Abstract

As the demand for oil and gas increases, operating companies have begun to explore alternatives to drilling with conventional rigs. One promising approach is to drill wells with coiled tubing rather than drill pipe. While great successes have been achieved with coiled tubing in small diameter re-entry projects with Orienters and Positive Displacement Motors (PDMs), their use for large diameter, directional wells has never been explored. Schlumberger and BP evaluated the feasibility of this concept by drilling a series of wells for BP America using a combination of Rotary Steerable System (RSS) and PDM in the San Juan Basin.

BP's projects in the San Juan Basin are gas wells, of between 2500 and 7500 ft. depth, depending on the target formation. The coiled tubing was used in two different applications. Firstly, to drill 6 ¼" hole to 2500 ft. to explore the shallow coal-seam gas and secondly to drill the 8 ¾" section to 4000 ft. as the intermediate section for deeper wells.

This demonstrated that drilling with the RSS and PDM assembly, in conjunction with Coiled Tubing, has several advantages over conventional drilling. The major benefits observed when drilling with coiled tubing are increased ROP due to the elimination of connection time, elimination of the need for controlled drilling and the ease of directional steering for different well profiles.

This paper discusses the pre-job engineering, the lessons learned whilst executing the job and the conclusions reached as to the benefits and capabilities of such a system.

Introduction

The San Juan basin is an area of approximately 9,000 square miles in northwestern New Mexico and southwestern Colorado (Fig 1).1,2 It is a monocline structure that dips steeply off to the east, west and north sides of the basin. Sedimentary rocks are present at depths of up to 14,500 feet; however, hydrocarbon production comes only from Upper-Cretaceous formations that have a maximum depth of approximately 7,500 feet.2 The remaining recoverable gas reserves are estimated to be 12.9 Tcf, with 8.5 Tcf of this being coal bed methane (CBM) reserves.3, 4 Thousands of wells have been drilled in the San Juan basin since the first well was drilled in 1901. Most of these wells targeted gas reserves in the Picture Cliffs, Mesa Verde, and Dakota formations, all of which are Cretaceous sandstones.3 However, starting in 1988, wells began to be drilled targeting CBM in the Fruitland Coal.1 Currently, the basin contains approximately 30,000 production wells, of which a majority are producing gas from the Fruitland Coal.1,15

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