Fluid pressure is a key factor in determining hydrocarbon productivity of unconventional plays. Most unconventional plays have experienced uplift accompanied by erosion after the maximum burial depth, which has altered the fluid pressure in both fluid dynamics and thermodynamics. In this study, we calculate the fluid pressure variation of unconventional plays during uplift, taking into account the factors of hydrocarbon compositions, thermal history, initial pressure, and sealing conditions. The calculation considers both fluid dynamics (Darcy flow and diffusion) and thermodynamics (pressure-volume-temperature relationships). The results show that uplift with an ideal sealing condition may enhance the overpressure of gas reservoirs and that of most volatile oil reservoirs with gas to oil ratio (GOR) > 1000 scf/bbl, and decrease the overpressure (or form underpressure) in black oil reservoirs with GOR < 500 scf/bbl. Good sealing layers with low permeability, high capillary pressure and low diffusivity are required to keep the abnormal pressure in unconventional reservoirs.
Liquid-rich unconventional plays have had an important contribution to new discoveries in recent years1. Because of the ultra-low permeability in tight hydrocarbon reservoirs (e.g., micro- to nanoDarcy scale for the Barnett shale2), high fluid pressure is beneficial for productivity. High fluid pressure is especially necessary for liquid hydrocarbon production because of its low mobility. Pressure higher than hydrostatic pressure at reservoir depth is defined as overpressure. More specifically, when the pressure gradient is more than 12 kPa/m (0.53 psi/ft) or the ratio between the fluid pressure and hydrostatic pressure is more than 1.2, it is referred to as overpressure. When the pressure gradient is less than 9.8 kPa/m (0.43 psi/ft) or the ratio to the hydrostatic pressure is less than 1.0, it is referred to as underpressure3. Some examples of abnormal pressure in unconventional reservoirs are summarized in Table 1.