Successful Application of Dual Lateral Junction Techonology to Develop a Marginal Gas Field in the Carboniferous Area of the UKCS Southern North Sea

Abstract

This paper describes the application of dual lateral, level 4 junction - technology to successfully develop a marginal field in the Carboniferous area of the Southern North Sea (SNS) on the United Kingdom Continental Shelf (UKCS). This is the first known use of this technology in this area of the SNS where significant drilling risks have previously led to relatively simple well designs to mitigate the risk of failure. The Rita Field straddles blocks 44/21b and 44/22c and lies 110km due east of the United Kingdom coastline and 35km west of the UK-Dutch offshore boundary. The field is composed of adjacent, tilted, Carboniferous fault block structures containing Westphalian reservoir sandstones sealed by Silverpit shale and halites at the regional Base Permian Unconformity. The NW-trending fault blocks are separated by a NE - striking normal fault. The eastern fault block was successfully tested by 44/22c-9 in 1996 whilst the western fault block was targeted by 44/21b-11 in 1998 but failed to find gas. Well results, however, indicated the likely presence of up-dip reservoir quality Westphalian sandstones, although the development risk was higher. The selected development scenario was a dual lateral well from a single subsea wellhead, accessing both Rita main fault blocks. Although this concept yielded the most attractive economic development scenario, it nevertheless set the multidiscipline team with many significant well design challenges, including the following: Directional planning to target a gap within the high pressure Plattendolomite rafting in the Zechstein evaporite sequence whilst accommodating the reservoir strict targeting objectives of each leg. Utilising the five separate liner hanger systems that would be required in this single dual lateral well. Conducting extensive directional drilling within the Silverpit evaporite sequence with low weight OBM drilling fluid. Horizontal drilling up to 3000 ft of Carboniferous reservoir in 6 in hole whilst managing directional, hole stability and formation damage objectives. Placing the junction within a very confining area of the Zechstein basal sequence and achieving full cement isolation. Deploying the long 4 in. sandscreen lower completions. Mitigating the risk of formation damage in the first isolated reservoir leg whilst the second leg was being drilled and completed. Developing and deploying the first HPHT gauge through a 13-5/8 in vertical subsea tree in the UKCS. Further, as the western fault block was seen as an exploration target, the well design had to accommodate the geological uncertainly due to the poor quality of the seismic data, of the NE - striking normal fault and the planned reservoir entry point being out of position. Successfully dealing with these engineering challenges resulted in several industry firsts which will be fully described within the text of the paper. On completion, production rates were better than expected with very good selective delivery from both legs of the well prior to co-mingling.