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.
Since most horizontal wells are completed open-hole, formation damage caused by drill-in fluids is of great concern. For perforated wells, the length of perforation tunnels normally exceeds the depth of damage caused by drill-in and completion fluids thereby bypassing the damage.
Removing damage caused by drill-in fluids in horizontal wells is difficult and expensive. In some instances it may be operationally impossible due to the inability to run coiled tubing far away from the heel of the well. Even in cases where chemical treatments are possible they are very expensive and the effectiveness of most of the clean-up procedures is questionable. Longeron et al1 showed that under laboratory conditions, breakers do not always efficiently clean up the damage caused by different fluids. Hence, it is important to prevent damage by designing appropriate drill-in fluids.
The Flow Initiation Pressure (FIP) is a laboratory-measured quantity that is related to the drawdown required to initiate flow into the well. The FIP determines whether the entire open-hole section contributes to flow into the well or only a portion of it.