The oil and gas industry is witnessing an increasing demand for more cost-effective well design and operations. Thus, the scope of the operator company's planned new processing capacity aims to attain a competitive cost and schedule. In this work, slim versus fat casing designs are evaluated in price and technical challenges, including removing rig/skidding, drilling, and ensuring well suspension.

Data from ten (five slim design and five fat design) wells in southern Iraq was quantitatively analyzed. Attaining the project's target requires that the well be drilled as a deviated well (S-type). The analysis includes the cost of CSG, lost circulation, lost curing, lost circulation materials, volume of the cement plugs to cure losses, non-productive time, stuck pipe and differential sticking, and cement bond quality. Moreover, a cost analysis is conducted by considering all of a project's relevant factors—including economic and technical considerations—to ascertain the likelihood of completing the project.

The finding emerged that the amount of lost mud and the average cost of addressing losses were higher in slim than fat designs. The slim design is associated with higher volumes of cement plugs for curing losses than the fat design. As per NPT analysis, the time required to fix losses emanating from slim design was 62% higher than fat design. A critical observation emerged from the study that while differential sticking failed to occur in both designs, stuck pipes happened in some of both designs. The cost analysis of slim and fat designs focused on the cost of drilling, CSG, wellhead, diesel, and fueling is also done. The total cost of the fat design amounted to approximately 53.67%, while the total cost of the slim design was about 46.33%. This made the slim design's cost savings ratio of roughly 7.34%. Meanwhile, given that similar issues may occur in the proposed well design, the following measures have been isolated to help tackle such problems. (1) Optimize mud design to inhibit Tanuma formation Clay swelling issues (2) Reduce OH time to avoid Tanuma's time-dependent clay swelling. (3) Reduce the inclination across Tanuma to 20 degrees.

Finally, this paper describes how two casing designs are successfully engineered and executed and serves as a guide for selecting proper candidates for this design. Also, it is an operational guide for two casing designs, slim and fat, to ensure that these challenging long open holes will be successfully and economically drilled while minimizing risks and ensuring compliance with the well delivery process.

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