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The Chalk Reservoir of the Ockley Accumulation

The Chalk Reservoir of the Ockley Accumulation

The abstract to the paper presented at the Geology of Geomechanics conference.
As a result of the collaboration between AGR, Maersk Oil, Axis Well Technology and PetroStars UK, a paper on the Chalk Reservoir of the Ockley Accumulation (Licence Block UKCS 30/1) was presented last year at the Geology and Geomechanics conference organised by the Geological Society. The paper focussed on in-situ stresses, natural fractures and implications for stimulation was published in the Special Publication of the Geological Society of London.
The Ockley discovery is a gas condensate accumulation contained within tight chalks of the Hod Formation. The faults and fractures observed 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 approximately 0.199 bar/m. The in situ stresses are estimated to all be within ~46 bar of each other indicating a near isotropic in situ stress system. Therefore, the orientation of SHmax 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 minimise extremely costly early screenouts.