Horizontal Drilling Techniques at Prudhoe Bay, Alaska
Authors: Wilkirson J.P. , Smith Jay H. , Stagg T.O. , Walters D.A. 
Published: December 2007

Three extended-departure horizontal wells have been drilled and completed in the Prudhoe Bay Unit, AK, by Standard Alaska Production Co. Horizontal slotted-liner completions of 1,575, 1,637, and 1,163 ft [480, 499, and 354 m] were achieved at an average vertical depth of 9,000 ft [2743 m]i. Over the three-well program, improvements in technology and operating procedures have resulted in a 40% reduction in cost per foot. When compared to conventional completions, production data from the horizontal wells indicated rate benefits up to 300% and a potential increase in ultimate recovery. This paper discusses the development of the techniques used to drill horizontal wells at Prudhoe Bay and reviews the horizontal drilling operations for each well.

Introduction

The application of horizontal drilling to the Prudhoe Bay field is driven by the goal of increasing ultimate field recovery. Conventional recovery methods are expected to leave an estimated 12 billion bbl [19 Gm3] of oil in place. Horizontal drilling in areas with gas and water coning potential and in regions of uneconomic oil column thickness was viewed as a potential method to maintain Prudhoe production levels and to increase ultimate recovery.

In the midfield area at Prudhoe Bay (Fig. 1), the oil column lies between an expanding gas cap and a water leg. Throughout the production life of a well, high formation permeability allows production life of a well, high formation permeability allows increasing gas and water fractions because of coning. The overall production rate of the field will eventually be limited by the gas production rate of the field will eventually be limited by the gas handling capacity because the produced gas and water must be reinjected. A horizontal completion, strategically placed between the gas cap and water leg, will allow increased production rates at lower drawdown pressures and, consequently, will reduce gas and water coning tendencies.

In the southwestern portion of the field where the oil column is less than 100 ft [30 m] thick, development using conventional wells is hindered by economic considerations. Because the gas cap is absent in this area, horizontal wells can be drilled along the top of the oil zone to expose longer reservoir sections that provide economical production rates while minimizing water production.

In Jan. 1984, Standard Oil Production Co. began work on a project to develop the technology to drill and to complete horizontal project to develop the technology to drill and to complete horizontal wells at Prudhoe Bay. The strategy chosen was to drill a test well in south Texas to provide a technical basis, followed by three Prudhoe Bay wells to optimize the drilling techniques. To maximize the Prudhoe Bay wells to optimize the drilling techniques. To maximize the transfer of technology, drilling supervisors and a geologist from Standard's Alaska staff were assigned to the test well. Key individuals of the team that had drilled four successful horizontal wells in Europe were employed as consultants.

The south Texas test well, Braune-Wieding No. 1, was designed to model closely the proposed Prudhoe Bay horizontal well profile. A 1,900-ft [579-m] horizontal section (inclination greater profile. A 1,900-ft [579-m] horizontal section (inclination greater than 84 degrees) was successfully placed in a 30- to 40-ft [9- to 12-m] -thick sand near 10,300 vertical ft (3140 m]. The well established and verified several planning and operational techniques. A top-drive system proved beneficial in dealing with anticipated directional drilling conditions, especially tight hole problems. Using, this system, two o three connections were eliminated and back-reaming while circulating 90-ft [27-m] sections was possible. To maximize torque available at the bit, a torque/drag simulator was used to design the wellbore profile and drillstring configuration. Experience was gained in high-angle directional drilling using steerable mud motors and conventional rotary assemblies. Measurement-while-drilling (MWD) systems were used extensively and proved indispensable for directional surveying and formation identification using gamma ray and resistivity sensors. Geologic markers were used to adjust and refine the directional profile continually while building angle to horizontal. Wellbore stability was a critical consideration in the high-angle sections of the well, and invaluable experience was gained in maintaining stability with high-weight, low-fluid-loss drilling fluids. Adequate hole cleaning was achieved with optimal mud properties and efficient solids removal equipment. The well was properties and efficient solids removal equipment. The well was logged with drillpipe-conveyed tools and completed using a slotted liner placed through the entire horizontal section. The completion incorporated a specially built liner hanger/packer and a circulating string that permitted drilling fluid displacement from the slotted liner.