Improved Horizontal Well Stimulations in the Bakken Formation, Williston Basin, Montana

Abstract

Abstract The Bakken formation in the Williston Basin of Montana has undergone rejuvenation thanks to the success of horizontal drilling and completions. Over the last several years there has been an evolution in the drilling and completion methodology, which has resulted in significant improvements in well productivity. Although the horizontal wells will produce at economic rates without stimulation, hydraulic fracturing increases well productivity significantly. For the program discussed in this paper, the wells are drilled for longitudinal fracture orientation. The original wellbores had cemented liners and employed the limited-entry technique to distribute the fracture treatment. This program now uses noncemented liners and a modified fracture treatment with diverter stages. This paper presents a review of the approach taken to evaluate the most effective way to complete and hydraulically fracture these laterals. It documents the evolution of the hydraulic fracture design and wellbore configurations. It also presents results showing the improvements in proppant distribution along the wellbore caused by changes in fracture treatment design and its impact on well productivity. Introduction The Sleeping Giant project is a stratigraphic middle Bakken play located in Richland County, MT (Fig. 1), covering approximately 400 square miles. It is bounded by a facies change and loss of porosity to the northeast and pinch-out to the southwest. During the late 1980s, there was a marginally successful horizontal play in the Bakken formation to the east of Richland County. The Bakken is present in only the subsurface of the Williston Basin. It is comprised of three distinct intervals, each of which are near termination in the project area. Fig. 2 shows a type log through the Bakken section. The three intervals are described as: The Mississippian - Upper Bakken Shale (Highstand) This "hot" gamma ray zone serves as the contact of the Bakken with the overlying Mississippian - Lodgepole. It is comprised of a black, organic-rich, pyritic shale with measured total organic content (TOC) up to 40%. It is between 8 ft and 12 ft thick in the project area, and serves as a good overlying seal. It is well within the mature oil envelope in the Sleeping Giant Project area, and is the primary source of hydrocarbons for the Middle Bakken and several other shallower reservoirs. The Devonian/Mississippian - Middle Bakken (Lowstand) This interval contains the main reservoir facies, composed of burrowed, silty and sandy dolomite with fair to good (8% to 15% density) porosity and heavy oil staining. This primary interval is up to 15 ft thick and laterally homogeneous. Fossils include brachiopods, pelmatozoan fragments, gastropods and trace fossils. Above and below this interval are the transgressive and regressive sequences, respectively, each becoming increasingly muddy and laminated approaching the bounding shales. The Devonian - Lower Bakken Shale (Highstand) This "hot" gamma ray zone is comprised of a black to brownish-black, fissile, non-calcareous, organic mudstone or shale. It has TOC of up to 21% as measured in area cores. It serves as the basal contact of the Bakken with the underlying Devonian - Three Forks, and is between absent and 6 ft thick in the project area. The organic matter appears to be distributed evenly throughout the member. Quartz is the dominant mineral with minor amounts of muscovite, illite and other clays. Pyrite is present in lenses, laminations, or is finely disseminated throughout. Fossils within the shale member include conodonts, algal spores, brachiopods, fish teeth, bones and scales. In this area the Bakken formation is found at a depth of approximately 10,000 ft. The fracture gradients range from 0.69 to 0.75 psi/ft. Reservoir fluid properties are: oil gravity of 42° API and 0.95 gas gravity with an initial GOR of 500 scf/bbl. The reservoir is slightly overpressured with an initial pore pressure gradient of 0.5 psi/ft and a bottomhole static temperature of 240°F.