Synthetic polymers for Enhanced Oil Recovery; Mechanical degradation, and alleviation thereof: Experimental study across scales in pipes, chokes, and porous media of regular HPAM, HPAM-ATBS co-polymers, and TAPs
Keywords:synthetic polymers, enhanced oil recovery, eor
This thesis and the related work concern synthetic polymers for enhanced oil recovery, their ability to reduce the mobility of the water phase during oil recovery, and their predisposition to mechanically break when subjected to certain flow conditions.
A major problem during water injection for pressure support and to push oil through the reservoir, is the adverse mobility ratio between water and oil. Water will usually have a lower viscosity than the oil, meaning it has less resistance to flow and moves more easily. It also flows more easily where the water saturation is high, giving a self-enforcing nature to viscous instabilities. In short, water will create low resistance flow paths for itself or flow in paths of low resistance, reaching the producer at a relatively early stage of injection and with large volumes of oil remaining to be produced. If the mobility difference between the injected phase and the oil is reduced, the oil will be displaced in a more piston-like manner, accelerating the oil production, increasing the volume of the reservoir contacted by the water, and delaying the water production.
The mobility difference can be improved by decreasing the mobility of the water phase through making it more viscous. This can be done by adding polymers. Polymer are large molecules composed of repeating chemical building blocks. The polymers used for Enhanced oil recovery (EOR) have molecular weight of millions of g/mol, so large that only a few hundred ppms added to the water is needed to increase the viscosity significantly.
Three categories of synthetic water-soluble polymers have been studied in this work, partly hydrolyzed polyacrylamide (HPAM), HPAM-ATBS co-polymers, and thermo-thickening associative polymers (TAPs). Their flow behaviour has been studied across scales in tubes, chokes, and porous media (unconsolidated sand, and Berea and Bentheimer sandstone).
HPAM is worldwide the most used polymer for Enhanced Oil Recovery (EOR), but there are several issues related to its use:
- Viscosity is reduced in saline water
- It thickens in porous media at high flow rates typical for the injection zone, resulting in pressure build-up, poor injectivity or the creation of fractures
- It is prone to mechanical degradation at harsh flow conditions, an effect amplified in saline water
Mechanical degradation is that the backbone of the polymer molecules ruptures, resulting in a polymer with a lower molecular weight and correspondingly lower viscosity, making the solution less effective in pushing oil to the producer.
HPAM-ATBS co-polymers are derivatives of HPAM that have been modified to be less influenced by salt in the mixing water. Due to a, on the molecular level, large appendix on the ATBS segments in the polymer, the polymer solutions’ viscosity is not influenced as much by salt as HPAM. This is because it, due to steric hindrance, remains expanded. Also, since it remains expanded, mechanical degradation will not increase with salinity of the mixing water.
TAPs are polymers mainly composed of HPAM and (H)PAM-ATBS (or other water-soluble polymers), that have been modified to be triggered by heat. At a certain temperature, segments of them will become hydrophobic and through hydrophobic interactions, they bind weakly to each other creating a reversible weakly cross-linked gel with increased resistance to flow. This effect is, in addition to being triggered by heat, also enhanced by salt and porous media flow, and is strongly shear-thinning. This makes TAPs very promising for EOR operations as they will have low resistance to flow at the high injection rates at the often cooled down injection zone, and high resistance to flow at the low rates of the warmer main part of the reservoir. As their thickening does not only rely on the molecular weight, lower molecular weight polymers can be used. Lower molecular weight polymers are less predisposed to mechanical degradation and injectivity problems.
Degradation experiments in cylindrical pipes of different dimensions also confirmed that salt accelerates mechanical degradation more for HPAM than for HPAM-ATBS, and that high molecular weight polymers are more prone to mechanical degradation than low molecular weight. It was also found that mechanical degradation scales with shear rate, and not with pressure drop, velocity or Reynolds number, and that even at turbulent flow, degradation will not increase with length of the tube. That is, degradation seems to be limited to the inlet of the flow constriction. Since pressure drop itself does not harm the polymer, polymer solutions can be choked (have its pressure reduced) as long at the shear rate is kept below a critical value. Three methods for choking with acceptable degradation were experimentally identified: 1) Take the pressure drop out over a longer distance (long tube), 2) Use several chokes in series, each below a critical pressure drop or 3) Choke concentrated solution.
Testing of TAPs with increasing content of the temperature-triggered segment in different porous media confirmed the assumption that a critical concentration of either polymer or the active segments are needed to achieve thermo- thickening. It was also confirmed that below their critical temperature, their flow behaviour is like the unmodified mother polymer, and this is also (for the most part) the case for bulk flow (tube flow), even at elevated temperature. These experiments were not performed to study mechanical degradation and were run below the critical shear rate for degradation, but their high flow rate behaviour will most likely be that of a medium molecular weight PAM-ATBS co-polymer, as that is the main constituent of these polymers. The experiments revealed that within the limited diversity of these tests, type of porous media did not significantly influence the thermo-thickening. At high salinity, homogenous and stable thermo-thickening was achieved in a 76 cm long sand- pack, but the build-up of resistance to flow was slow, and for the weaker systems tested (lower associative content or salinity), resistance to flow collapsed at very low velocities. These systems still show great promise regarding EOR, but a better understanding of the mechanisms guiding their behaviour is needed for optimum design for specific field conditions.
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