Natural or Induced Fracture? Methods to Distinguish Using Geomechanics

Abstract

Borehole geology has a long history of interpreting natural fractures from image logs. During image log interpretation it is common that drilling induced fractures of a variety of shapes are observed which can lead to confusion and misinterpretation. This presentation discusses conditions at which induced fractures can develop and emphasises the need for an integrated approach for interpreting image logs to distinguish natural and induced fractures. Well based geomechanical simulations and verification of induced fracture development are also covered. The key to understanding a borehole image log is the ability to distinguish between natural reservoir and drilling induced fractures. The addition of optimal drilling conditions (hole stability, mud parameters etc.) will allow good quality data capture and enhance accuracy of interpretation. An induced fracture (drilling induced tensile fracture, Figure 1) is the result of the redistribution of hoop stresses around the borehole during the drilling process. The shape and orientation of an induced fracture is dependent upon a number of factors, including: hoop stress distribution around the borehole, bore hole deviation/azimuth, w.r.t present day stress orientation, pressure overbalance, borehole cooling effect and drilling parameters (e.g. weight on bit, torque). To add to the complexity of borehole image interpretation, development of drilling induced fractures can be influenced by pre-existing natural fractures leading to the development of complex features in the image log.