ABSTRACT:

Due to the low permeability of many shale gas reservoirs, multi-stage hydraulic fracturing in horizontal wells is used to increase the stimulated volume. However, each created hydraulic fracture alters the stress field around it, and subsequent fractures are affected by the stress field from the previous fractures, which results in higher net pressures, smaller fracture widths, and diminished microseismic emissions. The results of a numerical evaluation of the effect of stress shadowing, as a function of natural fracture and geomechanical properties, are presented, including a detailed evaluation of natural fracture shear failure (and, by analogy, the microseismicity) due to a created hydraulic fracture using both continuum and discrete element modeling approaches. The results show the critical impact that a created hydraulic fracture has on the shear of the natural fracture system, which in-turn, significantly affects the success of the stimulation. Furthermore, the results provide important insight into the mechanisms that generate the microseismicity that occurs during a hydraulic fracture stimulation.

1. INTRODUCTION

US shale gas production was 5 tcf in 2010 (23% of US dry gas production) and is expected to hit 13.6 tcf by 2035 (or 49% of US dry gas production) (EIA, 2012). Shale gas became significant with the development of the Barnett shale in the late 90s (Navigant Consulting, 2008), which was driven by: 1) the application of horizontal wells; 2) the application of, and improvements in, hydraulic fracturing; and 3) significant natural gas prices (GWPC, 2009). Numerous authors (Frantz and Jochen, 2005, Harper, 2008, Lancaster et al., 1996, Chong et al., 2010, and King, 2010) have shown that hydraulic fracturing is one of the key drivers to shale gas development and the presence of, and ability to maintain flow in, natural fractures is critical to shale gas production.

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