Horizontal-Well Performance at Prudhoe Bay
Authors: Broman W.H. , Stagg T.O. , Rosenzweig J.J. 
Published: December 2007

Summary
The performance of Prudhoe Bay horizontal wells was evaluated after 1 to 5years of production. This paper describes the performance of horizontal wellsin the Western Operating Area of the Prudhoe Bay Unit (PBU), includingsurveillance and well-work achievements, new drilling and completiontechnology, and analytical evaluations.

Introduction
Production from most wells in the PBU is governed by a large Production frommost wells in the PBU is governed by a large expanding gas cap. As gasapproaches the productive intervals, coning necessitates restrictions onproduction well rate to control rate-sensitive GOR's because of facilitygas-handling constraints. Horizontal wells, called nonconventional wells(NCW's), initially were drilled to provide additional standoff from the gas capand greater oil rates and recoveries than the conventional-well (CW)offsets.
The peripheral areas of the PBU receive limited direct pressure support fromthe gas cap, and some sections are being pressure support from the gas cap, andsome sections are being waterflooded in a pressure-maintenance program. Theperipheral area outside the waterfloods was the next target for NCW applicationwhere thin light-oil columns require increased wellbore exposure for highproductivity. NCW's then were drilled as peripheral waterflood producers inboundary patterns. Current development has targeted the low-productivity zonesat the base of the upstructure reservoir. These zones are characterized by poorreservoir quality and reduced formation thickness.
In addition, the use of NCW's for gas injection in a peripheral extension ofthe field, the Eileen West End, maximized injectivity for a minimum number ofgas injectors. Existing vertical wellbores have been converted into NCW's withdrainhole technology.
NCW's will continue to be used to develop the upstructure base zones andperipheral areas of the PBU. Future potential applications of NCW's includeadditional waterflood development (as injectors and/or producers) and drainholecompletions to produce the thin oil lenses left behind by gas encroachment asthe field continues to mature.
There currently are 27 NCW's (Fig. 1) in the PBU, including two gasinjectors. BP Exploration (Alaska) Inc. operates 17 NCW's in the Western Operating Area of the PBU; Arco Alaska, Inc. drilled and operates the remainderof the NCW's in the Eastern Operating Area.

Horizontal-Well Types
Four types of NCW's are used in the PBU.Five horizontal wells have been drilled at angles of about 90 to verticalin areas known to be relatively free of vertical barriers to flow. This type ofwell provides maximum standoff from gas and/or water contacts while allowingenhanced productivity from the increased wellbore exposure.Fifteenhigh-angle wells were designed for areas where shales indicated potentialimpedance to vertical flow. These wells typically are drilled at 85 to 88 fromvertical and permit production from above and/or below any shales that wouldhave reduced recovery from a horizontal completion. High-angle wells have alsobeen used as gas injectors, where the longer wellbore increases injectivity.Four inverted high-angle wells have been used in areas where shales mayinterfere with the recovery mechanisms and where gas influx under the shales ispredicted. This technique involves drilling the entire light-oil column at ahigh angle and then turning the wellbore back up through the reservoir at anangle exceeding 90. The ability to plug back from encroaching gas influx at theend of the wellbore allows prolonged low-GOR production from this type of well.Three drainholes have been drilled, converting existing CW's into NCWcompletions by sidetracking a horizontal section into the productive intervalfrom the original wellbore. The drainhole completion provides potential costsavings because the existing wellbore is used down to the target reservoir. This reduces geologic uncertainty and eliminates a substantial portion of thedrilling investment for a replacement CW.

Selection Criteria
The major selection criteria for each type of NCW depend on the developmentobjectives, geology, reservoir fluid contacts, completion design, andeconomics. The geologic input required for each candidate should contain areservoir description, including the ratio of vertical to horizontalpermeability,; an accurate shale description; and an estimate of faultproximity.