The aggregation and deposition of asphaltenic material in reservoir rock are significant problems in the oil industry and can adversely affect the producibility of a given reservoir. To obtain a fundamental understanding of this phenomenon, we have studied the deposition and aggregation of colloidal asphaltene in capillary flow by experiment and simulation. For the simulation, we have used the stochastic rotation dynamics (SRD) method, in which the solvent hydrodynamics emerge from the collisions between the solvent particles, while the Brownian motion emerges naturally from the interactions between the colloidal asphaltene particles and the solvent. We compare our simulation results with flow experiments in glass capillaries where we use extracted asphaltenes only in toluene, re-precipitated with n-Heptane and also asphaltenes precipitated from the whole oil. In the experiments, the asphaltene precipitation and deposition dynamics were monitored in a slot capillary using optical microscopy under flow conditions similar to those used in the simulation. Maintaining a constant flow rate of 5µL/min, we found that the pressure drop across the capillary first increased slowly, followed by a sharp increase, corresponding to a complete local blockage of the capillary. Subsequently the pressure fell sharply as asphaltenes were re-entrained. This condition was confirmed by the visual observations that showed the slow build-up of asphaltenes deposit followed by the sudden erosion of a channel through the deposit at the time when the pressure suddenly decreased. We calculate the change in the dimensionless permeability as a function of time for both experiment and simulation. By matching the experimental and simulation results, we obtain information about 1) the interaction potential well depth for the particular asphaltenes used in the experiments, and 2) the flow conditions associated with the asphaltene deposition process. The data obtained will also be used as input parameters for a deep-bed filtration model.
Abstract Precisely ethoxylated oleyl sulfonates were prepared and studied as model surfactant candidates for EOR. They were found to yield low interfacial tensions (IFT's), to give high solubilization parameters, and to have high electrolyte tolerance. Unfortunately, as a class of compounds they have a tendency to form liquid crystals (rather than microemulsions), which must be overcome by adding cosolvents, elevating temperatures, or restricting the maximally ethoxylated species.
Abstract The foaming characteristics of a number of crude oils from a variety of sources were determined by Bikerman's pneumatic method. Extraction of these crudes with both alkali and acid indicated that the crude oil components responsible for the foam stability were removed by the alkali extraction. Further examination of the alkali extract revealed that after neutralization it was the chloroform-soluble part of this extract (0.02% wt% of the whole crude) that was responsible for the foaming properties of the crudes investigated. This latter point was confirmed by demonstrating that the surface rheological properties of one of the extracted crudes could be restored by adding back the chloroform-soluble portion of the neutralized alkali extract. Analysis of this extract indicated that the foam-stabilizing materials were short-chain carboxylic acids and phenols of molecular weight ≤400. In principle, such analytical information could be used to identify crude oils likely to present severe foaming problems in the field. Such information could enable the process engineer to take appropriate corrective measures early in the life of a new field, thus avoiding the need for high capital expenditure at a later stage.
Abstract To explore how the microscopic geometry of a pore space affects the macroscopic characteristics of fluid flow in porous media, we have used approximate solutions of the Navier-Stokes equations to calculate the flow of two fluids in random networks. The model pore space consists of an array of pores of variable radius connected to a random number of nearest neighbors by throats of variable length and radius. The various size and connectedness distributions may be arbitrarily assigned, as are the wetting characteristics of the two fluids in the pore space. The fluids are assumed to be incompressible, immiscible, Newtonian, and of equal viscosity. In the calculation, we use Stokes flow results for the motion of the individual fluids and incorporate microscopic capillary force by using the Washburn approximation. At any time, the problem is mathematically identical to a random electrical network of resistors, batteries, and diodes. From the numerical solution of the latter, we compute the fluid velocities and saturation rates of change and use a discrete timestepping procedure to follow the subsequent motion. The scale of the computation has restricted us so far to networks of hundreds of pores in two dimensions (2D). Within these limitations, we discuss the dependence of residual oil saturations and interface shapes on network geometry and flow conditions.
Abstract The Cerro Prieto geothermal field is located in Baja California, Mexico, in the Salton Trough–a rift basin filled mainly with Colorado River sediments. A comprehensive wireline log analysis was undertaken as part of a multidisciplinary study of this geothermal system. It established (1) the physical properties of the various sedimentary units; (2) the depositional environment and hydrothermal alteration of the units; (3) the location, attitude, and displacement of faults; and (4) the subsurface circulation of the geothermal fluids. Presented are the methodology that was used and the application of the results to further exploration and development of this high-temperature geothermal resource.
AbstractMeasuring the rheological properties of crosslinked fracturing fluids is difficult but important. Fluid properties play a key role in the determination of the final geometry of die created fracture and in the distribution of proppant within the fracture; therefore, an accurate knowledge of these parameters is necessary for optimum treatment design. The first paper1 in this series described a method to measure accurately and reproducibly the rheological properties of crosslinked fracturing fluids. The technique is the first that applies long-accepted mathematical methods to correct the measurements for the deviations in shear rate caused by the non-Newtonian nature of the fluids. This, in turn, allows the rigorous examination of mathematical fluid models to determine which, if any, best describes the flow properties of the fluids.
AbstractThis paper presents a critical analysis of some recently published papers on naturally fractured reservoirs. These publications have pointed out that for a matrix-to-fracture-gradient flow regime, the transition portion of pressure test data on the semilog plot develops a slope one half that of the late-time data. We show that systems under pseudosteady state also may develop a 1:2 slope ratio. Examples from published case studies are included to show the significant errors associated with the characterization of a naturally fractured system by using the 1:2 slope concept for semicomplete well tests.
Abstract The use of the acoustic well sounding (AWS) technique to determine bottomhole pressure (BHP) requires an estimate of the gas-void fraction (fg) in the liquid column of a pumping well annulus. Three correlations relating the annular superficial gas velocity to fg are available for saturated oil columns. These correlations were developed by Godbey and Dimon,1 Podio et al.,2 and Gilbert as reported by Gipson and Swaim.3 Use of these correlations for determining the BHP, either flowing or shut in, involves a stepwise numerical integration often performed by a computer. This work addresses three aspects of estimating the BHP from AWS data: (1) estimation of the superficial gas velocity, (2) development of analytical solutions for a single-step BHP calculation, and (3) comparison and interpretation of the predicted BHP's by use of the three correlations for the field examples. A mathematical model, based on the principle of mass balance of the annular, gas phase, is used to determine the superficial gas velocity. This model rigorously accounts for the time-dependent pressure, volume, and the gas deviation factor in the liquid-free annulus. Analytical solutions are obtained for both the Godbey-Dimon and Podio et al. correlations to calculate the BHP in a single step. These analytical solutions provide a significant improvement over the numerical stepwise integration technique, because a hand-held calculator can be used for the BHP calculations. The field examples studied indicate that both the pumping liquid column height and the superficial gas velocity play a key role in estimating the gas void fraction—an essential element in calculating the BHP. We observe that only the early-time shut-in pressures are affected by the presence of gas bubbles in the liquid column. Because the bottomhole flowing pressure (BHFP) is dependent on the correlation used to predict the fg, both skin and productivity index calculations are affected. Estimation of the permeability/thickness product and the static reservoir pressure, however, are independent of the fg correlation used.
Abstract A problem in formation evaluation of tight gas sands is that their permeabilities are sometimes surprisingly sensitive to variations in overburden pressure. Photomicrographs of pore casts show an interconnected system of sheet pores, which are somewhat like the surfaces of a randomized honeycomb. A mathematical relation for predicting the pressure dependence of flow rate in sheet pores has been derived from the dimensions of the pores and the elastic constants of the matrix. The equation has been validated by measurements on artificial media containing cracks of known dimensions in glass and concrete. The observed pressure sensitivity of the gas sands used in this study requires the aspect ratio of the pores (in this case, the ratio of average large dimensions to sheet thickness) to be greater than 100. Aspect ratios have been determined by taking the large dimension from photomicrographs of pore casts or grain size and the thickness from mercury injection pressure or the slope of a plot of apparent permeability vs. the reciprocal of mean gas pressure. The latter gives the diffusive contribution to gas flow from which the pore size can be calculated. The two methods for measuring pore size give satisfactory agreement. The aspect ratios for the sheet pores in tight gas sands are large enough to explain the dependence of permeability on overburden pressure.
Abstract Over the past 20 years, a number of studies have reported temperature effects on two-phase relative permeabilities in porous media. Some of the reported results, however, have been contradictory. Also, observed effects have not been explained in terms of fundamental properties known to govern two-phase flow. The purpose of this study was to attempt to isolate the fundamental properties affecting two-phase relative permeabilities at elevated temperatures. Laboratory dynamic-displacement relative permeability measurements were made on unconsolidated and consolidated sand cores with water and a refined white mineral oil. Experiments were run on 2-in. [5.1-cm] -diameter, 20-in. [52.-cm] -long cores from room temperature to 300°F [149°C]. Unlike previous researchers, we observed essentially no changes with temperature in either residual saturations or relative permeability relationships. We concluded that previous results may have been affected by viscous instabilities, capillary end effects, and/or difficulties in maintaining material balances.
Abstract This paper presents an empirically derived correlation for estimating the minimum pressure required for multicontact miscible (MCM) displacement of live oil systems by pure or impure CO2 streams. Minimum miseibility pressure (MMP) has been correlated with temperature, oil C5+ molecular weight, volatile oil fraction, intermediate oil fraction, and composition of the CO2 stream. The effects of temperature and oil C5+ molecular weight on pure CO2 MMP have been well documented. However, CO2 sources are rarely pure, and solution gas usually is present in reservoir oils. The correlation presented in this paper accounts for the additional effects on MMP caused by the presence of volatile components (methane, C1; and N2) and intermediate components (ethane, C2; propane, C3; butane, C4; hydrogen sulfide, H2S; and CO2) in the reservoir oil. This correlation also is capable of estimating MMP for a contaminated or enriched CO2 stream on the basis of the pure CO2 MMP.
Abstract The apparent viscosity of foam flowing through smooth capillaries was measured experimentally, and a mathematical model was developed. Foam texture (a measure of bubble volume) is a key parameter in determining the following properties of foam flowing through a capillary: (1) whether the foam exists as bulk foam or as a chain of bubbles where each pair of bubbles is separated by an individual lamella, (2) the number of lamellae per unit length of the capillary, and (3) the radius of curvature of the gas-liquid interface. The apparent viscosity is the sum of three contributions: (1) that from slugs of liquid between bubbles, (2) the resistance to deformation of the interface of a bubble passing through a capillary, and (3) the surface tension gradient that results when surface active material is swept from the front of a bubble and accumulates at the back of it. The sensitivity of both measured and calculated apparent viscosity is presented as a function of bubble size, capillary radius, ratio of bubble radius to capillary radius, velocity, quality, and surface tension gradient.
Abstract A second-generation density logging tool has been developed that uses a gamma-ray source and two Nal scintillator detectors for borehole measurement of electron density, pe, and a quantity Fpe that is related to the lithology of the formation. An active stabilization system controls the gains of the two detectors, which permits selective gamma-ray detection. Spectral analysis is performed in the near detector (two energy windows) and in the detector farther away from the source (three energy windows). This paper describes the results of laboratory measurements undertaken to define the basic tool response. The tool is shown to provide reliable measurements of formation density and lithology under a variety of environmental conditions.
AbstractNew methods for analyzing drawdown and buildup pressure data obtained at a well located in an infinite, naturally fractured reservoir were presented recently. In this work, the analysis of both drawdown and buildup data in a bounded, naturally fractured reservoir is considered. For the bounded case, we show that five possible flow regimes may be exhibited by drawdown data. We delineate the conditions under which each of these five flow regimes exists and the information that can be obtained from each possible combination of flow regimes. Conditions under which semilog methods can be used to analyze buildup data are discussed for the bounded fractured reservoir case. New Matthews-Brons-Hazebroek (MBH) functions for computing the average reservoir pressure from buildup data are presented.
Abstract In fractured reservoirs primary production of heavy oil is mainly from fractures (secondary porosity). Matrix oil can be produced only at a very low rate because of low oil mobility. In this paper, in-situ combustion is considered as an EOR method. Experiments are described which show that the burning process is governed by diffusion of oxygen from the fractures into the matrix. The main oil-production mechanisms were found to be thermal expansion and evaporation with subsequent condensation of the oil from the matrix. A semi-two-dimensional (2D) numerical simulator has been constructed for modeling the process. It incorporates the main physical mechanisms as found in the experiments; heat losses to cap or base rock and gravitational effects are not included. The model predicts that in-situ combustion in fractured reservoirs is feasible and will have a high recovery efficiency in the swept zone. Oxygen breakthrough was observed when the air-injection rate exceeded a critical value predominantly determined by the fracture spacing. This phenomenon and also the shape and width of the combustion zone can be explained by a simple analytical model, which is in fair agreement with the simulator results. The effect of heat losses on the velocity of the combustion front is estimated and a lower bound for the injection rate can be obtained in this way. Combining this lower bound with the upper bound for oxgen breakthrough leads to a maximum value for fracture spacing. This distance will be on the order of 1 m [3.28 ft]. The conclusion is that in-situ combustion appears to be a feasible process in a naturally fractured reservoir with a high recovery in the swept zone. However, for predictions for a particular reservoir, vertical sweep efficiency should be taken into account.
AbstractThis paper describes an automated laboratory system that can measure accurately the dielectric properties of core samples in the ultrahigh-frequency (UHF) range. The system consists of a precision coaxial-line sample holder, a network analyzer, a plotter, a printer, and a desk computer. The computer is for measurement control, data acquisition, and data analysis. A new method is developed to measure and to compensate for the error of the network analyzer system. This method uses a brass sample and does not require standard terminations. A procedure for core sample preparation is also recommended to ensure accuracy of the data.
Abstract When reservoir solids reversibly consume hydroxide, the impact on alkaline-waterflood performance can be significant. Only recently has this reaction been recognized as a principal factor influencing oil recovery rates and chemical-pulse depletion. This paper considers the origin of the reversible hydroxide uptake to be ion exchange of sodium for hydrogen ions. Using a simple, mass-action equilibrium model, we describe the alkali exchange isotherm. Because hydronium and hydroxide concentrations in water are never zero, hydroxide uptake must be reported relative to a reference pH and salinity. With the recognition of a reference state and with the mass-action model, we predict qualitatively the effects of pH, salt concentration, and temperature on the measured hydroxide uptake isotherms for the Wilmington, Ranger-zone sand. Mineral sites that exchange ions of sodium for hydrogen may also exchange calcium for hydrogen or for sodium. Using simple mass-action equilibria again, we demonstrate that reversible hydroxide uptake depends on hardness concentration and that calcium/sodium, exchange is pH dependent.
AbstractIn this paper we describe the use of a novel technique, laser Doppler anemometry (LDA), to obtain information on fracturing fluid behavior. This technique permits measurement of fluid velocity at any point in a flow system. By scanning across the flow geometry, it is possible to obtain the velocity profile, which is related, in turn, to the rheology of the fluid.At low shear rates, velocity profiles obtained for aqueous solutions of hydroxypropyl guar showed significant deviations from those calculated using known power law parameters. The investigation was extended by conducting a series of rheological experiments using rotational and capillary viscometers over a wide shear-rate range (10−2 to 2 × l03 seconds−1). The data have been fitted to a three-parameter Ellis model, and the velocity profiles calculated from these data agree well with experimental ones.The immediate results of this work are of interest in proppant transport modeling and correlate well with published data that show that apparent viscosities obtained from proppant settling velocities are lower than those obtained from power law parameters.
AbstractSurfactant flooding for EOR results in multiphase oil/brine/surfactant/alcohol mixtures. Because alcohol and surfactant do not partition among the phases in constant proportion, thermodynamic modeling of quaternary systems is necessary in an accurate treatment of phase equilibria in these systems. We describe here the quaternary version of a free-energy-based equation of state (EOS). The systematic computation of phase equilibria in a quaternary system begins on the binary edges and ternary faces and proceeds by numerical continuation through the higher dimensional composition space. We expedite the computation with knowledge of the limits of thermodynamic stability—the spinodal surfaces—and all critical points that lie on those surfaces. This preliminary calculation indicates those regions in which solution of the nonlinear equations of phase equilibria will suffer from slow numerical convergence. Multiphase equilibria are searched out by both parametric and compositional continuation. The results provide the basis for understanding the trends of three- and four-phase equilibria in oil/brine/surfactant/alcohol mixtures.
Abstract New analytical solutions for the response at a well intercepting a layered reservoir are derived. The well is assumed to produce at a constant rate or a constant pressure. We examine reservoir systems without in-terlayer communication and document the usefulness of these solutions, which enable us to obtain increased physical understanding of the performance of fractured wells in layered reservoirs. The influence of vertical variations in fracture conductivity is also considered. Example applications of the approximations derived here are also presented.
