Are the UK's fractured basement reservoirs ripe for exploitation? Hurricane Energy's Robert Trice explains why he believes they are.
The potential of fractured basement reservoirs is often overlooked by explorers, yet there are numerous examples of basement fields having been discovered and produced over the decades from around the world (See Fig.1).
Furthermore, the volumes of hydrocarbons found within fractured basement can be significant and production rates can be high. In some countries, for example Vietnam and Yemen, the contribution from fractured basement reservoirs has had a transformational effect on the country’s reserve creaming curve.
Basement potential in the UK
To date, basement on the UK Continental Shelf (UKCS) has largely been underexplored, despite the fact that numerous indications of hydrocarbons have been reported from basement in wells dating back to the 1970s. As production from the UKCS continues to decline, and with the exploration potential of the more traditional plays becoming increasingly mature, the potential from the overlooked and underrated basement play therefore warrants further exploration.
Over the past eight years Hurricane Energy has set out deliberately to explore this potential untapped resource, initially focusing on the West of Shetland area, where the extremely old (2.33 – 3.2 Ba) and fractured Lewisian basement forms extensive ridges and localized highs, which are the focal point for charge from the prolific Kimmeridge Clay source rock in the area.
However, other areas of the UKCS also offer basement potential, as now being promoted through the PILOT (an industry/government group) New Plays Initiative, as described in Sir Ian Wood’s February 2014 UKCS report Maximising Recovery Review (See page 25).
To date there is still no formal assessment from the government, geological survey, or oil industry on the resource potential of the UKCS Fractured basement play, however, Hurricane has made significant progress in evaluating the basement play West of Shetland. A summary of that progress is provided below.
Progress in opening the UK’s basement play
Data acquired by Hurricane Energy through the drilling and drill stem testing of over a kilometre 1km of basement encountered in three wells has provided an insight into the reservoir properties of the Rona Ridge fractured basement Play. (See Fig. 2).
Analysis of these data has allowed for the detection of productive fractures and characterization of fractured basement reservoir properties.
Fractured reservoirs require a different workflow to clastic reservoirs and basement is no exception. The workflow Hurricane has applied to exploring for hydrocarbon in basement West of Shetland is tried and tested, and has successfully generated 450MM boe of 2C resources. The approach used includes:
- Prospect generation through the integration of fieldwork, seismic and well data
- Successful well placement, targeting faults as defined on 3D seismic, with well prognosis of top basement and fault locations accurate to within meters
- Thorough review of offset well data and global analogues to establish optimum bottomhole assemblies, resulting in rate of penetrations (ROPs) in the basement of 5-7m/hr and the acquisition of quality logging while drilling (LWD) data sets
- Generation of confident static reservoir properties (e.g. fracture frequency, fracture orientation, and fracture porosity) from a combination of wireline and LWD data
- Establishing an understanding of effective fracture porosity, by comparing bulk porosity estimated through nuclear magnetic resonance and density neutron techniques, with discrete fracture porosity estimated from acoustic and electrical image logs.
- Identifying dynamic reservoir properties from gross flow zones, to flow rates estimated from single fractures, evaluated through image log and production logging tool (PLT) integration. Fluid typing established through bottomhole samples procured under variable choke size and flow rates.
- Establishing reservoir behavior away from the borehole environment through transient pressure analysis modelling combined with Petrel, sector and discrete fracture network (DFN) models.
The analytical components comprising Hurricane’s workflow are necessitated by the fact that fractured basement reservoirs are classified as Type 1 Fractured Reservoirs, in that the poroperm system is provided entirely by a natural fracture network. This means, gaining and developing an early understanding of the distribution and characteristics of the fracture network is essential. Specifically an understanding of the hydrodynamic fracture network is key for optimum well placement.
Consequently, wells are targeted to intersect faults which are identified from 3D seismic, the faults being considered important targets as their associated fault zones are considered to have enhanced reservoir properties. An example of this concept is seen in Fig. 3, which shows an interval of borehole that has been identified as a fault zone from 3D seismic an interpretation, corroborated by image log analysis and demonstrably preferentially productive as seen through PLT logging.
Hurricane has drilled two wells on the Lancaster discovery, the data from which supports a 2C resource estimate of 207MM boe recoverable.Hurricane’s next Lancaster appraisal well, scheduled for 2Q 2014, is designed to test the flow potential of the reservoir through the drilling and testing of a 1km-long horizontal basement section.
The well is targeted at evaluating the P90 oil volume that resides within structural closure (Fig. 5) by penetrating nine seismically mapped fault zones and undertaking a series of open hole tests, designed to evaluate flow rate, as well as reservoir response to shut in and build up. The 25-day testing program is planned to demonstrate a commercial flow rate, which, if achieved, will result in the well being suspended for use as a future producer.
The trapping of hydrocarbons within basement can include a significant stratigraphic element, with hydrocarbon columns extending below structural spill resulting in long hydrocarbon columns and flank accumulations. This hydrocarbon distribution is in part due to the presence of highly effective lateral seals, but is also due to permeability anisotropy and heterogeneity within the hydrodynamic fracture network. Such poroperm characteristics are influenced by long geological histories of subaerial exposure and tectonic activity, with fracture network permeability being enhanced through epithermal and hydrothermal dissolution/abrasion combined with tectonic reactivation of pre-existing joints and faults.
The Lancaster resource range so far identified is in part based on the potential of stratigraphically trapped oil identified through pressure surveys (modular dynamic tester) and gas returns measured as part of mud-logging acquisition. However further appraisal wells, including flank and deep penetrations, will be required to refine the stratigraphic upside and also the presence and characteristics of an aquifer.
If Hurricane’s appraisal efforts confirm the anticipated oil-in-place and reservoir deliverability, the Lancaster field is expected to be produced via a phased development, utilizing an existing suspended well and the planned 2014 horizontal well in the first stage of development. Early oil from such a development is anticipated for 2018.
Robert Trice is CEO and founder of Hurricane Energy. He has a PhD in Geology from Birkbeck College, University of London, and gained the bulk of his geoscience experience with Enterprise Oil and Shell. He has worked in field development, exploration, well-site operations and geological consultancy. Robert has held the position of Visiting Professor at Trondheim University (Norway) and has published and presented on subjects related to fractured reservoirs and exploration for stratigraphic traps.