Speaker
Description
Depressurization in hydrocarbon reservoirs can mobilize trapped gas in the presence of residual oil and water and lead to improved recovery. The effects of reservoir pressure cycling are also important for storage applications in depleted reservoirs, like temporary storage of natural gas and hydrogen, and in permanent CO2 storage, where reservoir pressure may drop temporarily due to fault activation or leakage. Traditionally, drainage and imbibition processes in the reservoir have been studied by fluid invasion and displacement at the pore scale, that may lead to trapping. Here, we will instead focus on the drainage and imbibition characteristics that occur due to the expansion and compression of the trapped gas in the presence of residual oil and water when the reservoir pressure changes.
To this end, we use a level set model for capillary-controlled displacement with local volume conservation as a basis for the investigations [1]. The model enforces volume conservation of disconnected ganglia by modifying their pressure to prevent volume changes, and it also conserves volume during ganglion splitting and merging. Thus, simulations predict the pressures of trapped ganglia, which is a prerequisite for describing pressure-volume behaviour of ganglia under various processes, such as Ostwald ripening of trapped gas [2]. Here, we extend the model to handle local mass conservation of a compressible gas, in the presence of incompressible oil ganglia and water, when the (uniform) reservoir pressure changes stepwise. The strategy is to first calculate the equilibrium gas pressures for trapped ganglia from which we calculate the number of moles of gas from an equation of state (EOS). Then, for each stepwise change in reservoir pressure, we combine the EOS with the volume conservation equation to find the gas pressure in each level set iteration that corresponds to the volume for the current reservoir pressure. In the case of cubic EOS, the resulting gas pressure equation is a fourth order polynomial which we solve numerically. The reservoir pressure is changed once a static three-phase fluid configuration is achieved.
Using the developed model, we perform quasi-static simulation of depressurization followed by re-pressurization on trapped gas configurations (using CH4 and CO2) in the presence of residual oil ganglia and water achieved from the simulation of a conventional gas-water invasion cycle on a 3D segmented micro-CT image of sandstone. We monitor changes in average ganglia capillary pressure as a function of trapped gas saturation and show the hysteresis behaviour. The simulations show that gas ganglia coalesce as they expand during depressurization, leading to oil displacement. Eventually, a percolating gas cluster forms and the critical gas saturation is calculated. Re-pressurization results in snap-off of large ganglia as they get compressed. The gas connectivity, quantified by the Euler characteristic, also displays hysteresis. Further, the hysteresis from reservoir pressure cycling is different from standard injection-displacement experiments due to the expansion and compression behaviour of the gas, which is further demonstrated by the comparison of fluid configurations in the two cases. Hence, reservoir pressure cycling calls for other hysteresis models in reservoir simulation.
| References | [1] Jettestuen, E., Friis, H. A., Helland, J. O. (2021). A locally conservative multiphase level set method for capillary-controlled displacements in porous media. Journal of Computational Physics 428, 109965. https://doi.org/10.1016/j.jcp.2020.109965. [2] Singh, D., Friis, H.A., Jettestuen, E., Helland, J. O. (2023). Pore-scale Ostwald ripening of gas bubbles in the presence of oil and water in porous media. Journal of Colloid and Interface Science 647, 331–343 (2023). https://doi.org/10.1016/j.jcis.2023.05.070. |
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| Country | Norway |
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