A Field Study in Optimizing Completion Strategies for Fracture Initiation in Barnett Shale Horizontal Wells
- Aaron A. Ketter (Devon Energy Corporation) | John L. Daniels (Schlumberger) | James R. Heinze (Devon Energy Production Co. LP) | George Waters (Schlumberger DCS)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- August 2008
- Document Type
- Journal Paper
- 373 - 378
- 2008. Society of Petroleum Engineers
- 1.7.1 Underbalanced Drilling, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 3.2.4 Acidising, 4.6 Natural Gas, 1.6.9 Coring, Fishing, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.1.1 Exploration, Development, Structural Geology, 4.1.2 Separation and Treating, 1.14.3 Cement Formulation (Chemistry, Properties), 1.6 Drilling Operations, 2.4.3 Sand/Solids Control, 2 Well Completion, 5.8.2 Shale Gas, 4.2.3 Materials and Corrosion, 2.5.1 Fracture design and containment, 3 Production and Well Operations, 5.6.5 Tracers, 1.14 Casing and Cementing, 2.2.2 Perforating
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The Barnett shale is an unconventional gas reservoir that currently extends over an estimated 54,000 sq miles. In an effort to improve well economics and to reduce the number of surface locations in populated areas, the number of wells being drilled and completed has rapidly increased. 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 encountered when completing horizontal Barnett shale wells. These difficulties have manifested themselves 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 are now 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 cemented vs. 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 an additional 25% per stage or more. Using these optimized strategies has reduced by 74% the number of stages in which fracture-initiation difficulties have been encountered.
The Barnett shale is a Mississippian marine shelf deposit that lies unconformably on the Ordovician Viola limestone/Ellenburger group and is overlain conformably by the Pennsylvanian Marble Falls limestone. The Barnett shale is within the Forth Worth basin, and the focus of our study will concentrate on wells within Denton, Wise, and Tarrant counties, which form 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 that range from 3 to 5%. The Barnett shale is believed to be its own source rock and is abnormally pressured in this area. Commercial production is achieved only with 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 caused by 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. Although well performance was not increased drastically with 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 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 stage isolation. Fractures in uncemented laterals are prone to grow in such a way that unstimulated volumes, or "gaps," are often left in the reservoir; this can equate to a smaller overall fracture area and reduced productivity (Fisher 2004), as illustrated in Fig. 1.
As drilling progressed outside the core area and acreage became more readily 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 that seen in uncemented laterals. In 2005, more than one in four cemented horizontals experienced fracture-initiation problems, as compared to one in 25 for uncemented laterals (Fig. 2). This overwhelming rate led to the optimized completion strategy offered in this paper.
Inefficient fracture initiation can be defined as the lack of sufficient fluid-injection rates that results in the inability to pump designed proppant concentrations, delivering an ineffective fracture network. The stimulation job typically will be characterized by high pumping pressures and, occasionally, abnormal fracture gradients. Fig. 3 displays an example of an inefficient fracture initiation, while Fig. 4 displays an efficient fracture initiation and propagation.
Inefficient fracture initiation can be related to cement design, perforation phasing, perforating lengths, cluster spacing, formation stresses, and hydraulic-fracture pad-stage design. The cost incurred because of these problems is quite significant, representing an additional 25% of a stage's total completion cost. The cost of an improperly placed stage also can be detrimental to the productivity of the well by reducing the overall fracture area. Each failure also provides a logistical problem by setting the fracturing schedule back a day or more, thereby reducing the efficiency of the completion program. The goal of this case study was to recommend an optimized completion strategy that would reduce the completion cost of cemented horizontals, increase stimulation coverage, and accommodate an aggressive drilling program's need to maintain an undisturbed fracturing schedule.
The case study was divided into two distinct segments. First is the problem-assessment segment, which evaluated 154 horizontal wells, 31 of which displayed inefficient-fracture-initiation issues. Correlations were developed by use of field data to recognize probable causes and possible solutions to overcome these challenges. The second segment included 102 horizontals in which these new strategies were implemented. This paper will discuss how fracture-initiation problems were reduced to 4.7% from 19.1, a 74% improvement.
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