wellbore stability in rocks affected by discontinuities

Maintaining a borehole stable during the life time of the well is one of the greatest tasks encountered in the oil and gas industry, since wellbore instability-related problems could be very challenging and costly, and also affects the operation plan. Therefore, wellbore stability analysis is an important part in design and planning stage in many operating companies. There are several factors that can impact the stability of the well and must be taken into account, such as rock and fracture intrinsic properties, pore pressure, drilling fluid weight, in situ state of stress, and also wellbore trajectory and orientation. Apart from the failure that can happen for intact rock, failure also can occur in rocks intersecting discontinuities. Faults, fractures, bedding planes, laminations and all kind of planes of weakness can cause failure and instability of the wellbore. Thus, in case of facing such formations some other considerations also must be taken into account, and an extensive wellbore stability must be performed to decrease the risk of instability to minimum. Several methods have been proposed by many authors to investigate the stability of a fracture while it is intersected by a well during drilling. In this study, the method which is suggested by Rasouli and Younessi (2010) has been studied and justified. This method is developed based on a rock engineering system introduced by Hudson (1991). Using an interaction matrix which the main parameters placed on the lead diagonal and the interrelation between these parameters will be placed in off-diagonal elements. The main parameters taken into account in this study are, dip and dip direction, roughness, aperture, in situ effective normal stresses, pore pressure and wellbore pressure. Then by using a semi quantitative method the cause-effect diagram was constructed and showed that the in situ stresses have the most significant effect on the fracture sliding potential. In the other hand, it showed that drilling fluid pressure is the most dominant parameter in the system, which is rewarding result since it is the only parameter that can be control by the operators among all other parameters. In next step in order to obtain the sliding potential index, two parameters of scaled relative interactive intensity (SRII) and (Ω) were introduced and coded for each parameter. FSPI classified in three classes, less than 30, between 30 and 60 and more than 60 which are corresponding to stable, nearly stable and unstable fractures respectively. A series of fracture sets have been used in order to justify the applicability of this method. Using data set for 20 fractures and obtaining the FSPI value for each of them and finally a comparison with Mohr-coulomb criteria shows a good agreement which can confirm the applicability of this method.