This reference is for an abstract only. A full paper was not submitted for this conference.


This poster presents the results from a case study that integrates detailed rock mechanics and swelling tests with information from petrophysical logs and core properties acquired to evaluate, define and predict the instability mechanism in this portion of the Khafji reservoir.

To achieve the objectives, 300 feet of preserved core were cut through the problematic shaly sand member using oil based mud. In addition, 200conventional plugs and 28 whole core samples from eleven wells were utilized for the purpose of developing geomechanical and pore fluid models. Both oil based mud (OBM) and water based mud (WBM) filtrate were used for the swelling and triaxial compression tests.

The development of a strength and stress profile for the well is the first step in understanding wellbore instability problems. These profiles are generated using rock properties, drilling experience, in-situ stress regimes and strength measurements on core samples. The results demonstrate that the in-situ stress in the Khafji reservoir can be characterized, and the critical azimuths of marked instability increase are discernible. Wellbore instability problems can be predicted and averted.

The optimum mud weight windows to drill horizontal wells have been identified using the geomechanical model. Wells oriented parallel and perpendicular to maximum horizontal stress (SHmax) require minimum mud weight of 80–84 pcf and wells drilled WNW-ESE require mud weight from 80–95 pcf.. The swelling test results point toward increased swelling in the presence of the WBM filtrate compared to the OBM filtrate and a decrease in formation compressive strength when in contact with the OBM. It may therefore be prudent to redesign the already "inhibitive" WBM to suit the formation and the clays.

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