MigriX uses a dynamic mesh (flowpath) technique to transport hydrocarbons from their generation location, via primary migration, secondary migration and into structural and/or stratigraphic traps. Tertiary leakage and the spill from traps processes are also simulated through time. Migration is modeled to occur in regular and/or irregular mesh elements and may be influenced by sealing faults in a model typically close to the seismic scales in resolution. Hydrocarbon saturations within the computing nodes are considered to be non-constant and hysteresis leakage effects are considered.
MigriX simulates the hydrocarbon generation and migration history of the basin as a series of timesteps and will visualize results interactively after each time step using a simultaneous simulation and visualization technique. For each time-step the geometry is reconstructed, the geological and flow properties are calculated, thermal model and the generation within the timestep is determined and migration is simulated for the expelled hydrocarbons using a multi-component representation of the petroleum system. A simplified and very fast multi-component PVT modelling using local pressures and temperatures is used.
Figure 2.1 Modelled migration flow-rates along a carrier bed
Migration process description
Primary migration is modelled within a high-resolution element description where each structural element is vertically subdivided into many sub-layers, each with its own permeability and entry pressure. The entry pressures will determine the vertical direction of flow, and hydrocarbons will flow in the direction of lower potential until a carrier is reached.
A carrier is in MigriX defined as a flow unit (below a vertical barrier) that has sufficient permeability to transport hydrocarbons laterally over the time steps modelled. MigriX will use the vertical sub-layer entry pressure structure to determine in which flow-units the hydrocarbons will migrate. The vertical saturation distribution is determined within each dynamic sublayer flow-mesh element and hydrocarbons are transported laterally into the next element.
Once the hydrocarbons have reached the trap, they will mix and the PVT modelling will determine the trap phases of each sublayer trap. Lateral spill of oil and gas is modelled between each sublayer trap. At the same time the properties of the seals above and/or below the traps are used to determine if capillary leakage occurs. The vertical entry pressure distribution between each trap and carriers above or below are used to determine the static and dynamic seals and when these are overcome by the trapped hydrocarbons leakage is initiated. The area of the leakage zone is determined and the saturations, permeabilities and entry pressures determine the rate of leakage from each trap. Re-migration between traps is thus modeled for each time step using the geometries and properties of the traps and seal at the reconstructed depths of the timestep.
When starting to model the next time-step the structural reconstruction of that timestep is used to remodel the shape of the traps and spill surplus volumes that no longer can be accommodated in the traps due to e.g. compaction. Thereafter, the generated volumes within the next timestep are added and migration is modelled
Model geometries
MigriX will load horizons (layers) from regular grids and combine these with fault-traces for each layer. The resulting geometrical model contains non-vertical fault planes built by combining the grids with the traces and producing a combination of regular elements away from the faults and non-regular elements around the fault traces. The regular eight-point mesh is split into mesh elements with fewer corners and with a more free orientation of the mesh sides. This allows MigriX to perform a more accurate description of fault planes and the intersection between faults and horizons. The latter is important in the fault seal modelling that can be done with MigriX. MigriX uses a dynamic flow-path modelling approach. A single flow-path are dynamic elements that are recomputed in direction and size for each time-step modelled. Each geometrical element of the model may be subdivided into a number of flow-path elements. MigriX has a very efficient numerical scheme for solving a first order differential equation for flow within each flow-path element, and is therefore able to compute migration stringer hydrocarbon saturations, column heights and migration velocities from the Darcy equation for two-phase flow.
MigriX considers the 3D volume to be split into a series of layers and sub-layers. Within each layer there can be a number of sub-layers. Users specify the properties of each layer, while MigriX will use algorithms to distribute these properties from the average values of the layers into the values of the sub-layers. MigriX makes full use of the sublayer description when modelling migration within low-permeability rocks. The same concept is used for the primary migration modelling and capillary leakage modelling. The numerical solutions differ, however, in order to make the computations more accurate and also faster. During leakage modelling, MigriX will also account for hysteresis effects. These effects may result in a long term reduction in the effective entry pressures of the seal once the seal has been breached, reducing the sealing potential of leaking traps to less than the entry pressure seal capacity.
Figure 2.2 Layers and sublayers in MigriX, colour-coded for Vshale (0-100%)
Fault seal
The modelling of fault sea (figure 2.3) is an important integrated part of the MigriX simulator and faults, or parts of faults, can be described as open, sealing or partly sealing to flow. The fault seal modelling allows for the modelling of hundreds or thousands of faults within a single migration model. When cross-fault migration within the same layer is modelled, all the trapped oil will spill before the gas starts to spill.
Figure 2.3 Examples of migration along partly sealing faults in the open UK CNS model.
PVT modelling
MigriX uses a fast PVT modelling technique to compute the phase properties of the hydrocarbon mixture during migration. PVT is computed for every mesh location that the hydrocarbons migrate through, and for all traps, during burial based on the local palaeo pressure and temperatures of the model. A four-component model (dry gas, wet gas, light oil, oil) is used for MigriX models. Models that have been created in the full Migri version can be simulated with more components.
