Opon Gas Field, Colombia: Part II - Drilling Case History
Authors: Greener J.M. , Trimble G.E. , Singer G.M. , Barnes M.J. 
Published: December 2007

Abstract
This paper describes the Opon Gas Field development drilling case history in the Middle Magdalena Basin of north-central Colombia, South America. World class levels of drilling fluid and cementing densities in excess of 22.0 ppg were required to control the extreme pressures encountered. A continuous improvement process is detailed in regard to casing, drilling fluid, cement and related drilling mechanics programs in a severely pressured and environmentally sensitive operation.

Introduction
Amoco Colombia Petroleum Company (ACPC) is preparing to drill its third operated well in the Opon Gas Field in Colombia (Fig. 1). Drilling engineering strategies derived from the first two wells are being employed to enhance program performance. Initially, the drilling team's primary objective has been to provide a usable wellbore for testing, evaluating and producing the severely overpressured gas reservoir, while maintaining safety and environmental standards.
Based on the results of the initial two wells and the planned field development, the Opon Drilling Team has established specific goals to satisfy the needs of their customer, ACPC. Due to the well numbering sequence and structural positioning, Opon #6 was selected as the third ACPC operated well. A comprehensive scorecard system has been developed for Opon #6 to measure performance and customer satisfaction (Fig. 2). Measurements include threshold, target, and maximum levels for: days versus depth; dry hole cost; case, complete and test; safety; and environmental protection goals. The Opon Gas Field presents numerous challenges for successfully drilling and managing an economically viable project. Drilling and completion costs have exceeded $25 MM USD per well due to the following factors which contribute to the extreme operating conditions:

Rugged and Remote Topography
The entire field is situated in an environmentally sensitive area with steep mountain slopes, dense jungle vegetation and unstable soil. Field elevation changes from 500 to 750 feet create problems managing the high volumes of water and debris run-off due to frequent, heavy rains.

Severely Abnormal Pressures
Due to the extreme abnormal pressures encountered in the Opon reservoir, proper design and maintenance of the mud system is critical to the success of the wells. Numerous problems associated with the management of rheological properties and wellbore stability has led to the implementation of an oil based drilling fluid.

Wellbore Stability
High angle formation dips &gt;60 degrees), numerous faults, abnormal stresses, and fractured formations amplify wellbore stability challenges resulting in casing and mud program design modifications.

Ultra-High Weight Cement Slurries
To control formation pressures and mitigate gas migration, cement slurry designs in excess of 23 pound per gallon (ppg) are required. Utilization of high density weighting agents and proper product-to-water ratios are essential to developing a manageable cement slurry.

Limited Offset Information
Data to design the Opon #3 drilling program was obtained from the Opon #1(1966), Opon #2 (1967), and Lilia #10 (1993) wells (Fig. 3). Information from the earlier wells was limited due to the inability to successfully drill, test, and complete a wellbore through the objective La Paz formation.

Security
The security situation in the Opon Gas Field can best be described as challenging.
P. 515