Directional Drilling On The Arctic North Slope Of Alaska
Authors: Murray Peter J. , Booth Neil R. 
Published: December 2007

The paper is a review of directional drilling undertaken by Sohio Petroleum Co. in the western half of the Prudhoe Bay Field on the Arctic North Slope of Alaska.

The first three sections cover well planning and include such topics as the necessity for directional wells, government restrictions and regulations, target and geological limitations, and the production of a directional map for drilling programs.

The next four sections describe operational procedures carried out to drill the well and fulfill procedures carried out to drill the well and fulfill requirements of the well program. Directional control tools and their specific use in these wells are covered, together with the various surveying techniques employed throughout the wells.

The final sections on surveying problems include a discussion of the work being undertaken to account for and eliminate the difference in results between the various surveying techniques.

Introduction

The Prudhoe Bay Field has estimated proved recoverable reserves of 9.2 billion bbl of crude oil and is being developed by two operators of which Sohio-BP operates the western half of the field. This paper is a review of present planning and drilling practices as applied by Sohio-BP.

The North Slope of Alaska is a barren desert of snow most of the year. In summer the Prudhoe Bay area is a mosaic of fragile tundra and lakes (Fig. 1). The western half of the Prudhoe Bay Field covers some 40,000 acres at the target depth 8,800 ft ss TVD. The initial development phase drains this half of the field from 72 wells (Fig. 1B). There are three gathering centers fed by 24 wells each. The present development program requires 60 additional wells that will develop the western half of the field to approximately 320-acre spacing. Future development may reduce the well spacing to 160 acres per well.

Drilling Pads

To minimize tundra damage, wells are drilled from a series of drilling pads, 15 in all, evenly distributed over this half of the field, from each of which up to 17 wells are to be drilled. The drilling pads are built of gravel excavated from nearby river beds and stand 5 ft above the ground level. This elevation above the tundra is standard for all the facilities and roads on the North Slope. The gravel protects the tundra, acts as insulation to the permafrost, and has a self-cleaning effect during the numerous snow blows of the winter. Pads, once built, are left in abeyance for one freeze-thaw cycle to allow the gravel to settle and freeze hard. Cellars 8 ft × 14 ft × 4 ft deep are dug out and conductor pipe set before the rig moves over the well. To reduce gravel and tundra subsidence during production due to thawing of the permafrost, insulated conductor pipe is used. This pipe is 20-in. casing with insulation and is set 80 ft into the ground as near vertical as possible and cemented in position.

Wells are spaced approximately 100 ft apart to allow room for a rig to operate between adjacent wells; hence, pads up to 1900 ft in length have been built (Fig. 2). The pad includes a section for the rig camp and a reserve pit or sump. The reserve pit adjacent to the pad is surrounded by a gravel berm built to the level of the pad. Gravel roads link pads and gathering centers. Flow lines between pads pads and gathering centers. Flow lines between pads and gathering centers are built parallel to the access roads.

The use of drilling or production pads in thus way ensures maximum efficient production with maximum environmental protection. Individual drilling sites would simplify the development drilling of this field in that vertical holes could be drilled, but the construction cost of individual pads, flowlines, and access roads would be prohibitive.