The evidence for large-scale structural features (lineaments/faults) affecting a hydraulic stimulation is much more compelling than for small-scale features (natural fractures). Large-scale features are weaker and have similar dimensions to a typical hydraulic fracture. But is it beneficial to stimulate these features and what are the potential consequences? An analysis of structural features from the Marcellus and Duvernay formations has been undertaken, with static characterization (seismic, image logs and outcrops), dynamic characterization (fracture diagnostics and well performance) and geomechanical modeling; ultimately to understand whether, in the presence of structural features, any field development decisions might get impacted.
Maps of structural features supported by seismic attributes are commonly challenged as to what they physically represent. Outcrop analogues demonstrate how strain is distributed in intrinsically layered media, such as shale. Therefore a shale may preferentially fold above a fault. Folding may result in strain partitioning, with bedding-parallel slip (shear) limiting the vertical extent and opening (dilation) of discrete fracture planes. Lineaments in Marcellus folds are either broad zones of axial kink-band deformation associated with higher bedding dips, or planar zones comprising reactivated natural fractures forming an inherited en echelon fabric. Lineaments in the Duvernay are zones of distributed deformation commonly associated with a subtle flexure above faults.
A novel interpretation method of microseismic events in time reveals how lineaments are involved during a hydraulic fracture treatment driven by changes in net pressure. Hydraulic half-length is limited when fracs intersect a lineament at a high angle. This was confirmed by geomechanical modelling showing that lineament dilation prevents the opposite branch of the bi-wing frac from propagating. Diagnostics from plays with lineaments oriented close to maximum horizontal stress indicate that the length-scale of hydraulic communication is increased, because tensile reactivation is facilitated. Tracer data have been used to calibrate the conductive length-scale of these features in the sub-surface and also confirm that external fluids may be brought into the well-bore from underlying formations.
Whether a lineament helps well productivity depends partly whether it is ‘contained’ or ‘uncontained’ within the over-pressured formation. In the uncontained case, stimulation efficiency and enhanced risk of external fluids needs careful monitoring. In the contained case, stimulation of a lineament may enhance productivity of a stand-alone well, but conversely this same lineament may exacerbate the Parent-Child impact once adjacent wells are drilled. A potential mitigation measure may be to modify the proppant or stimulation design to screen-out these high conductivity (or leak-off) pathways, rather than trying to stimulate them, thereby enhancing near-wellbore complexity. Paradoxically, the best way to handle large-scale structural lineaments may be to stimulate them in order to shut them off.