Gary A. Pope

A compositional simulator for modeling surfactant enhanced aquifer remediation, 1 formulation

We describe a three-dimensional, multicomponent, multiphase compositional finite-difference simulator for application to the analysis of contaminant transport and surfactant enhanced aquifer remediation (SEAR) of nonaqueous-phase liquid (NAPL) pollutants. Mixtures of surfactant, water and NAPL can form many types of micellar and microemulsion phases with a complex and important dependence on many variables of which the dilute aqueous solution typically assumed in SEAR models is just one example. The phase behavior model is central to our approach and allows for the full range of the commonly observed micellar and microemulsion behavior pertinent to SEAR. The other surfactant related properties such as adsorption, interfacial tension, capillary pressure, capillary number and microemulsion viscosity are all dependent on an accurate phase behavior model. This has proven to be a highly successful approach for surfactant enhanced oil recovery modeling, so it was adapted to SEAR modeling. However, there are many significant differences between petroleum and environmental applications of surfactants, so many new features have been added to model contaminant transport and remediation and these are described and illustrated for the first time here.

Partitioning Tracer Test for Detection, Estimation, and Remediation Performance Assessment of Subsurface Nonaqueous Phase Liquids

In this paper we present a partitioning interwell tracer test (PITT) technique for the detection, estimation, and remediation performance assessment of the subsurface contaminated by nonaqueous phase liquids (NAPLs). We demonstrate the effectiveness of this technique by examples of experimental and simulation results. The experimental results are from partitioning tracer experiments in columns packed with Ottawa sand. Both the method of moments and inverse modeling techniques for estimating NAPL saturation in the sand packs are demonstrated. In the simulation examples we use UTCHEM, a comprehensive three-dimensional, chemical flood compositional simulator developed at the University of Texas, to simulate a hypothetical two-dimensional aquifer with properties similar to the Borden site contaminated by tetrachloroethylene (PCE), and we show how partitioning interwell tracer tests can be used to estimate the amount of PCE contaminant before remedial action and as the remediation process proceeds. Tracer tests results from different stages of remediation are compared to determine the quantity of PCE removed and the amount remaining. Both the experimental (small-scale) and simulation (large-scale) results demonstrate that PITT can be used as an innovative and effective technique to detect and estimate the amount of residual NAPL and for remediation performance assessment in subsurface formations.

Surfactant enhanced remediation of soil columns contaminated by residual tetrachloroethylene

The ability of aqueous surfactant solutions to recover tetrachloroethylene (PCE) entrapped in Ottawa sand was evaluated in four column experiments. Residual PCE was emplaced by injecting t4C-labeled PCE into water-saturated soil columns and displacing the free product with water. Miscible displacement experiments were conducted before and after PCE entrapment to determine the influence of residual PCE on column dispersivities. The first two column studies involved the injection of a 4% solution of polyoxyethy!ene (POE) (20) sorbitan monooleate, resulting in the removal of 90% and 97% of the residual PCE from 20- 30- and 40- 120-mesh Ottawa sand, respectively. Although micellar solubilization of PCE was the primary mode of recovery in these experiments, ~his process was shown to be rate-limited based on: (a) the disparity between initial steady-state concentrations of PCE in the column effluent and equilibrium vah, es measured in batch experiments; and (b) the increase in effluent concentrations of PCE following periods of flow interruption. In the latter two experiments, surfactant solutions containing mixtures of sodium sulfosuccinates removed > 99% of the residual PCE from soil columns packed with 40-270-mesh Ottawa sand. Approximately 80% of the PCE was mobilized as a separate organic liquid after flushing with < 100 mL of these surfactant solutions. This study demonstrates the capacity of surfactant flushing to enhance the recovery of residual PCE from Ottawa sand and indicates that ultra-low inter- facial tensions (< 0.0Ol dyn cm -I) are not required to achieve significant PCE mobilization when buoyancy forces are important. The potential for displacement of dense nonaqueous- phase liquids as a separate organic phase should, therefore, be evaluated during the selection of surfactant formulations for aquifer remediation.

Simulation of surfactant-enhanced aquifer remediation

Surfactant-enhanced aquifer remediation (SEAR) is currently under active investigation as one of the most promising alternatives to conventional pump-and-treat remediation for aquifers contaminated by dense nonaqueous phase organic liquids. An existing three-dimensional finite-difference enhanced oil recovery simulator is adapted to model the SEAR process. This simulator incorporates the complex chemistry and multiphase transport behavior of surfactant/water/organic mixtures in permeable media. Model governing equations and parameter requirements are discussed, and simulations are employed to illustrate some important issues potentially affecting SEAR performance at the field scale. Simulations suggest that the total time for remediation could be reduced by more than an order of magnitude over conventional remediation approaches by employing SEAR. The assumptions, approximations, and conditions required to achieve such a favorable result are identified, and the importance of modeling as a quantitative tool for the assessment of SEAR is highlighted.

Chemical flooding compositional simulator

A one-dimensional, compositional, chemical flood simulator, which enables calculation of oil recovery as a function of several major process variables, has been developed. The principal phenomenological relationships which must be input include phase behavior and interfacial tensions, as a function of electrolyte and surfactant concentrations, and polymer viscosity as a function of electrolyte and polymer concentration. The emphasis to date has been on the study of the process itself, especially the complex interactions which occur due to 2- and 3-phase behavior, interfacial tension, fractional flow, dispersion, adsorption, cation exchange, chemical slug size, and polymer transport. (12 refs.)

Anionic surfactant remediation of soil columns contaminated by nonaqueous phase liquids

Abstract A variety of column experiments have been completed for the purpose of selecting and evaluating suitable surfactants for remediation of nonaqueous phase liquids (NAPLs). The various NAPLs tested in the laboratory experiments were tetrachloroethylene (PCE), trichloroethylene (TCE), jet fuel (JP4) and a dense nonaqueous phase liquid from a site at Hill Air Force Base, UT. Both Ottawa sand and Hill field soil were used in these experiments. Surfactant candidates were first screened using phase behavior experiments and only the best ones were selected for the subsequent column experiments. Surfactants which showed high contaminant solubilization, fast coalescence times, and the absence of liquid crystal phases and gels during the phase behavior experiments were tested in soil column experiments. The primary objective of the soil column experiments was to identify surfactants that recovered at least 99% of the contaminant. The secondary objective was to identify surfactants that show low adsorption and little or no loss of hydraulic conductivity during the column experiments. Results demonstrated that up to 99.9% of the contaminants were removed as a result of surfactant flooding of the soil columns. The addition of xanthan gum polymer to the surfactant solution was shown to increase remediation efficiency as a lower volume of surfactant was required for recovering a given volume of NAPL. Based on these experimental results, guidelines for designing highly efficient and robust surfactant floods have been developed and applied to a field demonstration.

Modeling Relative Permeability Effects in Gas-Condensate Reservoirs With a New Trapping Model

Many gas-condensate wells show a significant decrease in productivity once the pressure falls below the dew point pressure. A widely accepted cause of this decrease in productivity index is the decrease in the gas relative permeability due to a buildup of condensate in the near wellbore region. Predictions of well inflow performance require accurate models for the gas relative permeability. Since these relative permeabilities depend on fluid composition and pressure as well as on condensate and water saturations, a model is essential for both interpretation of laboratory data and for predictive field simulations as illustrated in this article.

Cation Exchange in Chemical Flooding: Part 1--Basic Theory Without Dispersion

The interfacial activity and mobility control of a chemical flooding process are affected by the concentration of the cationic and anionic species that travel with the surfactant and polymer. In this study, equations from the literature are used to describe the environmental composition changes resulting from cation exchange which occurs as a chemical flood traverses a reservoir. This work presents examples of 2 or 3 exchanging cations (calcium, sodium, and magnesium) with and without mobilized oil present and with up to 4 fluids in a typical chemical flooding sequence (connate water, preflood, slug, and polymer drive). The results indicate how cation exchange and adsorption may be incorporated into a chemical flood design. The general theory from which the results are developed is based on the concept of coherence. This theory allows any number of exchanging cations to be present and adsorption of surfactant, polymer, or other species and their interaction with cation exchange to be included. (13 refs.)

A Successful Methanol Treatment in a Gas-Condensate Reservoir: Field Application

A field test was conducted to investigate the effectiveness of methanol as a solvent for removing condensate banks that form when pressure in the near wellbore region falls below the dewpoint. Core flood experiments on Texas Cream Limestone and Berea cores show that condensate accumulation can cause a severe decline in gas relative permeability, especially in the presence of high water saturation. This can result in well productivity declining by a factor of 3 to 5 as bottom hole pressure declines below the dewpoint. PVT analysis performed on field samples taken from the Hatter’s Pond field in Alabama indicate retrograde condensate behavior. These high-temperature deep gas wells show low gas productivity and large skin. A preliminary analysis of the data indicated the possibility of condensate and water blocking due to the loss of water-based drilling fluids. Core samples were used to measure gas relative permeability. Compatibility tests were conducted to ensure that the injection of filtrate and methanol did not cause any damage to the core. Since the formation brine is very saline, tests were conducted to check for salt precipitation during methanol injection. Based on these laboratory results and a single-well numerical simulation, a field test was conducted. The well chosen for treatment was producing 250 MSCFPD with 87 BPD of condensate. A thousand barrels of methanol was pumped down the tubing at a rate of 5 to 8 B/min. Gas production increased by a factor of 3 initially and stabilized at about 500 MSCFPD. Condensate production doubled to 157 BPD. The well shows a skin of –1.9 after methanol treatment. The increase in gas and condensate production was observed to persist more than 10 months after the treatment. Several possible explanations are provided for the positive results obtained in this test. Some general conclusions are made for the design for future treatments. Introduction Gas production from reservoirs having a bottom hole flowing pressure below the dewpoint pressure results in an accumulation of a liquid hydrocarbon near the wells. This condensate accumulation, sometimes called condensate blocking, reduces the gas relative permeability and thus the wells productivity. Condensate saturations near the well can reach as high as 50-60% under pseudo steady-state flow of gas and condensate. Even when the gas is very lean, such as in the Arun field with a maximum liquid drop out of 1.1%, condensate blocking can cause a large decline in well productivity. The Cal Canal field in California showed a very poor recovery of 10% of the original gas-in-place because of the dual effect of condensate blocking and high water saturation. Several methods have been proposed to restore gas production rates after a decline due to condensate and/or water blocking. Gas cycling has been used to maintain reservoir pressure above the dewpoint. Injection of dry gas into a retrograde gas-condensate reservoir vaporizes condensate and increases its dewpoint pressure. Injection of propane was experimentally found to decrease the dewpoint and vaporize condensate more efficiently than carbon dioxide. Hydraulic fracturing has been used to enhance gas productivity, but is not always feasible or cost-effective. Inducing hydraulic fractures into the formation can increase the bottom hole pressure. Hydraulic fracturing successfully restored the gas productivity of a well that died after the flowing bottom hole pressure dropped below the dewpoint. Recently, a new strategy of using solvents was developed to increase gas relative permeability reduced by condensate and water blocking. Al-Anazi et al. found that methanol was effective in removing both condensate and water and restored gas productivity in both low-permeability limestone cores and high-permeability sandstone cores. Gas productivity decreased about the same extent in both low and high permeability cores due to condensate blocking. After methanol treatment, an enhanced flow period is observed in both low and high permeability cores. Condensate accumulation is delayed for a certain time. During this time, the productivity index is increased an order of magnitude in both low and high permeability cores. The duration of the enhanced flow period is controlled by the volume of methanol injected and its rate of mass transfer into the flowing gas phase after treatment. Methanol treatments remove both water and

Application of higher-order flux-limited methods in compositional simulation

A higher-order flux-limited finite-difference scheme has been implemented into a compositional simulator to discretize the convection terms of the component conservation equations and the relative permeability terms of the phase fluxes. Hartens total variation diminishing criteria are imposed directly to the finite-difference equations and the bounds of the flux limiters which are suitable for larger Courant numbers and nonuniform grid systems are obtained. A time-correction technique is applied to increase the time accuracy and improve the stability condition.The scheme has been tested for miscible and immiscible flow problems in one and two dimensions, and the results were compared with those using a third-order method without flux limiting and some available analytical solutions.It has been shown that the scheme effectively reduces numerical dispersion and results in superior resolution of concentration and saturation fronts compared to conventional schemes. The stability conditions are also improved by using a time-correction technique. The results of the scheme are in good agreement with the analytical solutions.These improvements were achieved with negligible increase in computational effort. The scheme can also be applied to simulation problems with nonuniform gridblock sizes.