Calibrating Log Derived Stress Profiles In Anisotropic Shale Gas Formations- Incorporating Lab and Field
Authors: Almarzooq Anas , Aljeshi Hussain , AlAkeely Abdullah 
Published: March 2015

The complex properties of the unconventional gas resources pose challenges to petrophysical evaluation techniques and tools. Data from standard logging tools and standard interpretation techniques produce high levels of uncertainties in the analysis, hence, limiting their reliability in producing thorough petrophysical solutions. Both tight and shale gas formations add multiple layers of complexity to the petrophysical evaluation with complex lithology and heterogeneity causing uncertainty in the hydrocarbon volume calculations and hydraulic fracturing completion designs. Without an accurate completions design, it would not be possible to produce at an economic rate or volume from these formations. Therefore, the need for accurate petrophysical and Geomechanical properties is critical for shale gas formations development.

This paper provides field examples with workflow for identifying the anisotropy, calculating the log derived stress profiles and demonstrating the use of lab and field data for calibrating the log measurement. The lab measurements include the elastic moduli conversion dynamic (from logs) to static (from laboratory), stiffness tensors utilizing the oriented velocities in addition to rock strength and related parameters. This part includes the use of oriented velocities from the lab to validate and correct the existing tensors' correlations (Annie). Correcting the logging tool's measurement for factors such as the gas content and the acoustic conversion models will also be illustrated. The field data include the integration of the pre-fracturing job or mini fracturing to calibrate the calculated minimum horizontal stress (closure pressure) and post fracture analysis to validate the models.

The result of these calibrations is a more accurate estimation of the formation stress profiles which improves the completion designs. Once these calibrations are done correctly, more accurate stress profile can be calculated in offset areas where cores or mini-fracturing measurements are unavailable. This paper shows the process for calibrating the log derived stress profile and goes through the components and uncertainty

Introduction

When a measurement of a property in a formation is identical in all direction, the formation is termed isotropic. On the other hand, when a property varies depending on the direction of the measurement, it is defined as anisotropic formation. Shale gas formation is known to be anisotropic at different levels which can be intrinsic and stress induced anisotropies. The intrinsic stress is observed at two levels due to bedding and deposition where fine-scale of plate like clay grains are layered horizontally. The macro-bedding of the clay plates seen in its structure; commonly through SEM images as shown in Figure 1.A. The second, at larger level, is due to the clay lamination which can be seen in cores or through image logs and crossed-dipole sonic tool which detect the shear-wave anisotropy. Figure1.B shows one example of shale gas core where the lamination is observed.

On the other hand, the stress-induced anisotropy is caused by the magnitude difference between the principle stresses resulting in natural fracture and induces fracturing during drilling. Stress induced anisotropy characterization can be done at different stages by core mechanical measurements and results can be verified by borehole image logs analysis where the tensile and shear failures are observed. Borehole breakout and induced fractures are indicative of the maximum and minimum stress direction.