Anderson, Iain (Heriot-Watt University) | Ma, Jingsheng (Heriot-Watt University) | Wu, Xiaoyang (British Geological Survey) | Stow, Dorrik (Heriot-Watt University) | Underhill, John R. (Heriot-Watt University)
This work forms part of a study addressing the multi-scale heterogeneous and anisotropic rock properties of the Lower Carboniferous (Mississippian) Bowland Shale; the UK's most prospective shale-gas play. The specific focus of this work is to determine the geomechanical variability within the Preese Hall exploration well and, following a consideration of structural features in the basin, to consider the optimal position of productive zones for hydraulic fracturing. Positioning long-reach horizontal wells is key to the economic extraction of gas, but their placement requires an accurate understanding of the local geology, stress regime and structure. This is of importance in the case of the Bowland Shale because of several syn- and post-depositional tectonic events that have resulted in multi-scale and anisotropic variations in rock properties. Seismic, well and core data from the UK's first dedicated shale-gas exploration programme in northwest England have all been utilized for this study. Our workflow involves; (1) summarizing the structural elements of the Bowland Basin and framing the challenges these may pose to shale-gas drilling; (2) making mineralogical and textural-based observations using cores and wireline logs to generate mineralogy logs and then to calculate a mineral-based brittleness index along the well; (3) developing a geomechanical model using slowness logs to determine the breakdown stress along the well; (4) placing horizontal wells guided by the mineral-based brittleness index and breakdown stress. Our interpretations demonstrate that the study area is affected by the buried extension of the Ribblesdale Fold Belt that causes structural complexity that may restrict whether long-reaching horizontal wells can be confidently drilled. However, given the thickness of the Bowland Shale, a strategy of production by multiple, stacked lateral wells has been proposed. The mineralogical and geomechanical modelling presented herein suggests that several sites retain favorable properties for hydraulic fracturing. Two landing zones within the Upper Bowland Shale alone are suggested based on this work, but further investigation is required to assess the impact of small-scale elastic property variations in the shale to assess potential for well interference and optimizing well placement.
The production of natural gas from unconventional sources in the United States, and more generally North America, has put a spotlight on an industry that has been developing in the US for more than 30 years. This industry is a complex web of companies that produce, process and transport gas and oil. It has reshaped the energy supply market in North America and created a new business model. Since 2010, interest in unconventional gas has spiked in Europe with several companies revealing their interest in shale gas. The production of shale gas is characterised by the drilling and fracking of a large number of wells creating a significant footprint on the natural and human environment. This footprint and the perceived environmental impacts associated with it, have transformed engagement between companies and its stakeholders including advocacy groups, NGOs, academics, scientists and politicians. Relationships with communities living close to, or affected by, these activities have also been influenced by the public debate, on the ground protests and widespread communication of ‘facts’ by those against shale gas. The emotional debate has even spilled over into the media: some journalists and publications have become anti-shale gas activists in their own right. Politicians’ opinion appears to be balanced between the pros and cons. Oil and gas companies are increasingly forced to compete for space to present technical approaches, current or future economic benefits as well as their approach to managing positive and negative impacts. This is not facilitated by the ambivalence of elected politicians and government staff towards shale gas exploration. While a heated debate and anti-shale gas activism took place in North America during production, the European protest was in full swing in the early exploration phase. Total has experienced the opposition to shale gas in several countries. In France, a highly charged and emotional debate resulted in a moratorium on hydraulic fracturation and the cancellation of licenses. Having learnt from this experience at home, Total has consistently sought to, or encourage its partners to, proactively engage all stakeholders particularly local communities as well as opposition activists, take part in public debates at national and local levels, and demonstrate transparency through the disclosure of environmental and social studies, and the early sharing of information. However, the force of opposition and criticism remains undiminished and appears to be extending to conventional activities.
A re-evaluation of the Roosecote-1 borehole, located in the northern part of the Craven Basin (northern England), places the Bowland Shale in a modern geological context and allows an improved understanding of basin evolution and shale hydrocarbon prospectivity. In northern England during the Lower Carboniferous, rapidly subsiding sub-basins developed between local highs in the Craven Basin. Subsequently, during the thermal relaxation phase, a period of regional subsidence allowed thick accumulations, locally in excess of 500 m, of hemi-pelagic mudstone to be deposited as a transgressive systems tract.
The Roosecote-1 borehole is a stratotype section for the Bowland Shale Formation, located close to the northern basin- to- shelf margin of the Craven Basin. The upper part of the Mississippian (Lower Carboniferous) Bowland Shale Formation is the principal potential shale gas play in the British Isles. The Bowland Shale and other argillaceous units of the Craven Group are rich in organic matter, have a similar depositional style to coeval units in the USA including the world-class Barnett Shale of the Fort Worth Basin, Texas, USA. Throughout England, Wales and Ireland it is currently being investigated by several exploration companies, with at least 4 dedicated shale exploration wells now drilled.
Rock evaluation indicates the TOC at between 1.76 - 3.72 %. Many samples indicate a mixture of type III and IV kerogens; there was no evidence for type I or II kerogens found. Thermal maturity for the sequence is within the oil window, with results indicating liquid oil and wet and dry gas have been generated. However, an estimate of the organic matter transformation ratio of 0.36 (36% kerogen conversion to hydrocarbons being possible) compares poorly to the Barnett Shale.
Consequently we conclude that the Bowland Shale within the Roosecote-1 borehole has some, but possibly lower, potential as a shale oil play.
UK's Gas Heritage May Become Its Legacy Tom Pickering, Independent Consultant Tom Pickering is an independent consultant to operators and service companies operating in European unconventional gas, and was chairman of the Unconventional Gas Aberdeen 2012 conference held in November. He was a cofounder of Composite Energy, an early entrant into the United Kingdom unconventional sector before it was sold to Dart Energy in a deal worth more than GBP 40 million last year. Earlier in his career, Pickering worked for Amerada Hess in Aberdeen and London in well technology and global asset performance, and as an executive assistant. He is a graduate of the University of Aberdeen Business School. The United Kingdom is attempting to emulate the unconventional gas bonanza that has transformed the United States from having an energy industry that was in its "sunset" era to a nation rich in energy resources.