Overview of PVT Models
This page explains in detail the differences between the four PVTmodels in 3DSL, but if you are still uncertain as to what model you need, please contact us at firstname.lastname@example.org.
The way to think of 3DSL's® PVT models is in terms of increased physics of the PVT description and flow properties such as relative permeabilities and viscosities. Thus, the Tracer model is the simplest model. The Three-Phase Immiscible model is the next step up and also contains the Tracer model. The Blackoil model is the most sophisticated model and contains all other models.
The PVT models used in 3DSL® account for two main features: (1) compressibility (pressure) and (2) miscibility (composition). There is no dependence on temperature. Although all real systems are compressible (i.e. for a given fluid element with fixed mass, the down hole volume is always different from the surface volume) and all system are miscible (there is always some components of the oil that can vaporize into the gas phase or diffuse into the water phase, for example) in practice the assumptions of incompressibility and immiscibility are used all the time, and give excellent engineering results if used correctly. As in any modeling of complex systems, there is a tradeoff between the the physics of the model and the speed of the simulation. The simpler the flow physics the faster 3DSL will run. Thus, the fastest model is the Tracer model, the slowest will be the Blackoil. The correct mix of physics and speed is problem dependent and must be determined by the user. As a general rule though, it is good practice to start with a simpler physical model and add more physics as it becomes necessary.
Incompressible or Compressible
In streamline-based simulation the assumption of incompressibility can make such a remarkable difference in speed such that converting a deck that was originally compressible to an "equivalent" incompressible deck is often worthwhile. Generally, when the flow behavior under investigation is being strongly affected by the system geology (where and how quickly is injected water going to breakthrough at producer wells, for example), system compressibility has a second-order effect and can be neglected in the first instance of a study. Compressibility, however only says how phase densities behave as a function of pressure. It says nothing about the composition of a phase. Compressibility adds a significant overhead to 3DSL® since the solution of the system is also dependent on the absolute pressure level of the system. Adding compressibility has been an significant step forward for streamline-base simulation.
The base version of 3DSL® includes incompressible PVT physics. All PVT modules described below, except the blackoil module, can be defined with incompressibility. Compressibility is a separately priced item to 3DSL®. For a list of prices see here.
In contrast to compressibility, miscibility does not impact the speed of 3DSL®. 3DSL® allows for three models in terms of miscibility: (1) completely immiscible flow, (2) first-contact miscible flow, and (3) the standard Blackoil model. Completely immiscible flow is the standard waterflood model, in which the assumption is that the water phase is composed of 100% water molecules and the oil phase is composed of 100% "oil" molecules. Since 3DSL® allows for three phase, immiscibility also says that the gas phase is composed of 100% "gas" molecules. In first-contact miscible flow, the assumption is that all the gas (or solvent) is immediately dissolved into the oil, regardless of pressure. The oil phase is therefore usually referred to as the "oleic" phase since its density and viscosity change with the amount of gas (solvent) dissolved. A first-contact miscible system is always a two-phase system: an oleic phase containing "oil" and "gas" molecules and a water phase containing only water molecules. For the original discussion of first-contact miscible flow, see the paper by Todd&Longstaff, JPT,July 1972,pp.874-882 and Koval,SPEJ,June 1963,pp.145-154. Finally, the Blackoil model is the standard "Rs" model in which the amount of gas dissolved in the oil is given as a function of reservoir pressure, where Rs is a ratio of standard condition volumes. Because Rs is a function of pressure, the standard Blackoil model only makes sense in the context of a compressible system.
3DSL's® Tracer model makes the assumption that the system is incompressible and immiscible. Additionally, the Tracer model makes the assumption that all relative permeabilities are straight lines with zero residuals, all phase viscosities are equal to the oil viscosity, and all densities are equal to the oil density. In other words, kr=S for all phases, vis=oil_vis for all phases, and dens=oil_dens for all phases. These assumptions make the system linear, meaning that the streamlines remain unchanged with time unless there is a change in the well conditions. This is the fastest model to run, because the velocity field (or pressure field) only requires updating when well events happen. Notice that because the densities of all the phases are identical and equal to the oil density there is no gravity segregation of the fluids.
The Three-Phase Immiscible model still assumes that the system is immiscible. But in contrast to the Tracer model, each phase (and rockregion) may now have its own relative permeability functions. Each phases viscosities and densities can also be functions of pressure, giving a compressible system. These properties all make the system nonlinear. Streamlines now change between well events simply because of gravity (density difference) and non-unit mobility ratios (viscosities and rel perms). The Three-Phase Immiscible model also contain the first-contact miscible model as a sub-model. In this case, the density and viscosity of the oleic phase change as a function of solvent dissolved in the oleic phase, but the two-phase system (oleic/water) remains immiscible. The first-contact miscible model is often more nonlinear than a strict Three-Phase Immiscible model because the mobility ratio can deviate significantly from 1, thereby requiring more frequent updates of the velocity field (i.e. streamlines).
3DSL's® miscible model assumes that the gas component is miscible with the oil phase at all pressures resulting in an oleic phase that contains both oil and gas components. The gas component can also dissolve into the water phase which is useful for CO2 simulations.
The extent of miscibility: how much of the gas component actually dissolves into the oleic phase (or water phase) is governed by a parameter omega (w). If w=1 then all the gas present resides in the oleic phase and the system is a two-phase (oleic + water phase) system. If w=0 then all the gas present resided in the gas phase and the systems becomes identical to the immiscible model and is three phase (oil/gas/water). For w-values between 0 and 1, the system is a three-phase system (oleic/gas/water). There is a free gas phase, but the gas saturation is smaller than it would be in the pure immiscible model. w is also a function of the total oil & gas mass fractions present in a grid cell and is described by a second variable beta (B). In other words, the amount of gas that can dissolved into the oil phase is also a function of how much oil is present. See Thiele et al, 2002 in our Publications Section. See the technical appendix of the 3DSL® manual for a detailed description of the model and equations.
3DSL's® blackoil model is the standard blackoil model used in many finite difference simulators. This means that it is compressible and allows gas to either come out of solution or dissolve into the oil depending on the amount of gas present and the pressure of the system. Alternatively, if Rs is zero, a dead-oil, compressible model can be used.