Reducing Well Placement Uncertainty and Improving Reservoir Understanding in Multi-Target Horizontal Wells Using Advanced Far-Field Petrophysics

Abstract One of the main objectives while drilling a production lateral is to place it in a zone with optimal formation properties at a safe distance from the oil-water contact to avoid early water production. For these purposes, in addition to standard LWD sensors, azimuthal deep and ultra-deep resistivity tools are often used. This article provides examples of far-field petrophysics in multilayered sand channels to identify, at a distance, preferred reservoirs for production. Trajectory corrections are usually made based on logging data, including azimuthal deep or ultra-deep resistivity measurements. Triple combo measurements are used to preferentially locate the wellbore in the target reservoir. The following qualitative intervals can be distinguished in clastic reservoirs: shale, clean sand, silty sand, and shaly sand. When drilling in multilayered sand channels, there is uncertainty associated with estimating the quality of reservoirs at a distance above or below the wellbore. An advanced far-field petrophysics methodology helps to evaluate the shale volume and water saturation in multiple sand channels at a distance. While drilling a horizontal well in multiple sand channels, shale, clean sand, silty sand and shaly sand intervals were identified based on qualitative estimates from gamma ray, resistivity, density, and neutron porosity logs cut-offs. Up to 7 layers of different resistivity were mapped by azimuthal ultra-deep resistivity using 1D and 3D inversions. An advanced far-field petrophysics methodology was implemented to estimate the shale content (using shale index ratio) and water saturation of sand channels located above and below the wellbore trajectory. Offset well data was used to define formation boundaries and petrophysical properties. Real-time data was used to confirm/adjust defined formation boundaries and petrophysical properties for lateral variations. Ultra-deep resistivity 1D/3D inversions provided Rh/Anisotropy Ratio. 3D anisotropy ratio inversion values were used to compute average shale index for lithology identification. Interpolated or kriged offset well porosity data corrected for actual volume of shale is projected onto the target wellbore and correlated against the inverted and measured resistivity and porosity data when different boundaries are crossed while drilling or observed in the far-field measurements. Far-field water saturation was derived for the mapped formation layers, with considerations of drainage and imbibition Archie parameters and variations of formation water salinities. This paper describes a methodology for multilayered sand channel characterization to reduce uncertainty and increase reservoir understanding while geosteering horizontal wells. As a result, in addition to qualitative assessment of intervals of shale, clean sand, silty sand and shaly sand near the wellbore, far-field sand channel characterization was performed to reduce uncertainty in identifying optimal zones in which to place the well.