Optimization of reaming size for salt-gypsum formations in the Piedmont structural belt of the Tarim Basin

During drilling operations in the Kuqa Piedmont tectonic belt of the Tarim Basin, drill string sticking and casing collapse frequently occur due to the creep behavior of salt-gypsum formations. To mitigate these challenges, reaming while drilling technology is proposed for application in salt-gypsum intervals. This study first determines the rock mechanical properties and creep characteristics of the salt-gypsum layer through laboratory experiments. Subsequently, a numerical simulation model for drilling and reaming operations in the salt-gypsum layer is developed, enabling optimization of reaming size by comprehensively accounting for the effects of formation creep and casing running feasibility. Finally, the effectiveness of the optimized reaming strategy is validated through field case applications. Results show that salt rock in the piedmont area exhibits both transient and steady-state creep stages, with the latter being predominant. The creep behavior is well represented by the Heard creep model. The maximum error of the salt-gypsum layer creep simulation model is less than 10%, confirming high model accuracy. Considering the combined effects of salt-gypsum rock creep and casing running feasibility, the optimal reaming sizes for 8 1/2‘, 9 1/2’, 12 1/4‘, and 13 1/8’ wellbores are determined as 9 1/8‘–9 1/2’, 10 1/4‘–10 1/2’, 13 1/2‘–14’, and 14‘–14 1/2’, respectively. In Well A, reaming while drilling technology was applied to enlarge the 9 1/2’section to 10 1/2’. Post-reaming evaluations revealed a stable wellbore diameter, and the 196.85 mm casing was successfully installed, demonstrating the technical effectiveness of reaming while drilling in the piedmont salt-gypsum formations. Field implementation of this technology has reduced the incidence of drilling stuck events in salt-gypsum intervals by 16–26%, confirming that the proposed method provides reliable guidance for reaming while drilling applications in similar piedmont structural settings.