Integrated geomechanical analysis for wellbore stability of silici-clastic and carbonate rocks using advanced logs, cores and drilling data.

Abstract

Geomechanics is the study of earth stress, rock strength and strain (deformation) due to stresses. It impacts the entire life cycle of a well. The theme of this paper is wellbore stability which is the technique of understanding and preventing brittle failure or plastic deformation of the rock surrounding the wellbore due to mechanical stress or chemical imbalance. Excessive cavings, and or stress induced breakouts, seen in the drilling process may be indications of well bore in-stability. In the extreme, rock stress may cause collapse of the wellbore resulting in stuck pipe or casing shear, potentially creating huge un-planned cost burdens to operations.

Prior to drilling, natural rocks are usually in a stable state. After drilling, the rock surrounding the wellbore undergoes changes in terms of stress regime. This can result in wellbore failure. Often there are two types of mechanical failures – tensile and shear. Differentiation of natural and induced fractures is necessary to understand the modes of failure. Prior to studying wellbore stability due to mechanical causes, chemical imbalance may have to be ruled out. The interpretation workflow in diverse lithologies (silici-clastics and carbonates) is different as this paper will demonstrate.

While prediction of borehole instability is possible, prevention is not until all the relevant data is acquired and analyzed in an integrated fashion to create operational best practices. This requires an iterative process of pre-planning, execution, data acquisition, evaluation, monitoring and re-assessment of that plan. This paper demonstrates such a workflow based on available log (Borehole image logs, dipole sonic, resistivity, density-neutron and NMR logs if available), core and stress tests, drilling and formation integrity test data to understand rock strength, pore pressure, fracture gradients and the resulting drilling induced fractures and breakouts. The industry standard Mohr-Coulomb and other failure criteria will be discussed and applied where adequate.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015