Considering Lithological Variability in Top Seal Analysis
Top seal capacity plays a key role in prospect risking and is often expressed in a maximum hydrocarbon column height the seal could hold at the crest of the trap. Top seal capacity is associated with two mechanisms: a) capillary sealing and b) mechanical sealing. In its general sense, capillary sealing describes the capability of a top seal saturated with water to prohibit water displacement by the buoyant reservoir fluid. The maximum capillary sealing capacity of a top seal is thus given by the ratio of the critical capillary entry pressure of the water-wet top seal and the buoyancy force of a hydrocarbon column height. The critical capillary entry pressure of a top seal is dependent on its pore throat size distribution and the properties of the hydrocarbon and water phases. Once the critical capillary entry pressure of the top seal is exceeded the seal leaks. Mechanical sealing reflects the buoyancy pressure exerted by a hydrocarbon column a top seal rock can hold before it fractures and releases the excess pressure and is a function of overburden, overpressure and the properties of the water and hydrocarbon phases. While mechanical sealing capacity is usually calculated as part of pore pressure prediction, capillary sealing capacities are derived from samples using mercury injection capillary pressure (MICP) experiments in the best case. However, in most cases MICP data is either not available for the top seal section and a different approach has to be taken, or is disassociated with the lithological variability of the top seal. Based on the combination of pore pressure prediction, conventional geophysical well logs and Monte Carlo simulation, we developed a workflow to estimate capillary and mechanical top seal capacities guided by local and general geological constraints. Hereby, we derive clay content distributions from calculated v-shale logs calibrated with typical clay content ranges for different depositional environments and, if available, utilize the density log to calibrate an overburden estimate. Subsequently, the workflow integrates established relationships from the public domain, which relate clay content and effective stress to porosity, permeability and critical pore throat size to calculate capillary and mechanical sealing capacities in terms of a maximum hydrocarbon column height distribution. The output can be directly used for top seal risking in prospect evaluation and/or input for further volumetric calculations.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014