|RITA GAS FIELD - FIRST UK CARBONIFEROUS DUAL LATERAL DEVELOPMENT|
|Field Developments and Case Studies|
1E.ON Ruhrgas UK North Sea Ltd
The Rita gas field in blocks 44/21b and 44/22c was developed by E.ON Ruhrgas UK E&P Ltd. and partner GDFSUEZ E&P UK Ltd. in 2008 – 09 by the drilling of the first dual lateral well targeted at Carboniferous reservoirs in the Silverpit Basin area of the UK Southern North Sea (Figure 1).
The reservoir intervals comprise the fluvio-deltaic sandstones of the Westphalian C/D-aged, Lower Ketch to Boulton interval (Figure 2). These dip generally westwards at 12 - 15° in two major north-trending fault blocks. These main fault blocks are further segmented by subsidiary east-trending faults. Field closure is dip-controlled to the south-west and fault-controlled to the north-east (Figure 3).
Gas was proven on the eastern fault block by well 44/22c-9. Dry hole 44/21b-11 proved reservoir quality Ketch sandstones below the assumed field-wide gas-water contact but gave encouragement for the presence of gas-bearing sandstones up-dip within the Rita structure.
Top seal is provided by the stratigraphic pinch-out of the Ketch reservoir sandstones below the shales and halites of the unconformably overlying Silverpit Formation at the Base Permian Unconformity and the conformably overlying Boulton Formation shales. The base seal comprises the shales of the underlying Westphalian B Cleaver Formation.
Reservoir quality, net:gross and connectivity are generally good in the Lower Ketch 2 sandstones but poorer in the underlying Lower Ketch 1 and overlying Upper Ketch and Boulton formations. It was, therefore, important in the development to penetrate the maximum possible thickness of all the intervals to optimise production. There was also uncertainty in the location of the reservoir targets in the Rita West block that was to be drilled due to alternative seismic interpretations.
Gas was sourced from the underlying Westphalian and Namurian coals and shales and contains 3.9% N2 and 1.3% CO2.
With four stratigraphic intervals and two main but compartmentalised fault segments, pre-drill estimates of GIIP and reserves were necessarily wide.
A drilling and production solution was required that minimised costs but maximised reservoir penetration and hence production, but that also provided flexibility to address the subsurface uncertainties and drilling difficulties recognised in the area.
The development plan comprised two 3000 feet horizontal well sections orientated approximately east-west targeted at penetrating the full stratigraphic interval from Lower Ketch to Boulton in each leg. Both stratigraphic and structural compartmentalisations of the reservoir were taken into account during well bore design. These two well sections were to be drilled from a top hole well bore with a dual lateral, level 4 junction set in the competent Basal Z1 Zechstein sequence at the base of the 95/8” liner. The potentially problematic Plattendolomite interval within the Zechstein was avoided by the recognition of a hole in its distribution from the interpretation of the 3D seismic data over the field.
The drilling programme called for the western lateral to be drilled first with a 7” liner to be set immediately beyond the main north-trending fault allowing for the potentially unstable Upper Carboniferous shales and the mobile thin halite sequences at the base of the Permian Silverpit Formation to be safely cemented behind the liner.
The 81/2” section of the western lateral was drilled to a horizontal section of 2741 feet, 871 feet greater than planned due to the uncertainty in the location of the main fault and consequent reservoir distribution (Figure 3).
Drilling performance through the 6” section through the reservoir was enhanced by the first UK use of a short sub, known as the “torque buster” immediately behind the bit that induced relatively high frequency rotational vibration when pumped through. The 2792 feet section was drilled in three bit runs with an average penetration rate of 13 feet/hour.
Seismic and geocellular models were continuously updated during drilling to ensure optimum stand-off from the gas-water contact, and wellsite chemostratigraphy was used to monitor stratigraphic position in real time. The complete sequence from Lower Ketch to Boulton, drilled up stratigraphy, was penetrated in the western leg which was suspended using an invert emulsion fluid to minimise formation damage before production start-up.
Following the setting of the level 4 junction, the eastern leg 81/2” section was drilled and the 7” liner set at the top of the Westphalian section. The 6” reservoir section was drilled close to the discovery well 44/22c-9 and penetrated the full Boulton to Lower Ketch sequence, drilled down stratigraphy, within 3000 feet (Figure 3).
Equipment-limited test rates for the well were 60 and 77 mmscfgd from the western and eastern legs respectively. Peak production is limited to 75 mmscfgd and the wells are performing as anticipated more than one year after production start-up. Post-well material balance estimates after one year’s production show connected GIIP and reserves to be close to the pre-drill P50 estimates.
The authors would like to thank E.ON Ruhrgas UK E&P Ltd. and GDFSUEZ E&P UK Ltd. for their support for the publication of this paper.