A Field Study Optimizing Completion Strategies for Fracture Initiation in Barnett Shale Horizontal Wells

Abstract

Abstract The Barnett shale is an unconventional gas reservoir that is currently estimated to extend over 54,000 square miles. In an effort to improve well economics and to reduce the number of surface locations in populated areas, there has been a rapid increase in the number of horizontal wells being drilled and completed. With this change in development strategy, operators and service companies alike have had to search for innovative solutions to overcome challenges faced in horizontal completions. Inefficient fracture initiation is the largest reoccurring problem seen when completing horizontal Barnett shale wells. These difficulties are manifested as high fracture initiation and propagation pressures, which lead to low injection rates and high treating pressures. These losses reduce the efficiency of proppant placement and stimulation. As drilling activity has increased over the past couple of years, fracture initiation problems have represented a substantial source of expense and downtime. This field study examines 256 horizontal Barnett shale wells in an effort to identify the causes of these near-wellbore issues and to offer corrective solutions for future completions. The goal of this study is to recommend an optimized completion strategy to minimize these near-wellbore problems, increase stimulation coverage and decrease unplanned completion expenses. In 2005, 19% of the stages in horizontal wells examined encountered near-wellbore difficulties. This field study inspects the major contributors to fracture initiation, specifically focusing on cccemented versus uncemented laterals, horizontal stress anisotropy, perforation strategy, cementing strategy and stimulation design. The paper offers statistics on which changes have had the greatest effect on stimulation placement. These problems can cost operators upwards of an additional 25% per stage. Using these optimized strategies has reduced the number of stages where fracture initiation difficulties have been encountered by 74%. Introduction The Barnett shale is a Mississippian-age marine shelf deposit that unconformably lies on the Ordovician-age Viola Limestone/Ellenburger group and is conformably overlain by the Pennsylvanian-age Marble Falls Limestone. The Barnett shale is located within the Forth Worth basin and the focus of our study will concentrate on wells within Denton, Wise, and Tarrant counties, the core area. The Barnett in the core area ranges from 300 to 500 ft in thickness. Permeabilities range from 0.00007 to 0.0005 md with porosities ranging from 3 to 5%. The Barnett is believed to be its own source rock and is abnormally pressured in this area. Commercial production is only achieved through hydraulic fracture treatments. Before 1997, Barnett 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 drastically increased by using slickwater, completion costs were reduced by approximately 65%. In 2002, horizontal wells were experimented with in an effort to increase the wellbore's exposure to the reservoir. The results of the first horizontal wells compared to vertical wells were three times the estimated ultimate recovery (EUR) at twice the well cost. Horizontal wells offered an economic solution to areas outside the core and reduced the number of surface locations needed near populated areas. 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 often left in the reservoir, which can equate to a smaller overall fracture area and reduced productivity1, this is illustrated in Fig. 1.