Abstract
The large jointed chalk reservoir situated at 3 km depth in the North Sea's Ekofisk field is undergoing major compaction after nearly 15 years of oil and gas production. Approximately 150 km3 of the overlying sediments, mostly shales, are involved in the subsidence. A maximum central subsidence of nearly 3 meters, and a maximum present rate of 45 cm per year has set in motion numerous studies of the phenomenon. Non-linear finite element (FEM) and non- linear distict element (DEM) analyses of the compaction and large scale subsidence were performed. Consistent differences between the continuum and discontinuum analyses were noted. In the latter, slip on hypothetical bedding planes and subvertical or vertical faults was allowed, which possibly gives a more realistic simulation of the real processes of subsidence in such a large body of rock. Laboratory studies of the reservoir joints included roughness measurement, JRC and JCS characterization, direct shear tests while saturated in heated Ekofisk oil, and coupled closure-shear-flow tests with heated oil followed by heated sea water. Discontinuum modelling using Cundall?s UDEC was performed on representative jointed assemblies (two sets of steeply dipping conjugate joints) to investigate the effect of a major reduction of pore pressure within the deformable matrix and along each joint. It was found that the large shrinkage deformation of the matrix allowed joint shearing to occur despite the constraint of uniaxial strain. Relative mass bulking due to small but widely distributed joint shear possibly explains the observed maintenance of excellent productivity despite large vertical strains in the reservoir.
Introduction
The Ekofisk field which is operated by Phillips Petroleum Company, is one of several hydrocarbon reservoirs associated with the Central Graben in the southern North Sea. The Maastrichtian and Paleocene (Tot and Ekofisk) chalks form an extensively jointed gentle anticlinal-domal structure, 300 meters in thickness at 3 km depth. The reservoir is pear- shaped in plan, with maximum dimensions of approximately 9 km (EW) by 14 km (NS). The higher porosity chalks (30 to 45 %) which are undergoing non-linear deformation have caused a central compacting zone measuring approximately 30 km2 in area (approx. 4 by 7 km). The area of seabed presently affected by the subsidence appears to be more circular in shape (approx. 7 by 9 km) and covers an area of approximately 50 km2 . Numerical modellets are therefore faced with the problem of predicting the subsidence of some 150 km3 of overburden (mostly shales), using laboratory samples of the chalk and shale as small as 15-30 cm3 ; a discrepancy in volume of sixteen orders of magnitude.
Compaction modeling using a non-linear continuum model
Non-linear finite element continuum analyses of the compaction process were performed with the CONSAX code (D'Orazio and Duncan, 1982). This program uses eight-noded isoparametric elements. Known distributions of porosity and fluid pressure time histories were modelled. A modified Cam Clay Model was used to simulate the non-linear void ratio-log effective stress curves, which represent the pore collapse behavior of the most porous chalk.
