Porosity and permeability of the overburden from wireline logs: a case study from offshore Malaysia

In the framework of a CO_2 storage feasibility study, we evaluate porosity and permeability of shale formations constituting the overburden of a hydrocarbon reservoir, where the gas is stored. These properties are required to perform fluid-flow simulations and analyze possible leakages from the reservoir to the surface. In this process, calibration with well logs is essential. Standard log-interpretation procedures on shales may induce errors, since the shale part is discarded due to its apparent negligible permeability compared to the clastic part. In this study, it is the “shale effect” that we consider by introducing its contribution to porosity and therefore to permeability, so that new expressions are developed to obtain porosity from density and traveltimes that consider this effect. Indeed, shales have non-zero porosity and finite permeability even if the flow rates are several orders of magnitude smaller than those in sandstones. Moreover, calibration does not mean to honour one single log profile but the interpretation should be compatible with all the profiles available. We obtain clay content, porosity and permeability from two wells offshore Malaysia using the linear and Stieber relations between gamma ray and shale volume. The results of using these relations are quite similar. However, the analysis shows that the density logs yield too small values of porosity. Instead, the sonic log gives a better estimate of porosity that honours the P-wave velocity. The verification is performed by using the Krief–Gassmann equations to obtain the P-wave velocity, either from the density-log or from sonic-log derived porosities. This is also confirmed by comparison to porosity obtained from the neutron log.