Chalk reservoir of the Ockley accumulation, North Sea: in situ stresses, geology and implications for stimulation

Abstract

Abstract The Ockley discovery is a gas condensate accumulation contained within tight chalks of the Hod Formation. The observed faults and fractures are a combination of features radial to the main periclinal structure and parallel to the local structural grain 100° from north. Most natural fractures appear to be healed or cemented. The pore pressure gradient at Ockley is c. 0.199 bar/m. The in situ stresses are estimated to all be within c. 46 bar of each other, indicating a near-isotropic in situ stress system. Therefore the orientation of S Hmax is hard to define and drilling optimally oriented wells to create transverse hydraulic fractures is difficult. The estimated intact rock fracture initiation pressure in a horizontal well would exceed all the in situ stress gradients. Therefore, even if a vertical planar induced fracture were created at the wellbore wall, it would probably exploit natural fractures and bedding planes, leading to complex fracture geometries. Acid fracture stimulations are plugged by the insoluble clay residue within the clay-rich Hod chalk, so this is not an optimal strategy. Proppant fracturing has more merit, but complex fracture geometries present significant challenges for successful treatment design while trying to avoid or minimize extremely costly early screen-outs.