Barnett Shale Refracture Stimulations Using a Novel Diversion Technique

Abstract

Gas production from the unconventional Barnett Shale reservoir now exceeds 3 Bcf/d, which is more than 5% of total U.S. dry gas production. Typically Barnett Shale wells exhibit a rapid production decline following the initial hydraulic fracture stimulation treatment, so that, within 5 years, an operator is normally faced with a well producing below its economic threshold. To keep up with current gas demand, operators have moved to an aggressive horizontal drilling and completion program. Additionally, in an effort to increase the productivity of existing wells and book additional reserves at reduced cost, operators have restimulated their older vertical wells, with demonstrable success. This success is providing compelling opportunities to enhance refracture treatment coverage by targeting bypassed and ineffectively stimulated zones in additional vertical wells and even some horizontal wells. Because of the heterogeneous nature of this unconventional gas reservoir, the restimulation of horizontal wells is problematic, and operators have demonstrated limited success using current stimulation techniques. This paper describes a new fracture diversion technique particularly adapted for horizontal well refracture stimulation. During the treatment, a fracture diversion system (FDS) is used to create a temporary bridge within the active fracture networks. That results in differential pressure increase and causes treatment redirection to understimulated intervals along the lateral. This technique enables both fracture diversion without mechanical intervention and, when enhanced with microseismic monitoring, real-time optimization of the fracturing treatment. Refracture stimulation case studies are presented in which this novel diversion technique is successfully applied to horizontal Barnett Shale wells. This paper demonstrates how real-time hydraulic fracture monitoring has enabled operators to make informed decisions that influence fracture geometry, increase lateral coverage, and improve gas recovery. To date, more than 20 fracture diversion designs have been successfully placed. The trial wells have included both cemented and uncemented completions, with drilled azimuths selected to encourage either transverse or longitudinal fracture fairway development. With a continuing optimization of the described refracturing technique, these FDS designs and placement strategies have evolved to the point where they are consistently exhibiting fracture diversion as evidenced by movement of microseismic activity and improved lateral coverage. While this engineered fracture diversion technique is ideally suited for re-fracture stimulations, it is also applicable for stimulation of new wells where the technique enables stimulation of larger wellbore intervals when used in the same fashion as for re-fracture stimulation applications. Introduction The Barnett Shale is a Mississippian-age marine shelf deposit that unconformably lies on the Ordovician-age Viola Limestone/Ellenberger group and is conformably overlain by the Pennsylvanian-age Marble Falls Limestone (Ketter et al. 2006). Formation thickness varies from 200 to 800 ft through the reservoir. The productive rock is typically a black, organic-rich shale with ultralow permeability in the range of 70 to 500 nanodarcy. To attain economically viable production rates, hydraulic fracture stimulation is a necessity.

The Environmental Costs and Benefits of Fracking

“…Refracturing of 15 oil wells in the Bakken Shale yielded a 30% increase in estimated ultimate recovery (EUR) (22). In the Barnett Shale, where natural gas production declines 3-to 5-fold within a few years, refracturing increased EUR by 20% (23). As the price for oil or natural gas rises, refracturing will become increasingly common.…”

“…The relative water intensity (L/GJ) of the hydraulic fracturing was ∼6 times higher for this later oil than for the earlier oil produced from the well. In the Barnett Shale, refracturing generated 7.1 million m 3 (0.25 Bcf ) or 0.26 GJ of additional natural gas per well, a 20% increase in EUR (23). The 7,600-9,500 m 3 (2-2.5 million gallons) of water used to refracture each well, however, resulted in a water intensity of 32 L/GJ, higher than coal for extraction and processing but still below the water intensity for electricity generation compared with coal (Tables 1 and 2).…”

Well Screen and Optimal Time of Refracturing: A Barnett Shale Well

“…In order to validate our simulation model, we modeled initial fractures and the refracturing process in a horizontal well in the Barnett Shale formation. The Barnett Shale is a Mississippian-age marine shelf deposit having a formation thickness that varies from 200 to 800 ft through the reservoir and ultralow permeability in the range of 70-500 nanodarcies [23]. The selected well is a cased, uncemented well in the formation.…”

“…The well is located in the lower Barnett Shale and drilled northwest to maximize fracture-network formation. It has five equally spaced initial fractures covering a depth interval from 7,900 to 10,106 ft [23]. The well was refractured after more than 4 years in four stages in the middle of the initial fractures.…”

Source-Mechanism Studies on Microseismicity Induced by Hydraulic Fracturing

“…Microseismic mapping is a technology that has proven valuable for monitoring stimulations in unconventional reservoirs, such as gas shales and tight-gas sandstones (Hart et al 1984;Warpinski et al 1990;Walker et al 1998;Fisher et al 2002;Maxwell et al 2002;Griffin et al 2003;Fisher et al 2004;Warpinski et al 2005;Cipolla et al 2005;Mayerhofer et al 2005;Wolhart et al 2005;Wolhart et al 2006;Shemeta et al 2007;Vulgamore et al 2007;Daniels et al 2007;King et al 2008;Potapenko et al 2009;Waters et al 2009). It is performed by fielding a large array(s) of highly sensitive and high-frequency-response three-component geophones or accelerometers in an offset well(s) in close proximity to an interval that is being stimulated.…”