Refracture Treatments Proving Successful In Horizontal Bakken Wells; Richland Co, MT

Abstract

Abstract The Bakken reservoir development in Richland County, eastern Montana, has been a center of activity and has received significant attention in the industry since the first horizontal well was completed there in 2000. For one operator the initial completion program utilized cemented liners in the laterals in wells oriented to promote longitudinal hydraulic fractures. As discussed by Wiley, et al.,1 radioactive tracer logs indicated that there were often significant intervals, especially in the heel section of laterals that were not stimulated using this technique. In late 2003, one operating company decided to attempt a refracture treatment in a well that showed long lengths of unpropped interval based on its tracer log. The approach was to emulate the completion technique then in use on new wells with uncemented liners. The result from this first refracture treatment was encouraging with a peak post-refracture treatment production rate in excess of the peak production rate after the original completion stimulation. As a result of this success, a refracture treatment program was initiated. To date, all but one of the initial cemented liner wellbores have had refracture treatments pumped into them. The overall result has been an increase of over 1,300,000 stock tank barrels (STB) in the estimated ultimate recovery (EUR) for the wells in this program. Results showing the improved coverage from tracer logs will be presented along with examples of pre- and post-treatment production curves. Over the course of this refracture treatment program, the fracture treatments have evolved. Treatment size has remained the same; however, the method of pumping the treatments has changed. This paper discusses how the refracture treatments were pumped and the results of the program. Introduction The Spring Lake/Elm Coulee development is a stratigraphic middle Bakken play located in Richland County, Montana (Fig. 1), covering approximately 500 square miles. It is bounded by a facies change with decreased porosity to the northeast and pinch-out to the southwest. During the late 1980s, there was a marginally successful horizontal play in the Upper Bakken shale formation to the east of Richland County. The Bakken is present in only the subsurface of the Williston basin. It is comprised of the following three distinct intervals, each of which is near termination in the project area.Mississippian-Upper Bakken shale (highstand)Devonian/Mississippian-Middle Bakken (lowstand)The Devonian-Lower Bakken shale (highstand) Fig. 2 shows a type log through the Bakken section. A previously published paper 1 describes the three intervals in detail. In this area, the middle Bakken formation is 6 to 15 ft thick and is found at a depth of approximately 10,000 ft. The fracture gradients range from 0.69 to 0.77 psi/ft. Reservoir fluid properties are: oil gravity of 42° API and 0.95 gas gravity with an initial gas/oil ratio (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. The permeability ranges from 0.05 to 0.5 md, with a Kv/Kh ratio of 0.1 and a porosity range of 8 to 12%. The overall reservoir accumulation for the Spring Lake/Elm Coulee development is informally known as Sleeping Giant among some of the original prospectors and participants in this play. At Sleeping Giant, the mid-Bakken dolomite has primarily been developed by long horizontal producing laterals that attempt to maximize contact with the reservoir to develop economic production rates from the very low-permeability reservoir. Open natural fractures are not believed to be significant in the primary recovery.