The Relationship Between Fractured Well Performance and Hole Deviation
Authors: Vincent M.C. , Pearson C.M. 
Published: December 2007

M.C. Vincent and C.M. Pearson

SPE Members

This paper presents the results of a field study investigating the relationship between the productivity and wellbore deviation of hydraulically fractured wells. Fractured, high angle wells are shown to have improved skin factors, higher productivity indexes and increased production rates compared to wells of lower deviation. The relationship of decreasing skin factor with greater wellbore deviation is shown to have a similar trend to the theory for a non-fractured well. No relationship is found between the wellbore azimuth and well productivity although the paper lists the design steps necessary to minimize the negative effects associated with "out of plane" fracture turning. The results of this study are contrasted to other published studies and reasons for differences are proposed.

The paper describes how this study was used to justify drilling higher deviation wells from existing drill sites at the Kuparuk River Field in Alaska, eliminating the expense and environmental impact of developing five additional peripheral drill sites.

Introduction

This study was initiated in 1992 to determine the best development strategy for peripheral areas of the Kuparuk River Field. Located in the Alaskan arctic, the Kuparuk River Field is one of the United States' largest producing oil fields (300,000 barrels oil per day), covering an area of approximately 115,000 acres. The field was discovered in 1969, but was not deemed economic for development until 1977, after completion of the Trans Alaska Pipeline. Figure 1 shows the site of the field location on the North Slope of Alaska. Field development was started in 1979 using gravel pads as central drill sites from which deviated wells were drilled to drain a four section area (4 square miles). Initial development was on 160 acre well spacing with some 80 acre infill locations.

The Kuparuk reservoir is a sandstone at approximately 6000 feet subsea with solution gas drive as the primary production mechanism. The majority of the field is under secondary recovery receiving pressure support through a combination of waterflood and water alternating immiscible gas injection. Previous reports have documented both the geological description and the reservoir mechanisms of the field.

Production occurs from two horizons within the Kuparuk sandstone. An upper sandstone interval, referred to as the C Sand, is present over most of the southern half of the field and consists of very coarse to very fine grained siderite and sandstone. With average permeabilities of 130 md to 2600 md, and net pay ranges up to 80 ft, the C Sand typically produces at rates from 1000 to 5000 barrels per day (BPD).

The A Sand, the lower producing zone, is present throughout the field and contains 65% of recoverable oil in place. The average net thickness is typically less than 30 feet, with permeability ranging from 20 to 100 md. It is a fine to very fine grained sandstone interbedded with shale and cemented with quartz and varying amounts of ankerite. The A Sand unit can be further divided into six distinct subzones. Each of these subzones has variable reservoir quality across the field and typically one or two of these subzones dominate the A Sand productivity in any given area. The resulting A Sand permeability-thickness (kh) of wells across the field averages 1000 md-ft. Unstimulated A Sand production rates of 300 BPD or less would be uneconomic in the high cost arctic environment.

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