Challenges of developing Carboniferous gas fields in the UK Southern North Sea

The Carboniferous, as one of the three major productive intervals in the UK Southern North Sea, has considerable future potential, particularly as national demand for gas exceeds production. Almost 3 × 10 12 SCF are under development in the southern Quad 44 area, more than 600 × 10 9 SCF reserves discovered await development and the potential for yet-to-find reserves is recognized. Most of the undeveloped assets have transferred ownership a number of times and only now are operators and partner groups going ahead with development. The subsurface, engineering and commercial challenges that face those operators are outlined. The Carboniferous reservoir sandstones are relatively thin and cannot be mapped directly using 3D seismic data. Because they are separated by non-reservoir shales and siltstones, understanding the three-dimensional distribution of these sandstones is critical to ensure optimal well placement, maximum exposure to the reservoir and, ultimately, recovery of gas. Trap definition and size depend upon accurate seismic time-to-depth conversion of a complex overburden. Halokinesis, lateral velocity variations, erosional unconformities and wedges of low velocity sediments all need to be addressed in order to generate the best time-to-depth conversion. Gas in Carboniferous reservoirs is richer in inert gases, CO 2 and N 2 , than is gas from the Rotliegendes fields. These two gases contribute to reducing its calorific and commercial value, necessitating blending with higher quality gas or the removal of the inert gases before acceptence for entry into onshore distribution networks. This relies on the ability, willingness and capacity of host facility operators to accept this gas through their infrastructure. Negotiations to allow blending and acceptance of lower calorific value gas, to no detriment to the user, and the removal of inert gases will result in continued development of the Carboniferous fields. It may also encourage further exploration of the Carboniferous once a market and an off-take route to that market are established.