Geomecanica en yaciemientos de petroleo

SPE/iSRM 47240 Constraining the State of Stress in Tectonically Active Settings
R. Plumb, SPE, 1PM Schlumberger, P. Papanastasiou, SPE, Schiumberger Cambridge Res., N. Last, SPE, BP Colombia

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Abstraot

This paper describes thehierarchical approach used to characterize the regiomd state of stress in tectonically active settings, The example featured here is the Cusiana field where wellbore instability was a serious problem. The Cusiana field is located, in the foothill of the Colombian Andes, South America. A key to solving the instability the was to identify what stress information was needed to improve drilling performance.Using drilling records, caliper logs, cavings samples, cores from the reservoir and a few meters of borehole image, an internally consistent working hypothesis of the state of stress was formulated, tested and refined. Simple calculations based on elastic-brittie model, confirmed also by drilling data, showed that the minimum principle stress is horizontal and the overburden was the intermediatestress. Elastoplastic modeling of borehole deformation in sands and shales based on core testing and finite element analysis placed tighter constraints on the relative principal stress magnitudes and the magnitude of the maximum horizontal stress.
Introduction

Wellbore instability was a serious problem in the Cusiarra field in the early 1990s. The Cusiana field is located in the tectonicallyactive foothills region of the Eastern cordillera of the Colombian Andes ‘ . Wellbore instability problems were caused by mechanical failure of the rock due to the action of highly non-hydrostatic stress field The stress field is controlled by the relative motion of fault blocks in response to the high compressive stresses that are generated in this tectonically active re@on. Under such conditionthe mechanical failure of the wellbore can only be partially controlled by the mud weight. Hole size was frequently greater than 4g” and solids 179

production was measured in units of truck-loads per hour. Because of the severe instability problems large hole sires, conventional log-based approaches for diagnosing and curing wellbore instability were not practical. Characterizing the contemporarystate of stress in the field was fundamental to identi~lng and implementing the practical solution to the problem. Last et rd.’, described the integrated approach that resulted in drilling cost reductions of as much as 50%. Characterizing the regional shess in tectonically active settings is more complicated than in relaxed basin settings mainly because one can not assume the stress field isregionally homogeneous. Moreover one does not have the luxury of having or obtaining cores, logs and stress measurements at the locations where they are most needed. Estimating the state of stress in Cusiana proved extremely challenging because methods commonly used in other oil fields were not practical in Cusiana mairdy because of difficulty in getting downhole measurements. These difficulties...