ABSTRACT Wellbore stability is believed to occur if the far-field stress state exceeds the strength on the wellbore wall. Such a stress calculations are normally performed subject to constant in-situ stresses gradient, i.e. fixed far-field stresses are imposed in the far-field around a wellbore. These stress gradients or far-field stresses however may not be constant in many cases, particularly near a faulting regions. Simulating a strike-slip fault, the in-situ stress perturbation in the surrounding the fault is conducted. The perturbation and direction changes of these stresses in the vicinity of the fault center are quantified. The magnitude of the perturbated in-situ stress and the rotation of the principal in-situ stresses directions are extremely important for wellbore stability and fracturing designs. Different critical mud weights and wellbore layout may be determined for wells in different locations and distances to the fault center.
1. INTRODUCTION Wellbore stability has been defined as a critical condition when the stress concentration exceeds the maximum strength of the formation on and in the vicinity of the wellbore wall. Such a stress concentration may be calculated based on the magnitudes of the total effective induced stresses due to the far-field stresses, pore pressure changes, thermal and chemical conditions. A constant stresses gradient or uniformly distributed far-field stresses is normally assumed in the fixed direction and also the principal in-situ stresses are assumed to be perpendicular to each other, i.e. maximum/minimum horizontal stresses and vertical stress are normal to each other. These assumptions of uniformly distributed far-field stresses or constant in-situ stresses gradient may not be valid in practice, particularly in a faulting region. Even for a rare case where the far-field stresses are uniformly distributed, stresses perturbation in the faulting region may alter both the magnitudes and directions of the uniformly distributed far-field stresses. Because the in-situ stress regimes (magnitude and directions) surrounding an oil or gas exploitation region can be one of the most important factors on wellbore stability, hydraulic fracturing design, completion and many other geomechanics related studies, a determination of the magnitude and directions of the far-field stresses become absolutely critical for the induced stresses calculations and wellboe integrity evaluations. Extensive studies in estimating stress perturbations near faults are conducted, focusing on regional stress rotations and magnitude changes. In-situ stress distribution from earthquake origin location, analyzing regional stresses from different faulting systems, measuring individual data from different wells and the determining critical slipping points from an entire fault system, respectively [Zoback and Richardson, 1996, Yamasaki et al., 2014, Homberg et al., 1997, Lin et al. 2010, Townend, 2004]. Often, the information on in-situ stresses is estimated from a single well or an average of those from multiple wells, so that a single stress gradient is generated and used for engineering purposes. Such a practice can be popular or practical but may not be acceptable when we plan a group drilling or fracturing wells in faulting regions. First of all, unlike those formation physical parameters, in-situ stresses distribution may only be defined based on equilibrium conditions or calculated based on forces or stresses from the tectonic plate boundaries. Secondly, depending on the far-field stress ratio, fault strike, and faulting zone cementation, the insitu stresses in different locations or distances to a fault zone center may display much different stress magnitudes and orientations. Thirdly unless a carefully design procedure utilized, those locations of these sample wells where in-situ stress data are extracted are normally not purposefully chosen from a stand point of structure geology. Finally the location of each well drilled in a faulting region normally is planned based on a constant stress gradient assumption, but stresses regime in the faulting region does not support such a constant average far-field stress gradient assumption, the projected far-field stress in each well varies accordingly. Consequently, attempting to determine the far-field stresses, the data from different wells should not be averaged as these data reflect different perturbed impacts from the fault, i.e. they should be determined based on their locations and characterized based their structure geological and geomechanics conditions. Also once a varied farfield stresses are determined, the external in-situ stress boundary conditions for each well should be calculated, different magnitude and direction of these in-situ stresses applied based on the location of these wells, subject to the regional characteristics of the geological structure. We must emphasize that determining the far-field stresses can be as important as quantifying the in-situ stress perturbation near a fault so that correct and accurate stress cloud may be generated near a fault. Thus for either a hydraulic fracturing or wellbore integrity design,