In horizontal well completions for unconventional reservoirs, multiple hydraulic fracturing stages, each often with multiple clusters, are used to provide sufficient stimulated area to make an economic well. Each created hydraulic fracture alters the stress field around it. When hydraulic fractures are placed close enough together, the well-known stress shadow effect occurs in which subsequent fractures are affected by the stress field from the previous fractures. The effects include higher net pressures, smaller fracture widths and changes in the associated ‘complexity’ of the stimulation. In this paper, we present the results of a numerical evaluation of the effect of multiple hydraulic fractures on stress shadowing for a horizontal well in North America and attempt to highlight the potential cause of the deviation between numerical predictions and field results. The study provides insight into understanding stress shadow effects under field conditions as well as the potential impact on hydraulic fracture stimulations. In addition, the results may help explain the relationship between stress shadowing and microseismic behavior seen in multi-stage horizontals.
The world-wide boom in shale gas, shale oil and other unconventional developments is now well-known, and the boom has had a marked impact on the balance of hydrocarbon production. For example, where states such as Texas and Alaska have been hydrocarbon production leaders for decades, the Energy Information Agency (EIA 2012) notes that North Dakota has become the second largest oil producer in the United States due to production from the Bakken shale. These shale developments, notably beginning in the Barnett in the 1990s, have been driven by:
the application of horizontal wells;
the application and improvements in multi-stage hydraulic fracturing; and
significant commodity prices (GWPC 2009 and King 2010).
In multi-stage hydraulic fracturing, dozens, and perhaps 50 or more, hydraulic fractures are created along a long horizontal wellbore. Due to the inflation of the hydraulic fracture, and placement of proppant to keep the fracture permanently open, the stress field around the hydraulic fracture is altered. As far back as Sneddon's work on the evaluation of stress near a crack (Sneddon 1946), numerous authors have looked at the impact of stress field changes around hydraulic fractures (Nagel & Sanchez-Nagel 2011, Rios et al. 2013 and Warpinski et al. 2012). The stress field change, principally the increase in the minimum horizontal stress, Shmin, caused by a hydraulic fracture (typically the final, propped hydraulic fracture) is called the stress shadow effect or simply the stress shadow.