Far-Field Petrophysics for Saturation Mapping Beyond the Wellbore

Summary Most conventional logging-while-drilling (LWD) tools acquire real-time measurements from a few inches to a few feet away from the wellbore for lithology, porosity, and saturation evaluation. Deep and ultradeep resistivity tools were developed primarily for well placement and geomapping applications and can detect resistivity variations farther away, in all directions around the wellbore. This article describes the integration of deep and ultradeep resistivity measurements for saturation mapping beyond the wellbore. Deep and ultradeep resistivity tools can map multiple formation layers and determine the resistivity of those layers far from the wellbore. Because saturation is dependent on porosity and resistivity, a workflow has been developed to combine real-time shallow measurements with inverted LWD resistivity measurements and logging data from offset wells. The geological model is updated based on deep resistivity measurements, and petrophysical parameters for each layer, combined with inverted resistivity values, are used to derive saturation. Uncertainty is defined based on weighted uncertainty in porosity from offset wells and variation in resistivity across the mapped layers. Azimuthal resistivity measurements were also used to validate the inverted values and reduce uncertainties in cases of boundaries close to the wellbore. The workflow has been tested on several wells and different reservoirs, utilizing a combination of shallow and deep azimuthal data along with offset logs. Layers with different formation characteristics and saturation profiles were crossed, and the data were utilized to validate the mapped saturation values compared with the nearby measurements. The resulting saturation profile provided critical information that enhanced understanding of the reservoir and gave better insight into variation in the far field. Such information is critical for completion-design and well-placement decisions and aligns with the industry’s direction toward increased data analytics combining offset wells with real-time data. A continuous improvement of the model is expected as more wells are drilled and the database increases.