How do you accurately measure gas permeability in reservoir core samples while conducting HPHT formation damage testing on cesium formate brine?
This was the challenge facing Corex Ltd., one of the world’s leading core analysis companies and experts in formation damage, when asked by Cabot to carry out HPHT core flood tests for a major North Sea operator. A review of their test procedures identified two areas of concern that could introduce errors into gas permeability determinations:
- Gas leakage from the cores through the surrounding elastomer seals at extreme temperature and pressure.
- Dehydration of core's fluid contents if the test gas is not fully saturated with water vapour under test conditions.
To solve the problem of gas leakage Corex came up with a gold-plated solution. The core was wrapped in a gas-impervious layer of 24-carat gold before fitting the elastomer sleeving and O-rings.
The second problem of dehydration was overcome through the installation of a pressurised high-temperature gas humidifier to ensure that gas entering the core was fully saturated with water.
Using this new test set-up Corex carried out two formation damage tests with cesium formate brine on cores from a major HPHT field located in the UK North Sea. The test temperature was 200°C (392°F) and the pore pressure inside the core was maintained at 5,800 psi. In one test, the gas used during drawdown and permeability measurements was passed through a pressurised humidifier held at room temperature. In the other test, the gas was passed through the new pressurised high-temperature gas humidifier.
Commenting on the outcome of the tests reported in SPE paper 1216491, Ian Patey, manager of Corex’s Formation Damage group, said: “The overall permeability of the reservoir core sample to gas was unaffected by injection of 10 pore volumes of cesium formate brine, followed by a 48-hour static soak at 200°C and back-production using 4 litres (1,000 pore volumes) of HPHT humidified gas under 100 psi drawdown. In the other test permeability was reduced by nearly 15%. This shows that full HPHT humidification of the gas phase results in higher gas return permeability when compared with a comparative test using gas humidified at room temperature and high pressure. Consequently, it’s critical that gases used in HPHT core flooding tests are fully saturated with water vapour at the test temperature and pressure to ensure correct and realistic results.”
This innovative use of gold-coated cores and HPHT humidified gas sets new levels of accuracy for core flood testing of high-density completion brines at high temperatures. It also helps match laboratory test results with operators’ experience of using cesium formate brines in HPHT wells over the last ten years.
1) Downs, J.D.: “Observations on Gas Permeability Measurements under HPHT Conditions in Core Materials Exposed to Cesium Formate Brine”, SPE 121649 presented at 2009 SPE European Formation Damage Conference, Scheveningen, The Netherlands, 27–29 May 2009.
Pictured above: 24-carat gold film prevents gas leaking from the core under hydrothermal conditions.
