Interpretation of borehole measurements acquired in laminated clastic sequences subject to mud filtrate invasion
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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 thesis 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 both laminated sandy and shaly sand rock formations, induction (electrical resistivity), nuclear (porosity), and formation pressure tester (pressure) measurements are significantly affected by both relative proportion of lithology and invasion of mud-filtrate. For a thinly laminated sandy rock formation, invasion of mud-filtrate gives rise to an average increase of bulk density of 5%. A decrease of approximately 24,000 ppm in the salinity of the mud filtrate gives rise to a 10% decrease in water saturation if the water saturation is calculated with resistivity readings acquired with conventional induction logging instruments. Water saturation in a laminated sandy rock formation can vary from 10% to 23%, when calculated with saturation and porosity exponents perpendicular and parallel to the bedding plane, respectively, and with the vertical resistivity reading acquired with a tri-axial induction instrument. Permeability calculated from dual-packer formation tester measurements can significantly depart from the effective average permeability in a composite and laminated sandy formation. The error in the estimated permeability is 12% in a laminated sandy formation with equal proportion of high and low permeability sands. In laminated shaly sand rock formations, the error in the estimation of water saturation is 25% lower when calculated with the vertical resistivity measured by a tri-axial induction instrument than when calculated with the resistivity measured by a conventional induction instrument. 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. For a laminated shaly sand formation with 50% volume of shale, the error in the estimated permeability is approximately 33%.