Case Study: Application of Azimuthal Resistivity, Azimuthal Density, and Resistivity Inversion to Geosteer in a Clastic Stringer, Saudi Arabia

Abstract

Geosteering within thin clastic reservoirs can be extremely difficult due to their varying thicknesses, lateral facies changes and changing channel geometry. The efficiency of traditional triple-combo tools does not help achieve this objective due to their lack of azimuthal sensitivity. This paper presents a case history of reservoir navigation within a clastic reservoir in Saudi Arabia utilizing deep azimuthal resistivity, reservoir navigation modeling, and multi-component while drilling (MCWD) resistivity inversion.A number of challenges needed to be overcome for this well to become a successful oil producer. Firstly, the development and lateral continuity of the reservoir sand was not guaranteed. Reservoir sand developed at the bottom of the formation in all nearby offset wells. The development in the upper part of the target formation however, varied throughout the field. In addition, further uncertainty was present regarding position of the oil-water contact (OWC).A reservoir navigation strategy was devised utilizing a full triple combination of logging while drilling (LWD) tools that included deep azimuthal resistivity and azimuthal density. In addition, MCWD resistivity inversion software was used to ascertain the resistivity of the shoulder beds through inversion of real-time resistivity and azimuthal resistivity curves. The reservoir navigation strategy comprised maintaining a well inclination of 85 degrees until a clean reservoir sand was penetrated. MCWD would provide the first indication of the approaching reservoir, and well inclination would be increased in anticipation of reservoir entry. Upon entry in the reservoir, the deep azimuthal resistivity data, used in conjunction with reservoir navigation, would keep the well in the reservoir until TD or until the sand pinched out. In addition, MCWD would be used to ascertain the lower conductive boundary resistivity, either shale or water.This strategy was successfully implemented and resulted in 1,400 ft of reservoir contact, achieving 100% Net: Gross. The deep azimuthal resistivity prevented reservoir exit in spite of higher-than-anticipated formation dip and a thinning reservoir towards the toe of the well. MCWD successfully proved that the lower conductive boundary was shale, not water, and confirmed the distance to the boundary results from the deep azimuthal resistivity tool.