Application-Appropriate Drilling Technologies Excel in Managed-Cost Mature-Asset Environments

Abstract

Production in most wells follows a predictable pattern dictated by the decline curve. Initially high production is quickly followed by a long, measured decline. This potentially long decline in production, together with an ever-increasing demand for energy, has resulted in many mature, low production fields. To stem the decline in production in these wells and extend the viable economic life of these assets, operators are increasingly turning to advances in technology. Technologies such as improved slimhole re-entry drilling bottomhole assemblies (BHAs), enhanced reservoir navigation systems, and improved conventional drilling techniques are successfully meeting the challenges of developing mature fields in established oil and gas basins around the world. This combination of techniques and technologies is being used in the Williston Basin to increase the recoverable reserves and improve the present economic value of each asset. Production from the Williston Basin declined from the time of its initial discovery through the early 1990s. The introduction of horizontal re-entry drilling technologies then revitalized this region. Targeting these thin beds typically requires extending 4½-in. laterals from existing or new vertical 7-in. cased wells. In the recent past, re-entry performance was confined by well placement restrictions, water and salt zones, and available drilling technologies. These requirements restricted the wellpath to relatively tight radius build sections. Together with the drilling difficulties associated with slimhole tubulars, these tight builds often resulted in high drag in the hole, limiting the lateral section to 3,000 to 4,000 ft due to weight transfer and drag constraints. However, in recent applications that did not suffer from such restrictions or the consequences of the water/salt formation hole instability issues, larger radius curves could be incorporated into the wellplan. This increased radius reduced drag in the well program, which together with improved motor technologies and experienced wellsite execution, allowed the 4½-in. re-entry section to achieve world record open slimhole multilaterals. The successful completion of such multilateral wells provide for over 10,000 ft of potential reservoir exposure in this thin bed. Introduction The Williston Basin is a large, roughly circular sedimentary basin covering several hundred thousand square miles along the eastern edge of the Rocky Mountains straddling the northern U.S.A. states of western North Dakota, eastern Montana, as well as southern Saskatchewan, and Manitoba in Canada, as shown in Fig. 1. The scope of this paper will concentrate on the North Dakota portion of this basin. Oil production began in earnest in the North Dakota Williston Basin in 1951 when Amerada Hess completed and produced its first commercial well. Drilling continued in a cyclical nature for the next 50+ years with production mainly based upon the Mississippian Madison Group formations, although some Mesozoic strata are productive (see Fig. 2). Currently, 3,300 oil wells are still producing in North Dakota. Additionally, there are over 14,000 wells existing in the North Dakota Williston Basin, having produced over 1.5 billion barrels of oil. Annual oil production peaked in 1984 at more than 52 million barrels of oil, followed by the expected and inevitable steep decline, as shown in Fig. 3. The decline was temporarily arrested in the late 1980s with the introduction of horizontal drilling of the Bakken formation. Subsequent advances in horizontal drilling techniques and technologies allowed for further exploitation of other formations to alter the production decline again in the late 1990s and mid-2000s. Although production will probably never again reach the peaks achieved 30 years ago, the reversal is so drastic that production in North Dakota is currently on the rise (see Fig. 3). The Birdbear (Nisku) formation oil-producing payzones are characterized by compartmentalized secondary porosity thin beds locked in deep muddy limestone and dolomite formations. The Nisku ‘A’ dolomite is encased between two impermeable anhydrite beds, creating a large regional stratigraphic trap, ideally suited for horizontal drilling and subsequent viable economic development. In this particular area of the Williston Basin, formation thicknesses vary from 2 to 4 ft (0.7 to 1.2 m) when exposed with vertical wells. Traditionally, the Birdbear is relatively low on the list of most prolific oil-producing zones, ranking 10th overall in Table 1.