Many drilling provinces throughout the world have high reserve potential but relatively low production rates from typical wells. This low productivity may preclude economic development drilling using conventional techniques. The increasingly marginal nature of finding and producing natural gas has increased the need for technologies that reduce the cost of drilling and completing wells beyond mere incremental steps. Slim-hole drilling and completion is one technology with the potential to make a substantial impact on costs.

In mid-1995, a major operating company, with support from the Gas Research Institute (GRI), undertook a three well slimhole drilling project in the Greater Green River Basin of Wyoming. Barriers to slim-hole drilling, both real and imagined, were overcome during the course of the project, and the performance capability of modern slim-hole technology was determined.

Slim-hole drilling and completion will allow additional economic development of marginal areas, particularly in natural gas provinces where there is minimal need for high-volume artificial lift.

Introduction and Background

Early in 1995, GRI began a major project to define "Emerging Resources in The Greater Green River Basin" and to determine the optimum means of developing the area's large tight gas resource. A primary objective of this project was to identify technology that could be utilized immediately to this end. Other work underway for GRI had already identified slim-hole technology as a promising approach for reducing drilling and completion costs while maintaining well productivity. An opportunity for a three well slim-hole drilling and completion project on the Cow Hollow lease in the Moxa Arch field of Wyoming (Figure 1) was presented by a major operator, and GRI agreed to participate.

The term "slim-hole" means many things to many people. Most commonly it is defined as one or more bit-sizes smaller than what is normally used in an application. The three slim production holes at Moxa Arch were all drilled with a 4 3/4-inch bit, which is slim-hole by almost anyone's definition.

Interest in slim-hole drilling and completions peaked in the United States in the early 1960s, but then declined due to the absence of strong economic forces to drive development of the technology. As producers began to seriously search for methods to reduce costs in the early 1990s, interest in slim-hole technology began growing again. From the mid-1980s to the mid 1990s, the percentage of wells completed as slim holes in the United States rose from 3 to 5 percent, and the percentage is even higher for gas wells.

An industry-wide survey to find barriers to slim-hole technology growth revealed that drilling was not perceived to be a large barrier. For barriers related specifically to drilling, the two topics perceived as the largest barriers were fishing and directional drilling in slim holes, as shown by the chart in Figure 2. Other topics perceived to be large barriers by the industry included mud motors, well control, and bits for slim holes.

The Moxa Arch project addressed three of the top six perceived barriers to slim-hole operations, and demonstrated that, in many cases, these are only perceptions. Technology has advanced to the point that expectations and perceptions based on past history are often no longer valid.

Slim-Hole Project Objectives

The two major objectives of the slim-hole work were to (1) determine the current performance capabilities of modern slimhole technology and (2) determine the cost savings potential of slim-hole drilling in the Greater Green River Basin.

Several areas of modern drilling technology were addressed by the project.

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