Flow testing is a key and normal part of evaluating the commercial development potential of a hydrocarbon discovery. Without it, the flow capacity of a reservoir can only be inferred from static well data. So, to more thoroughly evaluate any discovery well, it is necessary to equip the well to allow reservoir fluids to enter the wellbore and be safely conveyed to surface where they are metered, analysed and sampled.
The data recorded during flow testing of the Horse Hill-1 (HH-1) well is being used to help estimate the recoverable reserves potential of the Portland sandstone reservoir and Kimmeridge limestone oil play from an unstimulated vertical well. The information is also being used to assess the most appropriate potential development scenarios for both the Portland sandstone and Kimmeridge limestones, particularly with respect to whether horizontal wells or stimulation may be required to achieve commercial flow rates. In Horse Hill-1, the zones of interest lie behind a cemented casing. The objectives of the series of flow tests were to establish the flow capacity of the single Upper Portland sandstone layer and of two limestone layers (the Kimmeridge Micrites), and to recover oil samples. Each zone was perforated to allow flow into the wellbore. Flow passed up the well through a valve arrangement, known as a "Christmas tree", then on into the flow testing equipment at the surface. Extended flow testing was carried out and consisted of three separate flow tests: in the lower and upper Kimmeridge Micrites (limestones) and in the Upper Portland sandstone at depths of 900m, 840m and 615m below ground surface level respectively.
The flow tests were designed to test oil lying within conventional limestone and sandstone rocks, not shale oil. Importantly, our objectives contain oil and do not have any free gas component in the subsurface. Please note that the three Horse Hill-1 flow tests all took place above the current UK-wide ceiling of 1,000m (3,300ft) below ground level, above which massive hydraulic fracturing (fracking) is not permitted by law. Therefore, the flow test zones cannot be fracked.
Note: hydraulic fracturing is currently defined by UK law (the Infrastructure Act 2015) as involving the total aggregate injection of more than 10,000 cubic metres (approximately 2.2 million Imperial gallons) of water into a rock formation at a pressure above the fracture gradient of that rock formation. This limit corresponds to the minimum injected fluid volume typically required to enable hydrocarbons to flow commercially from shale rocks.
A generic flow test configuration from a recent UK flow test is shown in Figures 1 and 2. It should be noted that this was not a UKOG flow test and the equipment used in UKOG's HH-1 flow test was different. Note also that the physical footprint and activity level of the testing period were both significantly less than during the drilling phase. Figure 3 shows a schematic of a typical flow testing equipment arrangement. Oil was transported from site via road tankers, to a trans-shipment point.
No free water was produced. During each flow test a stable flow rate was metered, sample reservoir fluids were taken and then the well was shut in to record pressure. The number of road tanker loads was minimised during the flow test programme. The amount of time to execute the entire flow testing programme was around 50 days. This included mobilisation, setting-up and removal of all related equipment from the well site.
Heavy goods vehicle movements were arranged to minimise road traffic. Effective 31st December, 2015, the flow test had full regulatory consents from the Oil and Gas Authority, Health and Safety Executive and Environment Agency, together with the existing planning permission from Surrey County Council. The flow testing was conducted in strict accordance with the terms and conditions of the permits.
Figure 1: Layout in Recent UK Onshore Flow Test, View from Front
Figure 2: Layout in Recent UK Onshore Flow Test, View from Rear
Figure 3: Schematic of Typical Oil Gas Flow Test
Photos of the Flow Test operations can be found here.