UNDERBALANCED DRILLING

IADC-UBO panel adopts underbalanced well classification

Two years of work by an IADC committee have generated a classification system that provides risk management guidance and the basis for a reference database of underbalanced wells. Formal IADC adoption is slated for first-half 2001

Noel Monjure, Vice Chairman, Standards and Nomenclature work group, IADC-UBO, and Manager, Business Development, ABB Vetco Gray, Houston

he IADC Underbalanced Operations (IADC-UBO) Committee was formed in 1998, following several years of sponsorship by international oil companies. The committee’s mission is to promote safe, efficient execution of underbalanced operations worldwide.

Underbalanced operations are a field-proven process for increasing rate of penetration (ROP) during drilling, as well as dramatically increasing production rates by reducing or eliminating skin damage in reservoir sections. At the direction of the IADC-UBO Committee, workgroups were formed to address issues related to underbalanced operations that would promote this mission.

This article presents the results of work accomplished over the last two years, including the final classification system approved by the IADC-UBO Committee’s membership. This classification system will be presented for approval by IADC’s board of directors in early 2001.

Background

Standards and Nomenclature was one of the work groups formed in 1998. The group’s first objective was to seek input from all interested parties, and develop a method of classifying underbalanced wells that would provide risk management guidance to drilling engineers, management and regulatory authorities. Development of the classification system also would provide a basis for establishing a reference database of wells drilled with underbalanced technologies.

The Standards and Nomenclature work group held formal meetings involving operators and suppliers in Houston, Amsterdam, Caracas, Abu Dhabi and Jakarta. The purpose was to seek input from a broad group of personnel working in the underbalanced area. A core section of work group members incorporated comments from all of the meetings and conferences. They then developed the system’s final form.

IADC-UBO’s Final Classification System

After considering a number of different systems and methods, the work group proposed a tiered classification that addresses key areas of risk management and technology application. To assist with categorization of all wells drilled with UBD techniques, a secondary element denotes whether the well was truly drilled underbalanced, or just with a "low head." The system’s first part identifies the well, or well section, based on the projected risk associated with the wellbore. These levels of risk, 0 through 5, are listed below:

• Level 0 – Performance enhancement only; no zones containing hydrocarbons.
• Level 1 – Well incapable of natural flow to surface. Well is "inherently stable" and is a low-level risk from a well control point-of-view.
• Level 2 – Well capable of natural flow to surface, but conventional well kill methods are enabled, and limited consequences are possible in case of catastrophic equipment failure.
• Level 3 – Geothermal and non-hydrocarbon production. Maximum shut-in pressures are less than UBD equipment’s operating pressure rating. Catastrophic failure has immediate, serious consequences.
• Level 4 – Hydrocarbon production. Maximum shut-in pressures are less than UBD equipment’s operating pressure rating. Catastrophic failure has immediate, serious consequences.
• Level 5 – Maximum projected surface pressures exceed UBO equipment’s operating pressure rating, but are below BOP stack rating. Catastrophic failure has immediate, serious consequences.

A matrix is listed in the accompanying table, to easily classify the majority of known underbalanced applications. This system combines the risk management categories defined above (Levels 0 to 5) with "sub-classifiers," to indicate whether wells are drilled underbalanced or with a "low head" using underbalanced technology. To provide a complete method of classifying the type of technology used for one or more well sections (or multiple wells in a particular project), a third component of the classification system addresses the underbalanced technique used. A Glossary of Terms will follow the matrix in the final classification document (assuming approval), for explanation of the items shown below.

Example of system usage. A horizontal section of an oil well is drilled in a known geologic area, using a drilling fluid lightened with nitrogen gas to achieve an underbalanced condition through the reservoir section. The maximum predicted bottomhole pressure is 3,000 psi, with a potential surface shut-in pressure of 2,500 psi. This well would be classified as a 4-B-4, indicating Classification Level 4 risk, and UBD drilling with a gasified liquid.

System adoption. The IADC-UBO Committee and the Standards and Nomenclature work group recommended that operators and contractors using, or planning to use, underbalanced techniques, adopt the IADC-UBO classification system. This will provide a uniform basis for categorizing all underbalanced wells and a database for reference by drilling engineers and others interested in using UBD technologies to improve the economics of their projects. The IADC also will make available a standard tour report form for underbalanced operations that includes a space for entering the IADC-UBO classification system. Please contact IADC in Houston for more information, at info@iadc.org, or by phone at 1 (281) 578 7171.

Acknowledgment

The author wishes to thank ABB Vetco Gray and the IADC-UBO Committee for permission to publish this article on behalf of all the members of the Standards and Nomenclature work group. The IADC-UBO Committee also would like to thank World Oil for publishing the IADC-UBO classification system and helping to promote safe, efficient execution of underbalanced operations to its readers.