Fraccability determination of a Posidonia Shale Formation analogue through geomechanical experiments and micro-CT fracture propagation analysis

Gas resources in The Netherlands are steadily running scarce, therefore alternative gas resources are necessary. One of those alternatives present in the Dutch subsurface is shale gas, present in among others the Posidonia Shale Formation (PSF). Little is known about this formation and drilled cores are rare. This study therefore focuses on a time and depositional equivalent in the U.K.: the Whitby Mudstone Formation (WMF). Stimulating the production of a reservoir by hydraulic fracturing (fraccing) is necessary in order to produce the gas economically viable. To determine the prospectivity of this fraccing (fraccability), several concepts are assessed, e.g. Brittleness Index (B.I.) (Rickman et al. 2008), Fracture Toughness (KIC) (Lawn & Wilshaw 1975) and Fraccability Index (F.I.) (Jin et al. 2014). The fraccability of the WMF is determined through geomechanical experiments and X-ray micro-computed tomography fracture propagation analysis. Samples were taken along an exhumed outcrop 8 meter in height. To account for anisotropy, experiments were conducted at different angles to the bedding. The applied unconfined stress to create new fractures in repetitive tests, depends on the interplay of the already present fractures. Fractures that do not reach the sample’s edges, simulate a triaxial ellipsoid shape and follow a sublinear aperture-length ratio. The WMF results show low KIC-values, which is favorable, but scattered B.I. and F.I. values, which do not provide a clear view on the prospectivity of the WMF. In general, the B.I. and F.I. provide important insights in fraccability, though its significance is relative. Values can be compared with other datasets or intervals under the condition that the same limits of experimentally measured Young’s Modulus, Poisson’s Ratio and Fracture Toughness in these concepts are applied. The WMF thus exhibits a questionable fraccability. Near the formation’ top and bottom, fraccing is preferred opposed to the middle. But weathering and exhumation of the WMF samples has certainly had its effect on the quality and uncertainty of the results.Petroleum EngineeringGeoscience & EngineeringCivil Engineering and Geoscience