Influence of Residual Oil on Polymer Injectivity
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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 . 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  . 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 . 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 . 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 , 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 . 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 . 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.