Showing papers in "Spe Reservoir Engineering in 1990"

Experimental study of cocurrent and countercurrent flows in natural porous media

Abstract: This paper reports flow experiments involving cocurrent and countercurrent spontaneous water/oil imbibition performed on the same laterally coated sample of a natural porous medium with local saturation measurements and various boundary conditions. The experiments with countercurrent imbibition showed slower oil recovery, a smoother water/oil front, and slightly lower ultimate oil recovery than those with predominantly cocurrent imbibition. Numerical simulations revealed that the relative permeabilities that enabled good prediction of countercurrent oil recovery rate are about 30% less than the conventional cocurrent relative permeabilities at a given water saturation. Viscous coupling is assumed to be the origin of this difference. A new formulation of Darcy equations that uses a matrix of mobilities was found to be in qualitative agreement with experimental results.

A newly developed chromium(III) gel technology

Abstract: Laboratory testing of a recently developed chromium(III) (Cr(III)) gel technology is reported. The gels can be used in conjunction with a number of oilfield treatments. The single-fluid acrylamide-polymer/Cr(III)-carboxylate aqueous gels are formed by crosslinking acrylamide polymer with a Cr(III)-carboxylate-complex crosslinking agent. Representative gel compositions and associated gel properties are discussed.

A Pore-Level Investigation of Foam/Oil Interactions in Porous Media

Abstract: Direct visual studies of foam flow in etched-glass micromodels containing residual oil demonstrate that foam decays as a result of breakage of pseudoemulsion films. Foam films collapse whenever nearby thin aqueous films separating gas bubbles and oil rupture. Consequently, surfactant formulation for foam insensitivity to oil in porous media should be based on stabilizing pseudoemulsion films.

Hydrate Dissociation in Sediment

Abstract: An analytical model that describes hydrate dissociation under thermal stimulation in porous media is presented. The model views the dissociation as a process in which gas and water are produced at a moving dissociation boundary. The boundary separates the dissociated zone containing gas and water from the undissociated zone containing the hydrate. A similarity solution to the conservation equations is derived, and results are presented in graphical forms that are useful in numerical computations. In particular, heat fluxes, temperature profiles, and gas pressure distributions are presented for two cases that simulate saturated-steam and hot-water thermal stimulation. A parametric study showed that the dissociation rate is a strong function of the thermal properties of the system and the porosity of the porous medium. The energy efficiency of the dissociation process, defined as the ratio of the heating value of the gas produced relative to the heat input, was also computed. For hydrate thermal stimulation, an energy efficiency value of about nine was found, which appears encouraging for natural gas production from hydrate.

Effect of pressure and temperature on the phase behavior of microemulsions

Abstract: This paper reports that optimal salinity of three different anionic microemulsions was found to increase as a function of increased hydrostatic pressure. This is equivalent to a phase transition from an upper Winsor II(+)(WII(+)) microemulsion to a lower Winsor II({minus})(WII({minus})) microemulsion. Increased pressure induces a compressibility effect that is consistent with the observed phase transition. Increasing temperature also leads to increasing optimal salinity. Prediction of temperature effects is complicated by temperature-dependent interactions and entopic contributions caused by dispersion. Fluid models that account for temperature effects are needed; therefore, no attempt was made to develop a theoretical interpretation of this effect. The temperature range is 0 to 100{degrees}C, and the pressure was varied from 0.1 to 50 MPa.

Rheology of Gas/Water Foam in the Quality Range Relevant to Steam Foam

Abstract: Experimental results on the rheology of gas/water foam essentially show that if the gas flow rate is increased at an imposed water rate, the pressure gradient increases, reaches a maximum (the break point), and then decreases. A model for foam flow was formulated that explains this behavior semiquantitatively. It consists of a highly simplified model for foam in a single capillary coupled with a description for foam flow in a bundle of identical parallel capillaries. Below the break point, capillaries filled with foam occur with water-filled capillaries; above the break point, the water-filled capillaries are replaced by gas-filled capillaries. A consequence of the model is that if one plots the results in a special way, a universal curve is obtained, dependent on only the porous medium and the fluid properties. The experimental data more or less conform to this curve, which can be used (in the same way as relative permeabilities) to predict pressure gradients for gas/water rates other than those experimentally measured. Consequences of the foam model for numerical simulation of foam flow are discussed.

Xanthan Stability at Elevated Temperatures

Abstract: Xanthan stability is examined to define more clearly the polymer's temperature limitations as a mobility-control agent. Experiments were performed to probe the relative importance of hydrolysis, oxidation, and helix-coil transitions in xanthan degradation. In the absence of oxidizing agents (i.e., dissolved oxygen), results indicate that free-radical, oxidation/reduction reactions are not the dominant mechanism for xanthan degradation. Depending on the pH, acid-catalyzed hydrolysis and base-catalyzed fragmentation reactions may play important roles. With Arrhenius calculations, it was estimated that under ideal conditions (no dissolved oxygen, pH 7 to 8, and moderate to high salinities), a xanthan solution could maintain at least half of its original viscosity for a period of 5 years if the temperature does not exceed 75 to 80{degrees}C (167 to 176{degrees}F). New polymers will be needed for chemical floods where xanthan does not have sufficient stability.

MATHEMATICAL MODELING OF HIGH pH CHEMICAL FLOODING.

Abstract: This paper describes a generalized compositional reservoir simulator for high-pH chemical flooding processes. This simulator combines the reaction chemistry associated with these processes with the extensive physical- and flow-property modeling schemes of an existing micellar/polymer flood simulator, UTCHEM. Application of the model is illustrated for cases from a simple alkaline preflush to surfactant-enhanced alkaline-polymer flooding.

A Control-Volume, Finite-Element Method for Local Mesh Refinement in Thermal Reservoir Simulation

Abstract: This paper describes a control-volume, finite-element technique for coupling coarse grids with local fine meshes. The pressure is treated in a finite-element manner, while the mobility terms are upstream weighted in the usual way. This requires identification of the cell volume and edges that are consistent with the linear finite-element discretization of the pressure. To ensure that the pressure equation yields an M matrix, various conditions are required for the type of triangulation allowed. Because the form of the equations is similar to the usual finite-difference discretization, standard techniques can be used to solve the Jacobian. The local mesh-refinement method is demonstrated on some thermal reservoir simulation problems, and computational results are presented. Significant savings in execution times are obtained while predictions similar to global fine-mesh runs are given.

Low-Temperature Oxidation of viscous crude oils

Abstract: During in-situ combustion, oxygen can bypass the flame front and react with oil temperatures that are relatively low compared with those in combustion. These reactions, called low temperature oxidation (LTO), can adversely affect the crude's physical and chemical properties and result in lower oil recovery. This paper presents laboratory study results on the LTO of four oils, ranging from 31.1 to 10.1{degrees} API and from 14 to 54,300 cp dead-oil viscosity at 85{degrees}F. They include two biodegraded oils, a mature crude and an immature heavy oil. The LTO of these oils is reported as a function of reaction time, oxygen partial pressure, and temperature. LTO increased oil viscosity and density. For the 31.1{degrees} API oil, these increases are minor and should have insignificant effects on process performance. For the heavier oils, however, the viscosity of the oxidized oils increases exponentially with increasing extent of oxidation, X{sub O{sub 2}}. relatively minor oxidation (40mg O{sub 2}/g oil) results in a six-fold increase in viscosity. The rate of viscosity increase depends on an oil's API gravity, origin, and composition. Compositional changes accompanying LTO are also discussed.

Nitrogen miscible displacement of light crude oil : a laboratory study

Abstract: A comprehensive laboratory study of N{sub 2} miscible flooding for enhanced recovery of light crude oil was performed. The minimum miscibility pressure (MMP) of N{sub 2} is a major constraint to its EOR application, so an empirical correlation for MMP estimation was developed and found to be reliable. Supporting work included many in-house slim-tube displacement determinations of MMP and the compilation and analysis of previously published data. The reservoir fluid composition, especially the amounts of the methane and ethane-through-pentane fractions, was found to be the major determining factor for miscibility. High-pressure coreflooding tests with sandstone cores were performed to evaluate the effects of gravity stability and injection mode on the N{sub 2} miscible process. N{sub 2}-gas miscible flooding successfully recovered most of the oil from laboratory cores. Gravity-stable and gravity-unstable displacements gave different oil recoveries, as did secondary and tertiary N{sub 2} displacements.

Laboratory Measurements of CO2-Foam Mobility

Abstract: Mobility and control of high-pressure CO{sub 2} floods by use of foam is a promising oil recovery technique. Critical information for the use of CO{sub 2} foam as a thickened displacement fluid is the ratio of the combined CO{sub 2}/surfactant-solution flow rate to the pressure drop in the swept region. This foam mobility can be measured in laboratory experiments. In this work, a high-pressure apparatus has been designed and carefully calibrated to measure foam mobilities at various flow rates. These measurements were made during simultaneous flow of the dense CO{sub 2} and surfactant solutions through core samples. Dependence of CO{sub 2}-foam mobility on several variables was observed. The effect of aqueous surfactant concentration on CO{sub 2}-foam mobility has been explored thoroughly. The dependence in this case was such that foam mobility decreased steadily as surfactant concentration increased until a minimum mobility was attained at some particular concentration well above the conventional critical micelle concentration (CMC). The effect of foam volume fraction showed that mobility decreases with increasing fraction of surfactant solution.

Laboratory Studies of Capillary Interaction in Fracture/Matrix Systems

Abstract: Current study of fractured petroleum reservoirs often is based on the assumption of capillary discontinuity between matrix blocks. Both theoretical analysis and examination of field performance of some fractured reservoirs, however, indicate a degree of capillary interaction between matrix blocks. Experiments performed to gain an understanding of the capillary continuity across a stack of matrix blocks are described. Three matrix blocks with a total length of 3 ft (90 cm) were stacked and placed in a transparent core holder. Before the experiments with stacked blocks were conducted, rock properties, capillary pressure, relative permeabilities, and the aperture/overburden relationship were measured. Drainage capillary pressure data showed a threshold height of the same size as each of the individual matrix blocks. Experimental results show a strong capillary interaction (capillary continuity) between the neighboring blocks.

Interpretation of wellblock pressures in numerical reservoir simulation: Part 3; Off-center and multiple wells within a wellblock

Abstract: In reservoir simulation by finite differences, well models are used to relate the wellblock pressure, the actual wellbore pressure, and the flow rate of the well. These well models are based on an equivalent wellblock radius, {ital r}{sub 0}. Two methods for obtaining {ital r}{sub 0} exist: by numerical experiments and by an approximate analytical approach that assumes that the pressures of the blocks surrounding the wellblock satisfy the exact radial solution. By definition, the numerical approach gives the correct {ital r}{sub 0}. The ore commonly used analytical approach frequently gives a good approximation but, in some cases, it does not. Hence, an analytically derived {ital r}{sub 0} should always be checked by a numerical calculation. In this paper, two important well geometries are investigated numerically: a single well arbitrarily located within an isolated wellblock and two or more wells within a single wellblock. The numerical {ital r}{sub 0} for a single well is found to be independent of location, a result that contradicts the prediction of the analytical approach.

Evaluation of empirically derived PVT properties for Gulf of Mexico crude oils

Abstract: This paper evaluates several empirical PVT correlations for application in the Gulf of Mexico. Ideally, fluid properties are determined experimentally in the laboratory; however, these data are not always available. Correlations are consequently used to determine values for bubblepoint pressure, solution GOR, FVF, and viscosity. These values are necessary to compute oil reserves or for calculations involving flow through pipes or porous media. A review of the correlations is provided, along with the results of calculations made on 31 individual crude oil samples.

Composition Paths in Four-Component Systems: Effect of Dissolved Methane on 1D CO2 Flood Performance

Abstract: New measurements of phase compositions and densities are reported for a quaternary system containing CO{sub 2}, methane, butane, and decane and related ternary systems at 160{degrees}F and 1,250 psia (71{degrees}C and 8620 kPa). Measured values of phase compositions and vapor densities agreed well with values calculated with the Peng-Robinson equation of state (PREOS), through calculated liquid densities were less accurate. Composition paths for two-phase flow of four-component mixtures were calculated with the method of characteristics and the equation-of-state (EOS) representation of the phase behavior. Analysis of resulting paths indicates why displacement efficiency in 1D CO{sub 2} floods is insensitive to the addition of dissolved methane to the oil displaced.

An analytical solution for wellbore heat transmission in layered formations

Abstract: This paper presents an analytical method for determining wellbore heat transmission during liquid or gas flow along the tubing. The mathematical model describes the heat transfer of the flowing fluid in the wellbore and in the surrounding formation as one whole physical system. The transient heat transfer equations in the two regions with coupling at the sandface are solved simultaneously. Previous treatments of wellbore heat transmission are improved upon in several aspects. Non-homogeneous formations are treated which consist of several layers with different physical properties and arbitrary initial temperature distributions in the vertical direction. Closed form analytical solutions are obtained in real space and in Laplace space, which can be used to calculate the temperature distribution along the wellbore and in the formation, and to evaluate heat transfer rate and cumulative heat exchange between wellbore and formation. A more accurate formula is given for the widely-used transient heat conduction function f(t/sub D/) of thermal resistance. This is shown to differ from Ramey's approximate solution at early time, while approaching it at late time. 14 refs., 10 figs., 1 tab.

Extensions to Dietz Theory and Behavior of Gravity Tongues in Slightly Tilted Reservoirs

Abstract: This paper investigates accuracy implications in the modeling of unstable gravity tongues in slightly tilted, anisotropic reservoirs. An extended form of Dietz's equation is examined and its range of validity established by reference to high-resolution simulation. The theory is also used to investigate the onset of viscous fingering with a new viscous-to-gravity number.

Miscible displacement : recovery tests with nitrogen

Abstract: High-pressure slim-tube tests and phase-behavior studies were performed with reservoir fluid and three-component hydrocarbon mixtures. The amount of methane and intermediates in the reservoir fluid were varied while other parameters that affect the oil recovery were kept constant. The effects of displacement velocity, temperature, and length of the packed column on slim-tube recovery were also investigated. The results obtained show that the amount of methane and intermediates in the reservoir fluid has a significant effect on minimum miscibility pressure (MMP). The results suggest very strongly that a reservoir fluid with a low methane content needs a long path length to develop miscibility with N{sub 2} (a large fraction of the oil is bypassed in the first part of the column). The slim-tube oil recovery, and hence the average microscopic displacement efficiency of the process, is shown to increase significantly with slim-tube length when injection pressure and other variables are kept constant. The results from simulation tests of phase behavior and fluid flow in the slim tube confirm some of the conclusions made from the experimental results obtained in this work. An N{sub 2} MMP correlation was developed from displacement test data reported in the literature and in the present study.

Estimation of Multiphase Flow Functions From Displacement Experiments

Abstract: A method to obtain accurate estimates of relative permeability functions from low flood rate dynamic displacement data is presented. The simultaneous estimation of the capillary pressure function from this data is also discussed. These functions are estimated using a regression-based approach by parameter estimation with a numerical coreflood simulator. Pressure drop and production data are matched with the coreflood simulator. This estimation problem is solved through minimization of the appropriate weighted least-square objective function. Careful consideration is given to the functional representation of the relative permeability and capillary pressure functions so that the most accurate estimates of those properties can be obtained.

High-temperature relative permeabilities for Athabasca oil sands--

Abstract: Experimental study of Athabasca bitumen/water relative permeabilities. The study revealed no temperature effect on the relative permeabilities to water and bitumen over a range of 100 to 250 o C (212 to 482 o F). Comparable results were obtained with both steady- and unsteady-state relative permeability measuring techniques. It was determined that the oil-phase relative permeability curve was convex

Calculation of Parachors for Compositional Simulation: An Update

Abstract: Published parachor-correlation predictions diverge for high-molecular-weight (MW) components. This divergence stems from a lack of data. This paper incorporates parachor measurements for high-MW mixtures that have not been used in the preparation of previously published parachor correlations into new parachor correlations.

Relationship of the volume-balance method of compositional simulation to the Newton-Raphson method

Abstract: This paper provides an analytical exposition of the relationship between two fully compositional, isothermal, three-phase, numerical simulator types in the literature. The Newton-Raphson (NR) method is based on the standard NR solution of the mass-or-mole-conservation equations and associated constraints. The volume-balance (VB) method is based on the principle that, in each simulator block, the PV equals the combined fluid volume of all the phases. These two types of models have been regarded as different approaches to the same problem. An analytical demonstration, however, proves that, although the two model types were developed from fundamentally different viewpoints (i.e., mass conservation vs. volume conservation), they lead to the same matrix system of the finite-difference equations.

Appraisal of the use of polymer injection to suppress aquifer influx and to improve volumetric sweep in a viscous oil reservoir

Abstract: This paper presents an investigation of a way to use polymer injection to suppress the influx of a strong edge aquifer in a shallow, dipping reservoir containing a 15-cp (15-mPa {center dot} s) oil and thereby to improve volumetric sweep. Experimental studies were undertaken to determine the appropriate polymer properties, which were then used in simulation studies to establish oil-recovery profiles.

Discontinuous and mixed finite elements for two-phase incompressible flow

Abstract: The simulation of multiphase flow presents several difficulties, including the occurrence of sharp moving fronts when convection is dominating, the need for a good approximation of velocities to calculate the convective terms of the equation, and flow singularities around wells. To handle the first difficulty, the authors propose a Godunov-type higher-order scheme based on a piecewise linear approximation of the saturation associated with a multidimensional slope limiter. With respect to the second, the pressure equation is approximated by means of a mixed-hybrid formulation equivalent to the classic mixed formulation but yielding a positive-definite linear system. To solve the third difficulty, the authors introduce macroelements around wells. Numerical experiments illustrate the capabilities of the method.

Reservoir Simulation on a Hypercube

Abstract: Presentation of a testing of a 3D parallel implicit reservoir simulator for an Intel iPSC/2 hypercube with 16 vector processors. The simulator is based on an oil/water model. A correlation of computation efficiency with problem size and the number of processors demonstrates that up up to 96% of the available CPU time on the hypercube can be used. Such high efficiencies were achieved by developing special algorithms well suited for multiple processors and distributed memory.

Performance of horizontal wells subject to bottomwater drive

Abstract: This paper discusses the performance of a horizontal well subject to bottomwater drive and delineates conditions under which this completion mode is more appropriate. Information presented will enable the engineer to decide what productivity improvements may be expected from horizontal-well completions. The productivities of horizontal wells operating under bottomwater drive are discussed in terms of displacement efficiencies. Results and the discussions are also applicable to oil production by gas-cap drive.

Analyzing Injectivity of Polymer Solutions With the Hall Plot

Abstract: The Hall plot was originally used to analyze water-injection wells. This paper demonstrates that the Hall plot can also be used to analyze injection of polymer solutions. In particular, it is possible to determine the in-situ and residual resistance factors of a polymer solution from the Hall plot. The analysis methods developed are used to examine two field injection tests and one hypothetical example. The analytical results are verified with a reservoir simulator.

Effect of Gas Enrichment and Numerical Dispersion on Enriched-Gas-Drive Predictions

Abstract: This paper examines the effect of gas enrichment in reservoir-scale enriched-gas-drive displacements through predictions made with a compositional simulator and the Peng-Robinson equation of state (PREOS). The calculations examine behavior for a classical three-component condensing-gas drive and for a 12-component reservoir-fluid characterization that predicts condensing/vaporizing-gas-drive behavior. The predictions also illustrate behavior for two simplified reservoir analogues: a homogeneous cross section with a low vertical/horizontal permeability ratio and a stratified cross section.

Cold Lake cyclic steam stimulation : a multiwell process

Abstract: Study of cyclic steam stimulation (CSS) at Cold Lake (Alberta). Steam injectivity is achieved by fracturing the formation and considerable interwell communication is observed. The result is the watering out of a producer by condensed steam from a neighboring injector. The interwell interactions complicate steam-injections scheduling, but do not seem to reduce bitumen early-cycles production, but after wells start to interact, the steam stimulation process becomes more complicated. Steam-injection schedules for large commercial projects to increase the fieldwide oil recovery are exposed