The effect of mud-filtrate invasion on resistivity, nuclear, and formation tester wireline measurements is poorly understood when the formation of interest is laminated on a scale below the vertical resolution of borehole logging instruments. This paper describes a quantitative study to assess the effect of mud-filtrate invasion on well-log measurements acquired in thinly laminated clastic sequences.
Synthetic models are constructed with varying proportions of shale and two types of sand. These models are used to simulate the process of mud-filtrate invasion. Each type of sand is assumed isotropic, homogeneous, water wet, and saturated with oil to the level of irreducible water saturation. Simulations of the process of mud-filtrate invasion yield 2D cross-sections of water saturation, salt concentration, and electrical resistivity in the invaded rock formations. These cross-sections are then used to simulate wireline resistivity, density, neutron, and formation tester measurements. Using standard interpretation techniques, saturation of original oil in place is calculated with all the simulated log measurements.
Results indicate that for laminated sandy and shaly sand rock formations, induction (electrical resistivity), nuclear (porosity), and formation pressure tester (pressure) measurements are significantly affected by both the relative proportion of lithology and the invasion of mud filtrate. Water saturation in a laminated sandy rock formation can vary significantly when calculated with saturation and porosity exponents perpendicular and parallel to the bedding plane, respectively. Permeability calculated from dual-packer formation tester measurements can significantly depart from the effective average permeability in a composite and laminated sandy formation.
Conventional induction resistivity instruments cannot distinguish between resistivities in the flushed and invaded zones when the volume of shale is higher than 50%. Presence of shale also causes a significant error in the estimated permeability.