TX 75083-3836, U.S.A., fax 01-972-952-9435.

In July 2004 a Northern Oman Carbonate reservoir asset team embarked on an Underbalanced Drilling (UBD) campaign after a detailed feasibility study. The main objectives of the UBD campaign were to improve the Productivity Index (PI) and test the validity of real time reservoir characterizations (RTRC) in UBD operations. To date 16 wells have been successfully drilled UBD resulting in almost double the PI and better initial well production with a decline trend better or comparable to the offset conventional wells in the area. Real time reservoir characterizations (RTRC) helped identifing the features contributing to water production and enabled real time decision making to optimize well path and sometimes extension of the well length by correlating real time resistivity data with real time production. RTRC also enabled optimum completion selection to maximize production as well as ESP sizing and installation of proper water shut off mechanisms (the preferred water shut off method is expandable Open Hole Clad "OHC"). Reservoir and geological models were updated in accordance with this analysis. Other benefits seen from the UBD campaign include eliminating the need for stimulation, TLC logging (i.e. MPLT, FMI, etc...) and curing losses while drilling the reservoir section along with technology maturation of improved Down Hole Isolation techniques and the running of the OHC in UB situation. A key success factor was the assimilation of a multidisciplinary integrated team comprising of the asset team, well engineering, new technology, drilling and UBD contractors. They all contributed to the success of UBD campaign which resulted in new oil delivery ahead of time and within budget. As a result of the high success of the UBD campaign in 2004/2005 the asset team decided to drill all its oil producers underbalanced. However, the challenges that have arisen are mainly in the cost of UBD packages and services as well as improving the reliability of some of the well control tools such Down Hole Isolation Valves (DHIV) and the application of contingencies in case of their failure. Introduction to the field The field was first developed using widely spaced vertical wells. As the technology developed and field understanding grew horizontal wells were introduced in the early 1990's. Increased drilling activities have raised the gross fluid and net oil production rates in the field. The Northern Oman Carbonate reservoir field is located in NW Oman and is 26 km long, 12 km wide. The Reservoir forms a low relief anticline with NE-SW orientation with an overlying shale caprock. The Reservoir was deposited in a varying range of platform interiors, platform margins, slopes and basin environments resulting in a heterogonous reservoir characteristic. The reservoir is crossed by two major sets of faults. Seismic interpretation and FMI readings clearly indicate a NE-SW set and a NW-SE set. These faults act as water conduits when intersected during drilling resulting in early water break through and significant increase in BS&W. Maximum reservoir thickness is about 50 m with a large transition zone to water, the average log saturation shows the 50% water saturation level at approximately 1472 m tvdss and free water level (FWL) at 1490 m tvdss. For optimum drainage results the standard well path is designed to be three meter below the top of reservoir. Field production started in 1970 and progressively increased to reach peak production in 1973. After 1973 the production went through a gradual decline before it stabilized in the 1980's then slowly rising in the 1990's. In 2002 the production increased to reach its highest peak after 1973. Well performance in this reservoir is variable because of the heterogeneity of the reservoir facies. Field production is generally characterized by high initial gross production and rapid water cut development. Water production, in the reservoir, increased to 55% after the first four years of production. The average field water cut has been around 90% since 1982. In 2004 a prescreening study was conducted and identified this field as a high potential, low risk candidate for UBD.