Abstract
Nuclear magnetic resonance (NMR) logging is a well-established technology for estimating porosity, pore-size distribution, and permeability in conventional reservoirs. However, the uncertainty associated with these estimates can be significant in complex heterogeneous carbonate reservoirs, such as those with variable pore size and pore network connectivity. In such cases, distinguishing isolated pore space is often impossible using well-log measurements and conventional well-log interpretation methods, which makes permeability evaluation unreliable. This paper proposes a new application of the NMR log-inject-log method to improve assessment of permeability and to distinguish isolated pores from connected pores. We propose injecting manganese-bearing solution in the rock samples and simultaneously analyze NMR measurements before and after injection of the contrast solution. We introduced a method to eliminate the impact of isolated pore space from the NMR T2 distribution, which is then used for improved permeability assessment.
To confirm the feasibility of the proposed method for field applications, we conducted NMR laboratory measurements in two carbonate rock types. We injected manganese-bearing solution into rock samples using a coreflood experimental procedure and measured NMR T2 distributions before and after injection. We then estimated isolated porosity and interconnected porosity by taking into account the difference between the NMR T2 distributions acquired before and after the injection of the contrast solution. We introduced a method to calculate geometric mean of T2 distribution for interconnected pores, and also to obtain effective free fluid volume and bound fluid volume. Finally, we used the T2 distribution corrected for the impact of isolated pores in conventional NMR-based permeability models to improve permeability assessment. We cross-validated the NMR-based permeability estimates against Kinkenberg permeability measured by an unsteady-state gas permeameter. The results confirmed that the proposed method enables quantifying the isolated and connected pore volume, and finally improves NMR-based permeability assessment. The new method provided estimates of permeability with approximately 10% average error in eight carbonate rock samples, which was a significant improvement compared to the errors observed before correcting T2 distribution for the impact of isolated pores.
