Coreflooding and precision pressure control
24 August 2020
By Mitch Spronck
The process of coreflooding is typically performed in the oil and gas industry to determine various intrinsic properties of a rock sample. These include, but are not limited to: permeability, saturation, formation damage and hydrocarbon content. The fluid that is injected into the sample usually consists of different liquids and gases, such as methane, carbon dioxide, brine and oil. Other fluids such as acids, foam or other chemicals may also be used.
Process conditions in coreflooding systems
Coreflooding can be performed under a wide variety of parameters, which can range from very low pressures and ambient temperatures to pressures of several hundred bars (sometimes even several thousand bars) and high temperatures of up to 200°C. Also the flow rate can vary widely in the process of analyzing cores. The flow rate often needs to be truly zero, while at other times in the process several hundreds ml/min can be achieved or may be required.
Pressure issues related to coreflooding
What becomes clear from the aforementioned parameters and conditions, is that the Cv requirements of this process may have a very broad range and that several corrosive or caustic substances may be used. As a consequence of these broad Cv ranges, it can be challenging to assure that the pressure inside the system is kept in a precise way, especially at zero flow. Furthermore, conventional back pressure regulators cannot handle the large differences between the minimum and maximum flow and Cv. Also, chemical compatibility of materials can be an issue for these regulators.
How Pressure Control Solutions can help you manage your process conditions in coreflooding
To assure the pressure inside the coreflooding system with sufficient precision, the Equilibar Zero Flow regulators and Ultra Low Flow Series (UxL) – both part of the Research Series – are an optimal fit. Because these are designed as a dome loaded pressure regulator with a flexible, sensing diaphragm, it has no hysteresis, which ensures a very high precision in controlling pressure at zero or low flow.
The characteristics of Equilibar back pressure regulators in the Research Series will not only solve the challenge of precise pressure control at zero flow, but also the aforementioned problems that conventional back pressure regulators have:
Can handle large differences between the minimum and maximum flow and Cv:
The multiple-orifice-design permits the Equilibar to be used over very large Cv ranges. The back pressure regulators in the Zero Flow Series as well as the Ultra Low Flow Series have CV rangeabilities of over 10.000.000 (!), as their Cv ranges from 0 to 0,015 and from 1E-9 to 0,05 respectively. This enables the use of large pressure ranges, and will allow your process to operate at flows ranging from no flow to several hundred ml/min (or even over 1 l/min) as well as large temperature ranges of up to 300°C, with just one single regulator.
In terms of chemical compatibility, special attention is needed for the materials that your coreflooding instruments are made of: Since a coreflooding system may use several different fluids, some of which are corrosive to materials like stainless steel 316, the instrument may need other materials. Very suitable for coreflooding applications is Hastelloy and both the Equilibar Zero Flow Series and the Ultra Low Flow Series are available in Hastelloy. In case your application demands other materials, the ZF and UxL units are also available in e.g. zirconium, titanium, Monel and any other custom metal. Read more about material selection here.
Example of a coreflooding system set-up
In our P&ID schematic below, CO2, brine and oil are fed into the core using mass flow controllers (MFC). As mentioned, the composition of the fluids that are fed into the core can strongly vary. Downstream of the sample, an Equilibar® dome loaded back pressure regulator is placed in combination with a dual valve electronic pressure controller to keep the pressure stable in the sample compartment. In this case, the reference pressure is set in a 1:1 ratio with the desired upstream pressure. The reference pressure (also called: pilot pressure) is controlled electronically in our example, but could also be set with a manual forward pressure reducing regulator. A coreflooding set-up is usually equipped with pressure gauges or transmitters (PT) to monitor the pressure, but is also often equipped with mass flow meters (MFM) and liquid/gas separators. If you want to learn how to pilot your back pressure regulator, click on this link for further information.
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