This paper describes Saudi Aramco's initial experience introducing Underbalanced Drilling (UBD) technology in the Ghawar field. The intention was to prove that a step change in performance could be achieved by the application of this technology. Thus the intent was to optimize well design that will result in reduced unit well cost. The reasoning for applying this technology in power water injection wells (PWI) was the low risk involved in drilling these wells. The primary objective of drilling wells underbalanced was to eliminate formation damage and improve injectivity, thereby eliminating the need for acid stimulation. Further, increased rate of penetration (ROP) will result in reduction in drill time and cost. A major campaign was implemented in the Ghawar field to evaluate UBD in Saudi Arabia. Injection rates were dramatically improved and the need for acid stimulation was eliminated. ROP increased significantly when compared to conventional PWI wells. By the time this paper was completed, three oil wells had been drilled underbalanced. The drilling operations and some of the safety issues only will be highlighted. The operations and results are still being evaluated and hopefully will be presented in the near future.

Introduction and Background

Saudi Aramco drills power water injection (PWI) wells in the Ghawar Field to maintain pressure for optimum production of oil from the Arab-D reservoir. The Arab-D formation is a fractured, oil-bearing carbonate reservoir. Hydrocarbon recovery to date has been traditionally optimized through the drilling and completing of overbalanced vertical and deviated wellbores. These operations in some instances have been complicated by drilling related problems (i.e., loss of circulation and related stuck pipe problems). Traditionally, these wells are drilled with water based mud (WBM) requiring mud weight (MW) that exceeds the reservoir pressure by approximately 200 psi. This overbalance pressure results in mud filtrate invasion and drilled solids penetration into the micro & macro fractures of the carbonate formation. Consequently, this results in formation damage requiring extensive acid stimulation to bring back the injectivity of these PWI wells. Further, drillstring sticking and lost circulation results in excessive non-productive time (NPT).

Saudi Aramco has identified the minimization of drilling fluid losses into the reservoir, formation damage, and less operational problems as key well objectives. Saudi Aramco has begun a campaign to optimize well design utilizing new technology that will result in reduced unit well cost while maintaining safety as the bottom line of all operations. Underbalanced Drilling is considered an enabling technology that can potentially help Saudi Aramco to achieve its objectives. The long term goal is to evaluate whether the application of UBD can be extended in other areas safely and effectively to reap its potential benefits.

This paper summarizes Saudi Aramco's experience gained during the drilling of these wells and further discusses the observed well performance. Formation damage was minimized, thereby increasing initial injection rates. It also eliminated the need for massive and expensive acid stimulation. ROP was increased significantly, lateral length was able to be reduced, and this enabled several days of rig time savings.

The paper also describes various aspects of extending the application of UBD in other areas within Saudi Aramco. Some groundwork for moving forward to the execution phase in complex wells and pitfalls to avoid are also outlined.

Planning Phase

The geology of the area drilled is a carbonate/dolomite with varying degrees of porosity. The lenses in Zone 2A are the most prolific zones with the highest porosity. Zone 2B is a tighter zone and usually exhibits lower porosity values, and Zone 3 is the densest zone with the lowest porosity. The wells drilled cut a path through all three zones.

Due to the low risk involved in PWI wells, it was decided to first utilize UBD technology in drilling the PWI wells.

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