Fracture-Stimulation Technology for Gas-Storage Wells

Abstract

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction and Background Since introduction of Federal Energy Regulatory Commission (FERC) Order 636the fundamental role of the interstate gas-transportation business has changed from that of intermediary between producer and consumer to that of a service provider. Further, because these services are now unbundled and cannot be lumped into the supplier rate base, pipeline companies must now provide each of their services individually and at increasingly competitive prices. Some of these services include gas transportation, market-center hub services (e.g., balancing, compression, loaning, parking, peaking, trading), and gas storage. Gas storage is used for two primary purposes: to meet seasonal winter demands for natural gas (base-load storage) and to meet short-term peaks in demand(peaking storage), which can range from a few hours to a few days. For purely peak demand, cavern storage has become increasingly popular because of the extremely high deliverability that can be achieved, albeit only for short periods of time. Base-load storage facilities, for which depleted oil/gas fields are frequently used, can store larger volumes of gas but usually have smaller overall deliverability capacities. Nevertheless, minimum deliverability requirements exist for base-load storage facilities. One of the keys to providing gas-storage services on a commercially competitive basis is to minimize the cost of constructing and/or maintaining minimum deliverability requirements. In a study performed in 1993, the Gas Research Inst. (GRI) determined that, on average, the storage industry loses more than 5% of its deliverability capacity annually from base-load fields. The reasons behind deliverability loss are currently the subject of ongoing R&Dby GRI and the U.S. Dept. of Energy (DOE) even though they traditionally have been attributed to the introduction of foreign materials during gas-injection periods (e.g., compressor oils, rust, scale) and to the deposition of scale sand/or fines mobilization during periods of high-rate withdrawal, both of which can contribute to near-well damage. To counteract deliverability decline, the industry spends more than U.S. $100 million annually, primarily to drill new infill wells to supplement older wells that have experienced substantial deliverability decline. While new wells can be used to increase overall storage capacity of a field if sited appropriately, the stimulation (or restimulation)of existing wells is a considerably more cost-effective approach to maintaining deliverability. Gas-storage operators realize this. However, the traditional approaches to deliverability enhancement (blowing, washing, or mechanically cleaning the wellbore; reperforating; and acidizing, which combined account for over 80% of all storage-well enhancement activity) generally provide only limited, short-term results, not substantial, long-term well stimulation. Additionally, poor candidate-well selection, frequently based only on limited or inaccurate deliverability data, may also contribute to this outcome. The GRI study identified fracturing as a promising deliverability-enhancement technique for gas-storage applications; however, storage operators have historically avoided such methods because, in part, of concerns about reservoir seal integrity. Nevertheless, as a result of competitive pressures, fracturing of gas-storagewells is of increasing interest to storage operators. In response, the DOE (and more recently GRI) is sponsoring a joint program with industry to demonstrate application of new and novel fracture-stimulation technologies in gas-storagefields across the U.S. The ultimate objective of this program is to advance deliverability-enhancement technology to provide more efficient utilization of the country's gas-storage assets in response to the growing demand for a widespread, reliable natural gas supply. This article describes some of these fracture-stimulation technologies and advances made as part of that project. P. 61