Development of In-situ Measurements to Determine Reservoir Condition Critical Gas Saturations during Depressurisation

This paper describes the development of experimental methods to determine critical gas saturations and relative permeabilities relevant to the depressurisation of volatile oils. A series of reservoir condition coreflood experiments at pressures up to 415 bara and 123°C is described. The experiments were conducted with aged core and fluids and comprised of a waterflood followed by depressurisation at different rates. A key part of the laboratory data was the measurement of extensive three-phase in-situ saturations. These measurements were conducted at full reservoir conditions by adding discrete gamma-emitting radioactive tracers to the brine and oil phases. The experiments were complicated by the following features and successful solutions have been implemented: • high CO2 content of the oil • radioactive tracer adsorption on the rock • ultra-low flow rates The oil contained up to 22 mole% CO2 and compatibility trials were conducted with the gamma-emitting radioactive tracers. The oil phase tracer was ferrocene (C10H10Fe) containing iron-59 (Fe ), and the initial brine phase tracer was potassium cobalticyanide (K3Co[CN]6) containing cobalt-58 (Co ). The C10H10Fe was demonstrated to be compatible, but the initial brine phase tracer was replaced with caesium chloride (CsCl) containing caesium-137 (Cs). This Cs brine phase tracer adsorbed on the core and the magnitude of the adsorption was a function of the experimental conditions. It was necessary to calibrate the tracers at reservoir conditions and this required the development of a new flooding protocol and the measurement of in-situ porosity using a gamma-attenuation technique. In order to investigate the influence of depressurisation rate on critical gas saturation, a range of rates was investigated. The longest duration experiment involved depressurisation over a period of 141 days with an average flow rate of just 0.3 mL day. This low rate required extremely high standards of leak integrity. It was necessary to develop a new core sleeving arrangement that combined low gamma-attenuation with ultrahigh leak tightness. The high CO2 content did not favour the use of elastomer or epoxy resin coated cores during depressurisation, and a core sleeve involving PTFE and aluminium was developed. Produced fluids were collected at reservoir conditions in a visual cell and the very low flow rates could not overcome the static friction of the piston in the visual cell. This problem was solved with the novel use of a fluorinated hydrocarbon which acted as a ‘liquid-piston’. The development of new experimental procedures has enabled the measurement of three-phase, in-situ saturations during the depressurisation of volatile oils. This data has been used to determine critical gas saturations and derive relative permeabilities which can be used in reservoir simulations.