Geometry and Failure Mechanisms from Microseismic in the Duvernay Shale to Explain Changes in Well Performance with Drilling Azimuth

Abstract Non-contiguous acreage positions commonly lead to drilling azimuths parallel to section boundaries. Analysis of well-performance with drilling azimuth across plays in North America reveals that on-azimuth wells (parallel to σHmin) are typically better than off-azimuth wells. A simple, but elegant microseismic trial by Athabasca Oil Corporation has two wells on the same pad, thus minimizing geological variability; one on-azimuth and one 45°-off-azimuth, from which rock failure and connectivity can be assessed. Microseismic from the hybrid fracture treatments in the two wells shows a large contrast in event density, with the on azimuth well showing far fewer events, but double the productivity. There is also a difference in treatment fluid, with the on-azimuth well having 1/3 more gel and 1/3 less slickwater, although total injected volumes are equivalent. Events were filtered in multiple ways; by stage, every 5 minutes within a stage, by fluid type, and by magnitude to understand the azimuthal, stress and completions controls on the mechanics and the productivity. The treatments of the two wells have very different spatial patterns of microseismic events as detected by both surface and downhole arrays. The 45°-off-azimuth well has a well-developed longitudinal frac which is interpreted to facilitate re-stimulation of previous stages. In-situ stress calculations show that both shear failure and bedding parallel stimulation are more likely with an off-azimuth well. Furthermore, the 45°-off-azimuth well has two distinct domains during gel treatment; near-wellbore and far-field. The intensity of structural features is interpreted to be similar between the two wells, so is not thought to control the difference in event density. Filtering by event magnitude shows that the 45°-off-azimuth well also has more larger events in the far field, presumably enabled by the higher proportion of slickwater. Several hypotheses exist to explain the poorer 45°-off-azimuth well performance: 1) near wellbore frac complexity introducing tortuosity, measured by a pressure drop after breakdown; 2) out-of-stage re-stimulation causing inefficient treatments and potential over-flushing with a loss in near well-bore conductivity supported by generally lower breakdown pressures in the 45°-off-azimuth well, and; 3) planes of shear failure pinching out, resulting in areas of stranded connectivity. A conceptual model is developed that shows how shear and tensile failure may be preferentially developed in the off- and on-azimuth wells respectively, supported by in-situ stress calculations and an analysis of the S/P ratios from microseismic amplitudes from both wells. Other authors (e.g. Cipolla et al., 2014) recognized that the size of the event cloud is not necessarily proportional to productivity, but in this trial the inconsistency is striking. The interpretation herein, supports the assertion that tensile failure in the hydraulic fracturing process is largely aseismic (e.g. Maxwell & Cipolla, 2011; Warpinski et al., 2013) and that off-azimuth drilling has a higher risk for delivering lower quality fracture treatments.

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