Intelligent Wireline Formation Tester Evaluation of Low-Permeability and Low-Resistivity-Contrast Formation

Abstract

Abstract Exploration and development drilling in offshore China is extending to Paleogene formations that are characterized by low-resistivity-contrast and low-permeability rocks. These formations have become a focus for increasing reserves and production. During exploration activities, these low-resistivity, low-formation-contrast formations have been critical and challenging for formation evaluation because the geological structure and lithology are more complex than in previously discovered fields. Differentiating hydrocarbon from water using petrophysical interpretation has a large uncertainty in these formations. Confirming the fluid type using conventional formation testing technology has been extremely challenging because the produced fluid is mainly mud filtrate, which is no use for fluid confirmation. A new-generation intelligent wireline formation testing platform consisting of a focused radial probe inlet and a dual flowline with dual downhole pumps to enable flexible focused sampling was applied to three appraisal wells in offshore China. Given the larger flow area of the probe system, flow tests could be conducted in as low as 0.004-md/cP mobility zones (the tightest on record), and fluid identification could be performed in-situ while the fluid flowed through a group of sensors. Previous formation testing in these formations had been challenged because the water-based mud system caused suspension of solid particles (debris and mud solids). Filter and standoff accessories available with the intelligent wireline formation platform enabled designing a fit-for-purpose approach to overcome this challenge in a short time. This dedicated design resulted in increased efficiency in water sampling compared to previous testing done by the operator. Clean water resistivity, measured in situ, can now be applied to this new exploration block to recalculate the water saturation for reserve estimation. Whereas previous gas-water transition zone sampling was challenging because high water-based mud filtrate fractions masked the presence of formation water and formation hydrocarbon, the radial probe, combined with state-of-the-art resistivity measurements, allowed identification of gas and the measurement of formation water resistivity in a multiphase flow environment. The formation testing of these low-resistivity-contrast and low-permeability formations enabled acquisition of a 2% contaminated formation water sample in 140 minutes with formation mobility of 1 md/cP. The gas-water zone was confirmed from a dual flowline resistivity measurement and a hydrocarbon show in mobility of 1.4 md/cP.