Immiscible viscous fingering in porous media occurs when a low viscosity fluid displaces a significantly more viscous, immiscible resident fluid; for example, the displacement of a higher viscosity oil with water (whereμ_o > > μ_w). Classically, this is a significant issue during oil recovery processes, where water is injected into the reservoir to provide pressure support and to drive the oil production. In moderate/heavy oil, this leads to the formation of strong water fingers, bypassed oil and high/early water production. Polymer flooding, where the injected water is viscosified through addition of high molecular weight polymers, has often been applied to reduce the viscosity contrast between the two immiscible fluids. In recent years, there has been significant development in the understanding of both the mechanism by which polymer flooding improves viscous oil recovery, as well as in the methodologies available to directly simulate such processes. One key advance in modelling the correct mechanism of polymer oil recovery in viscous oils has been the development of a method to accurately model the “simple” two-phase immiscible fingering (Sorbie in Transp Porous Media 135:331–359, 2020). This was achieved by first choosing the correct fractional flow and then deriving the maximum mobility relative permeability functions from this. It has been proposed that central to the polymer oil recovery is a fingering/viscous crossflow mechanism, and a summary of this is given in this paper. This work seeks to validate the proposed immiscible fingering/viscous crossflow mechanism experimentally for a moderately viscous oil (μ_o = 84 mPa.s at 31 °C;μ_w = 0.81 mPa.s; thus, (μ_o/μ_w) ~ 104) by performing a series of carefully monitored core floods. The results from these experiments are simulated directly to establish the potential of our modified simulation approach to capture the process (Sorbie, et al., 2020). Both secondary and tertiary polymer flooding experiments are presented and compared with the waterflood baselines, which have been established for each core system. The oil production, water cut and differential pressure are then matched directly using a commercial numerical reservoir simulator, but using our new “fractional flow” derived relative permeabilities. The use of polymer flooding, even when applied at a high water cut (80% after 0.5 PV of water injection), showed a significant impact on recovery; bringing the recovery significantly forward in time for both tertiary and secondary polymer injection modes—a further 13–16% OOIP. Each flood was then directly matched in the simulator with excellent agreement in all experimental cases. The simulations allowed a quantitative visualisation of the immiscible finger propagation from both water injection and the banking of connate water during polymer flooding. Evidence of a strong oil bank forming in front of the tertiary polymer slug was also observed, in line with the proposed viscous crossflow mechanism. This work provides validation of both polymer flooding’s viscous crossflow mechanism and the direct simulation methodology proposed by Sorbie et al. (Transp Porous Media 135:331–359, 2020). The experimental results show the significant potential for both secondary and tertiary polymer flooding in moderate/heavy oil reservoirs.

Immiscible viscous fingering in porous media occurs when a high viscosity fluid is displaced by an immiscible low viscosity fluid. This paper extends a recent development in the modelling of immiscible viscous fingering to directly simulate experimental floods where the viscosity of the aqueous displacing fluid was increased (by the addition of aqueous polymer) after a period of low viscosity water injection. This is referred to as tertiary polymer flooding, and the objective of this process is to increase the displacement of oil from the system. Experimental results from the literature showed the very surprising observation that the tertiary injection of a modest polymer viscosity could give astonishingly high incremental oil recoveries (IR) of ≥100% even for viscous oils of 7000 mPa.s. This work seeks to both explain and predict these results using recent modelling developments. For the 4 cases (µ_o/µ_w of 474 to 7000) simulated in this paper, finger patterns are in line with those observed using X-ray imaging of the sandstone slab floods. In particular, the formation of an oil bank on tertiary polymer injection is very well reproduced and the incremental oil response and water cut drops induced by the polymer are very well predicted. The simulations strongly support our earlier claim that this increase in incremental oil displacement cannot be explained solely by a viscous “extended Buckley-Leverett” (BL) linear displacement effect; referred to in the literature simply as “mobility control”. This large response is the combination of this effect (BL) along with a viscous crossflow (VX) mechanism, with the latter VX effect being the major contributor to the recovery mechanism.

Summary: Immiscible fingering in reservoirs results from the displacement of a resident high-viscosity oil by a significantly less viscous immiscible fluid, usually water. During oil recovery processes, where water is often injected for sweep improvement and pressure support, the viscosity ratio between oil and water (μo/μw) can lead to poor oil recovery due to the formation of immiscible viscous fingers resulting in oil bypassing. Polymer flooding, where the injection water is viscosified by the addition of high-molecular-weight polymers, is designed to reduce the impact of viscous fingering by reducing the μ0/μw ratio. A considerable effort has been made in the past decade to improve the mechanistic understanding of polymer flooding as well as in developing the numerical simulation methodologies required to model it reliably. Two key developments have been (i) the understanding of the viscous crossflow mechanism by which polymer flooding operates in the displacement of viscous oil and (ii) the simulation methodology put forward by Sorbie et al. (2020), whereby immiscible fingering and viscous crossflow can be simply matched in conventional reservoir simulators. This publication extends the work of Beteta et al. (2022b) to conceptual models of a field case currently undergoing polymer flooding—the Captain field in the North Sea. The simulation methodology is essentially “upscaled” in a straightforward manner using some simple scaling assumptions. The effects of polymer viscosity and slug size are considered in a range of both 2D and 3D models designed to elucidate the role of polymer in systems both with and without “water slumping.” Slumping is governed by the density contrast between oil and water, the vertical communication of the reservoir and the fluid velocity, and, when it occurs, the injection of water channels along the bottom of the reservoir directly to the production well(s). It is shown that polymer flooding is very applicable to a wide range of reservoirs, with only modest injection viscosities and bank sizes returning significant volumes of incremental oil. Indeed, oil incremental recoveries (IRs) of between 29% and 89% are predicted in the simulations of the various 2D and 3D cases, depending on the slug design for both nonslumping and slumping cases. When strong water slumping is present, the performance of the polymer flood is significantly more sensitive to slug design, as alongside the viscous crossflow mechanism of recovery, a further role of the polymer is introduced—sweep of the “attic” oil by the viscous polymer flood, which is able to overcome the gravity-driven slumping, and we also identify this mechanism as a slightly different form of viscous crossflow. In slumping systems, it is critical to avoid disrupting the polymer bank before sweeping of the attic oil has been performed. However, as with the nonslumping system, modest injection viscosities and bank sizes still have a very significant impact on recovery. The conceptual models used here have been found to be qualitatively very similar to real field results. Our simulations indicate that there are few cases of viscous oil recovery where polymer flooding would not be of benefit.

Polymer flooding is a mature EOR technology that has seen an increasing interest over the past decade. Co-polymers of Acrylamide (AMD) and Acrylic Acid (AA) have been the most prominent chemicals to be applied, whereas sulfonated polymers containing 2-Acrylamido-tertiary-butyl sulfonic acid (ATBS) have been used for higher temperature and/or salinity conditions. The objective of this study was to generate guidelines to aid in the selection of appropriate polyacrylamide chemistry for each field case. Our main focus was in sandstone fields operating at the upper end of AMD-AA temperature tolerance, where it needs to be decided whether sulfonation is required. The performance of the polymer throughout the whole residence time in the reservoir was considered since the macromolecule can undergo some changes over this period. Several key properties of nine distinct polymer species were investigated. The polymers consisted of AMD-AA co-polymers, AMD-ATBS co-polymers and AMD-AA-ATBS ter-polymers. The polymers were studied both in their original state as they would be during the injection (initial viscosity, initial adsorption and in-situ rheology) as well as in the state which they are expected to be in after the polymer has aged in the reservoir (i.e. in a different state of hydrolysis and corresponding viscosity retention and adsorption after ageing for various time periods). We note that the combination of viscosity retention and adsorption during the in-situ ageing process has not been typically investigated in previous literature, and this is a key novel feature of this work. Each of the above parameters has an impact on the effectiveness and the economic efficiency of a polymer flooding project. The content of ATBS was limited to 15 mol%. Buff Berea sandstone was applied in the static and dynamic adsorption experiments. The majority of the work was carried out in seawater at temperature, T = 58°C. Under these conditions AMD-AA samples showed maximum viscosity and lowest adsorption when the content of AA was moderate (20 mol%). When the AMD-AA polymers were aged at elevated temperature, the AA content steadily increased due to hydrolysis reactions. When the AA content was 30 mol% or higher, the viscosity started to decrease, and adsorption started to increase as the polymer was aged further. Thermal stability improved when ATBS was included in the polymer structure. In addition, sulfonated polyacrylamide samples showed increasing initial viscosity yields and decreasing initial adsorption with increasing ATBS content. For most of the samples, the maximum observed apparent in-situ viscosity increased when the bulk viscosity and relaxation time of the sample solution increased. The information generated in this study can be used to aid in the selection of the most optimal polyacrylamide chemistry for sandstone fields operating with moderate/high salinity brines at the upper end of AMD-AA temperature tolerance.

In this work, we present a Darcy-scale study of immiscible unstable displacement in large (15 × 15 × 1 cm3) slabs of Bentheimer sandstone. Oil is displaced by water under two wetting conditions, a weakly water-wet system and a weakly oil-wet system (at two initial water saturations), with a viscosity ratio of μo/μw = 100. The system is directly visualized using x rays and the fluid distribution monitored throughout the displacement. Following a period of water flooding, the injection water is viscosified with glycerol to act as a pseudo tertiary polymer flood. The experiments are then modeled directly using numerical simulation applying a recently developed fractional flow approach, which has been shown to reproduce experimental immiscible fingering, before upscaling the results to the reservoir scale. From the observed results, we can conclude that typical wettability modification using crude oil aging may generate a wettability similar to that found in oil reservoirs, but this may still result in an exaggerated capillary dampening of the viscous fingers. However, as the system is scaled up in size to the field dimensions, viscous forces will dominate, and fingering will re-emerge due to the increase in the viscous/capillary force balance. But if the input relative permeabilities are constructed from an excessively capillary stabilized experiment, the shock front mobility of the fractional flow will still overestimate the stability of the system. Indeed, we derive the same water saturation above Swi for the three floods, but with different shock front mobilities.

In the design and optimization of an underground hydrogen (H2) storage facility in an aquifer or other reservoir system, the H2/water relative permeabilities (RP) are the most critical two-phase data for input to numerical simulation. In this paper, we present a critical analysis of the published experimental H2/water RP functions in the literature. We present fine-grid simulations of H2 displacing water (denoted H2 --> water) using three of the most widely cited steady-state RP datasets (13, 20, 38) in a mildly heterogeneous permeability field, at a field length scale of ~100m. Since the viscosity ratio between water and H2 is (µw/µH2) ≈ 70, then at some length scale above a few meters, it is inevitable that the system must show immiscible viscous fingering of the H2 into the water phase. Indeed, the emergence of viscous fingering at some length scale is a “sense check” that the input data used in the simulations are correct, especially the H2/water relative permeability functions. In fact, none of the three published H2/water RP curves leads to viscous fingering. Instead, they all show stabilized flood fronts. The reasons for this are due to shortcomings of the (conventional) gas/liquid experimental methods used to obtain the RP functions. These methods yield RP functions at the wrong force balance between the capillary, gravity and viscous forces. For fingering to emerge, it is necessary to derive the viscous dominated RP functions. An alternative, more physically appropriate set of viscous dominated “fingering RP functions” is proposed and applied. When applied at the core scale, these new RP functions show fully dispersed flow, as they do when applied in a vertical (downwards) gravity stable displacement. However, the viscous fingering emerges naturally in horizontal flow as the length scale of the system increases and viscous forces become dominant.