During drilling, oil shows from mud logging were identified in the Portland Sandstones and, additionally, oil shows, mineral fluorescence and elevated gas readings of readings of C1 through to C5 were detected whilst drilling (C1 to C5 Hydrocarbon gases from spectrometry show the presence of C1 (methane), C2 (ethane), C3 (propane), C4 (butane) and C5 (pentane)).
A preliminary most likely estimate P50 (A 50% probability that a stated volume will be equalled or exceeded) of 3.1 million barrels ("mmbbls") of gross in place hydrocarbon volume within the upper Portland was calculated and announced on 24 October 2014, with a further gross unrisked in place prospective hydrocarbon volume of 16.8 mmbbls of oil in a separate lower sand in the Portland interval located in an untested fault block to the south.
On 5 November 2014 it was announced that the well unexpectedly encountered hydrocarbons as it went through the Kimmeridgian clay sequence, which had been thought to be thermally immature in this area, and therefore not capable of producing hydrocarbons. This opened the potential for conventional limestone targets, among others, with a measured Total Organic Carbon content (TOC) of over 5% by weight in some samples.
On 17 December 2014 Alba announced that as a result of the analysis and integration of subsurface data from the HH-1 discovery well, the calculated estimates of the discovered most likely gross OIIP within the overall Upper Portland sandstone conventional reservoir in the HH-1 and CF-1 structure had increased by over 250% from the 3.1 mmbbls reported on 24 October 2014 to a current most likely OIIP of 8.2 mmbbls. Similarly, calculated estimates of P90 (estimated to be at least a 90% probability (P90) that the quantities in place equal or exceed this low estimate) discovered OIIP had increased by nearly 400% to 5.7 mmbbls and P10 (estimated to be at least a 10% probability (P10) that the quantities in place equal or exceed this high estimate.) OIIP volumes showed a 250% increase to 12.1 mmbbls.
The above table is adapted from a diagram from the publication, Onshore UK hydrocarbon prospectivity. A small but significant resource by Nigel Smith, Keyworth. 14 (British Geological Society) and also with data from https://www.gov.uk/guidance/oil-and-gas-onshore-exploration-and-production and Alba's own data from consultants NUTECH
Portland Recoverable Oil
On 8 April 2015, US-based Nutech estimated that the HH-1 well indicates a total OIIP of 158 mmbo per square mile. The amount excluded the previously reported Upper Portland Sandstone oil discovery. Nutech's report stated that this OIIP lies within a 653 feet aggregate net pay section, primarily within three argillaceous (shale-rich) limestones and interbedded mudstones of the Kimmeridge Clay Formation, and also mudstones of the Oxford and Lias sections. Nutech calculated that approximately 72% of the OIIP, or 114 mmbo, lies within the shallower Upper Jurassic Kimmeridge interbedded limestone and mudstone sequence.
An independent study of the Portland Sandstone reservoir using petrophysics was conducted by Xodus Group an international energy consultancy based in the UK, for HHDL, and where the results were published on 11 May 2015. The results showed that the Upper Portland Sandstone conventional reservoir contains a "Best Estimate" (P50) gross STOIIP of 21.0 million barrels ("mmbbls") entirely within PEDL137 and encompasses both the HH-1 well and the historic CF-1 well. This had previously been calculated at 12.8 mmbbls in December 2014.
On 5 June 2015 Schlumberger (one of the leading suppliers of technology, integrated project management and information solutions to customers working in the global oil and gas industry), acting in an advisory capacity to HHDL, independently evaluated the unconventional oil potential of HH-1 and estimated approximately 271 mmbbls per square mile for the Jurassic section. A total of 255 mmbbls gross OIP was estimated to lie within the low-porosity limestone and mudstone plays of the Kimmeridge, Oxford Clay and Lias (Upper Portland Sandstone discovery). The value of 255 mmbbls per square mile was comparable to the OIIP estimated by Nutech.
Nutech provided an additional independent report of the estimated OIIP contained within the Horse Hill licences (PEDL 137 and PEDL 246) on 18 June 2015. The new study calculated a best estimate (P50) OIIP of 9,245 mmbbls within the Kimmeridge, Oxford and Lias formations, with a calculated best estimate total Kimmeridge OIP of 5,230 mmbbls. The calculated OIIP figures estimated by Nutech do not include the OIIP for the Portland Sandstones. It is stressed that these values should not be construed as contingent resources or reserves.
On 26 August 2015 Schlumberger provided HHDL with an additional independent report of the estimated OIIP contained within the Horse Hill licences. The new report calculated a total OIIP of 10,993 million barrels, and was composed of 8,262 mmbbls within tight limestones and shales of the Kimmeridge Clay Formation, and 2,731 mmbbls associated with the shales of the Oxford Clay, and a further aggregate OIIP of 2,731 billion barrels within the Lias.
Results of Flow Test
On the 16 February 2016, the operator of the licences, Horse Hill Developments Limited (HHDL) reported Phase One Flow Test results from the HH-1 Well, where light, 40-degree API, sweet oil had flowed naturally to surface from an 80-foot zone within the Lower Kimmeridge limestone interval at a depth of approximately 900 metres below ground level. Flow commenced at an initial instantaneous rate in excess of 700 barrels per day using a 1-inch choke, in an approximate mix of 50:50 oil to water. The well was then choked back to 32/64 inches resulting in a steady early oil rate in excess of 463 barrels of oil per day over a further 7.3-hour period, in an approximate mix of over 99% oil and less than 1% water. Further, 40-degree API, sweet oil continued to flow naturally to surface from an 80-foot zone within the Lower Kimmeridge limestone interval at a depth of approximately 900 metres below ground level.
On 1 March 2016 HHDL reported results from the Phase Two Flow Test where 100% dry, 40-degree API, light, sweet oil had flowed freely to surface, without pumping, at an average stabilised rate in excess of 900 barrels per day from an 88-foot aggregate perforated zone within the Upper Kimmeridge limestone interval at a depth of approximately 840 metres below ground level.Restricted flow of 100% dry oil, through a 1-inch choke, using a rod pump began at an initial instantaneous rate in excess of 700 barrels per day ("bopd"). Later the well started flowing freely to surface, without pumping. The free flow rate increased to over 900 bopd. The flow of dry oil then stabilised at rates averaging in excess of 900 bopd.
Upper Portland Zone
On 9 March 2016 HHDL reported results from the Phase Three Flow Test where 37-degree API, light, sweet, dry (99-100% oil) crude had flowed to surface, via rod pumping, at an average stabilised rate of 168 barrels per day over a continuous 9-hour period. The flow was from an 103-ft aggregate perforated zone within the Upper Portland sandstone interval at a depth of approximately 615 metres below ground level. Maximum flow during this test period was restricted by the initial rod pump configuration. The flow test would continue with a pump configuration designed to permit optimal flow over a longer flow period.
On 21 March 2016 HHDL reported that further periods of flow using the same pump, re-seated immediately above the perforated zone, resulted in a similar stable dry oil flow over a two-day period. It was apparent that the rate was limited by the 7-ft stroke capacity of the pump. A larger 26-ft stroke pump was installed resulting in a maximum rate in excess of 360 bopd and an average stable rate of 323 bopd over 8.5 hours.
The Horse Hill-1 well has now produced at a combined average stable rate of over 1,688 bopd from the two Kimmeridge and Upper Portland zones. Highlights from the flow test:
- The final Portland test of 323 bopd, over an 8.5-hour, period is the highest stable dry oil flow rate from any onshore UK Portland well. It is likely that the rate can be further increased using a higher capacity downhole pump during the next planned test.
- Proof that the Kimmeridge limestones contain significant volumes of moveable light oil that can be flowed to surface at commercial rates
- The stable, natural dry-oil flow rate of 464 bopd from the Lower Kimmeridge Limestone is the first ever flow from this rock unit in the Weald Basin and onshore UK.
- Based on the analysis of published reports from all significant UK onshore discovery wells, HHDL has concluded that the well’s 1688 bopd is likely the highest aggregate stable dry-oil flow from any onshore UK new field wildcat discovery well.
- Based on the analysis of published reports from all significant UK onshore discovery wells, HHDL has concluded that the 901 bopd from the Upper Kimmeridge zone is likely the highest stable natural dry oil flow rate from a single reservoir in any UK onshore new field wildcat discovery well.
- High quality Brent Crude was produced: light, sweet oil (40 degrees API in Kimmeridge, 35-37 degrees API in Portland), with 1,940 barrels delivered to the Esso Fawley refinery
- Preliminary analysis confirms that the Lower and Upper Kimmeridge Limestone units are naturally fractured reservoirs with high deliverability.
- There is a strong possibility for further optimisation and increased flow rates from all three zones in future development and production wells, particularly through the use of horizontal wells.
The most recent Xodus calculation (February 2017) for Portland sandstone P50 Oil in Place (OIP) is 32 million barrels, a 53% increase on previous calculations. The base case Portland initial oil rate was estimated at 350 barrels of oil per day per well. Ultimate recovery could be increased by a further 8-14% of OIP via implementation of a water re-injection scheme.
Regulatory consents are now awaited for the start of long-term production testing at Horse Hill, with the aim of moving towards a declaration of commerciality and stable long-term production from the Portland and Kimmeridge by the end of 2018.
Horse Hill-1 well site with drilling rig