N. Last, SPE, British Petroleum Exploration Colombia; R. Plumb, SPE, Schlumberger Cambridge Research; R. Harkness, University of Southampton; P. Charlez, SPE, Total Exploration Production; J. Alsen, Triton Energy Corp.; M. McLean, SPE, British PetroleumAbstract. An integrated approach to evaluating the causes of severe wellbore instability in the Cusiana field is described. The field is located in a tectonically active region of Colombia. Deterioration of the hole during drilling operations has led to excessive nonproductive time and expensive wells. The scale of the problem is unprecedented in the world. In wells costing tens of millions of dollars, millions per well could be attributed to poor hole conditions.This paper describes how the problem was addressed and what actions were taken to improve operational performance, resulting in reduced drilling costs. The improved understanding has contributed to better well planning, and improved drilling performance, and has underlined the need to consider all aspects of the drilling process to achieve improved hole conditions in a difficult geological setting.Introduction. Nature of the Problem. Major drilling problems have been encountered in the Cusiana field. The field is located in the Llanos basin (Fig. 1), a tectonically active foothills region of the Eastern Cordillera in Colombia. The biggest problems stem from hole enlargement (Fig. 2), resulting in large amounts of cavings, often measured in truck loads per hour, which cause hole cleaning problems, stuck pipe, poor cementing, and often the need to sidetrack. The added complication of severe hole rugosity (alternating in-gauge and over-gauge sections) through interbedded sand-shale sequences. as in (Fig. 3) makes tripping, running casing and logging problematic, and renders large amounts of reaming necessary.The range of problems facing the drillers was formidable. Limited success was achieved by using experience from other parts of the world. For example, the use of higher mud weights, the conventional approach to solving hole enlargement problems, was unworkable because of the significant mud losses that occurred as the mud weight was raised. There was even some evidence that higher mud weights were accelerating the onset of poor hole conditions. Initially there was reluctance to accept the need for a new approach.Geological setting. The main features of the setting are illustrated in Fig. 1. Although the geological history includes an initial period of extension (including contemporaneous extensional growth faulting), the current tectonic environment is characterized by active thrusting toward the southeast, the assumed direction of maximum horizontal stress, with the thrust front marked by the Cusiana fault. To reach the reservoir in the hanging wall of the Cusiana fault, most production wells must penetrate the hanging wall of the Yopal fault and cross the Yopal fault. Although not detailed in Fig. 1, numerous smaller faults in the structure complicate the lithological sequence (sections are repeated) and produce abrupt and frequent changes in bedding dips. As an example, Fig. 4 is a detailed section through well X.Fig. 5 summarizes the generic lithological sequence. While problems have been encountered in most of the formations, by far the most problematic drilling has occurred in the alternating sand-shale sequence called the Carbonera In particular, losses and tight hole have occurred in the sandier units (C1, C3, C5 and C7), and wellbore enlargement has occurred in the shalier units (C2, C4, C6 and C8).Approach to Solving the Problem. Concerns over the increasingly high drilling costs and the lack of progress toward improved hole conditions led to the establishment of a multidisciplinary, multicompany team with the responsibility to help reduce costs associated with wellbore instability. This objective was challenging given the complexity of the problem, an aggressive drilling schedule and the absence of hard facts needed to make convincing arguments for change. In the end a relatively simple solution emerged: namely, manage the instability rather than trying to cure it completely.P. 147