Horizontal drilling was successfully applied in a mature waterflood at Occidental's North Wasson Clearfork Unit (NWCFU) as the first waterflooded longitudinal fractured horizontal in the Permian Basin Leonardian Series. The field is located on the northwestern shelf of the Permian Basin, where the Middle Clearfork (MCF) reservoir had been developed with 20-acre vertical well spacing. Recent geological models revealed a series of relatively thin, discontinuous lenticular extensions of "pay" quality porosity in areas of poor areal sweep efficiency. Although horizontal development is typically used for greenfield tight unconventional resources, this concept was evaluated as an alternative to traditional vertical infill drilling in this conventional, heterogeneous, waterflooded reservoir. Because the widely accepted practice of maximizing stimulated rock volume with closely spaced, transverse fractures would jeopardize the waterflood sweep efficiency, the horizontal well was drilled parallel to the in-situ maximum horizontal stress (SHmax), rather than perpendicular. A horizontal pilot well was drilled with the following objectives: 1) to prove the effectiveness of multistage, longitudinal fractures to increase oil production from a heterogeneous reservoir, thereby improving recovery efficiency in a mature waterflood, and 2) to investigate the ability to manage the well's decline with waterflood support from offset vertical injection wells. Initial results have shown that the pilot was successful in meeting both objectives. Furthermore, this horizontal well, which cost five times as much as a vertical well, had an initial peak rate that was seven times as much as a typical vertical well in the field, and in the first year, it produced about seven times as much as a typical vertical well. In this paper, we will discuss the successes and challenges of the pilot project and share the lessons learned.
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