Geomechanical Modeling for Wellbore Stability during Drilling Nahr Umr Shales in a field in Petroleum Development Oman

Abstract

Abstract Drilling through Nahr Umr shales has been a challenge in many fields in North of Oman due to the weak nature of these shales. Increased difficulty is faced during drilling if the knowledge of the geomechanical parameters is limited especially the in situ stress magnitude and direction in the formation drilled. Problems such as borehole collapse and stuck pipes cause major delays in drilling time due to borehole cleaning and sidetracking in this specific field. This time delay and other recovery measures undertaken has cost huge amount of money that can be significantly reduced if the geomechanical parameters such as rock strength and in situ stresses of Nahr Umr are known. This can be then used to obtain the appropriate mud weight to drill a well in any desired direction in order to minimize borehole failure. The geomechanical properties of Nahr Umr shales across North of Oman are not very well covered. Unfortunately acquiring such data always comes last on the list of the petroleum engineers due to lack of awareness of the importance of such data in appropriately calculating the drilling parameters. This specific field had some good data obtained through lab and field measurements that can be used in building a geomechanical model. This included parameters such as unconfined compressive strength of the rock (UCS), Poisson’s ratio, Young’s modulus, and in situ stress magnitude and direction. These parameters were used to build a geomechanical model for Nahr Umr shales for this specific area. This model was then used as input in wellbore stability analysis tool. The same geomechanical model can be used for Nahr Umr shale in any near by area if there isn’t any data available for that specific area however for more accurate analysis it is always better to acquire data for the area of interest. In this paper some problems during drilling the Nahr Umr shales will be discussed. Also, the workflow of building a geomechanical model in order to use it in wellbore stability analysis will be described. The output results will summarize the minimum required mud weight and unconfined compressive strength (UCS) for drilling a well in any direction in a form of lower hemisphere plots. The preferred direction for drilling inclined well through Nahr Umr shales in this field was found to be in the maximum horizontal stress direction (SHmax) which is the NE-SW direction. Drilling in the minimum horizontal stress direction (Shmin) which is perpendicular to SHmax was found to be more risky for which mud weight has be raised to avoid borehole failure. Understanding the geomechanics in this field, in addition to other drilling practices, has contributed in reducing well delivering times from 36.8 days on average in 2009 to 30.1 days in 2010 to date, mainly due to reduction in borehole failure related problems.