Optimizing Completion Strategies for Fracture Initiation in Barnett Shale Horizontal Wells

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 103232, "A Field Study Optimizing Completion Strategies for Fracture Initiation in Barnett Shale Horizontal Wells," by A.A. Ketter, SPE, Devon Energy; J.L. Daniels, SPE, Schlumberger; J.R. Heinze, SPE, Devon Energy; and G. Waters, SPE, Schlumberger, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24-27 September. The Barnett shale is an unconventional gas reservoir, estimated to extend over 54,000 sq miles. To improve well economics and reduce the number of surface locations in populated areas, there has been a rapid increase in the number of horizontal wells drilled and completed. Inefficient fracture initiation is the largest problem encountered when completing horizontal Barnett shale wells. This field study examined 256 horizontal wells to identify the causes of these near-wellbore issues and to recommend corrective measures for future completions. Introduction The Barnett shale is a Mississippian-age marine shelf deposit in the Fort Worth basin, and this study concentrates on wells in Denton, Wise, and Tarrant counties. The Barnett shale in the core area is 300 to 500 ft thick. Permeabilities range from 0.00007 to 0.0005 md with porosities ranging from 3 to 5%. The Barnett shale is abnormally pressured in this area. Commercial production is achieved through hydraulic-fracture treatments. Before 1997, Barnett shale wells were completed with massive hydraulic-fracture treatments consisting of crosslinked gelled fluids and large amounts of proppant. Because of difficulties with effectively cleaning up fracture damage from the crosslinked gel and the high cost of these massive stimulation treatments, the wells were not as economical as desired. In 1997, large-volume, high-rate slickwater fracture-stimulation treatments were sought as a less-expensive alternative. While well performance was not increased drastically by use of slick-water, completion costs were reduced by approximately 65%. In 2002, horizontal wells were introduced to increase well-bore exposure to the reservoir. The first horizontal wells had three times the estimated ultimate recovery at twice the well cost compared to vertical wells. In the early stages of horizontal completions, the wells were divided equally between uncemented and cemented laterals. Shorter laterals that required single stimulations were uncemented, and cemented laterals were implemented when the stimulation design required multiple stages because of an increased lateral length. Composite bridge plugs were used for zonal isolation. Fractures in uncemented laterals are prone to grow in such a way that unstimulated volumes, or "gaps," are left in the reservoir, which equates to a smaller overall fracture area and reduced productivity. As drilling progressed outside the core area and acreage was available to accommodate longer laterals, the number of cemented horizontals surpassed the number of uncemented horizontals. However, the increase in cemented laterals also yielded a higher rate of inefficient fracture initiation than seen in uncemented laterals. In 2005, more than one in four cemented horizontals experienced fracture-initiation problems compared to one in 25 for uncemented laterals.