4 Initial pressure
In PANTHER the initial pore pressure needs to be prescribed in the reservoir, seal, base, and in the fault. Overpressures can be accounted for, and to allow modeling of hydrocarbon reservoirs, it is possible to specify different pressure gradients within the reservoir. Note that these are relatively simple pressure scenario's, in reality the pressure gradient may be more complex depending on hydrocarbon column height, capillary pressure effects, water mobility, and fault and seal properties. On this page, first a description is given how pressure are prescribed in the rock formations, in a case where the formations are not offset. In the second section, a description is given on how these pressures are prescribed for an offset geometry, and how the pressures within the fault zone can be specified based on the formation pressures.
Default: 10.5 MPa/km
Gradient used to set the initial hydrostatic pressure. See Figure 1
p(z) = p_grad * z + p_offset
Figure 1 Hydrostatic pressure gradient specified by p_grad and p_offset
Default: 0 MPa.
Pressure offsetting the hydrostatic gradient. See Figure 1
Pressure offset can be positive or negative.
Default: 0 MPa.
Oftentimes, reservoirs and underlying formations are overpressured. Overpressure is defined as the excess pressure with respect to the hydrostatic gradient. A constant overpressure value can be assigned to the reservoir and base formation, which is added to the hydrostatic gradient, giving the water pressure gradient (blue line in Figure 2).
Figure 2 Water pressure profile for hydrostatic gradient and an overpressure p_over
Default: 10.5 MPa/km
In some cases a hydrocarbon is present in the reservoir formation, e.g. oil or gas. To account for the presence of a hydrocarbon, a different pressure gradient can be prescribed within the reservoir interval, p_grad_res. At the base of the reservoir this gradient will connect to the water pressure gradient. As the density of hydrocarbons is lower than that of water, this situation will lead to an additional overpressure at the top of the reservoir, the so-called buoyancy effect. Within the reservoir the water pressure is also computed, here assuming a mobile water phase. Optionally, the water pressure can be assumed in the fault, rather than the gas pressure (REF).
At the moment, it is assumed the reservoir fluid occupies the entire reservoir height. In a future release, it will be possible to specify a reservoir fluid-water contact depth within the reservoir.
Figure 3 Two examples of the pressure gradients (solid lines) for a different reservoir fluid density (green line). Left: pressure gradient for a hydrostatic gradient (blue) and different reservoir fluid gradient (green). Right: pressure gradient (solid line) for a water pressure gradient which is overpressure below the reservoir and different reservoir fluid gradient (green). Note that the water pressure gradient will also be present in the reservoir (dotted line)
In case the reservoir compartments are offset, and a non-zero p_over is prescribed, there will be a mismatch between pressures on either side of the fault. It is assumed that the water pressure is the same in both compartments between the lowermost extent of either reservoir block. In the example shown in Figure 4, this means that the pressure at the top of the FW block will be higher than that in the adjacent seal of the HW block.
Figure 4 Example of water pressure gradients in two offset reservoir compartments, for a non-zero overpressure. Solid blue line: water pressure gradient in the HW block. Dashed dark blue line: water pressure gradient in the FW block
Water pressure gradient and reservoir pressure gradient for offset reservoir compartments (p_res_mode)
Default: 'same' (opt: 'different')
The reservoir fluid - water contacts are variable across the fault, as it is assumed that the reservoir fluid occupies to full reservoir formation. This can lead to different reservoir pressures on either side of the fault. With p_res_mode the reservoir fluid - water contact can be specified, where 'same' sets the base of both the HW and FW reservoir fluid columns to the lowermost reservoir position. This effectively means that the reservoir fluid pressure continues into the top part of the seal on one of the sides of the fault, see example in Figure 5. p_res_mode = 'different' places the base of the reservoir pressure column at the base of the FW and HW, which may be at different depths across the faults and hence leads to a different reservoir pressure on either side of the fault.
Figure 5 Example of water pressure gradients in two offset reservoir compartments, for a non-zero overpressure and different reservoir pressure gradients. Solid blue line: water pressure gradient in the HW block. Dashed line: pressure gradient in the FW block. Dotted lines: water pressure gradient in the reservoir. Left: p_mode_res = 'same', right: p_mode_res = 'different'
Default: min (opt: max, mean, min_water, max_water, mean_water)
The pressure in the fault must also be specified. For variable pressures on either side of the fault, as is the case for offset reservoir compartments in combination with overpressure and/or different reservoir pressure gradients, this is non-trivial. Fault pressures may depend on fault zone lithology for example, with more clay-rich faults tending to have a higher capillary pressure and a higher water saturation, versus more sandy faults with larger grains having a lower capillary pressure, and can be largely gas-filled, in case gas is the reservoir fluid. Various pressure modes can be specified on how to set the fault pressure, depending on the water and reservoir pressures in the footwall and hanging wall.
- min: take the minimum of the pressure gradients on either side of the fault
- max: take the maximum of the pressure gradients on either side of the fault
- mean: take the mean of the pressure gradients on either side of the fault
- FW: take the footwall pressure gradient
- HW: take the hanging wall gradient
- min_water: take the minimum of the water pressure gradient on either side of the fault
- max_water: take the maximum of the water pressure gradient on either side of the fault
- mean_water: take the mean of the water pressure gradient on either side of the fault
- FW_water: take the footwall water gradient
- HW_water: take the hanging wall water gradient In case p_grad == p_grad_res, min and min_water, max and max_water, etc will be the same