A computer implemented method for determining an amount of oil recoverable from porous reservoir rock by a miscible gas flood, the method comprising: analyzing a digital representation of the porous reservoir rock generated from three-dimensional imaging of the porous reservoir rock to determine geometric characteristics of a pore space of the porous reservoir rock; identifying locations of oil within a volume of voxels, wherein identifying the locations of oil comprises executing a first numerical multiphase flow simulation using the determined geometric characteristics of the pore space of the porous reservoir rock as a representation of an original state of the porous reservoir rock to obtain predictions of flow behavior of oil in the presence of a waterflood of the porous reservoir rock; identifying locations of gas within the volume of voxels, wherein identifying the locations of gas comprises executing a second numerical multiphase flow simulation using the determined geometric characteristics of the pore space of the porous reservoir rock as the representation of the original state of the porous reservoir rock to obtain predictions of flow behavior for a gas flood of the porous reservoir rock; and determining voxels within the volume where oil is either: a) coincident with gas, wherein determining voxels within the volume where oil is coincident with the gas comprises performing a logical AND operation on data representing a state of the pore space after the waterflood and a state of the pore space after the gas flood, or b) connected to gas by a continuous oil path, wherein determining voxels within the volume where oil is connected to the gas by the continuous oil path comprises identifying voxels that have contributed oil in a direct displacement and identifying voxels that have connected hydrocarbon phases to one or more of the voxels within the volume where oil is coincident with the gas, wherein the voxels are determined to estimate the amount of oil that is recoverable from the porous reservoir rock, wherein an amount of recoverable oil removed by a direct displacement is determined based on voxels within the volume where oil is coincident with the gas, and an amount of recoverable oil removed by extraction and swelling is determined based on voxels within the volume where oil is connected to the gas by the continuous oil path. analyzing a digital representation of the porous reservoir rock generated from three-dimensional imaging of the porous reservoir rock to determine geometric characteristics of a pore space of the porous reservoir rock; identifying locations of oil within a volume of voxels, wherein identifying the locations of oil comprises executing a first numerical multiphase flow simulation using the determined geometric characteristics of the pore space of the porous reservoir rock as a representation of an original state of the porous reservoir rock to obtain predictions of flow behavior of oil in the presence of a waterflood of the porous reservoir rock; identifying locations of gas within the volume of voxels, wherein identifying the locations of gas comprises executing a second numerical multiphase flow simulation using the determined geometric characteristics of the pore space of the porous reservoir rock as the representation of the original state of the porous reservoir rock to obtain predictions of flow behavior for a gas flood of the porous reservoir rock; and determining voxels within the volume where oil is either: a) coincident with gas, wherein determining voxels within the volume where oil is coincident with the gas comprises performing a logical AND operation on data representing a state of the pore space after the waterflood and a state of the pore space after the gas flood, or b) connected to gas by a continuous oil path, wherein determining voxels within the volume where oil is connected to the gas by the continuous oil path comprises identifying voxels that have contributed oil in a direct displacement and identifying voxels that have connected hydrocarbon phases to one or more of the voxels within the volume where oil is coincident with the gas, a) coincident with gas, wherein determining voxels within the volume where oil is coincident with the gas comprises performing a logical AND operation on data representing a state of the pore space after the waterflood and a state of the pore space after the gas flood, or b) connected to gas by a continuous oil path, wherein determining voxels within the volume where oil is connected to the gas by the continuous oil path comprises identifying voxels that have contributed oil in a direct displacement and identifying voxels that have connected hydrocarbon phases to one or more of the voxels within the volume where oil is coincident with the gas, an amount of recoverable oil removed by a direct displacement is determined based on voxels within the volume where oil is coincident with the gas, and an amount of recoverable oil removed by extraction and swelling is determined based on voxels within the volume where oil is connected to the gas by the continuous oil path.