A New Diagnostic Approach to Identify the Causes of Borehole Instability Problems in an Offshore Arabian Field

Abstract

Abstract Many wellbore instability problems are being reported during development drilling at an offshore field in Saudi Arabia. It is a sandstone reservoir with shale intercalations at a depth of about 6000 ft. A few wells had to be abandoned because of stuck pipe and hole pack-off problems. Quite a number of wells had to be sidetracked, and drilled using oil-based mud (OBM). The aim of this study is to identify the causes of the instability problems encountered in this field. The well plan, daily drilling report, well completion report, and various logs of each well are carefully studied. The frequency of instability problems, length reamed and the mud weight used are being analyzed with respect to wellbore azimuth, inclination and TVD. A computer program is developed using linear elasticity behavior of the rock material and the Mohr-Coulomb failure criterion. The model predicts the lower and the upper bounds of the mud weight that will prevent the borehole from compressive and tensile failure, respectively. Thus, we generate the stable mud weight window using rock mechanical and in-situ stress data pertaining to the problem wells where instabilities have occurred. Then we compare the mud weight used during drilling to the predicted mud weight window. Besides the Mohr-Coulomb failure criterion, we have used three modes of the Drucker-Prager criteria in order to compare the results. An approach that combines statistical analysis and constitutive modeling to identify the causes of instability and predict safe mud weight window, is being used for the first time to the knowledge of authors. Some interesting results of the study indicating the possible causes of the wellbore instability are presented and discussed. Introduction Drilling of wells account for a major portion of the development cost of a field. To maximize sweep and drainage of reservoirs, the trend is towards drilling horizontal and extended reach wells. This not only helps to reduce cost but also allows a cluster of wells to be drilled form one point. In such a scenario, if for some reasons the driller is not able to reach the planned target or trajectory, it not only increases the cost of development of the field but also hampers the planned drainage of the reservoir. Hence, these interruptions are highly disruptive to the planned development of the field. The inability to reach a desired target because of wellbore instability could be due to two reasons: Mechanical and Chemical. The mechanical wellbore instability is the formation's response to a drilling process in which the mechanical properties of the rock and the prevailing in-situ stress are not properly taken care of. It is usually experienced instantaneously at the time of drilling. The chemical wellbore instability, on the other hand, occurs due to the mud-formation interaction and is experienced after a time lapse. Majority of the instabilities occur in formations where chemically reactive shale is present. Such formations are the most troublesome to drill. The field under study can be divided into three main lithological sequences. The upper part is predominantly shale with coal and sand stringers. The middle part is clean sand and the lower part has sand and shale stringers. The target zone for the development wells is the clean sand. As the emphasis is towards drilling horizontal wells, the trajectories of these wells in shale-coal-sand stringers of the upper zone are highly deviated. Since longer deviated trajectories have to be drilled, it takes more time to drill, giving rise to delayed problems like tight hole, hole pack off, and irremedial stuck pipe which require side tracking.