Secondary Imbibition Research Articles

The evolution of pore-scale fluid-saturation in low-permeability sandstone reservoirs

In many tight-gas basins of the western United States distinguishing between productive and non-productive low-permeability sandstones, and predicting relative amounts of gas and water production is difficult. Comparison of gas shows, calculated water saturations, and saturation-height profiles between gas-productive and non-productive sandstones of equal reservoir quality all appear similar. Capillary pressure derived height functions are difficult to apply, and classic rock-typing procedures lack the predictive capability that is common to more traditional reservoirs. Basin reconstructions suggest the timing of petroleum charge and migration preceded maximum burial and uplift. This initial charge was likely a primary drainage displacement with reservoir porosity greater by a factor of 2-3 relative to values found today and permeability greater by 1-3 orders of magnitude. These reservoir systems became low-permeability following initial charge reflecting continued diagenesis throughout burial, subsequent uplift and erosion. With burial, decreasing pore volume caused water saturations and gas columns to increase. During uplift and erosion gas columns adjusted to changing structural configuration. In some cases this led to gas accumulations being leaked and spilled. In other cases, structural readjustment resulted in capillary imbibition and, in some cases, secondary (or higher order) drainage and imbibition. Within trapped accumulations, gas expansion upon uplift further increased gas columns. In cases where gas columns were spilled or within migration pathways imbibition led to residual or near-residual water saturations. Conventional formation evaluation is fundamentally rooted in concepts associated with primary drainage displacement. Tight-gas reservoirs that have experienced late uplift following an earlier phase of charge are unlikely to be characterized by primary drainage and are much more likely to be characterized by imbibition or secondary (or higher order) drainage and possibly imbibition. The hysteresis between primary drainage and imbibition or secondary (or higher order) drainage and imbibition in tight-gas reservoirs is significant and unlike many more traditional reservoirs does not tend to converge on a narrow range of values. Estimates of water saturation are scalar values and do not contain information that allows the saturation history and displacement direction to be deciphered. Recognition that reservoirs are unlikely to be in primary drainage equilibrium is a fundamental paradigm shift that impacts petroleum evaluation at all scales ranging from basin potential to completion decisions within a given well. Although this paper is written from the perspective of tight-gas petroleum systems, the principles are equally applicable to low-permeability oil reservoirs.

Transthyretin Amyloidosis with Pulmonary Involvement in a Patient with Monoclonal Gammapathy

Pulmonary involvement in the course of systemic senile amyloidosis caused by non-mutated transthyretin is rarely described. We report on concomitant monoclonal gammapathy of undermined significance (MGUS) and amyloidosis with non-mutated transthyretin with diffuse lesions in lung parenchyma. A female patient, 67 years old, was admitted with dyspnoea, malaise, weight loss, and disseminated radiological lesions in the lungs. On lung HRCT, signs of pulmonary hypertension, alveolar and interstitial involvement, with thickening of septal lines were found. Echocardiography revealed severe pulmonary hypertension, and electromyography revealed sensoromotoric polyneuropathy with axon and myelin damage. Pathological assessment of lung specimens revealed nodular deposits of amyloid in the bronchial walls and lung parenchyma Congo red staining was positive. Specimens of colon mucosa confirmed amyloidosis. Stainings for AA, AL and beta2-microglobulin were negative but were positive for transthyretin. Bone marrow trepanobiopsy indicated monoclonal gammapathy of MGUS type; Congo red staining was positive. Transthyretin amyloidosis with vascular involvement, particularly of arteriovenous anastomoses, including pulmonary vessels and an insignificant amount of AL protein (perhaps secondary imbibition with AL protein from serum) was diagnosed in amyloid deposits. No mutations of the transthyretin gene (exon 1,2,3,4) were found. The patient was treated with methylprednisolone, melphalan and then with cyclophosphamide. Radiological examinations performed 1 and 2 month/s after initiation of therapy showed progression of pulmonary lesions. The patient died one month later; an autopsy was not performed.

Traveling Wave Solutions in a Generalized Theory for Macroscopic Capillarity

One-dimensional traveling wave solutions for imbibition processes into a homogeneous porous medium are found within a recent generalized theory of macroscopic capillarity. The generalized theory is based on the hydrodynamic differences between percolating and nonpercolating fluid parts. The traveling wave solutions are obtained using a dynamical systems approach. An exhaustive study of all smooth traveling wave solutions for primary and secondary imbibition processes is reported here. It is made possible by introducing two novel methods of reduced graphical representation. In the first method the integration constant of the dynamical system is related graphically to the boundary data and the wave velocity. In the second representation the wave velocity is plotted as a function of the boundary data. Each of these two graphical representations provides an exhaustive overview over all one-dimensional and smooth solutions of traveling wave type, that can arise in primary and secondary imbibition. Analogous representations are possible for other systems, solution classes, and processes.

Computation of the two‐phase flow properties of intermediate‐wet porous media: A pore network approach

AbstractA pore‐and‐throat network including fractal‐like roughness features along its surface is employed to simulate primary drainage and secondary imbibition by accounting for the quasistatic motion of menisci in pores and throats and varying the contact angle from 0° (strongly water‐wet conditions) to 180° (strongly oil‐wet conditions). The angle of sharpness of roughness features defines a range of contact angles within which the cross‐section of the throat or pore is occupied completely by the one fluid and conditions of intermediate wettability are established. In contrast, outside this range, both fluids may coexist in a pore or throat. Such differences on the fluid distribution at the pore level affect strongly the capillary, electrical and hydraulic properties of the porous medium and are reflected in the capillary pressure, resistivity index and relative permeability curves. The simulator is used to calculate the aforementioned two‐phase flow coefficients as the pore system transits from strongly water‐wet or strongly oil‐wet to intermediate‐wet. The capillary pressure curves are always sensitive to the wetting state and the particular value of the contact angle. The relative permeability and resistivity index curves for secondary imbibition are grouped in families of curves which are sensitive mainly to the wetting state (water‐wet, intermediate‐wet/water‐wet pores, intermediate‐wet/oil‐wet pores, oil‐wet) rather than to the particular value of the contact angle.

Measured electric responses of unconsolidated layered and brine-saturated sand and sand-clay packs under continuous fluid flow conditions

We have investigated the effects of pore solution concentration on the complex electric response of two different unconsolidated samples, layered sand and sand-clay in the frequency range from 30 kHz to 3 MHz. The electric parameters that describe the electric response of the samples—real part of permittivity, conductivity amplitude and phase—are obtained through two-electrode electric measurements. Plots of the conductivity amplitude and phase as a function of frequency show large variations with water saturation and NaCl concentrations. This sensitivity may be useful for the characterization of the vadose zone. Under continuous fluid flow conditions, first drainage and secondary imbibition cycles were conducted for the two three-layered samples saturated with saline water in three different NaCl solution concentrations at atmospheric pressure and temperatures between 21 °C and 22 °C. Electrode polarization distorted the measurements, particularly in the kHz range. The distortion becomes negligible above a limiting lower frequency, which depends on NaCl solution concentration and the kind of sample. To obtain the intrinsic behaviour of the samples, the electric permittivities were analyzed above the limiting frequencies. Analysis of the real part of electric permittivity versus saturation indicates that, with increasing salinity concentration, the real part of the electric permittivity increases. Also, the hysteretic effect, the difference between first drainage and second imbibition, becomes more pronounced and remains present at higher frequencies. For the two samples, we observed a different correlation between conductivity amplitude/phase spectra and pore fluid concentration for seven saturation levels, suggesting that conductivity amplitude/phase spectra contain information about water saturation, salt solution concentration, and geotechnical properties (e.g., fines content) of unconsolidated near-surface soils. A simplified five-parameter double Cole–Cole model could fit the experimental data. ► We measured the electric response of layered sand and sand-clay. ► Increasing the salt concentration increases permittivity and the hysteretic effect. ► A clay fraction reduces the hysteretic effect significantly. ► It also causes the imbibition curves to lie below the drainage curves. ► The extended Cole-Cole model fits the observed permittivity versus saturation.

Hysteresis in the nonmonotonic electric response of homogeneous and layered unconsolidated sands under continuous flow conditions with water of various salinities, 100 kHz to 2 MHz

[1] We measured the electric parameters for four different configurations of unconsolidated homogeneous and layered sands as a function of frequency, water saturation, and salinity under fluid flow conditions. Our objective is to determine if the effect of heterogeneities at scales much smaller than the skin depth can be captured by introducing effective frequency-dependent electrical values whose behavior can be described by simple functions. We employed the parallel plate capacitor technique to measure the complex impedance over a broad frequency range, from 100 kHz up to 3 MHz. We conducted main drainage and secondary imbibition cycles at atmospheric pressure and temperatures between 21°C and 22°C. The hysteretic effect in the real part of the effective complex permittivity at higher concentrations of NaCl is more pronounced for the homogeneous configurations than for the heterogeneous samples. Effective medium theory works well for dry and saturated layered sand, when the NaCl solution concentration is 1 mmol/l. It fails for fully saturated layered sands at salinities of 10 mmol/l or more. It also does not work for partially saturated sands, independent of salinity. A description of the electric properties of a layered sand at all saturation levels by means of an effective homogeneous medium will therefore require a dependence on frequency, saturation level, and salinity of the pore fluid. An extended version of the Cole-Cole model fits the nonmonotonic behavior of the real part of permittivity versus saturation.

Pore-scale two-phase filtration in imbibition process through porous media at high- and low-interfacial tension flow conditions

This study provides new insights into pore-scale two-phase filtration during imbibition process through porous media under the high- and low-interfacial tension (IFT) flow conditions. First, the distribution and configuration of imbibing wetting and non-wetting phases in primary imbibition (free spontaneous imbibition or wetting process) is depicted. Second, the detailed pore-scale topology, structure, distribution, and configuration of different phases together with the pore-scale displacement mechanisms in primary drainage (i.e., desaturation of continuous wetting phase or de-wetting process), secondary imbibition (i.e., controlled spontaneous imbibition or desaturation of continuous non-wetting phase in high-IFT flow condition), and tertiary imbibition (i.e., forced imbibition or mobilization of discontinuous trapped non-wetting phase in low-IFT flow condition), are expounded. Finally, the advance of the displacement front and flow pattern configuration in secondary and tertiary imbibition is demonstrated and discussed. Furthermore, in tertiary imbibition, the blob size distribution of the displacing wetting phase, formation of the secondary displacement front and wetting film before breakthrough of the displacing wetting phase, rate-dependency of the advance of secondary displacement front and wetting film, interruption of the wetting film flow within wetting film region, pore-level phenomena within the wetting film region, and role of wetting film in pore-scale displacement mechanism are elucidated.

A three-phase four-component streamline-based simulator to study carbon dioxide storage

We have extended an existing streamline simulator (Batycky et al., SPE Reserv Eng 12(4):246–254, 1997) that considered two phases (aqueous and hydrocarbon) and two components (water and oil) to handle three-phase (aqueous, hydrocarbon, and solid), four-component (water, oil, CO2, and salt) transport applied to CO2 injection. We solved CO2 transport equations in the hydrocarbon and aqueous phases along streamlines and in the direction of gravity. To capture the physics of CO2 transport, in the hydrocarbon phase, we used the Todd–Longstaff model (Todd and Longstaff, J Pet Technol 24(7):874–882, 1972) to represent subgrid-block viscous fingering. We implemented a thermodynamic model of mutual dissolution between CO2 and water with salt precipitation (Spycher et al., Geochim Cosmochim Acta 67(16):3015–3031, 2003; Spycher and Pruess, Geochim Cosmochim Acta 69(13):3309–3320, 2005). The resultant changes in porosity and permeability due to chemical reaction and salt precipitation were also considered. We accounted for two cycles of relative permeability hysteresis (primary and secondary drainage and imbibition) by applying two different trapping models: Land (SPE J 8:149–156, 1968) and Spiteri et al. (SPE J 13(3):277–288, 2008). Relative permeability changes and variations in the trapped nonwetting phase saturations due to hysteresis were updated on a block-by-block basis. We verified our simulator by comparing one-dimensional simulation results with analytical solutions. We then performed simulations on three-dimensional reservoir models. We first simulated dry CO2 injection in an aquifer to investigate the effect of salt precipitation. After 2 years of CO2 injection, the permeability reduced by approximately 20%. We then used the simulator to design CO2 injection strategies in aquifers to maximize CO2 storage and in oil reservoirs to optimize both CO2 storage and oil recovery. Simulations were conducted on a North Sea reservoir description. We propose to inject CO2 and water simultaneously, followed by chase brine injection, which could render the majority of CO2 injected immobile while giving a much higher storage efficiency than injecting CO2 alone.

Interfacial area measurements for unsaturated flow through a porous medium

Multiphase flow and contaminant transport in porous media are strongly influenced by the presence of fluid‐fluid interfaces. Recent theoretical work based on conservation laws and the second law of thermodynamics has demonstrated the need for quantitative interfacial area information to be incorporated into multiphase flow models. We have used synchrotron based X‐ray microtomography to investigate unsaturated flow through a glass bead column. Fully three‐dimensional images were collected at points on the primary drainage curve and on the secondary imbibition and drainage loops. Analysis of the high‐resolution images (17 micron voxels) allows for computation of interfacial areas and saturation. Corresponding pressure measurements are made during the course of the experiments. Results show the fluid‐fluid interfacial area increasing as saturation decreases, reaching a maximum at saturations ranging from 20 to 35% and then decreasing as the saturation continues to zero. The findings support results of numerical studies reported in the literature.

Investigation of phase trapping by secondary imbibition within Fontainebleau sandstone

Pulsed magnetic field gradient stimulated echo (PGSTE) NMR is used to investigate the simultaneous flow of two phases (an aqueous phase and an hydrocarbon phase) within a strongly water-wet sample of Fontainebleau sandstone. The Fontainebleau sandstone is prepared in increasing steady-state water saturations by a secondary imbibition process. The increase in the water saturation causes an increasing fraction of the oil phase (non-wetting phase) to become trapped within the sample. The stimulated echo dependence on the gradient pulse area, q, is used to derive the displacement probability, P( X), for a fixed observation time. These displacement probabilities clearly show the progressive trapping of the oil phase with increasing steady-state water saturations. Quantitative measurements of the fraction of the oil phase trapped were made from the echo attenuation function E Δ ( q), both as a function of water saturation and observation time.

Secondary imbibition in NAPL-invaded mixed-wet sediments

A previously developed pore network model is used here to study the spontaneous and forced secondary imbibition of a NAPL-invaded sediment, as in the displacement of NAPL by waterflooding a mixed-wet soil. We use a 3D disordered pore network with a realistic representation of pore geometry and connectivity, and a quasi-static displacement model that fully describes the pore-scale physics. After primary drainage (NAPL displacing water) up to a maximum capillary pressure, we simulate secondary imbibition (water displacing NAPL). We conduct a parametric study of imbibition by varying systematically the controlling parameters: the advancing contact angles, the fraction of NAPL-wet pores, the interfacial tension, and the initial water saturation. Once the secondary imbibition is completed, the controlling displacement mechanisms, capillary pressures, relative permeabilities, and trapped NAPL saturations are reported. It is assumed that NAPL migrates into an initially strongly water-wet sediment, i.e., the receding contact angles are very small. However, depending on the surface mineralogy and chemical compositions of the immiscible fluid phases, the wettability of pore interiors is altered while the neighborhoods of pore corners remain strongly water-wet-resulting in a mixed-wet sediment. Here, we compare three different levels of wettability alteration: water-wet (advancing contact angles (20° to 55°), intermediate-wet (55° to 120°), and NAPL-wet (120° to 155°). The range of advancing contact angles and the fraction of NAPL-wet pores have dramatic effects on the NAPL-water capillary pressures and relative permeabilities. The spatially inhomogeneous interfacial tension has a minor impact on the trapped NAPL saturation and relative permeability to NAPL, and a slight effect on the relative permeability to water. The initial water saturation has a slight effect on the two-phase flow characteristics of water-wet sediments; however, with more NAPL-wet pores in the sediment, it starts to have a profound effect on the water and NAPL relative permeabilities.

Secondary imbibition in NAPL-invaded mixed-wet sediments

A previously developed pore network model is used here to study the spontaneous and forced secondary imbibition of a NAPL-invaded sediment, as in the displacement of NAPL by waterflooding a mixed-wet soil. We use a 3D disordered pore network with a realistic representation of pore geometry and connectivity, and a quasi-static displacement model that fully describes the pore-scale physics. After primary drainage (NAPL displacing water) up to a maximum capillary pressure , we simulate secondary imbibition (water displacing NAPL). We conduct a parametric study of imbibition by varying systematically the controlling parameters: the advancing contact angles, the fraction of NAPL-wet pores, the interfacial tension, and the initial water saturation. Once the secondary imbibition is completed, the controlling displacement mechanisms, capillary pressures, relative permeabilities, and trapped NAPL saturations are reported. It is assumed that NAPL migrates into an initially strongly water-wet sediment, i.e., the receding contact angles are very small. However, depending on the surface mineralogy and chemical compositions of the immiscible fluid phases, the wettability of pore interiors is altered while the neighborhoods of pore corners remain strongly water-wet-resulting in a mixed-wet sediment. Here, we compare three different levels of wettability alteration: water-wet (advancing contact angles (20° to 55°), intermediate-wet (55° to 120°), and NAPL-wet (120° to 155°). The range of advancing contact angles and the fraction of NAPL-wet pores have dramatic effects on the NAPL-water capillary pressures and relative permeabilities. The spatially inhomogeneous interfacial tension has a minor impact on the trapped NAPL saturation and relative permeability to NAPL, and a slight effect on the relative permeability to water. The initial water saturation has a slight effect on the two-phase flow characteristics of water-wet sediments; however, with more NAPL-wet pores in the sediment, it starts to have a profound effect on the water and NAPL relative permeabilities.

Imbibition fronts in porous media: effects of initial wetting fluid saturation and flow rate

We report a series of imbibition experiments in a porous medium, in which the progression of the imbibing fluid (water) displacing oil was followed by means of CT-scan measurements. These experiments, carried out at various fixed injection rates (or capillary numbers), consisted in measuring the water saturation profiles as a function of time. At low rates, the advance of the saturation front proceeds in a hyperdispersive manner, i.e., the water front spreads more rapidly than a tracer obeying normal (Gaussian) dispersion. By contrast, a hypodispersive behavior is observed at high rates, with a front that spreads less rapidly than does a tracer. By analyzing the early time dependence of the saturation profiles, we are able to characterize the divergence, at low water saturation, of the saturation-dependent capillary dispersion coefficient. This coefficient diverges like a negative power of saturation, with an exponent that strongly varies with flow rate. Those results were obtained for secondary imbibition experiments, in which the initial presence of water in the porous medium seems to be responsible for the strong hyperdispersion observed at low rate.

Impact of wettability alteration on two‐phase flow characteristics of sandstones: A quasi‐static description

We describe a two‐phase pore network simulator of drainage and imbibition which integrates a realistic representation of pore connectivity and morphology, a quasi‐static description of fluid displacement mechanisms, and a sound representation of the wetting properties of a sedimentary rock and of their alteration. The simulator works with 3‐D disordered networks of cylindrical ducts with triangular, square, and circular cross sections obtained directly from the analysis of microfocused computed tomography (CT) images of rock samples. All pore‐level displacement mechanisms (piston type, snap off, and cooperative pore body filling) are considered with arbitrary receding and advancing contact angles. Bond invasion percolation description is used in primary drainage, while bond site invasion percolation with ordinary percolation on a dual network and compact cluster growth is used in secondary imbibition. In this paper, we resolve how to calculate the relative permeability of nonaqueous phase liquid (NAPL) in the quasi‐static approximation of imbibition and illustrate spatial distribution of the clusters of trapped NAPL using our generalization to disordered networks of the Hoshen‐Kopelman cluster‐labeling algorithm. To understand the impact of wettability alteration on the capillary pressure and relative permeability functions, we perform a series of drainage and imbibition simulations by changing the range of advancing contact angles. Our study indicates that in imbibition, transport properties of a permeable solid are quite sensitive to the hysteresis between the receding and advancing contact angle. This sensitivity reflects competition among the different displacement mechanisms, which shapes the relative permeabilities, capillary pressures, and the distribution of the trapped NAPL.

Magnetization Evolution in Network Models of Porous Rock under Conditions of Drainage and Imbibition

Surface-enhanced relaxation of nuclear magnetization in fully and partially saturated water-wet porous media is studied using a pore network simulator. The simulator is based on a description of the pore space in terms of a regular cubic lattice of pores and throats following respective size distributions. The latter are obtained from geometric characterization of 3D stochastic replicas of real porous rock samples. Concepts of percolation theory are used to simulate the pore-scale distribution of a strongly wetting phase under conditions of quasistatic drainage by or imbibition against a nonwetting phase. The results of these simulations compare favorably with experimental mercury intrusion/retraction curves. The equations governing magnetization evolution in a connected pore system are then solved by matrix diagonalization for different values of wetting-phase saturation along the primary drainage and secondary imbibition paths. Analysis of the corresponding spectra of decay rates provides new insights regarding the influence of pore structure (pore and throat size distributions, spatial correlation), surface relaxation strength, and fluid distribution on diffusive coupling between pores. For pore and throat size distributions and surface relaxation strength representative of sandstones, diffusive coupling is quite important, especially under conditions of partial saturation. Remarkably, simulations show that correlated heterogeneity is the main reason pores appear poorly coupled with respect to NMR relaxation—an assumption underlying the correspondence between pore size and relaxation time distributions. Finally, for a given value of wetting-phase saturation, the history of saturation change (drainage or imbibition) is shown to have a profound effect on the spectrum of decay rates.

Measurement and evaluation of saturations for water, ethanol and a light non-aqueous phase liquid in a porous medium by gamma attenuation

The gamma attenuation technique is generally used to determine the properties of porous media. In this experimental study, this technique was used to define porosity, fluid content and liquid saturation of a porous medium. 60Co (1332 keV) as the gamma source, the Ottawa sand as a homogeneous porous medium and water, ethanol, LNAPL (toluene) as liquids were used in the experimental set-up. Experiments were performed for all liquids at three stages: the primary imbibition, the gravity drainage and the secondary imbibition.

Experimental measurement of air‐water interfacial area during gravity drainage and secondary imbibition in porous media

A new experimental method was developed to determine air‐water interfacial area as a function of capillary pressure and water saturation in unsaturated porous media. The surfactant sodium dodecyl benzene sulfonate (SDBS) was used in equilibrium column adsorption experiments to estimate air‐water interfacial area for water saturations (milliliter water per milliliter void) ranging from 0.05 to 1.0 and pressures ranging from 0 to 20 cm of water. A comparison was made between columns which were equilibrated under gravity drainage versus columns equilibrated under secondary imbibition. Gravity drainage experiments showed the air‐water interfacial area decreased linearly with saturation, while imbibition experiments showed a more complex nonmonotonic relation to the saturation. The interfacial area data are then compared with existing network models.

A Pore-Level Investigation of Relative Permeability Hysteresis in Water-Wet Systems

Summary Oil-recovery efficiency is known to depend crucially upon the surface chemistry of the particular crude oil/brine/rock system under investigation. Mineralogy, brine chemistry/pH, crude-oil composition, and ageing conditions all serve to modify the effective contact angles operating within a mixed-wet, porous medium. Recent network-modeling studies have helped to clarify the important role played by wettability during multiphase flow in nonuniformly wet systems. The consequences of modified-contact angles during waterflooding have been described in terms of various regimes. These have, subsequently, been used to reconcile a wide range of apparently contradictory wettability experiments. The purpose of this paper is to apply these ideas to the study of hysteresis phenomena in both strongly and weakly water-wet, drainage/imbibition, relative permeability curves. Such hysteresis effects have long been recognized, but the precise direction of hysteresis seems to vary from study to study. Indeed, in water-wet media, even the degree of sample consolidation appears to play an important role. A three-dimensional (3D) network model is reported, which takes into account a great deal of the underlying pore-scale physics. Initial water saturation, wettability alterations, film flow, phase trapping, and realistic variations in advancing and receding contact angles are all incorporated into the simulations. More specifically, this study examines relative permeability hysteresis in systems from Regime IA, which corresponds to strongly and weakly water-wet, porous media (i.e., media where some degree of modification in the distribution of effective contact angles has occurred). The consequences of different combinations of pore-scale-displacement mechanisms (viz., snap-off and piston-like displacement) for relative permeability hysteresis has been calculated by means of the pore-scale simulator. The effects of combining these mixed mechanisms with other network properties, such as coordination number (z) and certain volume and conductivity exponents (ν and λ), are also reported. The nature of hysteresis is found to change quite dramatically as these parameters are varied, and all experimentally observed trends in relative permeability hysteresis are reproduced under suitable conditions. This may be the first time that a pore-scale model has been used to provide a clear interpretation of all experimentally observed hysteresis trends during primary drainage (PD), secondary imbibition (SI), and secondary drainage (SD) displacements in both strongly and weakly water-wet systems.

Pore-Level Mechanisms for Altering Multiphase Permeability with Gels

Abstract The pore-level mechanisms by which a crosslinked gel changes a porous medium's physical morphology and multiphase permeability are examined using relative permeabilities, pulse tracers, and micromodel visualization. Coreflood experiments indicate that to retain oil permeability while reducing water-permeability, the maintenance of oil-phase pathways after a gel treatment is crucial. However, because the gel changes the medium's morphology, the original pathways for oil are disrupted if the posttreatment fractional flow of water is nonzero. This redistribution of phases generally causes a loss of oil-phase permeability that cannot be regained under normal flow conditions. Relative permeabilities during drainage are not affected significantly by the gel, but during secondary imbibition, the hysteresis in relative permeability is much more pronounced. Tracer experiments indicate the reason for this behavior is that the gel reduces the connectivity of the medium and the fluids distribute themselves more inefficiently, causing a large fraction of dead-end or isolated nonwetting-phase fluid to exist. These conclusions are particularly important for water-control applications where water saturation increases with time. The mechanism suggests that even if a gel exhibits preferential permeability reduction, the loss of absolute permeability and the shift in saturation can be highly damaging to the oil-phase permeability of a treated zone.

Effect of Capillary Heterogeneity on Buckley-Leverett Displacement

Summary The Buckley-Leverett (BL) equation has been routinely used for the description of immiscible displacement in laboratory Dperations and for parameter estimation and prediction. Recent advances, however, have led to a serious challenge of the validity of many of the traditional theories. For the case of BL displacement, most studies have examined heterogeneity effets as they relate to. viscous fingering. In this paper, we investigate the effects of capillary heterogeneity induced by variations in permeability in the direction of displacement. We study a variety of heterogeneity profiles, both uncorrelated and correlated spatially. We find that capillary heterogeneity significantly affects the saturation distributions, which closely follow the heterogeneity variation. The saturation response is stronger at lower rates, at more unfavorable mobility, under drainage conditions, and for smaller spatial correlations. The results are in agreement with available experimental data for primary drainage and secondary imbibition. They may be useful in the interpretation of saturation profiles and the identification of heterogeneity.