Article: 6X – An Unconventional Simulator
The 6X simulator has features for modeling unconventional oil and gas that are being actively developed for the CDOT (Completion Design and Optimization Tool) project with industry partners.
Through the project, the simulator has so far acquired special features to help model the physics of unconventionals, including multi-porosity systems and vapor-liquid equilibrium in confined media, and many features to help in modeling fracture mechanics, several being above and beyond those found in other simulators and similar tools.
6X has multi-porosity capabilities, in which different porosity systems (e.g. fractures, kerogen, sandstone) can be arbitrarily connected in a hierarchical model, for example directly, in serial, or more randomly (see picture above).
The model can account for the dual porosity systems commonly modeled with other simulators, with full elimination of the matrix equations for efficiency. A mixed single/dual porosity capability can also be handled.
The presence of high capillary forces in very small pores in unconventional reservoirs results in suppression of the bubble point pressure. To correctly model this, 6X solves the vapor-liquid equilibrium problem consistent with capillary pressures to make this correction.
Stress Dependent Permeabilities
Fracturing events are captured in 6X using regional tables of permeability or transmissibility modifiers versus net stress, which are applied to dynamically modify the reservoir flows. The table modifiers are either applied directly, or conditionally using a Mohr-Coulomb model and an input stress field. Several hysteresis models are also available to model the closing of fractures.
In addition, special consideration is given to the effects on well PIs and flows between different porosity systems, and provision is made to model background effects in nearby zones with transmissibility compressibilities.
Gels and Proppants
The 6X simulator has a tracer capability in which extra fluid components are carried around with the reservoir fluids, but which do not on their own affect the flow solution. They can be used as markers, to trace the flow of injected or initially in-place fluids through the reservoir throughout a simulation run.
6X uses the tracer facility in unconventionals for modeling the effects on flow of gels and proppants. Gels and gel breaker properties can be specified and their combined effect on fluid viscosity modulated. Proppant density can also be accounted for, with special consideration given to the prevention of ingress into non-fracture matrix, and to the differential flow of proppant in a fluid due to the effects of gravity.
It is worth noting that the scripting capability of 6X means that many applications of tracers provided through keywords can also be implemented by users themselves in scripts, but potentially using different algorithms to those provided.
Rock compaction models include a standard rock compressibility input, or tables of compressibility versus net stress, including hysteresis effects.
6X has implementations of several features similar to those described above as they appear in Sensor^ and Eclipse*, particularly the stress-dependent transmissibilities. Please contact Ridgeway Kite for specific details.
^ Sensor is a mark of Coats Engineering
* Eclipse is a mark of Schlumberger