Saudi Aramco planned to drill three horizontal wells underbalanced in the Uthmaniyah Field in Saudi Arabia but concern was raised because the large volume of fluid produced would also contain a high concentration of H2S. A detailed engineering study was conducted of the well parameters and it was recommended that a diesel-based fluid system be used initially for compatibility, cuttings carrying capacity and pressure control. Since the underbalanced operation was based on the flow drilling technique, the drilling fluid system was diluted and became a produced oil system as the underbalanced drilling operation progressed.

An important part of the design and treatment program was the minimization of H2S exposure for personnel on the rig site. A preliminary review of the operation revealed the areas of greatest concern; and the equipment layout was designed to ensure efficient separation and permit fluid treatment at surface. Laboratory studies recommended a proprietary oil-soluble H2S scavenger to remove residual H2S in the circulating fluid, and treatment points were selected for optimal performance.

All three wells were successfully drilled in an underbalanced condition. There were some minor H2S releases on the first well at several open points in the system, requiring a reassessment of the separation layout and design, particularly in the solids control equipment, and in the fluid treatment program. Once this was accomplished, the subsequent wells proceeded more smoothly. H2S levels in both the liquid and gas streams were monitored and showed very good removal efficiency despite the high production of both oil and gas from the wells.

This paper describes the design of the fluid system and separation equipment for the three wells. It will also highlight results from all three wells, showing the learning experience and optimization of the drilling operation from a fluid engineering and treatment point of view, as well as recommendations for future wells.


Underbalanced drilling (UBD) has long been accepted as a viable well construction technique, as it offers a number of benefits. UBD can avoid or minimize some drilling issues like lost circulation and stuck pipe, it increases rate of penetration (ROP) over conventional drilling techniques, and it has enables formation evaluation while drilling 1–5. It also reduces or eliminates formation damage because there is no fluid lost to the formation in most cases, resulting in higher productivity wells, and possibly increases reserves.6–13

More recent techniques in evaluating candidates for underbalanced drilling have removed much of the uncertainty whether a particular well will benefit from UBD14, and the development of new equipment and fluids has improved the safety, efficiency and productivity of wells drilled or completed underbalanced.

The industry has learned through experience and testing that the simple fact of being underbalanced does not guarantee the prevention of formation damage 15, 16, leading to advances in engineering design and fluids systems. Challenges remain, however, in equipment, engineering and fluids technology, and these challenges must be addressed in order to demonstrate the benefits of underbalanced drilling and create a better return for operators using this technique.

Each major UBD project possesses a learning curve, and may require some optimization in both equipment and procedures to fully optimize the technique for a specific reservoir.17 One example of this is the recent underbalanced drilling program undertaken by Saudi Aramco in the Ghawar Field in Saudi Arabia. After an initial evaluation of underbalanced drilling for water injector wells 18, Aramco drilled three oil wells in the Uthmaniyah area of the Ghawar Field under challenging conditions.

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