Building realistic and reliable subsurface models requires detailed knowledge of both the rock and fluids involved. While the hydrocarbon volume estimation has a profound impact on the viability of a development, next to the permeability, saturation height models, free fluid levels and the hydraulic communication have a significant role in determining the recoverable reserves.
When in different parts of the same field different free fluid levels (leading to different fluid contacts for the same rock quality) are identified, the lateral hydraulic communication at the field level can be challenged. This aspect is of importance since the hydrocarbon volume distribution impacts the recovery factor. At the same time building and initializing a model based on different free water level positions (zero capillary pressure) is challenging.
In this work, we propose a new strategy in studying one process leading to different free water levels (FWL) known as "perched" water contacts. Perched water contacts are the result of water entrapment (behind barriers for lateral flow) during hydrocarbon migration in the reservoir. The fundamental controls that lead to the perched contacts formation are studied and shown to be the rock quality and relative permeability. Counterintuitively, the perching effect is not going to feature in poor quality rocks (sub-milli Darcy permeability) – the effects would be visible only for a considerable barrier height. Regarding transition zones, the results show no significant difference is expected above the perched zone when compared to the unconstrained parts of the field. Field observations and dynamic simulations are used to identify the perching controls and diagnostics for perched contact identification.
Saturation height models (SHM) combined with fundamental rock properties (porosity and permeability) and free fluid levels are the base for a volumetric analysis hence being determinant in assessing the viability of a project. In estimating hydrocarbon volumes one possible challenge arises with the measurement of multiple FWLs (leading to different HAFWL) across the field while the hydrocarbon column appears in equilibrium.