One of the major drilling problems encountered during oilfield exploration and development phases is wellbore instability. This largely occurs as a result of mechanical failure of the subsurface underlying rocks; with problems such as stuck pipe incidences, lost circulation, clay swelling accounting for about 5%-10% of total drilling expenses and resulting in non-productive time (NPT) and equipment loss and environmental concerns.
To address these concerns, a geomechanical model is defined. Geomechanical Modeling involves the study of in-situ stresses induced in rock, due to well drilling, completion, fluid injection, temperature changes and the associated deformations and failure that occurs. It takes into account the rock strength and the pore pressure whilst describing the behavior of rock under mechanical disturbance. Typically, a Geomechanical model comprises of six components: Unconfined Compressive Strength (UCS), Pore Pressure (Pp), Vertical Stress, Minimum and Maximum Horizontal Stress and the orientations of the horizontal stresses.
Log-based Stress Modeling offers a practical approach to characterize in-situ stress state profile of the subsurface formations. The knowledge of the mechanical properties and in-situ stresses of the subsurface formations is important, as it provides insights to the determination of optimal mud weight window, stable well trajectories, casing set points so as to minimize wellbore stability related problems. The data required is obtained from a suite of logs such as density, sonic, porosity, caliper and image logs. Stress magnitude information is needed as a continuous function of depth to proper characterization. Information about Stress orientation and constraining its magnitude is based on the observation of failure at the borehole wall which can also be detected by borehole logging tools.
This dissertation employs series of techniques to characterize the magnitude of in-situ stresses. A one-dimensional geomechanical model is developed, by combining the stress information obtained, rock mechanical properties, formation pressures, observation of wellbore failure and defining the failure criteria for both borehole collapse and fracture breakdown. A wellbore stability study was done to determine an optimal mud weight window and the sensitivity of the obtained mud weight window to the minimum and maximum horizontal stresses was investigated.