Proactive Geosteering in Thin Reservoir Bound by Anhydrite in Saudi Arabia

Abstract

TX 75083-3836, U.S.A., fax +1-972-952-9435. AbstractThin reservoirs of a few feet in thickness present a clear challenge to well placement. Drilling out of the target is a real possibility, and plugging back and reentry can be extremely difficult. Clearly, the best solution is to avoid exiting the target reservoir by detecting approaching boundaries as early as possible and by remaining at an optimal distance from the boundaries. This practice is known as "proactive geosteering." In recent developments, wave resistivity LWD and azimuthal wave resistivity sensors have been shown to effectively facilitate proactive geosteering. Their abilities to scan laterally several feet, up to more than 10 ft, around the wellbore and to identify the relative azimuth of approaching boundaries have been instrumental in recent successes.The challenge posed by the subject reservoir in this study is the combination of the thinness of the reservoir, approximately 3 ft, and the high resistive environments of the boundary, zero-porosity anhydrite formations, as well as the oil reservoir containing a low porosity dolomite layer. The challenge was met by carefully selecting the most appropriate measurements to send to the surface, interpreting them in real time, and using multi-boundary inversion. A series of pre-well simulations were run using offset wells. The simulation results showed that for the most likely scenario, shallow azimuthal wave resistivity curves and images provided the highest sensitivity to the approaching boundary. Medium and deep resistivity curves were less active and of lower resolution, but they contributed to the inversion for dual boundaries. Newly generated high resolution electric and density reservoir imaging and petrophysical logs were also interpreted in real time to assess the relative dip and provide finer control of the well angle. They helped to verify that the well remained within the reservoir through nearly its entire span, i.e., that the well was successfully placed with a high net-to-gross in a very thin reservoir in resistive environments.

Geosteering in Unconventional Shales: Current Practice and Developing Methodologies

“…Real-time dips, corrected for borehole size and depth of investigation, enable the determination of local apparent dip and the trajectory of the well relative to that dip. This answers the question of whether the well is going up structure, down structure, or paralleling dip (Al-Hajari et al 2009). …”