Studies conducted earlier indicate the importance of solids invasion on determining the magnitude of Flow Initiation Pressure (FIP). Other studies have shown that relative permeability effects can play an important role in determining the FIP. In this paper we clearly identify the conditions under which solids invasion and relative permeability effects can play important roles.

Experiments are conducted with brine saturated cores and cores containing a residual brine saturation. FIP measured by flowing back with brine and with oil are reported. For low permeability rocks and small mobility ratios, relative permeability effects appear to play a dominant role. For higher permeability rocks (>100 md) and large mobility ratios, solids invasion becomes increasingly significant. A key difference between these two components of FIP is that while relative permeability effects will dissipate with time, internal filter cakes may not.

A model to calculate the FIP is presented which allows us to evaluate the relative magnitude of the pressure drop caused by relative permeability effects versus that caused by solids invasion. The results of this study clearly show that both solids invasion and filtrate invasion can play an important role in determining the FIP. In addition factors such as the relative mobility of water and oil and the depth of invasion of solids are important parameters. A method is presented that allows us to estimate FIP values in the field based on static filtration tests conducted on cores.