone wells were adversely affected by the use of acids.
This paper will discuss various conditions that should be evaluated when considering the use of acids for remediation or stimulation in gas storage wells. These conditions include mineralogy, completion type, and operational considerations such as the well's capacity to return fluids to the surface, types of acids and their associated chemistry, and reservoir characteristics. This paper will also evaluate the use of acids with respect to the most frequently observed types of damage mechanisms. These mechanisms were recently determined in a Gas Research Institute damage-mechanism study involving 32 wells and 12 gas storage reservoirs.
Evidence of at least two wells that have been adversely affected by acid will be discussed, and general guidelines for the use and types of acids and appropriate placement techniques will be given.
In 1993, a GRI-funded project reported on the deliverability enhancement technology currently being used by the gas storage industry. Part of this review indicated that most gas storage operators experience an average deliverability loss of approximately 5% over 1 year in the more than 400 U.S. storage reservoirs representing approximately 15,000 individual wells. The study also estimates that tens of millions of dollars are spent each year to recover or replace deliverability and to maintain the current rate of deliverability. The most widely used deliverability enhancement techniques are washing, acidizing, reperforation, and infill drilling. More than 100 acidizing treatments in all types of reservoirs were evaluated in this survey. Acidizing was performed 50% more frequently on carbonates than on sandstones, and all results were reported as good to excellent. Of the 63 sandstone treatment responses evaluated, only 15% had excellent results, and 20 to 30% had poor results. Deliverability enhancement costs for acidizing are estimated at 4 to 20 million dollars. The conclusions drawn from this data are that acidizing a carbonate is more effective than acidizing a sandstone, and that a lack of knowledge about acidizing sandstones contributes to the lower success rate in these formations.
Considerations for Acidizing Gas Storage Formations Damage Mechanisms. The first consideration before applying any type of deliverability enhancement technique should be the type of damage that is to be removed. A clear understanding of the damage mechanism is needed before a proper remedial/enhancement technique can be applied. Without this understanding, ineffective and/or inappropriate methods may be chosen, and remedial dollars may be wasted. A second GRI-funded research initiative was undertaken to provide a fundamental understanding of the formation damage mechanisms responsible for deliverability loss and their cause-effect relationships to deliverability loss in a broad spectrum of gas storage reservoirs.
For this study, researchers used well testing, downhole samples taken with bailers, rotary sidewall coring, and downhole video as their primary investigative tools, and identified the following eight major damage mechanisms as potential problems in gas storage reservoirs:
inorganic precipitates such as salts, carbonates, sulfides, etc.
hydrocarbons, organic residues, and production chemicals - particulate plugging
completion/stimulation fluid effects