Drilling-Fluid Design for Challenging Wells in the Andean Mountain Region

Abstract

Wells drilled in the Andean Mountain region of South America present significant challenges as a result of both operational and environmental factors. Wells located in the foothills along the basin are particularly difficult due tectonic stresses and unstable, probably, micro-fractured shales. Operators have experienced difficulties drilling wells using both water-based and oil-based muds (OBM). Environmental regulations hinder the use of OBM in many of those areas due to the potential environmental impact and costs associated with waste disposal. In many cases OBM has not prevented wellbore instability problems. This paper explains how a lack of understanding of regional geology and the practice of using successful drilling fluid design and drilling practices from other areas has led to wellbore problems. The water phase salinity of OBM and the use of the appropriate inhibitors in the drilling fluid play a key role in the minimization of wellbore problems. Although reactive clays are present in all the shales along the basin, they represent only 30 to 40 percent of the clay fraction, while non-expandable kaolinite clays are the major clay components. This paper explains how physical/mechanical effects are more important than inhibition in controlling these shales. Moreover, in some cases "excessive inhibition" due the presence of shale inhibitors such us potassium and high water phase salinity in OBM exacerbate the problems. Pore pressure transmission caused by fluid invasion is a major contributor to the observed problems. A combination of operational practices and improved fluid design minimizes mud and filtrate invasion. Troublesome shales in the Andean basin include, from north to south, the La Rosa and Icotetea in Venezuela, the Carbonera, Leon and Villeta in Colombia, the Napo in Ecuador, the Chonta in Peru and the Los Monos in Bolivia and Argentina. Case histories involving these shales are presented. Contrary to experiences in many other parts of the world, high water phase salinity OBM and potassium based water-based mud (WBM) are not the answer to shale stability problems. Rather, mud sealing properties, correct chemical composition and appropriate drilling practices are the key factors in maintaining wellbore stability. Introduction The challenges of drilling in the Andean Mountain region of South America are well documented. The presence of tectonic stresses combined with over pressures1 makes this a particularly challenging region. The stresses in this region were generated by the Andes Mountain orogeny. The geology is typified by steeply dipping sand/shale sequences. Many faults have been documented in this area. Claystones and shales dominate the lithology in this region. These clays and shales can be "sticky"2 at times, requiring the use of inhibitive drilling fluids to minimize the associated problems. The factors discussed above lead to wellbore stability being a major challenge when drilling in the Andean mountain region. These problems present the greatest challenge when drilling directional wells3. The drilling problems experienced in Colombia are well documented and include stuck pipe, high torque and drag, tortuous wellbores, twist-offs, poor cementing, and unplanned back-offs. Many of the difficulties encountered have been attributed to poor hole cleaning in enlarged hole resulting from wellbore instability. The cavings generated during the hole enlargement process have also presented hole cleaning challenges. Success in drilling wells in this region has been attributed variously to simplifying well design, understanding the tectonic stresses and their orientations, drilling fluid design4 and sound drilling practices.

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The Incorporation of Aluminum Hydroxide in Water Based Muds to Stabilize Marcellus Shale Drilling in West Virginia

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