Development of a well placement strategy for the Bowland Shale, UK, considering complex basin structure, wireline-derived petrophysical and rock properties, geomechanical models and the impacts of fracture stimulations

Abstract

The aim of this thesis is to assess the quality of shale intervals for gas production and to consider where horizontal wells could be drilled with respect to geological faults. The Bowland Shale, located in a region of NW England, is investigated where it is a potential shale gas play. The shale is situated within a complex geological setting, which necessitates an integrated analysis of fault patterns, reservoir and geomechanical properties, and hydraulic fracture characteristics to realise these aims. We ask to what extent the structural geology places limitations on where horizontal wells can be drilled. Mapping of key stratigraphic surfaces and structures using a high-fidelity 3D seismic dataset allows the geological history to be reconstructed, and the detrimental effects of Variscan compression realised. The results suggest that faulting does restrict the available sites where horizontal wells can be drilled. Short, vertically stacked wells may therefore be required to avoid major faults whilst ensuring gas can be extracted commercially. We investigate if there is an opportunity for this in the Bowland Shale by assessing both reservoir and geomechanical properties over the entire stratigraphic section at a single well. From this, we find that the upper section exhibits particularly good reservoir properties. These intervals of good reservoir properties also exhibit a distinctive rock physics response that can be modelled and visualised using templates. Within these rock types, three sections are highlighted that present the ideal combination of good reservoir and completion properties. Importantly, they are also separated by highly stressed intervals that may also mitigate risk of vertical fracture interference between stacked wells. However, to assess the risk of stimulated fractures propagating near major faults, further study is required of the Bowland Shale’s typical hydraulic fracture characteristics. While this is typically performed whilst stimulations are ongoing through analysis of microseismic events; prior to stimulation, a model is required to attempt to predict fracture geometries for a formation of specified rock properties and a specified treatment schedule. Using a simulation model, we ask how far the fractures propagate laterally, what their geometries are and if there is a risk of vertical interference between each proposed landing zone. It is determined that there is minimal risk of vertical fracture interference and potential fracture barriers behave effectively. Within the pre-defined landing zones, the fractures observed are relatively simple in geometry and propagate laterally over large distances between 500 m and 1000 m. This latter observation does further limit the areas where completion stages could be placed without risk of fractures interfering with the faults mapped from seismic data. Bringing together the results of the mapping and hydraulic fracture modelling, we can map a maximum of 13 well locations in the study area where horizontals could be drilled, but some of these wells are very short (~ 500 m). There remains excellent production potential in the area however, and by combing our results with production rates using an analogue play, we estimate ~ 300 Bcf of gas production could potentially be achieved.