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 Bowland Shale is a Carboniferous formation of Asbian to Yeadonian age located in Northern England. It is estimated to have a shale gas section with more than 5,000 ft thickness holding over 1300 TCF of total original gas in place. Drilling in the Bowland Basin started in August 2010 and by the end of 2011, three vertical wells (PH-1, GH-1 and BS-1) were drilled to a depth of 8,860 to 10,500 ft with objective of logging and coring the potential shale gas formations including Upper Bowland, Lower Bowland, Hodder Mudstone and Sabden Shale. All the drilled wells encountered several borehole stability problems, such as tight-hole, pack-off, overpull and excessive cutting, causing significant non-productive time (NPT) during drilling. Specifically, in GH-1 and BS-1, side-tracking was required to reach the target depth which imposed significant cost to the project. Careful investigation of the recorded drilling problems showed that majority of them were associated with formation collapse due to insufficient drilling fluid pressure. Fluid losses also occurred in some of the formations due to either too high of downhole pressure or presence of critically stressed natural fractures. These incidents implied that the applied casing design and mud weight program were not appropriate for the current-day state of stress and rock properties.
A comprehensive experimental and analytical geomechanical study was carried out to develop a reliable borehole stability model that can firstly explain the observed drilling incidents and secondly provide guidance for design and drilling of the planned wells. The plan was to drill a S-shape appraisal well (PNR-1) in the Preston New Road exploration site to log and core the Bowland Shale sequence and select the optimum landing depths for subsequent horizontal sections (PNR-1z and PNR2) to be completed for multi-stage hydraulic fracturing. The study recognized intrinsic shale anisotropy as a primary causative factor for borehole stability issues and formation collapses in the offset wells. Other important factors were identified to be the abnormal pore pressure regime and the presence of a tectonic strike-slip stress regime with large horizontal stress anisotropy. The anisotropy of the Bowland Shale was characterized in both laboratory and field scales, and anisotropic wellbore stability models were developed for offset and planned wells. As a result of this study, the PNR-1, PNR-1z and PNR2 wells were successfully drilled and completed with no notable borehole stability problems despite the presence of narrow operating mud weight window in several stratigraphic intervals. This paper presents a summary of the conducted borehole stability analysis aiming at risk-free and successful drilling of horizontal wells in the Preston New Road exploration site with emphasis on the effect of shale anisotropy.
Privately held Cuadrilla Resources is set to hydraulically fracture the UK’s first two horizontal shale wells. Fracturing work on the first well is expected to begin at its Preston New Road site in Lancashire next week, the operator said in a statement. Both wells, which intersect the Bowland Shale, are slated to be completed over 3 months, with their flow rates tested over 6 months. An initial assessment on recoverable gas from the wells is expected in next year’s first quarter, at which point Cuadrilla will determine the site’s commerciality, said Francis Egan, the company’s chief executive officer. The Bowland Shale contains 1,329 Tcf of gas in place, according to a 2013 estimate from the British Geological Survey.
Competing companies often have different names for the same new thing. Which of these is not a name used by companies fracturing many unconventional wells at generally the same time and place? Explanation: Encana's Cube label is the most used one, while others call them tanks or optimized sequence development. Supersized pads was a description used by the author of this JPT story. The British Royal Engineers designed and installed what offshore first after the Normandy invasion as part of Operation Pluto?
Cuadrilla’s Preese Hall drill site where fracturing last occurred in the UK in 2011. Minor earthquakes resulting from the operations halted fracturing in the country indefinitely. Cuadrilla Resources has started drilling the UK’s first horizontal shale well at its exploration site at Preston New Road, Lancashire, and hopes to begin hydraulic fracturing of two wells in the second quarter. No wells have been fractured in the UK since 2011, when a Cuadrilla well triggered small earthquakes in Lancashire. Advancement of the drilling program comes as the operator recovered data suggesting “excellent rock quality for hydraulic fracturing and a high natural gas content in several zones” of the Bowland shale.
Recent drilling results have highlighted the potential for the development of Jurassic source rocks of southern England as a shale oil play. Sustained natural oil flows have been reported by UKOG (2015) from the tight, Lower Kimmeridge limestones in the Horse Hill-1 well. According to the operator, this discovery is naturally fractured and can be produced without hydraulic fracture stimulation.
The occurrence of shale gas in the UK has been known of since the nineteenth century, but development of this resource attracted very little interest until recent years (Selley, 2012; Andrews, 2014). The first exploration well in the United Kingdom that was specifically drilled for shale gas was Preese Hall-1 in northwest England in 2010. This well was hydraulically fractured in the Bowland Shale, but operations were suspended following reports of repeated seismicity caused by the injection of fluid during hydraulic fracture treatment (Green et al., 2012). Assessments of the Carboniferous shale gas potential of northern England and Scotland and of the Jurassic shale oil potential of southern England have been published by the BGS/DECC (Andrews, 2013, 2014; Monaghan, 2014). These studies listed the various criteria for evaluation of shale plays and provided broad descriptions and resource estimates for the Carboniferous and Jurassic shale plays in the United Kingdom.
This paper presents the results of an integrated petrophysical and geological assessment of the Jurassic sequence in the south of England. The study area stretched from the Weald and Vale of Pewsey Basins in the north to the onshore parts of the Portland–Isle of Wight Basin on the Dorset coast in the south (Figure 1). The evaluation focused on the Kimmeridge Clay Formation, the Oxford Clay Formation, the Downcliff Clay Member, Charmouth Mudstone Formation and the Blue Lias Formation.
The stratigraphic framework used for the study is based on the extrapolation of the well-known outcrop stratigraphy on the Dorset Coast to the study wells. Wireline log data and new sedimentological core description results were used to constrain facies mapping. Detailed sedimentological core description was carried out on three of the twelve study wells. From the trends observed in the wireline log data, the lithofacies and level of oxygenation, 14 initial facies associations were assigned over the cored intervals ranging from restricted shallow marine through shoreface to shelfal environments. These facies associations were grouped into seven combined facies associations which were used as input for the electrofacies analysis and facilitated the extrapolation of facies to intervals that lacked core data Additionally this workflow provided a useful template for estimating Total Organic Carbon TOC from logs using the CARBOLOG® equation and this resulted in a significant improvement in the correlation between the laboratory measured TOC values and the log-based TOC estimates. Results from the mineralogical analysis of core and cutting samples were utilised to calibrate and improve the petrophysical interpretations and to assess the elastic properties of the rocks in the intervals of interest. The petrophysical data, elastic properties and the facies interpretations were used to evaluate and map the development potential of the Jurassic source rock intervals as unconventional reservoirs.
An extensive geochemical database was combined with new analyses to characterise the source rocks. This data was integrated into 1-D basin models to identify and map effective source kitchen areas. The organic matter in the analysed interval is dominated by Type II kerogen, with significant input of Type III kerogen towards the London-Brabant Massif. The Upper Jurassic Kimmeridge Clay and the Oxford Clay are within the early oil window, while the Lower Jurassic Downcliff Clay Member, Charmouth Mudstone Formation and the Blue Lias Formation have reached peak oil maturity in the deeper parts of the Weald Basin. The source richness and kerogen types were combined with the maturity maps to create generation risk maps.
The risk for ground water contamination from hydraulic fracturing was also evaluated. These results were combined with the reservoir and generation risk maps to produce common risk segment maps in order to identify the sweet spots in the study area.