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Changing the Mix: A New Approach to Polymers for Offshore EOR

Source: Chevron

Polymer flooding is a longstanding, popular tool for improving oil recovery, and over the years a number of researchers have worked at improving the quality of commercially available polymers. However, polymer mixing in offshore applications can present a number of challenges. An SPE Annual Technical Conference and Exhibition session discussed a new approach to better understanding the drivers for polymer hydration and the design of optimal field mixing systems. Researchers hope the information gathered in testing this approach could help with enhanced oil recovery (EOR) in offshore.

Do Hoon Kim, a polymer flooding consultant at Chevron, presented a paper (SPE 191391) cowritten with several colleagues from the Chevron Energy Technology Company and Chevron Upstream Europe that presented a new parameter—specific mixing energy (SME)—that can be used to scale-up laboratory mixing. The paper also covered Chevron’s development of a laboratory mixing process for novel liquid polymers that provided an acceptable viscosity yield, filtration ratio, and nonplugging behavior during injectivity tests in a surrogate core.

The use of polymers is a proven technology and commercially viable EOR process. Most recently, the powdered version of hydrolyzed polyacrylamide polymers has become a popular application for polymer flooding onshore due to the availability of space to store, mix, and hydrate them into viscous solutions for injection. However, Kim said polymer flooding with liquid polymers is an attractive technology for simple deployment in remote locations such as offshore reservoirs due to space constraints and potentially hostile environments.

“When you’re going to apply polymers for offshore, there’s a limit in space for mixing and in facilities. You cannot fit a tank over there, and also there are some environmental safety issues with having a powder in the offshore conditions,” Kim said.

Kim cited a previous paper (SPE 179657) that established the criteria for selecting a proper liquid polymer for deploying in offshore reservoirs, tailoring the polymer packages to improve polymer activity, filtration, and injectivity. He said his paper, and subsequent presentation, build on that conclusion by looking into the energy required for optimum mixing of novel liquid polymers from laboratory-scale to field-scale.

The Chevron team used a synthetic brine as the base brine for all laboratory and field tests. They conducted polymer filter ratio tests to identify the effectiveness of the polymer mixing which, in effect, shows how effectively that polymer can be injected through a porous medium without plugging or retention. Rheological measurements, long term injectivity experiments, and polymer loop yard tests were also performed. For the field, they developed a portable measurement unit to measure polymer rheology, filterability, and long-term core injectivity for on-site surveillance.

The results from the field and yard tests demonstrated the ability to mix and hydrate the neat liquid polymer in one step. Low flow rates across static mixers correspond with low pressure drops and lead to high filtration ratios, while high flow rates correspond with high pressure drops and lead to low filtration ratios. Kim said this validated the need for critical pressure across static mixers for optimal polymer mixing and hydration. The critical pressure drop is a proxy for the energy imparted to the fluid by flow through the static mixers, and SME is a term derived for a critical defining parameter for polymer mixing. SME attempts to quantify the amount of energy imparted during the fluid mixing process, and as such it could be used to find trends against key fluid performance parameters.

Using the SME approach, Kim said the performance of mixing and hydration of liquid polymers in a laboratory-scale overhead mixer and in a field-scale inline static mixer could be successfully interpreted. SME can also be applied for the mixing of powder polymers, though further experiments were needed to validate the scale-up. The quality of polymer solutions measured by viscosity and filtration ratio correlates to an SME, and Kim said the desired operating window range from the lab to the field is a function of SME. This information can be used to bolster EOR applications with liquid polymers in offshore environments.

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