Proactive Geosteering in Thin Reservoir Bound by Anhydrite in Saudi Arabia
Authors: Al-Hajari Abdrabrasool , Soremi Adeyinka , Ma Shouxiang Mark , Julaih Ali , Thompson Troy William , Saghiyyah George , Lotfy Amr , Bayrakdar Mohammed Aref , Bittar Michael S. , Chemali Roland E. 
Published: May 2016

Abstract
Thin 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.

Introduction
Drilling a horizontal well and maintaining it within the best section of reservoir present multiple and diverse challenges. For very thick reservoirs, large errors in well placement may not affect the initial production, but the overall sweep efficiency during the life of the field and the ultimate recovery of hydrocarbon are likely to be affected (Chemali et al. 2008). In general, the optimal location for a well in a thick reservoir is selected on the basis of future performance. The preferred well path is often defined with reference to geological boundaries, including overlaying conductive or resistive boundaries (such as shale, anhydrite, and oil-water contact). As a result, geosteering methods in thick reservoirs have consisted of monitoring the distance between the well path and a reference boundary, and adjusting the well direction to maintain that distance within prescribed limits (Iversen 2003).
For thin beds, the challenges generally consist of remaining within the boundaries of the reservoir and of experiencing as few exits as possible. When they occur, reservoir exits should be short with a rapid return to the pay zone, but severe doglegs must be avoided. Doglegs interfere with drilling or with running completion, including screens or casings. Clearly, every interval drilled out of the reservoir rock is a non-productive interval. In many cases, out of zone intervals are also less mechanically competent than the reservoir rock, giving rise to drilling problems. If an exit cannot be avoided, it is important to rapidly define a return path into the reservoir.