When the water-cut is too high for economical production, the field is usually abandoned with significant quantity of oil and gas still remaining in the reservoir. It is likely that in some cases a significant amount of gas and residual oil can be produced by the process of depressurisation.
The depressurisation process includes several stages: supersaturation, gas nucleation, bubble growth, critical gas saturation and finally oil and gas production. When the pressure of saturated oil falls below the bubble point pressure, there comes a point at which stable nuclei are formed in the supersaturated oil. The difference of pressure at which the first bubble is formed during depressurisation and the saturation pressure is called the critical supersaturation.
Formation of a new gas/liquid interface requires energy; therefore the liquid has to be at a pressure lower than the bubble point pressure, before a nucleus can form. The required work to create a gas bubble in the saturated oil phase is a function of the interfacial tension between gas and liquid. When the gas/oil interfacial tension is high the required work to create a gas nucleus will also be high. The extent of critical supersaturation is affected by dynamic conditions as well as departure from equilibrium conditions. Hence increasing the depletion rate, that is, lowering the time at which a certain level of supersaturation is maintained will increase the value of critical supersaturation. The limit of supersaturation may be determined by thermodynamic considerations. Once gas bubbles are formed, and depressurisation is continued, bubbles keep growing and displacing oil and water. As the nucleation mechanism has a direct impact on the gas evolution and hydrocarbon recovery during the depressurisation process, the nucleation phenomenon is of great interest.
The underlying physics of the above complex three-phase flow is not well understood to allow reliable predictions to be made for economic evaluation of the process.
Description
The main objective of this work was to provide a clearer understanding of the physical processes occurring during depressurisation. During the two phases of this study, which lasted for six years, we have conducted flow visualization at the pore level using high-pressure glass micromodels to identify the key features of the process. Depressurisation tests were conducted on model fluids simulating black and volatile oil samples at several depletion rates. Micromodels with realistic pore patterns and different wettability characteristics, including water-wet, oil-wet and mixed-wet, were used in the tests. A series of experiments were performed to investigate the effect of fluid properties, depletion rate, saturation conditions, and wettability and saturation history on the nucleation process, gas evolution and hydrocarbon movement. Tests were conducted on water-flooded oil as well as virgin oil.
Figure 1 shows the contact angle of water/gas interfaces when water was initially injected into a clean and dry mixed-wet micromodel, indicating that at some gas-wet locations. When this system is pressurised with water, some gas could be trapped in the gas-wet pores and increasing the pressure, within the operating pressure range, cannot remove all the trapped gas. Subsequent depressurisation of this system activates these micro-bubbles and gas bubbles are formed in their vicinity at the glass surface in both oil and water phases. Unlike the bulk nucleation, gas nuclei stick to the glass surface and cannot move easily towards the top section of the oil ganglia by the buoyancy force. The value of critical supersaturation in systems that contain pre-existing micro bubbles is low and it is related to the pore geometry and the filling process rather than the thermodynamic conditions.
Our observation highlighted that gas nucleation was dependant on fluid properties, particularly the gas/oil interfacial tension (IFT). In high IFT systems, the nucleation behaviour and the degree of supersaturation were affected by the saturation history and system wettability. In volatile oil systems with low interfacial tension between gas and oil, nuclei could form homogeneously and independently of wettability conditions and saturation history. Critical gas saturation and subsequent gas/oil production are strongly affected by the critical value of supersaturation and the number of bubbles that are formed during the depressurisation process. Two generalized correlations were developed to predict the critical supersaturation and the number of bubbles. The correlations reliably predicted independent experimental data reported in the literature. The data generated by the study provides information on gas nucleation sites, the gas nucleation rate and build up of the gas phase, which are essential in field development planning and estimation of oil and gas recovery. The information can also be used to verify the validity of results by network model simulators, before applying them to estimate important parameters such as the relative permeability for reservoir studies.
Figure 1. Water/gas interfaces in a mixed-wet micromodel when water is injected into a clean, dry micromodel, saturated with air (shown as gas).1, 2 & 3 indicate gas-wet, water-wet and neutral-wet interfaces, respectively.
Publications
2005
- K.S. Nejad, A. Danesh “Visual Investigation of Oil Depressurisation in Pores with Different Wettability Characteristics and Saturation Histories”, SPE 94054, Proceedings of the SPE Europec Conference, June 2005.
- K. Shahabi-Nejad and A. Danesh, P. Cordelier and G. Hamon, Total “Pore-Level Investigation of Heavy-Oil Depressurisation,” Proceedings of the SPE/PS-CIM/CHOA International Thermal Operations and Heavy oil symposium, November 2005.