Modelling Multiphase Flow in Downhole Valves

Hovedmålet med dette arbeidet er å finne metoder for å modellere to-fase strømning i en ventil med kompleks geometri med flere enkeltmotstander og tverrsnittsoverganger. Målet ble oppnådd ved å koble 3-dimensjonal data-simulert (CFD) en-fase strømning med 1-dimensjonale modeller for to-fase strømning. 3D modellen benyttes for å beregne strømnings-karakteristikken for de enkelte deler av ventilen, og dermed inngangsdata for en 1-dimensjonal (1D) ventil-modell. Denne metoden har gitt lovende resultater. Avviket mellom beregnet og målt strømningstap for 1-fase strømning er ca. 2% for både CFD-simulering og 1D-modell. Arbeidet har også omfattet design, bygging og instrumentering av en fullskala ventilmodell. En serie forsøk ble utført med luft, vann og to forskjellige typer olje. Dette gav en verdifull database for to-fase strømning i en typisk nedihulls innstengningsventil. Videre ble en 1D simulering-modell implementert og validert. Den viktigste måloppnåelsen er implementering av modeller for to-fase strømning i 1D-modellen. Her er to ulike utgangspunkter testet. Først ble klassiske korrelasjoner uten tilknytning til strømningsmønster utprøvd. Deretter ble en oppdatert modell-formulering med predikering av strømningsmønster implementert. Denne gav best resultat med et avvik i beregning av to-fase strømningstap på ca. 10%.

After finding oil in an oil well, it is essential to map the size of the reservoir. For oilfields in the Middle East, tests have traditionally been performed with a topside shut-in. A valve on the well head is then used to shut in the well, and when closi ng the valve, temperature and pressure build up is monitored. This method involves flaring off huge quantities of gas and oil. The purpose of this project is being able to design and operate downhole valves for well testing, run on wire line rather than d rillpipe. Downhole testing means that the test valve will be placed close to perforation of the well casing. Downhole shut-in test system is an alternative to the traditional topside shut-in test, where the test valve is placed on the wellhead. The benefi ts from the downhole shut-in test are: a. Large reduction in the need for flaring off test gas, and thereby large reduction in CO2 emissions. b. Improved accuracy of the estimate of the reservoir size, as the uncertainty in the gas content in the well cas ing is eliminated c. Lowering costs a. Drill pipe or coiled tubing is redundant. Existing valves on the market today for slick-line operation in sour wells are battery operated, and the time for closing the valve is too long, 40-60 seconds. A new valve is needed that closes very quickly. Issues in this context are: 1. predicting flow patterns in a multiphase flow downhole, with high temperature and high pressure 2. calculating pressure drop across valves and other obstacles with multiphase flow 3. finding maximum pressure loads due to water hammer effect during fast closing 4. ensuring a safe operation of the valve a. fast closing operation, without damaging the valve or anchor mechanism b. quick opening operation, without damaging the valve, sealings or anchor mechanism The outcome of the PhD project will be knowledge about multiphase downhole flow, making us able to design and safely operate downhole valves. An optimal valve is currently not available.