Many Unconventional reservoirs have a structural fabric composed of faults, fracture corridors, or large-scale kink-bands. Such large structures can create a strong planar, typically steeply-dipping, anisotropy, and it is often unclear how to manage this in a development plan. For the purpose of this paper we will refer to all long planar discontinuities as faults, but the concept is generally applicable to any planar structural feature that connects wells, irrespective of the formation mechanism.
A novel concept, " Structural Stage Spacing", has been introduced into the operation of Shell's Marcellus acreage to provide guidelines around how to optimally frac in faulted rock. Steeply dipping (70–90?) structural discontinuities, referred to as faults, that are parallel to the in-situ maximum horizontal stress (ENE-WSW) are avoided with frac stages by 75ft on each side, providing a 150ft perforation-free zone in the well-bore, the aim of which is twofold:
reduce the likelihood of stimulating (reactivating) faults by dilation and thereby lower the frequency of inter-well communication, and
enable optimal stimulation of non-faulted shale either side of the fault. All other orientations of faults can effectively be ignored, with frac stage spacing being uniform across them, so as not to drop perforations unnecessarily.
Evidence for the transport of frac fluid along faults parallel to in-situ stress is supported by chemical tracer data in multiple wells, showing a higher likelihood of inter-well communication where a fault is present. Evidence for sub-optimal stimulation of shale in faulted stages comes from PLT data, where inflow is constrained in stages with faults. Diagnostics of fault dilation are determined from Frac Pump Pressure plots, in which screen-outs or a low Instantaneous Shut-Down Pressure gradient (ISDP gradient) may be recorded as faults are fracced across.
HSE risk also increases when fraccing in the vicinity of faults that connect multiple well-bores. Consequences may range from water influx (kick) during drilling across faults " charged" by fraccing adjacent wells, liquid-loading and/or sand production in producers adjacent to a fracced well, with the added operational activity to bring wells back online, and in the extreme case, well-integrity issues.
Implementation of Structural Stage Spacing is a win-win-win technique, whereby costs are not incurred due to loading of adjacent wells, and the overall HSE risk is reduced; both achieved with zero net impact, or even uplift, in EUR.
URTeC 1577021
