Mechanistic Modelling of Radial Polymer Flow in Porous Media

Polymer flooding is one of the most successful chemically enhanced oil recovery (EOR) methods, and has primarily been implemented to accelerate oil production by sweep improvement. However, research from the last couple of decades have identified several additional benefits associated with polymer flooding. Firstly, improved polymer properties have extended their use in reservoirs with high temperature and high salinity. Secondly, improved understanding of the viscoelastic flow behavior of flexible polymers have revealed that they may in some cases mobilize capillary trapped oil as well. Despite of the recent progress, extensive research remains to quantify the appropriate flow mechanisms and accurately describe polymer flow in porous media. Simulations and history match operations performed in this thesis are aimed at improving the modelling of radial polymer flow in porous media. This has been achieved by (1) evaluating the accuracy and robustness of two different history match methods which are used to estimate the in-situ rheology of non-Newtonian fluids in radial flow, (2) investigating potential rate and memory effects (at the Darcy scale) of viscoelastic polymer solutions in radial flow, and (3) quantifying polymer in-situ rheology and polymer injectivity. The accuracy and robustness of both history match methods which are used to estimate the in-situ rheology of non-Newtonian fluids in radial flow was clearly demonstrated in radial flow experiments where effective (or cumulative) error was below 5 % of the maximum preset transducer pressure range. Thereby, the observed shear-thinning behavior of partially hydrolyzed polyacrylamide (HPAM) at low flux in porous media could not be attributed to insufficiently accurate pressure transducers during in-house flow experiments, as suggested by some researchers. The estimation of polymer in-situ rheology showed invariance between excluding and including the polymer pressure data outside the near-wellbore region. Thus, it was proposed that the polymer in-situ rheology is mainly defined by the pressure data originating from the near-wellbore region during radial polymer flow. Results showed that not only could the polymer in-situ rheology be (quantitatively) estimated from measurements of stabilized pressure, but could also be (qualitatively) identified from the pressure build-up during radial flow experiments. Consequently, the anchoring data from the pressure build-up during radial polymer flow was proposed as an additional tool for history matching field injectivity tests. Rate and memory effects (at the Darcy scale) of several HPAM polymers were investigated in flow through Bentheimer sandstone discs. Results showed that no rate effects occurred for mechanically undegraded polymer (Flopaam 3330S). However, rate effects were observed for mechanically degraded polymers (Flopaam 3630S and Flopaam 5115SH) where the onset of shear-thickening increased with volumetric injection rate. While memory effects (at the Darcy scale) were absent for the mechanically undegraded and relatively low molecular weight polymer, Flopaam 3330S, the mechanically degraded and relatively higher molecular weight (18 MDa) polymer, Flopaam 3630S, exhibited memory effects in which apparent viscosity decreased with radial distance. As mechanical degradation is suggested to be confined to the near-wellbore region in radial polymer flow, the memory effect was proposed to originate from the elastic properties of the polymer. In accordance with recent literature, the in-situ rheology of HPAM was shown to depend on flow geometry. During single and two-phase polymer flow, the shear-thinning behavior of HPAM was much more pronounced, and the extent of shear-thickening significantly reduced in radial compared to linear flow. Furthermore, the onset of shear-thickening during single-phase flow occurred at significantly higher velocities in radial relative to linear flow. However, this behavior was not consistent during two-phase flow as the onset of shear-thickening during linear and radial polymer flow coincided. Moreover, comparative studies of polymer flow in radial versus linear flow geometries during single and two-phase flow revealed that the impact of oil was to reduce apparent in-situ viscosity of HPAM. The low-flux in-situ rheology behavior was addressed and showed Newtonian behavior in linear flow while significant shear-thinning was observed during radial flow. Thus, both flow geometry and presence of oil were suggested to be key factors for estimating polymer in-situ rheology

Polymer Injectivity Test Design Using Numerical Simulation

Polymer flooding is an enhanced oil recovery (EOR) process, which has received increasing interest in the industry. In this process, water-soluble polymers are used to increase injected water viscosity in order to improve mobility ratio and hence improve reservoir sweep. Polymer solutions are non-Newtonian fluids, i.e., their viscosities are shear dependent. Polymers may exhibit an increase in viscosity at high shear rates in porous media, which can cause injectivity loss. In contrast, at low shear rates they may observe viscosity loss and hence enhance the injectivity. Therefore, due to the complex non-Newtonian rheology of polymers, it is necessary to optimize the design of polymer injectivity tests in order to improve our understanding of the rheology behavior and enhance the design of polymer flood projects. This study has been addressing what information that can be gained from polymer injectivity tests, and how to design the test for maximizing information. The main source of information in the field is from the injection bottom-hole pressure (BHP). Simulation studies have analyzed the response of different non-Newtonian rheology on BHP with variations of rate and time. The results have shown that BHP from injectivity tests can be used to detect in-situ polymer rheology.publishedVersio

Influence of Residual Oil on Polymer Injectivity

Because a significant fraction of the world’s light oil reservoirs has already been produced, the technical and economic challenges of developing heavy oil fields is now unavoidable [1]. Water flooding has proven to be a less than adequate recovery mechanism for heavy oil fields, due to the unfavorable mobility ratio between water and oil, resulting in poor volumetric sweep efficiencies and low ultimate recovery factors. Consequently, reservoirs containing viscous crudes have reported recoveries of less than 20% when utilizing water flooding as the recovery mechanism. Therefore, it is evident that there exists a huge potential for additional recovery by implementing improved oil recovery (IOR) projects, especially by employing enhanced oil recovery (EOR) methods. Polymer flooding has gained increased attention during the last decades. Several field scale pilot projects have been implemented in heavy oil reservoirs with a varying degree of success [2] [3]. During polymer flooding, polymers are added to the injection brine to impart a viscosity increase of the corresponding polymer solution. This will increase the mobility of the drive fluid, and the mobility ratio between the displacing and displaced fluid will be more favorable. However, there are challenges concerning polymer flooding. The viscosity increasing feature of the polymer solution frequently induce large injection pressures and consequently may have detrimental effects on injectivity. Injectivity damage resulting from polymer flooding will in some instances require drilling of additional wells. This may negatively affect the economic feasibility of a polymer flood project [4]. Also, polymer loss due to retention mechanisms is a major economic expenditure in polymer flooding projects and will in most instances influence the decision making in relation to polymer flood implementation [5]. The synthetic polymer partially hydrolyzed polyacrylamide (HPAM) has been extensively used for polymer flood projects due to its low production costs and beneficial rheological properties [3], and is the only polymer investigated in this thesis. The rheological properties of HPAM in porous media have been investigated extensively, where mechanical degradation has been established as very probable when being subjected to the high shear rates experienced in an injection well [6]. Most of the research on HPAM in porous media have been carried out in linear core plugs, where the results from these studies have been considered transferable to flow in radial geometries. Recent research however, suggest that polymer flow is significantly different in linear versus radial models [7]. HPAM injectivity in porous media may therefore be underestimated based on linear core studies. The principal aim of this thesis is to investigate effects of residual oil on polymer injectivity in radial models. In addition, the relationship between polymer concentration and rheological behavior in presence of residual oil will be assessed and compared to conditions in absence residual oil. In this thesis, a polymer flood experiment conducted in a radial disc saturated with residual oil will be history matched. These results will subsequently be compared to previously history matched results from experimental conditions in absence of residual oil. Three water floods and two polymer floods was history matched, whereas two different polymer concentrations (800 and 2000ppm) in the semi-dilute regime were investigated. A sequential order of alternating water and polymer floods enabled a permeability estimation both prior and post polymer flooding. History match results were obtained using two different simulator tools: STARS and MRST, respectively. Results from both simulator tools was consistent and thus will increase the confidence of obtained results. A sensitivity analysis was conducted to investigate the effect of different parameters on the STARS simulation tool. Several simulation model parameters such as grid block length and maximum timestep was investigated. In addition, polymer and fluid flow properties were assessed, which include: Molecular weight, viscosity, adsorption, adsorption reversibility, inaccessible pore volume, concentration, residual resistance factor and endpoint relative permeability. Current literature suggests that if porous media is first contacted with a low concentration HPAM solution that satisfies retention, no significant additional retention occurs when exposed to higher concentrations. In contrast, permeability determination both before and after polymer flooding revealed that additional retention occurred when the porous media was exposed to a higher concentration solution. In agreement with previously reported retention results in presence versus absence of residual oil, the amount of retention in presence of residual oil was reduced. However, this reduction was far greater than previous literature suggests. Both polymer concentrations exhibited strong shear thinning and shear thickening behavior in presence of residual oil. The highest polymer concentration was mechanically degraded during porous media propagation and the shear thinning behavior was inconsistent with previous literature. An effect of concentration on HPAM rheology was to shift the onset of shear thickening towards higher values with increasing polymer concentrations. This occurrence is not in agreement with previously obtained results. A comparison of bulk and in-situ viscosity of the highest concentration polymer was performed. Results revealed that in-situ viscosity was below bulk viscosity in the lower shear rate region. This is inconsistent with previously reported literature in the semi-dilute regime. An increase in HPAM concentration resulted in reduced injectivity values. However, this reduction was expected based on polymer rheology and presence of residual oil did not amplify the effect of concentration on injectivity. Simulation results showed that even though the absolute viscosity values of HPAM was severely reduced in presence of residual oil, the permeability decrease experienced during two-phase flow dominated. Thus, the overall effect of residual oil was to reduce injectivity of HPAM when varying flow conditions of the two experiments was not taken into consideration. However, since the isolated effect of residual oil on polymer injectivity was of primary concern in this thesis, injectivity in both absence and presence of residual oil was scaled according to corresponding brine injectivities, thus excluding experimental condition effects. The isolated effect of residual oil was to increase injectivity of HPAM significantly. Based on results obtained in this thesis, it may seem that injectivity estimation based on results from core floods in absence of residual oil may underestimate polymer injectivity

Mechanistic Modelling of Radial Polymer Flow in Porous Media

Polymer flooding is one of the most successful chemically enhanced oil recovery (EOR) methods, and has primarily been implemented to accelerate oil production by sweep improvement. However, research from the last couple of decades have identified several additional benefits associated with polymer flooding. Firstly, improved polymer properties have extended their use in reservoirs with high temperature and high salinity. Secondly, improved understanding of the viscoelastic flow behavior of flexible polymers have revealed that they may in some cases mobilize capillary trapped oil as well. Despite of the recent progress, extensive research remains to quantify the appropriate flow mechanisms and accurately describe polymer flow in porous media. Simulations and history match operations performed in this thesis are aimed at improving the modelling of radial polymer flow in porous media. This has been achieved by (1) evaluating the accuracy and robustness of two different history match methods which are used to estimate the in-situ rheology of non-Newtonian fluids in radial flow, (2) investigating potential rate and memory effects (at the Darcy scale) of viscoelastic polymer solutions in radial flow, and (3) quantifying polymer in-situ rheology and polymer injectivity. The accuracy and robustness of both history match methods which are used to estimate the in-situ rheology of non-Newtonian fluids in radial flow was clearly demonstrated in radial flow experiments where effective (or cumulative) error was below 5 % of the maximum preset transducer pressure range. Thereby, the observed shear-thinning behavior of partially hydrolyzed polyacrylamide (HPAM) at low flux in porous media could not be attributed to insufficiently accurate pressure transducers during in-house flow experiments, as suggested by some researchers. The estimation of polymer in-situ rheology showed invariance between excluding and including the polymer pressure data outside the near-wellbore region. Thus, it was proposed that the polymer in-situ rheology is mainly defined by the pressure data originating from the near-wellbore region during radial polymer flow. Results showed that not only could the polymer in-situ rheology be (quantitatively) estimated from measurements of stabilized pressure, but could also be (qualitatively) identified from the pressure build-up during radial flow experiments. Consequently, the anchoring data from the pressure build-up during radial polymer flow was proposed as an additional tool for history matching field injectivity tests. Rate and memory effects (at the Darcy scale) of several HPAM polymers were investigated in flow through Bentheimer sandstone discs. Results showed that no rate effects occurred for mechanically undegraded polymer (Flopaam 3330S). However, rate effects were observed for mechanically degraded polymers (Flopaam 3630S and Flopaam 5115SH) where the onset of shear-thickening increased with volumetric injection rate. While memory effects (at the Darcy scale) were absent for the mechanically undegraded and relatively low molecular weight polymer, Flopaam 3330S, the mechanically degraded and relatively higher molecular weight (18 MDa) polymer, Flopaam 3630S, exhibited memory effects in which apparent viscosity decreased with radial distance. As mechanical degradation is suggested to be confined to the near-wellbore region in radial polymer flow, the memory effect was proposed to originate from the elastic properties of the polymer. In accordance with recent literature, the in-situ rheology of HPAM was shown to depend on flow geometry. During single and two-phase polymer flow, the shear-thinning behavior of HPAM was much more pronounced, and the extent of shear-thickening significantly reduced in radial compared to linear flow. Furthermore, the onset of shear-thickening during single-phase flow occurred at significantly higher velocities in radial relative to linear flow. However, this behavior was not consistent during two-phase flow as the onset of shear-thickening during linear and radial polymer flow coincided. Moreover, comparative studies of polymer flow in radial versus linear flow geometries during single and two-phase flow revealed that the impact of oil was to reduce apparent in-situ viscosity of HPAM. The low-flux in-situ rheology behavior was addressed and showed Newtonian behavior in linear flow while significant shear-thinning was observed during radial flow. Thus, both flow geometry and presence of oil were suggested to be key factors for estimating polymer in-situ rheology

Qualification of New Methods for Measuring In-Situ Rheology of Non-Newtonian Fluids in Porous Media

Pressure drop (ΔP) versus volumetric injection rate (Q) data from linear core floods have typically been used to measure in situ rheology of non-Newtonian fluids in porous media. However, linear flow is characterized by steady-state conditions, in contrast to radial flow where both pressure and shear-forces have non-linear gradients. In this paper, we qualify recently developed methods for measuring in situ rheology in radial flow experiments, and then quantitatively investigate the robustness of these methods against pressure measurement error. Application of the new methods to experimental data also enabled accurate investigation of memory and rate effects during polymer flow through porous media. A radial polymer flow experiment using partially hydrolyzed polyacrylamide (HPAM) was performed on a Bentheimer sandstone disc where pressure ports distributed between a central injector and the perimeter production line enabled a detailed analysis of pressure variation with radial distance. It has been suggested that the observed shear-thinning behavior of HPAM solutions at low flux in porous media could be an experimental artifact due to the use of insufficiently accurate pressure transducers. Consequently, a generic simulation study was conducted where the level of pressure measurement error on in situ polymer rheology was quantitatively investigated. Results clearly demonstrate the robustness of the history match methods to pressure measurement error typical for radial flow experiments, where negligible deviations from the reference rheology was observed. It was not until the error level was increased to five-fold of typical conditions that significant deviation from the reference rheology emerged. Based on results from pore network modelling, Chauveteau (1981) demonstrated that polymer flow in porous media may at some rate be influenced by the prior history. In this paper, polymer memory effects could be evaluated at the Darcy scale by history matching the pressure drop between individual pressure ports and the producer as a function of injection rate (conventional method). Since the number of successive contraction events increases with radial distance, the polymer has a different pre-history at the various pressure ports. Rheology curves obtained from history matching the radial flow experiment were overlapping, which shows that there is no influence of geometry on in-situ rheology for the particular HPAM polymer investigated. In addition, the onset of shear-thickening was independent of volumetric injection rate in radial flow

Qualification of New Methods for Measuring In-Situ Rheology of Non-Newtonian Fluids in Porous Media

Pressure drop (ΔP) versus volumetric injection rate (Q) data from linear core floods have typically been used to measure in situ rheology of non-Newtonian fluids in porous media. However, linear flow is characterized by steady-state conditions, in contrast to radial flow where both pressure and shear-forces have non-linear gradients. In this paper, we qualify recently developed methods for measuring in situ rheology in radial flow experiments, and then quantitatively investigate the robustness of these methods against pressure measurement error. Application of the new methods to experimental data also enabled accurate investigation of memory and rate effects during polymer flow through porous media. A radial polymer flow experiment using partially hydrolyzed polyacrylamide (HPAM) was performed on a Bentheimer sandstone disc where pressure ports distributed between a central injector and the perimeter production line enabled a detailed analysis of pressure variation with radial distance. It has been suggested that the observed shear-thinning behavior of HPAM solutions at low flux in porous media could be an experimental artifact due to the use of insufficiently accurate pressure transducers. Consequently, a generic simulation study was conducted where the level of pressure measurement error on in situ polymer rheology was quantitatively investigated. Results clearly demonstrate the robustness of the history match methods to pressure measurement error typical for radial flow experiments, where negligible deviations from the reference rheology was observed. It was not until the error level was increased to five-fold of typical conditions that significant deviation from the reference rheology emerged. Based on results from pore network modelling, Chauveteau (1981) demonstrated that polymer flow in porous media may at some rate be influenced by the prior history. In this paper, polymer memory effects could be evaluated at the Darcy scale by history matching the pressure drop between individual pressure ports and the producer as a function of injection rate (conventional method). Since the number of successive contraction events increases with radial distance, the polymer has a different pre-history at the various pressure ports. Rheology curves obtained from history matching the radial flow experiment were overlapping, which shows that there is no influence of geometry on in-situ rheology for the particular HPAM polymer investigated. In addition, the onset of shear-thickening was independent of volumetric injection rate in radial flow

Qualification of New Methods for Measuring In Situ Rheology of Non-Newtonian Fluids in Porous Media

Pressure drop (ΔP) versus volumetric injection rate (Q) data from linear core floods have typically been used to measure in situ rheology of non-Newtonian fluids in porous media. However, linear flow is characterized by steady-state conditions, in contrast to radial flow where both pressure and shear-forces have non-linear gradients. In this paper, we qualify recently developed methods for measuring in situ rheology in radial flow experiments, and then quantitatively investigate the robustness of these methods against pressure measurement error. Application of the new methods to experimental data also enabled accurate investigation of memory and rate effects during polymer flow through porous media. A radial polymer flow experiment using partially hydrolyzed polyacrylamide (HPAM) was performed on a Bentheimer sandstone disc where pressure ports distributed between a central injector and the perimeter production line enabled a detailed analysis of pressure variation with radial distance. It has been suggested that the observed shear-thinning behavior of HPAM solutions at low flux in porous media could be an experimental artifact due to the use of insufficiently accurate pressure transducers. Consequently, a generic simulation study was conducted where the level of pressure measurement error on in situ polymer rheology was quantitatively investigated. Results clearly demonstrate the robustness of the history match methods to pressure measurement error typical for radial flow experiments, where negligible deviations from the reference rheology was observed. It was not until the error level was increased to five-fold of typical conditions that significant deviation from the reference rheology emerged. Based on results from pore network modelling, Chauveteau (1981) demonstrated that polymer flow in porous media may at some rate be influenced by the prior history. In this paper, polymer memory effects could be evaluated at the Darcy scale by history matching the pressure drop between individual pressure ports and the producer as a function of injection rate (conventional method). Since the number of successive contraction events increases with radial distance, the polymer has a different pre-history at the various pressure ports. Rheology curves obtained from history matching the radial flow experiment were overlapping, which shows that there is no influence of geometry on in-situ rheology for the particular HPAM polymer investigated. In addition, the onset of shear-thickening was independent of volumetric injection rate in radial flow.publishedVersio

Polymer flow in porous media: Relevance to Enhanced Oil Recovery

Polymer flooding is one of the most successful chemical EOR (enhanced oil recovery) methods, and is primarily implemented to accelerate oil production by sweep improvement. However, additional benefits have extended the utility of polymer flooding. During the last decade, it has been evaluated for use in an increasing number of fields, both offshore and onshore. This is a consequence of (1) improved polymer properties, which extend their use to HTHS (high temperature high salinity) conditions and (2) increased understanding of flow mechanisms such as those for heavy oilmobilization. A key requirement for studying polymer performance is the control and prediction of in-situ porous medium rheology. The first part of this paper reviews recent developments in polymer flow in porous medium, with a focus on polymer in-situ rheology and injectivity. The second part of this paper reports polymer flow experiments conducted using the most widely applied polymer for EOR processes, HPAM (partially hydrolyzed polyacrylamide). The experiments addressed highrate, near-wellbore behavior (radial flow), reservoir rate steady-state flow (linear flow) and the differences observed in terms of flow conditions. In addition, the impact of oil on polymer rheology was investigated and compared to single-phase polymer flow in Bentheimer sandstone rock material. Results show that the presence of oil leads to a reduction in apparent viscosity

Polymer Injectivity Test Design Using Numerical Simulation

Polymer flooding is an enhanced oil recovery (EOR) process, which has received increasing interest in the industry. In this process, water-soluble polymers are used to increase injected water viscosity in order to improve mobility ratio and hence improve reservoir sweep. Polymer solutions are non-Newtonian fluids, i.e., their viscosities are shear dependent. Polymers may exhibit an increase in viscosity at high shear rates in porous media, which can cause injectivity loss. In contrast, at low shear rates they may observe viscosity loss and hence enhance the injectivity. Therefore, due to the complex non-Newtonian rheology of polymers, it is necessary to optimize the design of polymer injectivity tests in order to improve our understanding of the rheology behavior and enhance the design of polymer flood projects. This study has been addressing what information that can be gained from polymer injectivity tests, and how to design the test for maximizing information. The main source of information in the field is from the injection bottom-hole pressure (BHP). Simulation studies have analyzed the response of different non-Newtonian rheology on BHP with variations of rate and time. The results have shown that BHP from injectivity tests can be used to detect in-situ polymer rheology

Polymer Injectivity Test Design Using Numerical Simulation

Polymer flooding is an enhanced oil recovery (EOR) process, which has received increasing interest in the industry. In this process, water-soluble polymers are used to increase injected water viscosity in order to improve mobility ratio and hence improve reservoir sweep. Polymer solutions are non-Newtonian fluids, i.e., their viscosities are shear dependent. Polymers may exhibit an increase in viscosity at high shear rates in porous media, which can cause injectivity loss. In contrast, at low shear rates they may observe viscosity loss and hence enhance the injectivity. Therefore, due to the complex non-Newtonian rheology of polymers, it is necessary to optimize the design of polymer injectivity tests in order to improve our understanding of the rheology behavior and enhance the design of polymer flood projects. This study has been addressing what information that can be gained from polymer injectivity tests, and how to design the test for maximizing information. The main source of information in the field is from the injection bottom-hole pressure (BHP). Simulation studies have analyzed the response of different non-Newtonian rheology on BHP with variations of rate and time. The results have shown that BHP from injectivity tests can be used to detect in-situ polymer rheology