Liquid CO2 and Tip-Screenout Fracturing as Techniques for Restimulating Gas Storage Wells
Authors: Reeves S.R. , Pekot L.J. , Weida S.D. , Ammer J.R. 
Published: December 2007

Abstract
This paper summarizes the first-year results of a four-year R&D demonstration program sponsored by DOE/METC entitled "Field Verification of New and Novel Fracture Stimulation Technologies for the Revitalization of Existing Underground Gas Storage Wells". Operations at three of nine planned test sites have been completed or are nearing completion, with highly encouraging results with respect to deliverability enhancement. At one site, fracturing with liquid CO2 and proppant is being tested, while tip-screenout fracturing is being tested at each of the other two sites.
Liquid CO2 fracturing is being tested at the Galbraith Field in Pennsylvania operated by National Fuel. Pre- and post-treatment pressure transient tests indicated that substantial reductions in skin factor were achieved with the restimulations, resulting in deliverability improvements of up to seven-fold. Important criteria were also identified regarding the application of liquid CO2 fracturing to gas storage reservoirs, including possible limitations posed by high formation permeability together with high in-situ stresses.
Tip-screenout fracturing is currently being tested at the Huntsman Field in Nebraska operated by KN Energy. The preservation of caprock integrity by controlling fracture height growth required that precise, low-volume treatments be designed and executed. Pre-treatment pressure transient tests and fracture-height surveys on one well have been performed, and indicate deliverability improvement can be achieved while successfully avoiding excessive fracture height growth. Final work, including fracturing the two remaining test wells and post-stimulation pressure transient tests, is now being conducted at this site.
The third test site, where tip-screenout fracturing is also being performed, is the Donegal Field in Pennsylvania operated by Columbia Gas. Pre-stimulation testing and the first treatment have been performed at the site. Analyses indicate that horizontal fracturing is evident, suggesting that an alternative approach (other than hydraulic fracturing) may be required for effective well stimulation.

Introduction
An improved, more efficient natural gas storage system is essential for supporting the growth in US gas demand in the coming decades. A high priority therefore exists to increase current domestic storage capability and offset the persistent 5.2% average annual loss in well deliverability from gas storage fields. Although storage field operators have extensive experience with current well remediation technology, recently published case studies demonstrate the shortcomings of traditional, non-fracturing well revitalization methods. In 1994, CNG Transmission fracture-stimulated 30 wells in five Oriskany gas storage fields using conventional techniques, demonstrating that highly encouraging results can be achieved with fracturing. However there still remains considerable potential for using new and novel fracture stimulation technologies to restore injection and withdrawal capabilities in gas storage wells where conventional fracturing may not be applicable, for example where fracture fluid sensitivities and height growth concerns exist, or in high-permeability reservoirs. Thus a comprehensive evaluation of alternative fracture stimulation techniques is required to demonstrate that certain technologies can be effectively applied to increase well deliverability in these settings.
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