With the growing shortage of oil resources along with inherent challenges of finding new oil fields, enhanced oil recovery (EOR) technology has become an attractive prospect. When standard techniques are used, just 20 to 40% of oil is obtained and remaining 60 to 80% of oil is left in the recently drilled oil reservoir. However, when EOR technology is used a greater amount of oil can be extracted from the oil reservoir.
Hydrogel and surfactant polymers play a major role in the usage of the EOR technology. Polymer flooding is a well-established technology that has been specifically developed for EOR applications. In this method, a solution is injected into porous rocks to release the remaining oil. When compared to traditional Newtonian fluids, fluids containing considerable viscoelastic properties enhance displacement efficiency as well as recovery yields.
In order to interpret the rheological behavior of such fluids, the Institute of Petroleum Engineering, in collaboration with Malvern Panalytical, are working on a new project. The study aims to understand the rheological behavior of fluids under the specific conditions of the oil recovery process. So far, EOR polymer systems were the main focus of the study. These systems have a high solution concentration and molecular weights, and hence, tend to have a non-Newtonian shear rate-dependent viscosity and viscoelasticity.
These polymer systems are capable of storing and dissipating energy based on the time and speed of deformation. In addition, they have an excellent resistance to high extensional viscosity or stretching in contrast to that seen under shear viscosity. This feature is of major significance because the fluid needs to accelerate and decelerate to penetrate pores through narrow connecting channels. This ensures that a steady volumetric flow rate is maintained. This phenomenon creates extensional forces along the flow axis and the fluid near the walls is simultaneously exposed to shear forces.
In order to study this complicated flow behavior, complementary test protocols and measurement methods have to be used. A rotational rheometer can be used to obtain data regarding the flow characteristics of polymer solutions. This can be done by determining the shear viscosity as a function of its shear rate or by measuring the material’s linear viscoelastic behavior under non-flow conditions using oscillatory testing.
Also, rotational rheometry can be used studying a material’s non-linear viscoelastic behavior under flow. This can be done by determining the usual stresses produced under rotational shear. Flow induced anisotropy contributes to these normal stresses. For instance, polymer coils can turn ellipsoidal under a shear field and stretches at higher rates of deformation because of the curved flow path produced in a rotational rheometer. As a result, a restoring tension or force is produced in the streamlines translating to a normal stress that acts on the upper geometry.
This article describes a case study where the elements of interest are studied to understand how the performance of oil recovery is affected by EOR polymers’ viscoelastic properties. Here, non-linear viscoelastic measurements performed on a rotational rheometer to explain the viscoelastic polymer behavior in the porous media and how such a behavior is affected by environmental conditions.
Materials and Methods
For this experiment, a Kinexus rotational rheometer as well as a microfluidic rheometer (eVROC) using a microfluidic channel can be used to definine hydrolyzed polyacrylamide (HPAM) solutions. The microfluidic channel features a contraction zone to measure the extensional viscosity under geometric conditions that are analogous to those seen in the reservoir.
Factors like hardness, temperature, and the concentration of reservoir brines were assessed in detail. Glass-silicon-glass micromodels (GSG) were also used as Core analogues so as to evaluate the pressure requirements. This pressure factor is required to ensure that the EOR fluids are forced via a rock section at variable flow rates. The GSG is based on a CT scan of actual core samples.
Depending on these pressure measurements, the viscosity in-situ based on Darcy’s diffusion law can be easily determined to make a comparison against the estimated rheological information.
Findings and Discussion
In Figure 1, shear viscosity data is plotted against the shear rate data for a HPAM polymer solution in brine. The solution has a molecular weight of about 26 MDa and a concentration of 2000 ppm. The Kinexus rotational rheometer was used to determine the solution and the same was measured with Darcy’s law depending on the flow via the GSG micromodel at variable flow rates.
Figure 1. Steady shear viscosity measured using a Kinexus rheometer compared with apparent viscosity measurements obtained with GSG micromodels for HPAM polymer solution at 2000ppm in 4g/l brine, 22°C.
It can be seen that there is good agreement between the Kinexus data and the GSG micromodel data in the mid shear range. These results show that Darcy’s law can be used for measuring on-site viscosity for porous structures.
Conversely, differences can be seen with respect to rotational rheometry at higher shear rates and lower shear rates, where the beginning of shear thickening was seen and higher viscosities were determined respectively. Based on comparable flow experiments performed on porous media, other authors have reported this shear thickening behavior.
The lack of such apparent shear thickening behavior in rotational rheometer measurements indicates that this phenomenon corresponds to the geometric arrangement pattern of the rock as well as range of joined channels and pores. This kind of geometric configuration can promote extensional stresses due to fluid’s acceleration and deceleration via the structure of pores and channels. Such geometric arrangements can be important for polymers with high molecular weights.
A plot of initial normal stress difference along with the shear stress determined with cone and plate configuration as a function of shear rate is shown Figure 2.
As the shear rate increases, the shear stress also increases, but the shear stress displays a non-linear profile that suggests non-Newtonian-shear thinning behavior, as shown in Figure 1.
While the normal stress (N1) profile is different, N1 remains constant up to a shear rate of 50 per second and displays a continual increase over this, suggesting tension or force in the polymer chains. The beginning of this increase takes place at a parallel shear rate where shear thickening was seen in the GSG micromodel, indicating that the extra pressure needed to sustain the EOR fluid flow in the pore structure over 50s seconds is the result of elastic effects and not due to increase in the shear viscosity.
Another group used Malvern Panalytical rheometers, and reached the same conclusions. Measurements made with the eVROC microfluidic rheometer reinforced these conclusions, where it is possible to determine extensional stresses under conditions that are comparable to those faced in the rock structure.
Figure 2. Steady shear stress and first normal stress difference measured using a Kinexus rheometer for HPAM polymer solution at 2000ppm in 4 g/l Brine, 22°C.
Figure 3 shows the plots of extensional stress measurements made using the eVROC and the normal stress measurements made using the Kinexus rheometer. While the stress magnitudes are completely different, both measurements match with the increase in normal and extensional stresses that take place at similar rates of deformation, regardless of shear or extension.
To make a comparison, the extent of the shear rate can be assumed to be equal to √3 x extension rate, as used to measure the Trouton ratio. Upon comparing N1 for different EOR polymer solutions, a strong dependence was seen on the salinity and concentration of polymers.
Figure 3. First normal stress difference measured using a Kinexus rheometer compared with extensional stress measured using a microfluidic extensional rheometer for HPAM polymer solution at 2000ppm in 4 g/l brine, 22°C.
This can be attributed to the volume taken up by the polymer in aqueous solution, which affects viscoelastic behavior, and is susceptible to solvent concentration, molecular weight, and quality. The results are further being studied with several Malvern Panalytical methods like Microrheology (Zetasizer Nano) and Permeation Chromatrography (Omnisec). The former technique will be used to explore the relaxation behavior and short-time dynamics of polymer systems and the latter will be used to study the sequence of polymer chains during the course of polymer injection process.
With the help of a rotational rheometer, it is possible to determine non-linear viscoelastic data for polymers from normal stress measurements. In the measured HPAM solutions, increased normal stress was seen that was well over a significant shear rate of 50 per second. This data matches with the increase in the pressure which was needed to sustain the fluid flow at a similar shear rate in a rock core micromodel structure.
Conversely, normal stress data shows that the increase in pressure is due to improved elasticity and this elasticity is not due to viscosity, but rather due to polymer chain stretching in the flow field. Extensional viscosity measurements support these conclusions, which demonstrate that the increase in normal stress corresponds with an increase in extensional stresses at equivalent rates of deformation. In addition, the comparison of various polymer solutions for normal stress reveals that environmental conditions strongly influence the performance of polymers used for EOR in the reservoirs.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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