Physical constraints on hydrocarbon leakage and trapping revisited

Abstract

In a water-wet petroleum reservoir with a water-wet seal, a continuous water phase will extend from the reservoir into the seal, and the pressure difference between the water phase in the uppermost pores of the reservoir and the water phase in the lowermost pores of the seal can therefore only be of an infinitesimal magnitude. This implies that any overpressure in a water-wet reservoir will not contribute to pushing the hydrocarbons through a water-wet seal, and overpressured water-wet reservoirs should therefore not be considered more prone to capillary leakage than normally pressured reservoirs. Within a water-wet petroleum reservoirs, the overpressure in the hydrocarbon phase relative to the water phase is balanced by the elastic forces at the fluid interface (interfacial tension). The overpressure in the hydrocarbon phase relative to the water phase therefore does not increase the risk of hydrofracturing the reservoir's seal. This implies that the risk of hydrofracturing should not be increased as a function of hydrocarbon column height, and should not be considered to be higher for gas than it is for oil. When an upward-directed hydraulic gradient is present from a reservoir unit into the overlying seal, water will continuously move upwards from the reservoir unit and into the seal if both rocks are water-wet. This movement of water may lead to exsolution of gas above the reservoir unit, and the presence of free gas may be detected as gas chimneys on seismic sections. This mechanism will operate regardless of whether or not a hydrocarbon accumulation is present below the gas chimneys, and fracturing of the reservoir unit's seal or capillary leakage of hydrocarbons are therefore not necessary conditions for the development of gas chimneys.