· General
Purpose Research Simulator (GPRS)
· Semi-analytical
Modeling of Non-conventional Wells
· Fast
Phase Equilibrium Computations Using Reduced Method
· Modeling
of Asphaltene Precipitation and Inhibition
· Modeling of
Wax Precipitation
· Complex
Multiphase Equilibrium Calculations by Direct Minimization of Gibbs Free Energy
· Modeling of
the Solubility of Hydrogen Sulfide in Brines Using Equation of State
· Aqueous
and Mixed-solvent Electrolyte Solutions
GPRS is a
next generation of research simulator developed in our research group. GPRS
uses modular object-oriented design and is easy to extend. GPRS software is
written by standard C++ with object-oriented programming.
GPRS incorporates all
reservoir simulation models and techniques and can be used by multiple
researchers with various purposes of reservoir engineering and management. The
major features of GPRS are:
·
Structured
or unstructured grid
·
Black-oil
or compositional fluid
·
Two-point
or multi-point flux
·
Arbitrary
choice of primary variables
·
Variable
implicit levels (FIM, IMPES, IMPSAT, AIM…)
·
Direct Lapack solvers and several iterative linear solvers with
many different pre-conditioners
·
Multiple
segment well with chokes, drift-flux wellbore flow
model
GPRS has grown very fast
with multiple developers. I am the technical leader to manage GPRS development team
which consists of research associates, post-docs, Ph.D. and MS students. The
ongoing development projects are:
·
Thermal
simulation with efficiently solving coupled energy and flow equations
·
Coupling
with multiple segment well with downhole control
devices, multiphase wellbore flow with drift-flux
model
·
CO2
sequestrations in saline aquifers
·
Parallel
with MPI in Linux and Unix operation systems
·
Efficient
linear solver with block data structure
· Efficient phase equilibrium computations in the compositional
simulations
Smart well
configurations by using surface adjustable downhole
chokes offer great potential for the efficient production of oil (or gas)
reservoirs because chokes can be set to provide a more uniform inflow profile,
and therefore the breakthrough of water or gas is delayed.
Although complex well
configurations can be done by existing finite difference reservoir simulators,
such models can be time consuming to build and the accuracy of the results
depends on the grid and the well model. The semi-analytical methods (based on
Green’s functions) were used to model the smart well configurations under
single-phase flow in SUPRI-HW for several years by several students. I
integrated all of the results into a final product: AdWell2.0 software.
AdWell2.0 is faster and simpler compared to finite difference models.
In using AdWell2.0, the
wells can consist of any number of segments, coupled through wellbore hydraulics. Three types of well segments have been
implemented
1.
inflow segments: these take inflow from the reservoir.
2.
pipe segments: these connect inflow segments and take no inflow from
reservoir.
3.
choke segments: these provide downhole
inflow control.
The major features of
AdWell2.0 are:
·
provide
inflow and pressure profiles for any number of wells in any number of
reservoirs
·
automatic
switching capability between constant rate or BHP control
·
smart
wells with chokes
·
multiple
rate production simulations
·
well
index computations
·
account
for reservoir heterogeneity using s-k*
method
I manage and develop the
AdWell2.0 software.
The phase
equilibrium computations can take as much as 70% of simulation time for a
compositional simulator. Increasing the speed of phase equilibrium computations
can significantly reduce the execution time of compositional simulator. For a
mixture of N components, the conventional method needs the N independent
variables for flash computation and N-1 for stability test. The N value is
usually larger than 10.
In the proposed approach,
for stability test the number of iteration variables is reduced to two to five
regardless of the number of components in a mixture. The number increases with
the increase of the number of non-hydrocarbon components (CO2, N2
and H2S) in the mixture. The stability test computations become
extremely robust with the global convergence of
The key
feature of asphaltenes in crude is that they
self-associate to form the micelles. The resins play an important role in
stabilizing the micelles.
A thermodynamic micellization model is proposed to describe the structure
of asphaltene micelles and precipitation in crudes. The asphaltene micelles
are assumed to consist of an asphaltene core
surrounded by a solvated shell. The theoretical framework of this model is
composed of the standard Gibbs free energy of micellar
formation and the Gibbs free energies of the petroleum liquid and the
precipitated phase. The standard Gibbs free energy of the micellar
formation is modeled as the sum of various contributions including the
association between asphaltene molecules in the core,
the deformation of the asphaltene and resin molecules
in the micelle and the adsorption of the resin molecules on the micellar core. The Gibbs free energy of the petroleum
liquid phase includes the standard, mixing and interacting contributions. The
precipitated phase is assumed to be a liquid mixture of asphaltene
and resin at high temperatures, and a solid mixture at low temperatures.
The direct minimization of
Gibbs free energy of the liquid-liquid (or solid-liquid) system is used to
calculate the micellar size and composition in the
petroleum liquid phase and the amount of precipitated asphaltene
and resin in the precipitated phase. The asphaltene
precipitation from different reservoir crudes is
calculated by the model and the predicted results are in good agreement with
experimental data. The effect of pressure, temperature and composition on the precipitation
is properly predicted by the model.
Aromatic solvents and
oil-soluble amphiphiles are recognized as asphaltene precipitation inhibitors in oil production and
transportation. The micellization model shows that
the aromatic solvents are concentrated in the micellar
shell, and the interfacial tension between the asphaltene
core and the shell is reduced as the micelles become stabler.
The amphiphiles behave like resin species of the
crude and coadsorb onto the micellar
core with resins. The adsorption enthalpy of an amphiphile
plays the most important role in stablizing the asphaltene micelles which can be used to screen the
efficient amphiphile. The model also predicts the
amount of the amphiphile required to inhibit the
precipitation.
Please see the papers 3, 5
and 6 in Publications for more
detail.
Wax
precipitation is often studied using stock tank oils. The effect of pressure
and composition on the precipitation is not very clear. In this study, we
divide heavy hydrocarbons into normal paraffins(P), iso-paraffins and naphthenes(N)
and aromatics(A). We developed the correlations to estimate the fusion
properties (i.e. melting-point temperature and enthalpy of fusion) and critical
properties (critical temperature, critical pressure and acentric
factor) for these PNA species. We assumed that the solid phase is composed of
mutual insoluble pure solid substances. The calculated results of this model
are in good agreement with four measured data for the cloud-point temperatures
and amount of precipitated wax. The effect of pressure and composition on wax
precipitation is then studied using this model. The cloud-point temperature
increases with the increase of pressure at fixed composition. However, the
composition effect is much more complicated. The study of the solubilities of a heavy normal alkane
in light normal alkane solvents (for example, solubilities of n-C36 in n-C5 to n-C12) revealed that the
solubility increases first, then decrease with the increase of the carbon
number of solvent, the maximum solubility is in n-C7 or n-C8. Our model also
predicted that the cloud-point temperature of a crude has similar phenomenon.
Mixing crude with light hydrocarbon or gas (N2 and CO2) decreases the
cloud-point temperature. The cloud-point temperature decreases first with the
increase of carbon number of solvent up to n-C7, then it increases. When the
solvent is n-C10, the cloud-point temperature of the crude and solvent (70:30
mole) mixture is higher than it is in the original crude. The results showed
that any model which only account for the effect of pure dilution on the
cloud-point temperature is incorrect.
The model results indicate
that the mixing liquid natural gas (C4 and C5) has the big effect on the
decrease of the cloud-point temperature. Reservoir fluids which have high
content of n-C10 to n-C15 may have more possibility of the wax deposition
because the n-C10 to n-C15 are incompatible with the heavy paraffins
in crudes.
Please see the paper 8 in Publications for more detail.
Global
minimum of Gibbs free energy is a sufficient and necessary condition of a
stable system. This is the second law of thermodynamics. Conventional algorithm
using fugacity-equality criteria cannot guarantee a true solution because the
fugacity-equality is only a necessary condition for stable state. Actually, many
false solutions have been found in LL and VLL equilibria
by the conventional algorithm.
We developed an algorithm
to compute multiphase equilibria by direct
minimization of Gibbs free energy. The simulated annealing algorithm was used
to perform the minimization. After extensive test, we found this method is very
reliable. We have tested the VLE of synthetic oil/CO2 mixtures in critical
regions, VLLE of sour-gas (hydrogen sulfide) mixtures and CO2-crude oil
mixtures and VLSE of crude oils.
Please see the paper 7 in Publications for more detail.
The Peng-Robinson EOS modified by Stryjeck
and Vera (PRSV EOS) is extended to model the solubility of hydrogen sulfide in
aqueous sodium chloride solutions at elevated temperatures. The fugacity
coefficient of hydrogen sulfide in liquid phase consists of two term: an EOS
term (short-range interaction) and a Debye-Huckel
electrostatic term (long-range interaction between ions). The van der Waals one-fluid mixing rule
is employed. The extended PRSV EOS retains its original cubic form in volume.
The interaction coefficient between water-hydrogen sulfide is treated as
temperature-dependent. The interaction coefficients between water-sodium
chloride and hydrogen sulfide-sodium chloride are determined as
temperature-independent and temperature-dependent, respectively. With the
temperature-independent interaction coefficients, the model can predict the
solubility with good accuracy up to 500K and 6 molar salt solutions. If the
temperature-dependent interaction coefficients are used, the modeling results
agree excellently with measured data up to 620K and 6 molar salt solutions. For
the oil-field formation brines, the model with the temperature-independent
interaction coefficients is recommended.
The
activity coefficients and solubilities of sodium
bromide and lithium sulfate at different concentrations of ethanol(or
methanol)-water mixtures were measured at different temperatures. The Pitzer equations with association were used to modeling of
the results.
Phase diagrams and solubilities were computed for the Na+, K+//Cl-, CO32- , SO42--H2O system at 250C by use of Pitzer's equations. When the product of ionic activities of a salt is large than the equilibrium constant of the salt, solid exists with solution. We solve the equations to get the equilibrium concentration of the salt, i.e. the solubility.