Relationships between Observed Hydrocarbon Column Heights, Occurrence of Background Overpressure and Seal Capacity within North West Europe

Abstract

When reservoir pore pressures approach the fracture pressure the risk of hydrocarbon leakage increases as a result of mechanical failure. Here, relationships between overpressure, hydrocarbon column heights and seal capacities (fracture pressure – pore/aquifer pressure) will be explored, and best practice criteria for assessing seal capacity is discussed.
The term overpressure (OP) can be applied to any formation with pore pressures higher than the hydrostatic pressure. From a 129 field database, critical shear failure pressures, fault reactivation pressures and tensile failure pressures were derived. Calculating the difference between such pressures and the aquifer pressures at structural crests within fields allows quantification of the envelope between the reservoir pressure and failure pressure - a term coined by Swarbrick et al. (2010) as “aquifer seal capacity” (ASC).
Conventional approaches suggest that fractures will form when the pore fluid pressure equals that of σ3 plus the tensile strength of the rock (Gaarenstroom et al. 1993; Converse et al. 2000; Nordgård Bolås & Hermanrud 2003; Winefield et al. 2005). However, there remains the distinct possibility that it is aquifer pressure that governs failure (Bjørkum et al. 1998; Swarbrick et al. 2010).
This research suggests a convergence of the fracture pressure gradient and Sv at 14,500 ft within the Central North Sea. At this point, the principal stresses switch. Furthermore, a general reduction in hydrocarbon column height (HCH) is observed with a) increasing aquifer OP and b) a reduction in ASC. The possibility of applying upper bound cutoff lines is explored, i.e. column heights are not expected greater than χ ft given an aquifer OP. The HPHT Shearwater field is an exception for all trends within the research. The concept of a protected trap is suggested as an explanation for this apparently anomalous trap integrity.
Understanding the nature of how, when and why pressure-related seal failure occurs can help alleviate drilling dry holes and unsuccessful exploration ventures.