SUMMARY
An evaluation of prospective shale gas reservoir intervals in the Bowland Shale is presented using a wireline log data set from the UK's first shale gas exploration well. Accurate ...identification of such intervals is crucial in determining ideal landing zones for drilling horizontal production wells, but the task is challenging due to the heterogeneous nature of mudrocks. This heterogeneity leads to stratigraphic variations in reservoir quality and mechanical properties, and leads to complex geophysical behaviour, including seismic anisotropy. We generate petrophysical logs such as mineralogy, porosity, and organic content and calibrate these to the results of core studies. If ‘reservoir quality’ is defined by combined cut-offs relating to these parameters, we find that over 100 m of reservoir quality shale is present in the well, located primarily within the upper section. To examine the link between geophysical signature and rock properties, an isotropic rock physics model is developed, using effective medium theories, to recreate the elastic properties of the shale and produce forward-looking templates for subsequent seismic inversion studies. We find that the mineralogical heterogeneity in the shale has a profound impact on modelled elastic properties, obscuring more discrete changes due to porosity, organic content and water saturation and that the best reservoir quality intervals of the shale bear a distinctive response on rock physics cross-plots. Finally, we consider the density of natural fractures in the shale by developing an anisotropic rock physics model to reflect high-angle fractures observed on micro-imagery logs. We invert crack density using shear wave splitting well log data and find a crack density of up to 4 per cent which correlates well with micro-imagery observations. Our work further supports previous authors’ core-based studies in concluding that the Bowland Shale holds good reservoir characteristics, and we propose that there are multiple intervals within the shale that could be targeted with stacked horizontal wells, should those intervals’ mechanical properties also be suitable and there be adequate stress barriers between to restrict vertical hydraulic fracture growth. Finally, our rock physics templates may provide useful tools in interpreting pre-stack seismic data sets in prospective areas of the Bowland Shale and picking the best locations for drilling wells.
The production performance of a shale reservoir is directly affected by the geomechanical characteristics of the formation. A target shale interval will ideally develop hydraulic fractures upon ...stimulation that stay open with the aid of injected proppant. However, shales are geomechanically complex due to heterogeneities in their rock properties such as mineralogy and porosity and the extent to which they may be naturally fractured. These characteristics can complicate the task of identifying the ideal target interval for placing horizontal wells. Whilst the Bowland Shale is the UK's most prospective shale gas target, long horizontal wells are generally not feasible or practical in the Craven Basin, due to the existence of many, large-offset reverse faults and high bedding dips. An alternative to this approach could include drilling shorter, stacked horizontal wells targeting different stratigraphic intervals. However, it is unclear if there are enough intervals within the stratigraphic section with the desired geomechanical properties to target with stacked horizontal wells, nor if there are adequate intervals that can limit vertical hydraulic fracture growth between those wells. The absence of the latter may ultimately lead to well interference and reduced production. These issues were addressed by the creation of a series of wireline log-based geomechanical logs at well Preese Hall-1, calibrated to pressure test data. Aided by the results of a cluster analysis model, the upper section of the Bowland Shale was classified into geomechanical zones to identify the optimal intervals for hydraulic fracturing and barriers to vertical hydraulic fracture growth. Three intervals are highlighted with low effective stress, low fracture toughness and high brittleness which may form excellent landing zones. Importantly, these landing zones are also separated by intervals of high effective stress that may limit vertical hydraulic fracture growth and mitigate the risk of well interference.
•Geomechanical models reveal best landing points in the Bowland Shale stratigraphy.•In some intervals, the Bowland Shale is overpressured, brittle and exhibits low effective stress.•Cluster analysis aids in picking optimal combination of reservoir and mechanical properties.•Highly stressed intervals between landing points may limit vertical fracture propagation and facilitate stacked drilling.
Hydraulic fracture modelling is a key component of a shale reservoir well placement strategy as it provides an indication of the typical lengths and heights of stimulated fractures and of the changes ...to the stress environment in which these are propagating. However, spatial and stratigraphic variations in the stress and geomechanical properties of shales make accurate modelling a challenging task. For the UK Bowland Shale, stacked horizontal wells targeting multiple stratigraphic intervals could be used to avoid large offset faults in a geologically complex area. However, it is not known how these intervals may respond to hydraulic fracturing and predicting the height and length of hydraulic fractures is necessary in order to assess the likelihood of vertical fracture interference across landing zones or propagation towards major faults. In the case of the former, intervals of high effective stress may be key to containing fractures within their desired target. Using a planar hydraulic fracture simulator, and a 3D geomechanical model incorporating dipping stratigraphy, the issue of predicting hydraulic fracture geometry in the Bowland Shale was assessed through a series of modelling exercises using well Preese Hall-1 and horizontal pseudo-wells. When pre-defined landing zones were targeted, narrow and long transverse fractures around 1 km from the well were simulated. When the simulation design mimicked perforation clusters placed at 12 m intervals along horizontal pseudo-wells, the effects of stress shadowing were acute and resulted in irregular fracture geometries. Furthermore, high effective stress intervals performed efficiently as barriers to vertical hydraulic fracture propagation, reinforcing the feasibility of using stacked production for the Bowland Shale. The modelling results were then used to discuss the possible placement of horizontal wells in a mapped, 100 km2 region around well Preese Hall-1, where up to 13 sites could be positioned, with a horizontal well length of around 1.5 km. Finally, by drawing on a well-established analogue for the Bowland Shale, it was estimated that up to 195 Bcf of gas could be produced from the 13 locations in the area if three stratigraphic intervals are produced from one location.
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•Fracture simulations help develop a well placement strategy for the Bowland Shale.•Modelled fractures are long and contained by vertical stress barriers.•Results are used to place theoretical horizontal wells whilst avoiding faults.
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