Abstract Due to low permeability of tight gas shale, production at commercial level requires conducting effective hydraulic fracturing and applying horizontal drilling technologies. Therefore, successful production from such complex reservoirs is heavily dependent on the selection of an appropriate completion technology requiring sufficient knowledge of borehole shape. Borehole breakouts provide valuable information for evaluation of in-situ stresses and verification of geomechanical models. Caliper and image logs, the regularly used methods to identify breakout zones, have several limitations such as not having a full coverage of the borehole wall, being affected by different parameters, pad width limitation, low resolution, and complicated processing procedure. In addition, good quality image logs are not usually available for shaly formations because of the requirement of using oil-based mud. This paper discusses a new approach to identify borehole breakouts in tight gas shale using some petrophysical logs. The used approach employs number of data processing techniques to determine borehole intervals with maximum likelihood of enlargement. The applicability and the generalization capability of the approach were supported by having significant accuracy. Case study on Burnett shale is presented in this paper. Introduction Tight gas, coal bed methane and gas hydrate are considered as unconventional sources of gas. These unconventional reservoirs have insufficient permeabilityand difficult accessability. Nowadays with decline of production of conventional reservoirs and increase in demand of hydrocarbon fuels, production from unconventional gas reservoir is becoming economic and has been brought into the forefront of energy future. This unconventional energy source is a fast-growing market and is recognised as a huge future potential for production. Economic production of gas from shale gas reservoir is a big challenge for the industry. Due to low permeability of shale, production in commercial quantities requires effective and economic hydraulic fracturing and horizontal drilling technologies. Therefore, having a good understanding of shale mechanical and chemical properties and the state of stresses in the earth's crust (i.e., geomechanical model) is one of the most critical steps toward reservoir evaluation and ultimately development of this sort of resources. In addition, production from such a complex formation with a typical poorly-defined gas-water contacts, natural fractures, and very low matrix permeability, is heavily dependent on the selection of appropriate completion technology (Jacobi, Gladkikh et al. 2008). Consequently, having sufficient knowledge of borehole shape (enlargement zones) is the key point toward a successful production. The integrity of the wellbore plays an important role in petroleum operations including drilling, completion and production. Wellbore failure occurs principally through changes in the original stress state due to drilling the rock that concentrates stresses around a wellbore. If the new stresses exceed the rock strength then breakouts form around the borehole. Breakouts are valuable information to estimate the magnitude and orientation of earth stresses and to calibrate and validatethe geomechanical model. In addition, they provide final borehole shape which is a critical factor in completion and production optimization. This study aims to identify borehole enlargement zones in gas shale formations from common petrophysical logs using some data processing techniques such as Bayesian classification, wavelet signal decomposition and data fusion instead of caliper and image logs that are associated with many limitations.

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