Abstract - There is a lack of consensus in the literature on the significance of hydrodynamic effects on hydrocarbon column heights trapped by capillary seals over geological timescales. The efficiency of the water drive mechanism in production is evident from the ability of high, production induced, water pressure gradients and flow rates to displace hydrocarbons. However, in the area of hydrocarbon exploration, it has been suggested that hydrodynamic effects can be effectively ignored in the evaluation of seal capacity over geological timescales. It is argued that, for the relatively low water flow rates which occur naturally, the relative permeabilities to water of trapped oil columns are low enough that viscous forces are negligible and only the capillary properties of seals need to be considered. This conclusion has largely been reached by consideration of thick top seals. When considering fault seal, and in particular the efficiency of membrane seals arising from the presence of high capillary threshold pressure fault rock, it is not clear whether pressure compartmentalisation in the water-phase can be ignored when estimating trapped column heights.
To examine the effect of across-fault pressure differences on trapped column heights we have solved the two-phase flow equations including capillary, viscous and gravity forces in 1D for a dipping carrier. We assume an oil column is trapped on the down-dip side of the fault and allow water flow to be either up or down dip, hence the trapped oil column is on either the upstream or downstream side of the seal. For a constant water flow rate we solve for the stable trapped column height which provides a dynamic balance between viscous, gravity and capillary forces. Results show that for realistic fault seals, naturally occurring across fault pressure differences can be significant and can impact the column height both positively and negatively depending on the water flow direction. Our modelling also indicates that realistic hydrodynamic flow rates can also influence the column heights supported by relatively thin or high permeability top seals as well as fault seals.
Abstract of talk given to:
Structurally Complex Reservoirs, Geological Society of London, February 2006.