Maintained pressure management and fracture network mapping are key issues for successful well construction in fractured carbonate reservoirs. Prediction and characterization of the fracture network prior to drilling is very difficult and challenge optimized well direction and placement in respect to the fractures. Avoiding productivity impairment by loss of drilling fluids is another major challenge.

Later stimulation of a micro-fractured reservoir invaded by drilling mud is extremely difficult or even impossible. Open hole completion is generally preferred in these type of reservoirs. Underbalanced drilling (UBD) is therefore an interesting option for enhanced data acquisition, avoiding reservoir impairment and achieving completion contingency for optimized well production and added value.

Locating and characterizing the natural fractures while drilling is a vital issue for appropriate well placement and selection of completion options during the field development strategy. Depending on the opening and extension of the fractures, the mud loss rates and volumes can differ from low to heavy. The associated fracture network is the main parameter contributing to well production.

Underbalanced drilling is one of the main recommended technologies for further development of fractured carbonate reservoirs in South-West Iran. Flow drilling, allowing the hydrocarbons to flow to surface while drilling, will be advantageous. With sufficient reservoir pressure, this method enables better fracture characterization and a dynamic decision process can be applied for drilling horizontal wells with optimized well trajectory intersecting more fractures. Formation damage can be avoided and dramatically reduce unsuccessfully drilled wells.


UBD is a drilling practice where the dynamic wellbore pressure intentionally is less than the formation pore pressure. Underbalanced drilling may improve drilling efficiency and enhance well productivity. However, for operational safety overbalanced drilling is generally preferred and the value of UBD is not yet fully acknowledged. At the same time UBD is increasingly used in many parts of the world. The technology involved is matured and the experience is increasing. UBD may be the only solution to unlock further drilling in depleted reservoirs with abnormal pressure ramps. UBD is applied for a variety of reservoir types at different complexity levels1.

For field development projects in Iran, underbalanced drilling has a great potential and the National Iranian Oil Company (NIOC) has initiated a UBD technology program. UBD projects are related to some partially depleted fields in the south of Iran. The motivation is to increase the drilling efficiency and to enhance production. Normal drilling procedures, even with minimum conventional mud density (0.8 s.g) is not possible in many of the fields due to heavy to complete mud loss. In practice, drilling horizontal wells with heavy mud loss creates two severe drilling challenges:

  • Stuck pipe due to cutting accumulation

  • Loss of data transmission

In fractured carbonate reservoirs commonly found in Iran, non-producing wells are a major challenge either due to formation damage by loss of drilling fluids or not appropriate well placement and direction to intersect the fractures. The key aspects for promoting UBD in Iran are;

  • Improved drilling performance

    1. Avoiding drilling fluid loss

    2. Improved ROP

    3. Less bit wear and tripping time

  • Improved reservoir fracture productivity

    1. Early and improved fracture detection

    2. Avoiding reservoir impairment

    3. Allowing open hole completion

    4. Reduced need for well stimulation

This paper discusses the above mentioned challenges to Iran and in relation to underbalanced drilling as a tool for optimized drilling and completion contingency in fractured carbonate reservoirs. Data and experience from West-Zagross fields located in South-Western part of Iran have been used as a basis for the study.

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