A 2D geomechanical model of an offshore gas field in the Bredasdorp Basin, South Africa

Abstract

Abstract This case study involves the unique application of density correction software applied to density data, prior to the estimation of geopressure gradients. The K-R gas field was discovered in 1983 about 50 km west, off the F-A gas field offshore South Africa. During exploration; gas discoveries were made in well KR-1 and KR-8, potentially commercial gas and encouraging oil flow rates in well KR-2, KR-4 and KR-6, a dry well for KR-5 and a dry well with encouraging oil shows in KR-3. The aim of this study is to create a model that evaluates the geomechanical behaviour of the upper shallow marine reservoir (USM) of the Bredasdorp Basin, South Africa and provide a safe drilling mud window for future work in the area. The K-R field has a strong NW–SE fault trend, resulting in a maximum horizontal stress orientation of 125°, determined from structural depth maps. All geopressure gradients were modelled using the drillworks@ software at the top (TUSM) and bottom (BUSM) of the reservoir. The Eaton method, that can predict pore pressure from either velocity or resistivity, was used to calculate both pore pressure and fracture gradient and then calibrated using “real” data from well completion and driller’s reports. The pore pressure and fracture gradient are what set the upper and lower mud weight limits. These values range between 8.46 and 9.60 ppg and 10.12–15.33 ppg, respectively. The rock mechanical properties (Friction angle, cohesive strength and uniaxial compressive strength) were empirically derived and show a similar trend for all wells. The drilling mud window becomes more constricted at depths below 2600 m, to the TD of the well.

Keywords

• Bredasdorp Basin
• Geo-pressure or Geopressure
• Pore pressure
• Overburden
• Fracture gradient
• KR-field