The goal is to help maintain and enhance the deliverability of the nation's natural gas storage system. This will be accomplished by characterizing the geochemical conditions of underground natural gas storage reservoirs and injection/withdrawal wells for a selected set of damage mechanisms and designing and successfully demonstrating practical and cost effective remedial techniques for these damage mechanisms.
Schlumberger Technology Corporation (Holditch Reservoir Technologies) – project management
Pennsylvania State University – laboratory testing, protocol development
Pittsburgh, Pennsylvania 15275
The gas storage industry is a significant contributor to the stability of gas supply in the United States. It has been estimated that deliverability losses of approximately 3.2 billion standard cubic feet per day (Bscf/d) every year occur in the storage industry due to a number of wellbore damage mechanisms. Previous studies have estimated that up to one hundred million dollars are spent each year to recover or replace this lost deliverability. Thus, restoring deliverability in existing wells, reducing deliverability enhancement costs, and/or developing technology to mitigate or eliminate damage from occurring are important to maintaining a stable, reasonably-priced gas supply.
Prior studies have identified the likely mechanisms that are responsible for deliverability loss in gas storage wells, and have defined procedures for identifying potential damage mechanisms. Although these studies generally discussed the possible remedial actions that need to occur to treat the damage, they did not address the exact cause of the damage, determine when it occurs (injection or withdrawal cycle), or suggest how to prevent or mitigate the damage. This project expands these previous efforts to identify both the specific damage mechanisms and the optimal remedial and/or preventative measures.
This study has increased industry's understanding of how inorganic precipitates (specifically siderite) develop and identified potential sources of chemical components necessary for siderite formation. This has resulted in an effective lab protocol designed to assess the extent of damage due to inorganic precipitates. A remediation technique has successfully restored deliverability to storage wells damaged by the inorganic precipitate siderite (one well had a 10-fold increase in deliverability) and can possibly be applied to other wells with similar damage. The study has also shown that non-darcy damage in gas storage wells can be significant and should be addressed.
To begin the project, a number of geochemical/geological environments were selected for study, including National Fuel Gas's (NFG) Summit Field, located near Erie, PA. Historical and operational data related to a selected study well (NFG's Summit 1527 well) were collected and reviewed (i.e., past pressure transient testing results, back pressure testing results, results of analyses of well bore fluids and pipe solids, and past stimulation information). Multi-rate pressure transient test analyses were then performed in nine wells suspected of having damage based on a comparison of current and original deliverability. Based on this testing and other data, four Summit wells (1527, 1522, 1524, and 1589) were identified as stimulation candidates.
Rotary sidewall cores from Well 1527 were collected in fall 2000 and analyzed. From this analysis, it was determined that most of the damage had occurred close to the well bore. A comparison of cores from an old section of the well bore with those from a newly drilled section suggested that prior stimulations had been reasonably effective in removing iron-related damage. Nonetheless, some iron-related mechanical damage remained. It was observed that the color of the entire core differed depending on whether it was taken from virgin formation or previously exposed formation. While this may suggest that some degree of damage extends deeper into the formation, the majority of the visible damage appears to occur within a few millimeters of the well bore.
Based on this testing and analysis, two working hypotheses were developed: (1) deposition of scale may occur primarily as a result of the dehydration of water solutions and (2) deposition of scale above the producing formation and in the casing occurs as a result of the entraining of solution droplets in the upward flowing gas stream. Next, conceptual strategies to mitigate or eliminate the damage mechanisms were developed. A team of industry experts (with representatives from the gas storage, university, service company, and consulting sectors) was assembled and the potential interactions among the affected part of the storage well, the reservoir, and the well bore environment were determined. A laboratory testing methodology was then developed for use in a field setting. This improved remediation technology for iron precipitates was successfully applied in NFG's Summit Field Wells 1524, 1589, and 1522 in Fall 2002.
Post treatment multi-rate pressure transient test analyses indicated that the treatment was successful in two wells. In one well, the treatment resulted in a 10-fold increase in deliverability (Q100) in the first year followed by a significant decrease in the Q100 the second year. Although the current Q100 is still higher that the pre-treatment Q100, test results suggest that damage may be recurring in this well. Although the amount of improvement in the second successfully treated well could not be quantified, its post-treatment Q100 is similar to the other successfully treated well.
The third well treated did not show any improvement in Q100 after treatment. In fact, it would appear that the well's performance is deteriorating with time, as evidenced by a significant decrease in the Q100 from the first post treatment test to the second post-treatment test.
A database was developed and used to study non-darcy damage in storage wells. The database contains information on 103 treatments (64 in sandstones and 39 in carbonates). Data was available for 22 treatment types, which were grouped into 13 treatment categories for analysis. Study data suggests that fewer treatments types have historically been used in carbonates reservoirs than in Sandstones reservoirs. In carbonates, only one treatment category (acidizing) was available for inclusion in the study database.
Non-Darcy damage is ubiquitous in storage wells and results in a very significant amount of non-productive energy loss (i.e., pressure drop) that could be used to achieve additional deliverability if reduced or eliminated. Given the prevalence and magnitude of non-darcy damage in USGS wells, the ability to reduce or eliminate non-darcy pressure drop in these wells represents a huge potential for increased deliverability from USGS wells.
The project has been completed.