Abstract
This paper is a case study of a Coiled Tubing Drilling (CTD) project in which a lateral extension was drilled from an existing wellbore into a gas storage reservoir. The project took place in the Donegal Storage Field located in Southwestern Pennsylvania. The gas storage reservoir is the Gordon Stray Sandstone at a depth of approximately 2,680 feet true vertical depth (TVD) in the project well (Donegal #4621). The focus of this case study is on the geological and engineering considerations as part of pre-planning and candidate selection, drilling operations, problems encountered during the project, and the deliverability enhancement that was achieved.
The drilling of a lateral extension using CTD technology was considered as a method to improve deliverability for several reasons: the failure of previous attempts to stimulate wells in the field using conventional and unconventional methods, geological considerations (including the extremely limited reservoir thickness of approximately four feet), the wellbore configuration, and the reduced footprint (environmental impact).
Drilling operations used several technologies that are considered unconventional in the Appalachian Basin. Build rates and steering were controlled via an umbilical within the coiled tubing. The umbilical contained an electric line for the transmission of Logging While Drilling (LWD) data, and hydraulic lines to manipulate the direction of the bit. The well was also drilled in a constant state of underbalance using on-site generated nitrogen to reduce mud column weight and four-stage separation to control flow rates at the surface.
Although several problems were encountered, and are discussed at length in the body of this paper, the lateral extension on Donegal #4621 was successful. A 698 foot lateral was drilled with approximately 450 feet located in the storage reservoir. Initial testing indicates that the horizontal extension has yielded over an 8,000% increase in deliverability from this well.
