oa Geosteering horizontal wells in the mid-Cretaceous Shu’aiba Formation, Oman

Petroleum development Oman (PDO) has been using geosteered horizontal wells in the Qarn Alam area to maximize oil-recovery rates, production and minimize attic oil. The Shu’aiba Formation is a mid-Cretaceous carbonate platform in this area, and its top is represented by a four million year old erosional surface. To complicate matters, the formation is also faulted, fractured and has erosional channels cut into its top surface. The mid-Cretaceous Nahr Umr shale conformably overlies the Shu’aiba and provides the seal for the reservoir. Unfortunately the log characteristics of the Shu’aiba carbonate and the Nahr Umr shale are very similar with very low resistivities of 1.0–4.0 Ohm for the Shu’aiba oil reservoir and 0.6–1.0 Ohm for the Nahr Umr shale. The objective of geosteering is to stay within a zone 0.5–1.5 m below the top Shu’aiba erosional surface where the reservoir characteristics are best, without exiting into the overlying shale. Currently this technique has been tried in three fields in the Qarn Alam area using three<br>different log-while-drilling (LWD) contractors and five different LWD tools. The resistivity response is the main geosteering tool used; gamma-ray and rate of penetration (ROP) are also important. Geosteering has so far resulted in wells being drilled within the 1.0 m target-window more than 80% of the time, and exits into the overlying shale are less than 0.4 exits per km. This presentation will concentrate on the continuing development of the technique and logresponse characteristics for geosteering the wellbore.