Abstract
Unconventional reservoirs such as gas shales and tight gas sands require
technology-based solutions for optimum development. The successful exploitation
of these reservoirs has relied on some combination of horizontal drilling,
multi-stage completions, innovative fracturing and fracture mapping to engineer
economic completions. However, the requirements for economic production all
hinge on the matrix permeability of these reservoirs, supplemented by the
conductivity that can be generated in hydraulic fractures and network fracture
systems. Simulations demonstrate that ultra-low shale permeabilities require an
interconnected fracture network of moderate conductivity with a relatively
small spacing between fractures to obtain reasonable recovery factors.
Microseismic mapping demonstrates that such networks are achievable and the
subsequent production from these reservoirs supports both the modelling and the
mapping. Tight gas sands, having orders of magnitude greater permeability than
the gas shales, may be successfully depleted without inducing complex fracture
networks, but other issues of damage and zonal coverage complicate recovery in
these reservoirs. As with the shales, mapping has proved itself to be valuable
in assessing the fracturing results.
Introduction
Unconventional reservoirs provide a significant fraction of gas production
in North America and increasing amounts in some other regions of the world.
Such reservoirs include tight gas sands, coalbed methane (CBM), and gas shales;
in 2006 these reservoirs provided 43% of the US production of natural gas
[Kuuskraa(1)]. Because of their limited permeability, which is foremost among
many other complexities, some type of stimulation process (and/or dewatering in
the case of CBM) is required to engender economic recovery from wells drilled
into these formations. The focus of this paper is on gas shales, with
particular emphasis on how these reservoirs perform relative to tight gas
sands. The important role of natural fractures in both the stimulation and
production processes, the importance of conductivity in the developed fracture
or fracture system, and the critical influence of the matrix permeability are
investigated using both mapping and modeling results.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
5 January 2008
- Meeting paper published:
11 February 2008
- Revised manuscript received:
4 November 2008
- Manuscript approved:
24 March 2009
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