Under-Balance Drilling (UBD) has been used in several carbonate fields of the Cluster, North Oman at Petroleum Development Oman (PDO). The reasons have been to improve productivity and injectivity, real-time reservoir characterization and to reduce/eliminate acid clean out. By using UBD in several fields, a better picture of well candidate selection and benefits has been derived.

In this paper we will present the results of successful utilization of UBD that may be applied to other fields. The results include an improvement in productivity, real-time decision making and a better understanding of reservoir flow characteristics. In summary the success of UBD depends on the selection of the fields which we elaborate on using several examples. These examples will demonstrate in details the history, application and benefits of UBD. The presentation will also illustrate future work planned using UBD.


The cluster of fields located in the north of Oman where all the fields are within 35 kilometres of each other (Fig. 1). PDO is split into the two main sections i.e. the North and South. The sections are then subdivided into Clusters which are made up of geographically-related fields. There are three main reservoir units in the Shuaiba carbonate reservoir in the cluster of fields: Upper Shuaiba, Lower Shuiaba and Kharib. UBD started in the cluster of fields in December 2003 targeting the Lower Shuaiba reservoir following a feasibility study which indicated the possibility of benefits using UBD.

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