Kamy Sepehrnoori

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.

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.

A Mechanistic Model of Wormhole Growth in Carbonate Matrix Acidizing and Acid Fracturing

A mathematical model that describes the growth and competition of wormholes during ann acidizing treatment in a carbonate formation was developed. The model is initialized with the distribution of largest pores. Wormhole characteristics (size, length, and distribution) were found too be controlled by acid-injection, diffusion, and fluid-loss rates.

A New Generation EOS Compositional Reservoir Simulator: Part I - Formulation and Discretization

A fully implicit equation-of-state (EOS) compositional simulator for large scale reservoir simulation is presented. The simulator uses a multiblock, domain decomposition approach; that is, the reservoir is divided into non-overlapping subdomains that are solved locally in parallel (inner iteration). The subdomain grids are defined independently of each other and their connections are attained through a global interface problem (outer iteration) formulated in terms of appropriate equations that guarantee continuity of total component fluxes. Parallel, iterative techniques are employed to solve the resulting nonlinear equations. The model formulation has been successfully tested for a dry gas cycling process on a single fault block. The numerical results show that the simulator and fluid-related calculations can be conducted efficiently and robustly. Promising results have been obtained using the proposed multiblock approach for nonmatching grids between fault blocks for two-phase flow problems. This work is presented in two parts. In Part I we outline the mathematical formulation and discuss numerical solution techniques, while in Part II we address framework and multiprocessing issues.

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.

A higher-order finite-difference compositional simulator

Abstract A third-order finite-difference method was applied to a new three-dimensional, four-phase, equation-of-state, compositional simulator. The third-order scheme was tested for first-contact miscible flow, waterflooding, immiscible, and multiple-contact miscible condensing gas displacements. The authors show that this method agrees with a two-dimensional, analytical solution. The authors also show that the higher-order method has less grid orientation effect than either one-point or two-point upstream weighting methods. A grid refinement study shows that it also has less numerical dispersion. This method was easy to implement and requires only a small increase in storage. Computation time can be reduced since the reduction in numerical dispersion allows larger grid blocks to be used while maintaining an accurate solution. For example, for a one-dimensional multiple-contact miscible displacement the computational time was reduced by a factor of 34.5 compared to the same accuracy using one-point upstream weighting.

Forecasting Gas Production in Organic Shale with the Combined Numerical Simulation of Gas Diffusion in Kerogen, Langmuir Desorption from Kerogen Surfaces, and Advection in Nanopores

We introduce a new numerical algorithm to forecast gas production in organic shale that simultaneously takes into account gas diffusion in kerogen, slip flow, Knudsen diffusion, and Langmuir desorption. The algorithm incorporates the effects of slip flow and Knudsen diffusion in apparent permeability, and includes Langmuir desorption as a gas source at kerogen surfaces. We use the diffusion equation to model both lateral gas flow in kerogen as well as gas supply from kerogen to surfaces. Slip flow and Knudsen diffusion account for higher-than-expected permeability in shale-gas formations, while Langmuir desorption maintains pore pressure. Simulations confirm the significance of gas diffusion in kerogen on both gas flow and stored gas. Relative contributions of these flow mechanisms to production are quantified for various cases to rank their importance under practical situations. Results indicate that apparent permeability increases while reservoir pressure decreases. Gas desorption supplies additional gas to pores, thereby maintaining reservoir pressure. However, the rate of gas desorption decreases with time. Gas diffusion enhances production in two ways: it provides gas molecules to kerogen-pore surfaces, hence it maintains the gas desorption rate while kerogen becomes a flow path for gas molecules. For a shale-gas formation with porosity of 5%, apparent permeability of 59.7 μD, total organic carbon of 29%, effective kerogen porosity of 10%, and gas diffusion coefficient of 10 -22 m 2 /s, production enhancements compared to those predicted with conventional models are: 9.6% due to slip flow and Knudsen diffusion, an extra 42.6% due to Langmuir desorption, and an additional 61.7% due to gas diffusion after 1 year of production. The method introduced in this paper for modeling gas flow indicates that the behavior of gas production with time in shale-gas formations could differ significantly from production forecasts performed with conventional models.

CO2 Flow Patterns Under Multiphase Flow: Heterogeneous Field-Scale Conditions

A finite-difference, equation-of-state (EOS), compositional simulator has been used to study CO[sub 2] flooding. First, unstable first-contact-miscible (FCM) displacements were simulated with a fine mesh to investigate the transition from gravity override to viscous fingering. Next, a direct comparison was made for FCM and multiple-contact-miscible (MCM) displacements under the same conditions to investigate effects of phase behavior on the growth of viscous fingers. Then, the effects of gravity, physical dispersion, capillary pressure, phase behavior, and heterogeneity were combined and simulated for CO[sub 2] flooding on a field scale with stochastic permeability fields.

Description of an Improved Compositional Micellar/Polymer Simulator

This is one of three companion papers describing a micellar/polymer or chemical flood simulator and comparing it to experimental data. A three-dimensional version of this simulator has been developed, but in this paper the authors describe only the one-dimensional (1D) flow equations because they focus on comparisons with linear coreflood data and related physical properties. All physical property models are the same in both versions, however. The property models described in this paper are (1) inaccessible PV, (2) permeability reduction, (3) polymer-phase viscosity, (4) adsorption, (5) residual phase saturations as a function of capillary number, and (6) relative permeabilities.