Successful underbalanced drilling (UBD) operations are strongly dependant on the understanding of in-situ stress condition in addition to rock mechanical properties. Wellbore integrity, which plays an imperative role in all the oil and gas operations, requires accurate geomechanical modeling and wellbore stability analysis. Borehole failure problems, which are very likly specially when drilling underbalanced, cost the petroleum industry several billions of dollars each year. Prevention of these problems requires clear understanding of the interaction between formation strength, in-situ stresses and drilling practice. Since in-situ stress and rock strength are not controllable parameters, adjusting the drilling practices, i.e. selecting optimal trajectory and bottom-hole pressure, is the common way to inhibit wellbore failure, which can be achieved by performing specialized geomechanical studies.

Drilling through some challenging formations in Kurdistan, has been always associated with several wellbore stability problems such as mud losses, borehole washout, stuck pipe, extra cutting/caving, and tight holes, which caused numerous nonproductive time to the drilling program and required drilling sidetracks in some of the wells. Review of the drilling reports and dual-caliper logs from 11 offset wells in the area revealed a huge amount of washouts in these formations. These events put a question mark on the feasibility of UBD in these intervals and required conducting a geomechanical modeling and wellbore stability study.

In this study, data from offset wells was analyzed to estimate the local in-situ stress magnitudes and orientations, in addition to pore pressure. The mechanical properties of the formations 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 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 borehole failures in the selected offset wells to verify and calibrate the geomechanical model.

Finally, an operating mud weight window was defined, and the optimum profile of the mud weight was recommended for drilling through each formations. This study showed that underbalanced drilling is feasible through the carbonate intervals, however, will encouter severe wellbore stability problems in the shaly and silty formations.

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