Effect of Positive Rate Sensitivity and Inertia on Gas Condensate Permeability at High Velocity

The authors have previously reported that gas condensate relative permeability will increase with increasing velocity when conducting steady-state measurements. The increase in relative permeability was referred to as ‘positive rate sensitivity’ or the ‘positive coupling effect’. A systematic series of core tests has since been conducted, generating data at IFT values ranging from 0.015 to 0.78 mN m−1, using core types ranging in permeability from 11 to 350 mD. The results confirmed that the positive coupling effect existed in low permeability cores and different lithologies at low and high IFT. The maximum tested velocity was in the region of 75 m per day, which was estimated to be at the boundary above which inertia would be significant.

To investigate the competition between inertia and the positive coupling effect, subsequent core tests have been conducted at velocities ranging from 7 to 700 m per day, with the higher velocity being in the region of fractions of a metre from the wellbore. The maximum test velocity was one order of magnitude above the velocity boundary with significant inertia. The test programme has generated previously unpublished data, showing that the positive coupling effect initially reduces the effect of inertia before becoming dominant in the core tests conducted using gas condensate fluids. Such unique data provide valuable information on the forces governing production at the wellbore to reservoir engineers.