Use of Longitudinally Fractured Horizontal Wells in a Multi-Zone Sandstone Formation
Authors: Pearson C.M. , Clonts M.D. , Vaughn N.R. 
Published: December 2007

Between 1992 and 1994, four horizontal wells were drilled, completed and hydraulically fractured in the multi-zone A Sand of the Kuparuk River Field, Alaska. This application of horizontal well technology is unique because the wells are drilled in the direction perpendicular to the minimum principal stress field in order to propagate longitudinal hydraulic fractures to effectively drain the thin bedded sands. Pertinent results from the initial simulation study are provided showing projected performance and the need to effectively fracture the entire length of the horizontal section. The project plan is presented with four different well designs having horizontal sections from 700 to 2000 feet and with either single or multiple completion intervals. Field results include drilling, completion and stimulation details, and production to date. An economic evaluation compares the performance of these wells to offset conventional wells and highlights the need for a simple/low cost completion and stimulation program in order for hydraulically fractured horizontal wells to be a viable development option.

Of the four wells, two perform significantly better than offset comparable conventional wells; one has comparable performance; and one has underperformed to date due to problems with the completion.

Introduction

The use of horizontal wells has been well documented in the industry. One application has been their use to exploit naturally fractured formations by orienting the well perpendicular to the fracture direction so as to maximize the number of fractures intersected. A number of case histories have been reported on these wells where the horizontal section has been hydraulically fractured. In another application, in relatively homogenous formations, the horizontal well is oriented in a similar direction and multiple transverse fractures induced by hydraulic fracturing. In both of these applications the wellbore is drilled in the direction of minimum principal stress. Hydraulic fractures extending away from the wellbore will orient themselves perpendicular to the minimum principal stress direction (and the wellbore) to produce a series of transverse fractures.

The completion design for initiation of transverse hydraulic fractures typically uses a high density of perforation shots limited to an interval less than twice the diameter of the well. While this may be effective at generating the required transverse fracture away from the wellbore it's likely that an axial or longitudinal fracture will be generated, as determined in some field studies. The reason for generation of the longitudinal fracture is the lower pressure required to overcome the hoop stress and "split" a wellbore along its axis rather than "part" the wellbore across its circumference.

Application of a longitudinal fracture presents an opportunity to increase productivity from moderate to high permeability formations which are hydraulically fractured. In such formations, the well productivity is controlled by the fracture conductivity. The horizontal well can therefore be viewed as an infinitely conductive fracture provided there is similar vertical communication. Also, if the horizontal well is drilled in the direction of maximum principal stress it will be parallel to any hydraulic fracture initiated from a conventional well.

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