Pressure Control in Balanced and Underbalanced Drilling in the Andarko Basin
Authors: Grace Robert D. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the Oklahoma City SPE Regional Meeting, to be held in Oklahoma City, Okla., March 24–25, 1975. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussions may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
Classic pressure control procedures are generally well established; however, procedures need to be established for situations other than classic such as in balanced and underbalanced drilling.
In the midcontinent region may areas are plagued by random abnormally pressured intervals. Generally, productivities are low in the abnormally pressured intervals. This combination of characteristics has prompted operators to drill into the pressured interval without protective pipe in order to improve the economics of stratographic exploration. When drilling problems result from the underbalanced condition, some operators and contractors, through their experience, are very adept at bringing the situation under control. However, most operators and contractors do not have the experience or equipment to properly control the gas kick or threatened blowout and procedures among those with the experience vary.
This dissertation establishes the fundamental technology necessary to properly control underbalanced drilling problems, establishes procedures, and reflects on the many questions that arise when potential disaster strikes. Development of the procedures outlined through field experience can significantly procedures outlined through field experience can significantly improve the economics of stratographic exploration in the midcontinent region.

Introduction
Pressure control procedures are classic in most areas of the world; however, some areas are subject to unusual circumstances. For example, in West Texas, the Texas Panhandle, and western Oklahoma, pressure gradients vary. That is, abnormally low pressures can be followed by abnormally high pressures, which, in pressures can be followed by abnormally high pressures, which, in turn, can be followed by normal pressures. To further complicate matters, certain geologic intervals are normally pressured in one area and abnormally pressured in other areas.
In the Delaware Basin of West Texas, the productivity of the abnormally pressured zones proved to be extremely low in most instances. Consequently, operators adopted the practice of drilling into the abnormally pressured zones underbalanced as much as 5 to 6 lb/gal. The small volumes of gas were routinely removed with surface separators and degassers and flared. This operation received the appropriate name "controlled blowout," and resulted in significant savings in drilling costs.
In the Anadarko Basin of the Texas Panhandle and western Oklahoma, the Morrow sands of Pennsylvanian age can be abnormally pressured while shallow formations are almost always subnormally pressured while shallow formations are almost always subnormally pressured. The routine practice in the early 1960's was to set an pressured. The routine practice in the early 1960's was to set an intermediate string of casing on top of the Morrow interval in order that any abnormal pressures might be properly controlled.