Application of Horizontal Wells at Prudhoe Bay

Abstract

Summary. Horizontal wells have been shown to increase productivity, to reduce coning tendencies, and to improve recovery. Given the potential applications in the Prudhoe Bay field, a project was initiated in 1984 to evaluate benefits and to drill and test three trial wells. This paper reviews production and reservoir engineering aspects of the trial program. It includes the objectives of the test program, the planning and drilling of three wells, the forecasting of production rates and recoveries, and the testing and analysis of actual well performance. In summary, the three wells have been successfully drilled and completed, with each well costing less than its predecessor. The wells have exhibited productivities two to four times that of conventional comparison wells, and increased oil recovery is anticipated. Horizontal wells can improve production rates and recoveries by a variety of mechanisms. At their most basic, the long wellbores allow longer completed intervals and therefore increased production rates. In reservoirs overlying an aquifer or located under a gas cap, the increased standoff from the fluid contacts can improve the production rates without causing coning. Additionally, the longer wellbore length serves to reduce the drawdown for a given production rate and thus further reduces coning tendencies. Fractured reservoirs can also benefit from horizontal wells. Long wellbores are likely to intersect more fractures and hence improve both production rate and ultimate recovery. Furthermore, the application of horizontal wells early in a project may allow development with fewer wells because of the larger drainage area of each well. In some fields, the advantages of horizontal drilling may allow development where conventional techniques would be uneconomical. The development of the Prudhoe Bay oil field, on the North Slope of Alaska, has been extensively reviewed in published literature. For reference, a field outline showing waterflood and non-waterflood area (Fig. 1) and a gamma ray log section depicting the various zones (Fig. 2) are included. Two areas of the field offer some of the potential advantages of horizontal drilling previously outlined.1. The midfield area generally contains a thick remaining oil column overlain by an expanding gas cap. Because Prudhoe Bay production will become constrained in the near future by the ability to compress and to reinject produced gas, rather than by oil productivity, immediate field production rate increases will result from reducing gas coning.2. The extreme downstructure part of the Prudhoe reservoir is undeveloped. The thin oil column and potential problems caused by water coning from the underlying aquifer have made development relatively unattractive to date. Horizontal wells offer the possibility of reduced coning and increased production rates, perhaps increasing the attractiveness of development. Projected remaining oil at abandonment for the entire field is about 12 billion STB [1.91 × 109 stock-tank m3], a significant target for any improved recovery scheme such as horizontal drilling. Given this potential target, well locations were screened for application as horizontal wells. A three-well program was selected to address both target areas and to develop a broad experience base. The selected well locations are shown in Fig. 3 and are discussed here. First Well Location (JX-2). This 80-acre [32-ha] infill location is in a structurally simple part of the field. Stepout from the gravel pad (from which all Prudhoe Bay wells are drilled) is less than 5,000 ft [1524 m], allowing a reasonable directional profile to the larger depth at 8,915 ft [2717 m]. The well is located at the base of the oil column to maximize standoff from the gas cap. Drilling technology and drainage area considerations limited the design completion length to 1,500 ft [457 m]. Second Well Location (B-30). The second midfield horizontal well was not spudded until after the initial well was tested. This second well increased drilling experience before the more difficult drilling associated with the Y-20 location. This experience factor and the additional reservoir performance data justified two horizontal wells in the midfield area. Third Well Location (Y-20). Y-20 was recommended as the third horizontal well and the first in the peripheral area. This would be an extended-reach well completed at the top of the reservoir to maximize standoff from the aquifer. The thin oil column in the periphery and longer stepout made this a more difficult, higher-risk well to drill. The stepout to the beginning of the horizontal section was approximately 8,300 ft [2530 m]. This exceeded the 5,000-ft [1524-m] stepout restriction placed on the first well. Information from the first two wells, however, was designed to permit a stepout of this magnitude. Because of the increased stepout, design length for this completion was limited to 1,000 ft [305 m]. Evaluation of Benefits Empirical, analytic, and numerical simulation methods were used to forecast the benefits of the horizontal wells planned for Prudhoe Bay. The benefits considered are productivity, critical coning rates, and recovery. Productivity. The productivity of a conventional well is proportional to the permeability-thickness product. Low productivities result from low values of permeability or formation thickness (or both). This can be compensated for in horizontal wells where the length of the horizontal section is not imposed by nature but chosen. The permeability-length product in horizontal wells plays a role similar to that of the permeability-thickness product of conventional wells. P. 1417^