Conductive faults and sealing fractures in the West Sole gas fields, southern North Sea

Abstract

AbstractUnderstanding the distribution and geometry of sealing faults and conductive natural fractures is important at all stages of field development and infill drilling. Sealing faults or fractures reduce contacted hydrocarbon volume per well by compartmentalizing the reservoir and reduce rate by locally impairing permeability. Open fractures or conductive faults enhance well productivity and contacted volume, but cause drilling difficulties and may lead to early aquifer influx or injection water breakthrough. With their lengthy drilling and production history, the West Sole area gas fields provide a natural laboratory for the study of sealing and conductive fractures and their inter-relationships. Conductive fractures were recognized early in field life, but the poor performance of some wells indicated that fractures were not uniformly distributed. As development continued, entire field segments were found to be unfractured. Subsequent appraisal drilling identified undepleted and partially depleted compartments, demonstrating that sealing faults or fractures are also present. More recent development of the satellite fields Newsham and Hoton has confirmed the presence in a single field of both conductive and sealing fractures, although there is a general inverse spatial relationship between the two. The structural history of the reservoirs provides an explanation for this phenomenon. Faulting at high pressures and temperatures during Jurassic rifting gave rise to sealing lithified cataclasite fault rocks. Inversion to form the hydrocarbon traps took place at lower pressures and temperatures. The associated brittle deformation breached sealing faults and generated open fractures.