Abstract
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.
Highlights
The exploitation of tight mudrocks, or shales, for hydrocarbon pro duction, revolutionized the global energy industry during the early 21st Century and posed unique challenges not typically encountered when producing from conventional, higher permeability reservoirs
This paper presents new findings around the geomechanical stra tigraphy of the Bowland Shale in northern England, where it forms a prospective shale gas target
Wireline logs from the Preese Hall-1 exploration well, located on the Fylde peninsula, Lancashire, were used in a series of geomechanical models
Summary
The exploitation of tight mudrocks, or shales, for hydrocarbon pro duction, revolutionized the global energy industry during the early 21st Century and posed unique challenges not typically encountered when producing from conventional, higher permeability reservoirs. In many of the prolific USA shale plays this involves drilling horizontal wells in the target formation and stimulating these wells at multiple stages. Such wells can be very long and, in 2019, the average horizontal well length in the USA reached 4.5 km (EIA, 2020b). The Bowland Shale Formation is a Carboniferous (Visean-Serpu khovian), predominantly fine-grained unit that was deposited in a series of basins across northern England. These basins formed in response to Late Devonian to Lower Carboniferous extension and were separated by broad highs; a configuration often referred to as ‘block and basin’ topography (Waters and Davies, 2006). Shale gas prospectivity exists in the west portion of the basin where Carboniferous sediments exposed at the Ribblesdale Fold Belt plunge beneath a Permo-Triassic cover, though inversion-related deformation persists in this area (Anderson and Underhill, 2020)
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