Study of Microscopic and Macroscopic Displacement Behaviors of Polymer Solution in Water-Wet and Oil-Wet Media

Performance of a polymer flood process requires the knowledge of rheological behavior of the polymer solution and reservoir properties such as rock wettability. To provide a better understanding of effects of polymer chemistry and wettability on the performance of a polymer flood process, a comprehensive experimental study was conducted using a two-dimensional glass micromodel. A series of water and polymer flood processes were carried out at different polymer molecular weights, degrees of polymer hydrolysis, and polymer concentrations in both water-wet and oil-wet systems. Image processing technique was applied to analyze and compare microscopic and macroscopic displacement behaviors of polymer solution in each experiment. From micro-scale observations, the configuration of connate water film, polymer solution trapping, flow of continuous and discontinuous strings of polymer solution, piston-type displacement of oil, snap-off of polymer solution, distorted flow of polymer solution, emulsion formation, and microscopic pore-to-pore sweep of oil phase were observed and analyzed in the strongly oil-wet and water-wet media. Rheological experiments showed that a higher polymer molecular weight, degree of hydrolysis, and concentration result in a higher apparent viscosity for polymer solution and lower oil–polymer viscosity ratio. It is also shown that these parameters have different impacts on the oil recovery in different wettabilities. Moreover, a water-wet medium generally had higher recovery in contrast with an oil-wet medium. This experimental study illustrates the successful application of glass micromodel techniques for studying enhanced oil recovery (EOR) processes in five-spot pattern and provides a useful reference for understanding the displacement behaviors in a typical polymer flood process.     

Mechanical study of the effect of fractional-wettability on multiphase fluid flow

Fractional wettability has been widely recognized in most of the oil reservoirs and it is a crucial factor that controls the fluid flow behavior in porous medium. The overall effect of the proportion of oil-wet grains on the fluid flow properties has been well discussed. However, recent studies found that the random distribution and coordination of oil-wet and water-wet grains could make multi-phase flow behaviors extremely complicated in such media. The multiphase flow mechanisms in fractional wettability media remains unclear. In this study, oil imbibition experiments were systematically conducted using glass cylinders packed with fractional-wet glass beads. To study the effect of fractional wettability on multiple-phase flow properties, samples with different oil-wet grain proportions were prepared, and fifteen repeated experiments were conducted for each oil-wet proportion. The experimental results showed that oil imbibition was largely dependent on but not strictly a function of the proportion of oil-wet grains in the medium. The imbibition behaviors of samples with the same fractional proportion could vary significantly, as some samples exhibited complete oil migration, while others did not. This probabilistic phenomenon is likely due to the random distribution of oil-wet and water-wet grains. A pore throat may behave as oil-wet or water-wet depending on the relative proportion of oil-wet grains the pore throat contains. When the grains that comprise the pore throat are dominated by oil-wet grains, the throat behaves as oil-wet, and vice versa. Only when these oil-wet pore throats are connected to form a complete oil-wet pathway throughout the medium can the oil continuously imbibe into the medium. Therefore, the extent of oil imbibition depends on the completeness of the oil-wet pathway, which is controlled by the proportion of oil-wet grains in the medium. The higher the proportion of oil-wet grains in the medium, the larger the number of oil-wet pore throats that can form; thus, the higher the possibility that those oil-wet pore throats can connect to form continuous oil-wet pathways.     

Displacement mechanisms of enhanced heavy oil recovery by alkaline flooding in a micromodel

Enhanced oil recovery (EOR) by alkaline flooding for conventional oils has been extensively studied. For heavy oils, investigations are very limited due to the unfavorable mobility ratio between the water and oil phases. In this study, the displacement mechanisms of alkaline flooding for heavy oil EOR are investigated by conducting flood tests in a micromodel. Two different displacement mechanisms are observed for enhancing heavy oil recovery. One is in situ water-in-oil (W/O) emulsion formation and partial wettability alteration. The W/O emulsion formed during the injection of alkaline solution plugs high permeability water channels, and pore walls are altered to become partially oil-wetted, leading to an improvement in sweep efficiency and high tertiary oil recovery. The other mechanism is the formation of an oil-in-water (O/W) emulsion. Heavy oil is dispersed into the water phase by injecting an alkaline solution containing a very dilute surfactant. The oil is then entrained in the water phase and flows out of the model with the water phase.     

Network Modeling of EOR Processes: A Combined Invasion Percolation and Dynamic Model for Mobilization of Trapped Oil

A novel concept for modeling pore-scale phenomena included in several enhanced oil recovery (EOR) methods is presented. The approach combines a quasi-static invasion percolation model with a single-phase dynamic transport model in order to integrate mechanistic chemical oil mobilization methods. A framework is proposed that incorporates mobilization of capillary trapped oil. We show how double displacement of reservoir fluids can contribute to mobilize oil that are capillary trapped after waterflooding. In particular, we elaborate how the physics of colloidal dispersion gels (CDG) or linked polymer solutions (LPS) is implemented. The linked polymer solutions consist of low concentration partially hydrolyzed polyacrylamide polymer crosslinked with aluminum citrate. Laboratory core floods have shown demonstrated increased oil recovery by injection of linked polymer solution systems. LPS consist of roughly spherical particles with sizes in the nanometer range (50–150 nm). The LPS process involve mechanisms such as change in rheological properties effect, adsorption and entrapment processes that can lead to a microscopic diversion and mobilization of waterflood trapped oil. The purpose is to model the physical processes occurring on pore scale during injection of linked polymer solutions. A sensitivity study has also been performed on trapped oil saturation with respect to wettability status to analyze the efficiency of LPS on different wettability conditions. The network modeling results suggest that weakly wet reservoirs are more suitable candidates for performing linked polymer solution injection.     

Coreflooding Studies to Investigate the Potential of Carbonated Water Injection as an Injection Strategy for Improved Oil Recovery and CO<Subscript>2</Subscript> Storage

Carbonated water injection (CWI) is a CO₂-augmented water injection strategy that leads to increased oil recovery with added advantage of safe storage of CO₂ in oil reservoirs. In CWI, CO₂ is used efficiently (compared to conventional CO₂ injection) and hence it is particularly attractive for reservoirs with limited access to large quantities of CO₂, e.g. offshore reservoirs or reservoirs far from large sources of CO₂. We present the results of a series of CWI coreflood experiments using water-wet and mixed-wet Clashach sandstone cores and a reservoir core with light oil (n-decane), refined viscous oil and a stock-tank crude oil. The experiments were carried out to assess the performance of CWI and to quantify the level of additional oil recovery and CO₂ storage under various experimental conditions. We show that the ultimate oil recovery by CWI is higher than the conventional water flooding in both secondary and tertiary recovery methods. Oil swelling as a result of CO₂ diffusion into the oil and the subsequent oil viscosity reduction and coalescence of the isolated oil ganglia are amongst the main mechanisms of oil recovery by CWI that were observed through the visualisation experiments in high-pressure glass micromodels. There was also evidence of a change in the rock wettability that could also influence the oil recovery. The coreflood test results also reveal that the CWI performance is influenced by oil viscosity, core wettability and the brine salinity. Higher oil recovery was obtained with the mixed-wet core than the water-wet core, with light oil than with the viscous oil and low salinity carbonated brine than high-salinity carbonated brine. At the end of the flooding period, an encouraging amount of the injected CO₂ was stored in the brine and the remaining oil in the form of stable dissolved CO₂. The experimental results clearly demonstrate the potential of CWI for improving oil recovery as compared with the conventional water flooding (secondary recovery) or as a water-based EOR (enhanced oil recovery) method for watered out reservoirs.     

Microscopic Roles of “Viscoelasticity” in HPMA polymer flooding for EOR

A polymer solution with a transient network structure due to the entanglement of long chain molecules exhibits a viscoelastic behavior when it flows through a tortuous and diverging/converging channel in porous media. A constitutive equation is first developed to represent the viscoelastic behavior of polymer solutions in this article. Then a 3D viscoelastic polymer flooding model is established to examine the effect of elasticity of polymers on EOR (enhanced oil recovery). The model is validated in comparison with laboratorial coring data. The simulated results show that the oil recovery of viscoelastic polymer flooding can be enhanced by larger displacement efficiency due to its microscopic roles. In the meanwhile, the injection pressure required increases correspondingly if the elastic effect is significant. Relaxation time as a major characteristic parameter of viscoelastic polymer plays a decisive role, and therefore the HPAM (partially hydrolyzed polyacrylamide) with evident elastic property is recommended in chemical flooding.     

Pore-Scale Imaging and Analysis of Wettability Order,Trapping and Displacement in Three-Phase Flow in Porous Media with Various Wettabilities

Three-phase flow in porous media is encountered in many applications including subsurface carbon dioxide storage, enhanced oil recovery, groundwater remediation and the design of microfluidic devices. However, the pore-scale physics that controls three-phase flow under capillary dominated conditions is still not fully understood. Recent advances in three-dimensional pore-scale imaging have provided new insights into three-phase flow. Based on these findings, this paper describes the key pore-scale processes that control flow and trapping in a three-phase system, namely wettability order, spreading and wetting layers, and double/multiple displacement events. We show that in a porous medium containing water, oil and gas, the behaviour is controlled by wettability, which can either be water-wet, weakly oil-wet or strongly oil-wet, and by gas–oil miscibility. We provide evidence that, for the same wettability state, the three-phase pore-scale events are different under near-miscible conditions—where the gas–oil interfacial tension is ≤?1 mN/m—compared to immiscible conditions. In a water-wet system, at immiscible conditions, water is the most-wetting phase residing in the corners of the pore space, gas is the most non-wetting phase occupying the centres, while oil is the intermediate-wet phase spreading in layers sandwiched between water and gas. This fluid configuration allows for double capillary trapping, which can result in more gas trapping than for two-phase flow. At near-miscible conditions, oil and gas appear to become neutrally wetting to each other, preventing oil from spreading in layers; instead, gas and oil compete to occupy the centre of the larger pores, while water remains connected in wetting layers in the corners. This allows for the rapid production of oil since it is no longer confined to movement in thin layers. In a weakly oil-wet system, at immiscible conditions, the wettability order is oil–water–gas, from most to least wetting, promoting capillary trapping of gas in the pore centres by oil and water during water-alternating-gas injection. This wettability order is altered under near-miscible conditions as gas becomes the intermediate-wet phase, spreading in layers between water in the centres and oil in the corners. This fluid configuration allows for a high oil recovery factor while restricting gas flow in the reservoir. Moreover, we show evidence of the predicted, but hitherto not reported, wettability order in strongly oil-wet systems at immiscible conditions, oil–gas–water, from most to least wetting. At these conditions, gas progresses through the pore space in disconnected clusters by double and multiple displacements; therefore, the injection of large amounts of water to disconnect the gas phase is unnecessary. We place the analysis in a practical context by discussing implications for carbon dioxide storage combined with enhanced oil recovery before suggesting topics for future work.

     

The Effect of Reservoir Wettability on the Production Characteristics of the VAPEX Process: An Experimental Study

The impact of fractional wettability on the production characteristics of a VAPEX process at the macroscale was investigated. Conventional VAPEX experiments were conducted in a 220 Darcy random packing of glass beads in a rectangular physical model and n-pentane was used to recover the Cold Lake bitumen from the oil-saturated model in the absence of connate water. The composition of oil-wet beads in the packed bed was altered from completely water-wet beads to completely oil-wet beads at different proportions of oil-wet beads mixed with water-wet beads. A substantial increase (about 40%) in the production rate of live oil was observed during the VAPEX process when the wettability of the porous packing was entirely oil-wet beads. A critical oil-wet fraction of 0.66 was found for the heterogeneous packing of water-wet and oil-wet beads of similar size distribution. Above this critical composition, the live oil production rate was not affected by further increase in the proportion of the oil-wet beads. It is believed that above this critical composition of the oil-wet beads, the crevice flow process is dominated by the continuity of higher conductivity live oil films between particles through the oil-wet regions. Below this critical composition, the live oil production rate increased linearly with the fraction of the oil-wet beads in the packing. The oil-wet regions favor the live oil drainage compared to that of the water-wet regions as they enhance the rate of imbibition of the live oil from the oil-filled pores to the vacated pores near the nominal VAPEX interface. These two factors enhance the live oil production rate during the VAPEX process. The solvent content of the live oil, the solvent-to-oil ratio (SOR), and the residual oil saturation did not correlate strongly with the proportion of the oil-wet beads in the packing. The average solvent content of the live oil and the residual oil saturation were measured to be 48% by weight and 7% by volume respectively.     

A Parametric Model for Predicting Relative Permeability-Saturation-Capillary Pressure Relationships of Oil–Water Systems in Porous Media with Mixed Wettability

A parametric two-phase, oil–water relative permeability/capillary pressure model for petroleum engineering and environmental applications is developed for porous media in which the smaller pores are strongly water-wet and the larger pores tend to be intermediate- or oil-wet. A saturation index, which can vary from 0 to 1, is used to distinguish those pores that are strongly water-wet from those that have intermediate- or oil-wet characteristics. The capillary pressure submodel is capable of describing main-drainage and hysteretic saturation-path saturations for positive and negative oil–water capillary pressures. At high oil–water capillary pressures, an asymptote is approached as the water saturation approaches the residual water saturation. At low oil–water capillary pressures (i.e. negative), another asymptote is approached as the oil saturation approaches the residual oil saturation. Hysteresis in capillary pressure relations, including water entrapment, is modeled. Relative permeabilities are predicted using parameters that describe main-drainage capillary pressure relations and accounting for how water and oil are distributed throughout the pore spaces of a porous medium with mixed wettability. The capillary pressure submodel is tested against published experimental data, and an example of how to use the relative permeability/capillary pressure model for a hypothetical saturation-path scenario involving several imbibition and drainage paths is given. Features of the model are also explained. Results suggest that the proposed model is capable of predicting relative permeability/capillary pressure characteristics of porous media mixed wettability.     

Dissipative particle dynamics simulation of wettability alternation phenomena in the chemical flooding process

Wettability alternation phenomena is considered one of the most important enhanced oil recovery （EOR） mechanisms in the chemical flooding process and induced by the adsorption of surfactant on the rock surface. These phenomena are studied by a mesoscopic method named as dissipative particle dynamics （DPD）. Both the alteration phenomena of water-wet to oil-wet and that of oil-wet to water-wet are simulated based on reasonable definition of interaction parameters between beads. The wetting hysteresis phenomenon and the process of oil-drops detachment from rock surfaces with different wettability are simulated by adding long-range external forces on the fluid particles. The simulation results show that, the oil drop is liable to spread on the oil-wetting surface and move in the form of liquid film flow, whereas it is likely to move as a whole on the waterwetting surface. There are the same phenomena occuring in wettability-alternated cases. The results also show that DPD method provides a feasible approach to the problems of seepage flow with physicochemical phenomena and can be used to study the mechanism of EOR of chemical flooding.     

Characterizing the Role of Shale Geometry and Connate Water Saturation on Performance of Polymer Flooding in Heavy Oil Reservoirs: Experimental Observations and Numerical Simulations

Many heavy oil reservoirs contain discontinuous shales which act as barriers or baffles to flow. However, there is a lack of fundamental understanding about how the shale geometrical characteristics affect the reservoir performance, especially during polymer flooding of heavy oils. In this study, a series of polymer injection processes have been performed on five-spot glass micromodels with different shale geometrical characteristics that are initially saturated with the heavy oil. The available geological characteristics from one of the Iranian oilfields were considered for the construction of the flow patterns by using a controlled-laser technology. Oil recoveries as a function of pore volumes of injected fluid were determined from analysis of continuously recorded images during the experiments. We observed a clear bypassing of displacing fluid which results in premature breakthrough of injected fluid due to the shale streaks. Moreover, the results showed a decrease of oil recovery when shales’ orientation, length, spacing, distance of the shale from production well, and density of shales increased. In contrast, an increase of shale discontinuity or distance of the shale streak from the injection well increased oil recovery. The obtained experimental data have also been used for developing and validating a numerical model where good matching performance has been observed between our experimental observations and simulation results. Finally, the role of connate water saturation during polymer flooding in systems containing flow barriers has been illustrated using pore level visualizations. The microscopic observations confirmed that besides the effect of shale streaks as heterogeneity in porous medium, when connate water is present, the trapped water demonstrates another source of disturbance and causes additional perturbations to the displacement interface leading to more irregular fingering patterns especially behind the shale streaks and also causes a reduction of ultimate oil recovery. This study reveals the application of glass micromodel experiments for studying the effects of barriers on oil recovery and flow patterns during EOR processes and also may provide a set of benchmark data for recovery of oil by immiscible polymer flood around discontinuous shales.     

Effect of Organosilicon-Acrylic Emulsion Treatment on Wettability of Porous Media

To investigate the influence of the organosilicon-acrylic on wetting properties of porous media, contact angle tests were performed on two different sandstones. In addition, the effectiveness of the emulsion on wettability alteration of porous media was validated by capillary rise and spontaneous imbibition tests. The results of wettability tests showed that the wettability of two sandstones was altered from water-wet to gas-wet after treatment with the emulsion. The principle that the critical radius of pore throats and wettability of porous media affect liquids flow was derived analytically and verified experimentally. Coreflood results demonstrated that the latex resulted in increasing the water permeability through altering the rock wettability to gas-wetting, then decreasing the friction drag between liquids and rocks surface. Thereby, the emulsion treatment could increase the flowback rate of trapped liquids. Experimental results were in good agreement with the theoretical analysis. In conclusion, all results indicated that the emulsion could alter the wettability from water-wet to intermediate gas-wet and enhance water permeability in porous media. It was extrapolated that the emulsion had the tremendous potential to be applied in field conditions, enhancing gas productivity through the cleanup of trapped water in the vicinity of the wellbore.     

EM Heating-Stimulated Water Flooding for Medium–Heavy Oil Recovery

We report a study of heavy oil recovery by combined water flooding and electromagnetic (EM) heating at a frequency of 2.45 GHz used in domestic microwave ovens. A mathematical model describing this process was developed. Model equations were solved, and the solution is presented in an integral form for the one-dimensional case. Experiments consisting of water injection into Bentheimer sandstone cores, either fully water saturated or containing a model heavy oil, were also conducted, with and without EM heating. Model prediction was found to be in rather good agreement with experiments. EM energy was efficiently absorbed by water and, under dynamic conditions, was transported deep into the porous medium. The amount of EM energy absorbed increases with water saturation. Oil recovery by water flooding combined with EM heating was up to \(37.0\%\) larger than for cold water flooding. These observations indicate that EM heating induces an overall improvement in the mobility ratio between the displacing water and the displaced heavy oil.     

Numerical simulation of pore-scale flow in chemical flooding process

Chemical flooding is one of the effective technologies to increase oil recovery of petroleum reservoirs after water flooding. Above the scale of representative elementary volume (REV), phenomenological modeling and numerical simulations of chemical flooding have been reported in literatures, but the studies alike are rarely conducted at the pore-scale, at which the effects of physicochemical hydrodynamics are hardly resolved either by experimental observations or by traditional continuum-based simulations. In this paper, dissipative particle dynamics (DPD), one of mesoscopic fluid particle methods, is introduced to simulate the pore-scale flow in chemical flooding processes. The theoretical background, mathematical formulation and numerical approach of DPD are presented. The plane Poiseuille flow is used to illustrate the accuracy of the DPD simulation, and then the processes of polymer flooding through an oil-wet throat and a water-wet throat are studies, respectively. The selected parameters of those simulations are given in details. These preliminary results show the potential of this novel method for modeling the physicochemical hydrodynamics at the pore scale in the area of chemical enhanced oil recovery.     

非牛顿流体在非均质油藏渗流压力场实验

在非均质油藏模型上进行非牛顿流体流动物理模拟实验，对比研究水驱、聚合物驱和交联聚合物对提高石油采收率的影响．通过布置高精度的压差传感器测量不同驱替过程模型中的渗流压力场的动态变化，成胶后的交联聚合物封堵了高渗条区，改变了油藏内流体流动方向，驱替出低渗区内油，提高了采收率．     

Microscopic Mechanisms of Oil Recovery By Near-Miscible Gas Injection

As gas flooding becomes a more viable means of enhanced oil recovery, it is important to identify and understand the pore-scale flow mechanisms, both for the development of improved gas flooding applications and for the predicting phase mobilisation under secondary and tertiary gas flooding. The purpose of this study was to visually investigate the pore-level mechanisms of oil recovery by near-miscible secondary and tertiary gas floods. High-pressure glass micromodels and model fluids representing a near-miscible fluid system were used for the flow experiments. A new pore-scale recovery mechanism was identified which significantly contributed to oil recovery through enhanced flow and cross-flow between the bypassed pores and the injected gas. This mechanism is strongly related to a very low gas/oil interfacial tension (IFT), perfect wetting conditions and simultaneous flow of gas and oil in the same pore, all of which occur as the gas/oil critical point is approached. The results of this study helps us to better understand the pore-scale mechanisms of oil recovery in very low-IFT (near-miscible) systems. In particular we show that in near-miscible gas floods, behind the main gas front, the recovery of the oil continues by cross-flow from the bypassed pores into the main flow stream and as a result almost all of the oil, which has been contacted by the gas, could be recovered. Our observations in high-pressure micromodel experiments have demonstrated that this mechanism can only occur in near-miscible processes (as opposed to immiscible and completely miscible processes), which makes oil displacement by near-miscible gas floods a very effective process.     

Analytical and Experimental Study to Predict the Residual Resistance Factor on Polymer Flooding Process in Fractured Medium

The major objectives of this study are to analytically and experimentally determine the residual resistance factor in the fractured medium based on the polymer solution properties and operational conditions. The parameters considered in this study are the polymer concentration, power law constitutive equation parameter, and salt concentration, sulfonation content of polymer, temperature, and molecular weight of the water soluble polymers which are used in polymer flooding for enhanced oil recovery. The results indicated that residual resistance factor in fractured medium is dependent on the coil overlap parameter and power law equation parameter of polymer. The coil overlap parameter is a dimensionless number consists of intrinsic viscosity and polymer concentration. Since intrinsic viscosity is a function of polymer diameter in medium conditions, to predict the residual resistance factor in fracture medium, an experimental correlation is generated for determination of the molecular diameter of polymer based on polymer molecular weight, temperature, salt concentration, and sulfonation content.     

利用三维网络模型研究滞留聚合物微观分布规律

再利用地下滞留聚合物是聚合物驱之后进一步提高原油采收率的新途径。基于油水两相流网络模拟模型,综合考虑聚合物渗流机理建立起聚合物驱微观模拟模型。用微观数值模拟手段研究了地下滞留聚合物分布规律及影响因素,为有效地开采聚合物驱后剩余油、合理利用滞留地下聚合物提供必要的依据。模拟结果表明,由于吸附和捕集作用将引起大量聚合物滞留在孔喉中,滞留聚合物占注入聚合物的61.7%。总体上看,大孔喉中聚合物滞留量较大,但滞留聚合物浓度较小。孔喉半径和形状因子为聚合物滞留的主要影响因素,孔喉滞留聚合物浓度与孔喉半径和形状因子平方根的乘积成反比。     

Fundamental Study of Pore Scale Mechanisms in Microbial Improved Oil Recovery Processes

A fundamental study of microscopic mechanisms and pore-level phenomena in the Microbial Improved Oil Recovery method has been investigated. Understanding active mechanisms to increase oil recovery is the key to predict and plan MIOR projects successfully. This article presents the results of visualization experiments carried out in a transparent pore network model. In order to study the pore scale behavior of bacteria, dodecane and an alkane oxidizing bacterium, Rhodococcus sp. 094, suspended in brine, are examined for evaluating the performance of bacterial flooding in the glass micromodel. The observations show the effects of bacteria on remaining oil saturation, allowing us to get better insight on the mechanisms. Bacterial mass composed of bacteria and bioproducts growth in the fluid interfaces and pore walls have been recorded and are presented. No gas is observed throughout any of the experiments. The biomass blocks some pores and pore-throats, and thereby changing the flow pattern. As a consequent, the flow pattern change together with the previously proposed mechanisms, including the interfacial tension reduction and wettability changes are recognized as active mechanisms in the MIOR process.