The integrity of the wellbore plays an important role in petroleum operations. Hole failure problems cost the petroleum industry several billions of dollars each year. Prevention of wellbore failure requires a strong understanding of the interaction between formation strength, in-situ stresses, and drilling practices. As in-situ stress and rock strength cannot be easily controlled, adjusting the drilling practices is the usual way to inhibit wellbore failure.

Drilling in the problematic Kolosh formation in Kurdistan has always been associated with several wellbore stability problems. This has caused large amounts of non-productive time to drilling programs and the drilling of sidetracks in some of the wells in this field. A review of the drilling reports and dual-caliper logs from offset wells in the area revealed large amounts of washouts in the middle Kolosh section. These indicators demonstrated the requirement for performing a geomechanical modeling and wellbore stability study to mitigate such problems in future drilling operations.

In this paper, local in-situ stress magnitudes, orientations, and formation pressures were characterized. For this purpose, data was analyzed from offset wells (e.g., borehole breakout data, bulk density logs, wireline formation tests, drillstem tests, pressure build-up tests, formation pressure data in this area). The mechanical properties of the formation (including dynamic and static Young's modulus, Poisson's ratio, and rock strength) were evaluated using sonic, density, and gamma ray logs. A rock mechanical properties database and data management software was applied to correlate the calculated dynamic elastic properties to the most appropriate static rock strength and stiffness parameters for a base case wellbore stability model and subsequent sensitivity analyses. 2D elastoplastic and 3D linear elastic models were used to back-analyze the hole collapse and enlargement in the selected offset wells to evaluate and calibrate the geomechanical model. Wellbore stability software was used for this purpose. Finally, a mud weight window was defined, and the optimum profile of the mud weight was recommended for drilling through the Kolosh formations. Due to a narrow mud weight window, additional potential problems were investigated including the possibility of fracturing at the top of Kolosh formation. Finally, relevant solutions were presented.

You can access this article if you purchase or spend a download.