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 oil mobilization. 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.
During the last decades colloidal dispersion gels (CDG) have been applied as an EOR method, providing sweep improvement in reservoirs with unfavourable mobility ratio. Our core flood results also indicate an improved microscopic sweep or microscopic diversion by CDG. This paper investigates the oil mobilization properties of nano-sized silica particles in comparison to nano-sized CDG particles and discusses the underlying mechanisms of microscopic flow diversion.Improved microscopic displacement efficiency has traditionally been coupled to changes in capillary number. However, this approach is insufficient to describe processes like low salinity injection, colloid dispersion gels, and microbial enhanced oil recovery.This paper presents a new concept of EOR by improved microscopic displacement defined as microscopic diversion. This method involves pore blocking and diversion of injection fluids. Multi-phase flow experiments using nano-sized silica particles attempts to investigate the effect (inelastic) nano-sized fines migration can have on oil mobilization by microscopic diversion.The oil mobilization properties were investigated by core floods using well-defined nano-sized silica particles in comparison with injection of CDG, polymer or silica particles dispersed in a polymer solution.. Core floods were performed on water-wet Berea sandstone cores with permeabilities of approx. 500 mD. The comparison of inelastic silica particles, polymer solutions and nano-sized CDG particles allowed an evaluation of the importance of viscoelastic properties for microscopic diversion.
Polymer injectivity is a critical parameter for implementation of polymer flood projects. An improved understanding of polymer injectivity is important in order to facilitate an increase in polymer EOR implementation. Typically, injectivity studies are performed using linear core floods. Here we demonstrate that polymer flow in radial and linear models may be significantly different and discuss the concept in theoretical and experimental terms. Linear core floods using partially hydrolyzed polyacrylamides (HPAM) were performed at various rates to determine in-situ viscosity and polymer injectivity. Radial polymer floods were performed on Bentheimer discs (30 cm diameter, 2-3 cm thickness) with pressure taps distributed between a central injector and the perimeter production well. The in-situ rheological data are also compared to bulk rheology. The experimental set up allowed a detailed analysis of pressure changes from well injection to production line in the radial models and using internal pressure taps in linear cores. Linear core floods show degradation of polymer at high flow rates and a severe degree of shear thickening leading to presumably high injection pressures. This is in agreement with current literature. However, the radial injectivity experiments show a significant reduction in differential pressure compared to the linear core floods. Onset of shear thickening occurs at significantly higher flow velocities than for linear core floods. These data confirm that polymer flow is significantly different in linear and radial flow. This is partly explained by the fact that linear floods are being performed at steady state conditions, while radial injections go through transient (unsteady state) and semi-transient pressure regimes. History matching of polymer injectivity was performed for radial injection experiments. Differences in polymer injectivity are discussed in the framework of theoretical and experimental considerations. The results may have impact on evaluation of polymer flood projects as polymer injectivity is a key risk factor for implementation.
Polymer flooding is a mature EOR technique successfully applied in both sandstone and carbonate reservoirs. ADNOC has developed a new EOR roadmap with the objective to identify and mature EOR options to improve displacement and sweep efficiency in carbonate reservoirs. Polymer based EOR was identified as one of the main EOR options. These options include polymer injection, simultaneous injection of miscible gas and polymer (SIMGAP), simultaneous injection of water and polymer (SIWAP), low salinity polymer, etc. However, the conditions of the reservoirs in Abu Dhabi are beyond the industry experience for the application of polymer which pose a significant challenge to polymer based EOR processes. The reservoirs are at high temperature (~100-130 °C), the formation brine is of high salinity (~200,000 ppm), the brine also has high concentrations of divalent ions (~18,000 ppm of Ca++ and Mg++) and the reservoir formation is carbonate where there is little experience in the industry for polymer injection. The stability of polymers is known to be severely affected at such conditions of high temperature, salinity and divalent ions concentration. Therefore, the main challenge for polymer based EOR processes in ADNOC reservoirs (such as polymer flooding, SIMGAP, SIWAP, etc.) is to find a polymer that are stable under such extreme conditions and can be injected in carbonate reservoirs. ADNOC has lunched a number of studies to experimentally investigate 1- the stability of polymers at such adverse conditions of high salinity, high temperature and high divalent ions, 2- injectivity of polymers in carbonate rocks using both outcrop and reservoir core plugs, 3- bulk and in-situ rheology of polymers, 4- effects of various parameters on the polymer performance, such as polymer concentration, shear rate and presence of oil on polymer retention and in-situ rheology and 5- impact of H2S and Oxygen on polymer stability. In this paper, we report the results of these different studies that can enlarge the application envelope of polymer flooding to high-temperature, high-salinity and light oil Middle Eastern carbonate reservoirs. The main conclusions of the studies are: 1- A polyacrylamide based polymer with high content of ATBS (SAV 10) was identified as stable at Abu Dhabi reservoir conditions, 2- SAV 10 polymer is also stable in the presence of H2S (500 ppm) and/or Oxygen up to 150 ppb, 3- The polymer has good injectivity in a wide range of injection rates ranging from 1ft/day to 120ft/day and wide range of permeability, 4- The polymer showed a shear thickening behavior with an increase in flux without any signs of mechanical degradation, noted by the stable viscosities of the effluents, 6- The presence of crude oil had significant impact on injectivity, in-situ rheology and adsorption in carbonate core material. In summary, the ATBS polymer showed a promising injectivity behavior which can be modulated for injection in the target reservoirs. In addition, as the required viscosity increase for both SIMGAP and SIWAP processes to work is moderate, we find the results to be very promising and they open the door for field testing and piloting.
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