Application of horizontal wells to a tight-gas sandstone reservoir: A case history

The Anglia gas field is located in the southern sector of the U.K. Continental Shelf(UKCS) in Blocks 48/18b and 48/19b. The reservoir is contained within the Permian Rotliegendes sandstone and is of small to medium size by U.K. southern North Sea standards. The field is divided into an east and west area with the gas initially in place distributed equally. Appraisal-drilling results indicated that the eastern portion of the field exhibited good productivity, whereas wells drilled into the western area produced at subeconomic rates. Development studies showed that to make the field development economically viable, well rates would have to be increased in the poorer western region. Conventional stimulation techniques were explored, particularly hydraulic fracturing; however, these proved unsuccessful owing to the proximity of the water leg. Attention, consequently, focused on the then emerging technique of horizontal drilling as a means of increasing well productivity. Development of a geologic model identified that the reservoir could be zoned into six reservoir units and that to maximize productivity, placement of the well into the predominantly aeolian zones was necessary. Reservoir simulation studies showed that a minimum 70-ft stand-off from the gas/water contact (GWC) was required to maximize ultimate recovery. This paper describes the analysis methods used and the results of applying horizontal wells in the field development and shows how, at a small cost premium, the method enabled a marginal field to be developed successfully.