An Approach to Simulating the Effects of Water-Induced Compaction in a North Sea Reservoir (includes associated papers 73134 and 73135 )
- C.C. Cook (Amerada Hess Norge AS) | M.A. Andersen (Amoco Norway Oil Co.) | G. Halle (Elf Petroleum Norge AS) | E. Gislefoss (Enterprise Oil Norge Ltd.) | G.R. Bowen (GeoQuest)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2001
- Document Type
- Journal Paper
- 121 - 127
- 2001. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.5 Reservoir Simulation, 5.1 Reservoir Characterisation, 5.1.5 Geologic Modeling, 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 5.1.1 Exploration, Development, Structural Geology, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.6.9 Coring, Fishing, 5.4.1 Waterflooding, 5.1.2 Faults and Fracture Characterisation, 5.5.1 Simulator Development, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.3.4 Integration of geomechanics in models, 1.2.2 Geomechanics, 5.6.4 Drillstem/Well Testing, 4.3.4 Scale, 2.4.3 Sand/Solids Control, 5.8.7 Carbonate Reservoir, 3 Production and Well Operations, 6.5.2 Water use, produced water discharge and disposal, 4.5 Offshore Facilities and Subsea Systems
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Rock-compaction drive under waterflood re-pressurization has not been accounted for previously 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 seawater into the chalk formation. Platform subsidence measurements taken during the pilot waterflood also 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 assess 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.
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.1 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 et al.,2 laboratory tests on Valhall cores indicate vertical compaction caused by the introduction of water under constant stress conditions. Piau and Maury3 relate the disciplines of soil mechanics and petroleum engineering relative to the weakening/induced compaction effects of water on chalk. Further, as reported by Chin and Prevost,4 the weakening effect of water on chalk compaction may make waterflooding 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 in both the Valhall and Ekofisk fields.1,5 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.4
A Physical Picture.
Fig. 1schematically illustrates in simple terms the Valhall chalk reservoir in the form of a cube in its initial state, followed by primary depletion and then waterflood.Primary Depletion.
Under primary depletion, the chalk cube's temperature remains constant while its pressure is lowered (see Fig. 1). Plastic deformation occurs, which at Valhall is believed to be caused by pore collapse. The chalk cube shrinks and the natural fractures heal (i.e., permeability reduction).Waterflood.
The chalk cube is then injected with cool seawater, whereby its temperature is lowered and pressure is increased. According to Perkins and Gonzalez6 and Teufel and Rhett,7 the stress state of the core is altered so that 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, Fig. 1 shows that the waterflood result is a further collapse of the chalk cube. However, permeability is slightly increased owing to induced fracturing.Physics.
As reported by Maury et al.,8 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. The compaction is localized to the flood front, but decays slowly with time after the front passes. The effect is greater if the chalk is in a plastic state. We do not know the microscopic physical mechanism associated with the compaction, but it is believed to be associated with capillary pressure effects. The increased water saturation disturbs the capillary forces, destabilizing the chalk and causing it to compact. The additional deformation induced by water saturation has been described in a constitutive theoretical analysis of chalk.8Fractures.
As described by Andersen9 and illustrated in Fig. 2, when the flood front passes through, the chalk may compact and fracture. Perkins and Gonzalez10 describe the similar cool-water fracturing as secondary fractures resulting from changes in the in-situ stress. Teufel and Rhett7 report large increases in reservoir permeability measured in well tests conducted before and after waterflooding, indicating the extensive nature of waterflood-induced fracturing at Ekofisk.Hydraulic Fractures.
The fractures resulting from water-induced compaction are distinct from the hydraulic fractures created as a result of injecting above formation parting pressure. Both types of fracturing significantly contribute to flow conductivity, but hydraulic fractures are confined to the injection well areas and are believed to open and close based on bottomhole injection pressure.Thermal Effects.
Charlez et al.11 explain the two main offsetting effects of cooling on the mechanical behavior of chalk during water injection. The first effect is a stiffening or strengthening of the material with decreasing temperature. The second effect is thermal contraction of both the solid and the fluid, which induces thermal stresses. The overall thermal waterflood effect on the mechanical behavior of North Sea chalk is not conclusively determined and warrants further investigation.
The Valhall field is located within the North Sea Central graben consisting of late Cretaceous Age rock. The field structure is an asymmetrical dome with a relatively steep western flank resulting from basin inversion along a regional fault system known as the Lindesnes fault. Reservoir depth is 2400 m subsea with a free water level at 2630 m subsea. Initial reservoir temperature and pressure were 195°F and 6600 psia, respectively.
The reservoir consists of two formations, the Tor and Hod. Initial rock porosities in the Tor generally range from 40 to 50%; porosities in the Hod formation are lower, ranging from 25 to 40%. The main reason for the preservation of such high porosity is believed to be the early migration of oil into the coccolith pore space before significant burial. One might say that over the years the oil has acted as an embalming fluid, protecting the chalk body from porosity decay.
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