Simulating the Effects of Water-Induced Compaction in a North Sea Reservoir
Authors: Cook C.C. , Andersen M.A. , Halle G. , Gislefoss E. , Bowen G.R. 
Published: December 2007

Simulating the Effects of Water-Induced Compaction in a North Sea Reservoir C.C. Cook, SPE, Amerada Hess Norge AS; M.A. Andersen, SPE, Amoco Norway Oil Co.; G. Halle, Elf Petroleum Norge AS; E. Gislefoss, Enterprise Oil Norge Ltd.; G.R. Bowen, SPE, GeoQuest

Abstract
Rock compaction drive under waterflood re-pressurization has previously not been accounted for in our flow model studies for a Valhall waterflood. However, field observations from pilot waterfloods indicate an increase in permeability with the injection of cool sea water into the chalk formation and platform subsidence measurements taken during the pilot waterflood provide evidence of a chalk-water interaction. Laboratory experiments on reservoir core samples indicate an accelerated compaction effect as the flood front passes through the sample. To asses the value of a large scale waterflood at Valhall we have developed a new approach to simulate the possible effects of water-induced rock compaction in our black oil flow models.

Introduction
Rock compaction drive induced by pressure decline is estimated to contribute 50% of the oil recovery from the Valhall Cretaceous age chalk reservoir under primary depletion. The tremendous natural energy coming from pressure-induced rock compaction drive has led to a delay in waterflood plans at Valhall.
However, as reported by Andersen and Foged, laboratory tests on Valhall cores indicate vertical compaction caused by the introduction of water under constant stress conditions. Piau and Maury relate the disciplines of civil and petroleum engineering relative to the weakening/induced compaction effects of water on chalk. Further, as reported by Chin and Prevost, the water weakening effect on chalk compaction may make waterflood more economically favorable for improving oil recovery from some North Sea chalk reservoirs.

Water-Induced Rock compaction
Compaction drive from pressure depletion significantly contributes to oil recoveries for both the Valhall and Ekofisk fields. It was previously believed that the mechanism to invoke compaction was exclusively related to pressure depletion. However, field and laboratory experience point to the fact that compaction may also occur from chalk/water interaction, even at constant stress. We no longer believe that reservoir pressure maintenance with water injection will arrest compaction. The question now is whether water weakening only accelerates compaction or increases ultimate compaction.
A Physical Picture. To describe the physical changes of the Valhall chalk, Figure 1 schematically illustrates in simple terms the chalk reservoir in the form of a cube in its initial reservoir state, followed by primary depletion and then water flood.
Primary Depletion. Referring to Figure 1, under primary depletion, the chalk cube's temperature remains constant while its pressure is lowered. Plastic deformation occurs which at Valhall is believed to be due to pore collapse. The chalk cube shrinks and the natural fractures heal (i.e. permeability reduction).
Waterflood. The chalk cube is then injected with cool sea water whereby its temperature is lowered and pressure is increased. According to Perkins and Gonzalez and also Teufel and Rhett, the stress state of the core is altered whereby the average effective stress decreases while maintaining a constant shear stress (i.e. weight of the overburden). The decreased stress state may be compared to a loss of strength. Again, referring to Figure 1, the result from the waterflood is a further collapse of the chalk cube. However permeability is slightly increased due to fracturing. Physics. As reported by Maury et. al., typical chalks from Valhall Field are very pure, made up of 98 to 100% calcium carbonate, without any secondary minerals. When a waterflood passes through this type of chalk it can generate compaction. P. 147^