Assessing relative permeabilities in geothermal reservoirs using theoretical relations, laboratory measurements and field data

Abstract

Liquid dominated reservoirs are a common type of hydrothermal reservoirs. They consist of fractured rock and reservoir fluid which may obey Darcy’s law. Steam can form in such reservoirs either through heat input which causes temperature increase or through pressure reduction which causes flashing of the fluid. The modified Darcy’s law for two phase reservoirs uses relative permeabilities to account for the area reduction of the phases. Darcy’s law was used in this thesis to calculate the relative permeabilities for a fluid which flashes in a hydrothermal reservoir and a comparison was made for two flow cases, horizontal and vertical flow. The results show that there is a significant difference in the relative permeabilities between the two flow cases although they share identical transport and thermodynamic properties. This effect of gravity on the relative permeabilities was also observed when measurements were made using water and air as the two phases. Laboratory measurements were also performed to calculate the relative permeabilities of water and steam of geothermal origin. The resulting relative permeabilities showed a tendency to follow the Corey curve to some extent. The so called Shinohara method was used on data from two phase geothermal wells to calculate the relative permeabilities in the reservoirs. The reservoirs consist of a fractured medium rather than porous and the relative permeabilities there show less interaction than in the experiments. The results are an important contribution to understanding the behaviour of two phase flow through a geothermal reservoir. They can be used for geothermal modelling, thus contributing to more detailed modelling of geothermal reservoirs.

Votgufusvæði sem eru algeng tegund jarðhitasvæða samanstanda af sprungnu bergiog jarðhitavökva sem flæðir m.a. samkvæmt Darcy lögmálinu. Gufa getur myn-dast í slíku jarðhitakerfi við varmainnstreymi sem veldur hitastigshækkun eða meðþrýstingslækkun vökvans sem veldur hvellsuðu. Darcy jöfnur fyrir tveggja fasa flæðiinnihalda hlutlektir sem taka mettun fasanna með í reikninginn. Í þessari ritgerðvar Darcy lögmálið notað til að reikna hlutlektir vökva sem hvellsýður í jarðhi-takerfi og samanburður var gerður á tveimur flæðistilfellum, láréttu og lóðréttu.Niðurstöðurnar sýna að munur er á hlutlektunum í þessum tilfellum þrátt fyrir aðeiginleikar þeirra séu að öðru leyti þeir sömu. Þessi áhrif þyngdarafls á hlutlektirnarsáust einnig þegar mælingar voru gerðar á vatni og lofti. Tilraunir voru einniggerðar þar sem mældar stærðir voru notaðar til að reikna hlutlektir vatns og gufuúr jarðhitavökva. Hlutlektirnar fylgdu Corey ferlunum að nokkru leyti. SvokölluðShinohara aðferð var notuð á gögn frá borholum jarðhitasvæða til að reikna hlutlek-tir í tveggja fasa jarðhitakerfunum. Jarðhitakerfin samanstanda frekar af sprungnufrekar en gljúpu bergi og hlutlektirnar benda því til minni víxlverkana milli fasannaheldur en tilraunirnar sýndu. Niðurstöðurnar auka skilning á hegðun vatns og gufuí jarðhitakerfum og stuðla þær að bættum reiknilíkönum fyrir jarðhitakerfi.