The Taq Taq Field is located within an anticline in the gently folded zone of the Zagros mountains, northeastern Iraq, approximately 50 km ESE of Erbil. The main reservoirs are fractured limestones and dolomites of Late Cretaceous age, with an oil column exceeding 500 m in thickness. Eocene limestones and dolomites at shallow depth form a subsidiary reservoir. The structure is a gentle thrust-related fold which has also been affected by dextral transpression. A pervasive fracture system is present within the reservoirs, giving good connectivity and deliverability. Initial discovery and appraisal was made in 1978 when three wells were drilled. The recent appraisal programme started in 2005 and by the end of 2008 two seismic surveys had been acquired and eight additional wells had been drilled. Mapping has incorporated a seismic principal component analysis for horizon and lithology identification. Modelling of the fractures has utilized a comprehensive data set derived from core and image logs. Special core analysis has been directed towards the understanding of the pore system and its interaction with the fractures. Synthesis of all these elements is performed in a dual-media dynamic model which is currently in use for development planning.

The integrity of the wellbore plays an important role in petroleum operations (e.g., drilling, completion, production). 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 (i.e., selecting optimal trajectory and bottom-hole pressure) 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 (e.g., hole washout, stuck pipe, extra cuttings/cavings, tight holes). 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.