N.R. Fairchild, Jr. and D.D. Wood
This paper presents a field case study on the application of advanced stimulation technologies (AST) to the East Ohio Gas (EOG) Stark-Summit Gas Storage Field, Clinton Formation, near North Canton, Ohio. The study demonstrates benefits in two ways:
increased deliverability; and
improved restimulation candidate selection.
Using 3-D hydraulic fracture modeling and reservoir simulation, one of the actual (AST) treatments pumped during the study was compared to the typical 75-quality nitrogen foam treatment used in the 1996 restimulation program. This comparison showed that the new treatment improved proppant placement and fracture conductivity. Over a 150 day withdrawal cycle this increases the cumulative gas production by 29,900 Mscf and improves initial gas rates by 30%. In addition, actual flow test data showed that post-fracture deliverability was more than doubled over pre-fracture values, and was 187% higher than any previous post-fracture results in the study well.
The case study also demonstrated the importance of proper restimulation candidate selection. By using improved techniques for pre-treatment flow test evaluation, better estimates of existing permeability and current effective fracture half-length can be determined. Identifying wells with little potential for increased deliverability due to extremely high permeability or an existing effective hydraulic fracture prevents these wells from being restimulated which results in cost savings. If these wells are not properly identified, up to $20,000 can be spent on a restimulation treatment that has little chance of significantly improving deliverability. In addition, better estimates of permeability will enable hydraulic fracture treatments to be designed and optimized on an individual well basis.
To aid in the optimization of hydraulic fracture candidate selection, design, execution, and evaluation, the following technologies were used: one-point well test analysis, 3-D fracture design, reservoir simulation, real-time fracture treatment diagnostics, post-fracture welltest analysis, and isochronal flow test analysis. Two test wells were used to evaluate the application of the different technologies which led to significant changes in EOG's treatment schedules and proppant concentrations. The case study was conducted in cooperation with the Gas Research Institute and its Advanced Stimulation Technologies deployment program.
EOG operates the Stark-Summit storage pool in north east Ohio near North Canton (Fig. 1). To maintain pool deliverability, EOG hydraulically fractures approximately 20 wells per year. Although these wells have been hydraulically fractured in the past, proppant flow back, paraffin buildup, and compressor oil sludge can reduced the effectiveness of these treatments.
Wells in the Stark-Summit storage field are completed in the Clinton Sandstone which is Silurian in age. The field is lenticular in nature and dips 800 ft from its western edge to its eastern edge. Average depth of the field is 4,200 ft with an initial reservoir pressure of approximately 1,500 psi. The field covers 27,900 acres with an average net pay of 23 ft. The boundaries of the field are defined by permeability pinch-outs which produce dry holes and low volume wells at the periphery of the field. Permeability varies widely over the field (1 to 20 md) and may be enhanced by natural fractures.
The field was put into production in 1927 and produced 140.3 Bcf of gas until 1941 when it was converted to storage. At the time of conversion, it was calculated that 22.6 Bcf of gas remained in the field. When storage operations began, there were 339 active and 120 abandoned wells. During storage development, in-fill drilling was conducted to obtain an average well spacing of 40 acres, and there are currently over 700 active wells in the field.