A New Evaluation Method for Premium Threads Based on Downhole Load Conditions and Time Effects

In deep and ultra-deep wells of PetroChina Southwest Oil & Gas Field, Ø196.85 × 16.83 mm 155 V production casings are adopted. To enhance performance under complex downhole conditions, premium clearance coupling threads are used, featuring an outer diameter 13 mm smaller than standard couplings. These threads adopt dual sealing structures for both external and internal pressures, and the manufacturer guarantees that the internal pressure resistance of the threads is 85% of the pipe body’s internal yield pressure. According to the API RP 5C5 standard, evaluations of gas-tight premium threads under actual downhole loading conditions require the conversion of coexisting internal and external pressures into an effective pressure. However, the pure internal pressure acting on the threads may exceed their bearing capacity, potentially causing sealing failure. Thus, a new evaluation methodology is required—one that incorporates both downhole loading conditions and time-dependent effects. The research includes the following aspects: (1) Analysis of downhole load conditions during drilling and completion operations, including casing running, temperature rise from cement hydration, cementing pressure bump, wellhead pressure testing, gas kick scenarios, and extreme loads under fully evacuated conditions; (2) Identification of risk load points based on yielding and sealing failure criteria; (3) Establishment of a safety-margin-based test load spectrum in accordance with casing string design factors; (4) Finite element analysis (FEA) based on structural and sealing integrity failure criteria; (5) FEA considering the stress relaxation of materials at high temperatures to assess sealing life; (6) Development of a theoretical integrity verification and evaluation method; (7) Optimization of thread selection based on experimental evaluation. Through FEA incorporating actual downhole conditions with coexisting internal and external pressures, the safe application of premium clearance coupling threads in ultra-deep wells is ensured. When internal pressure alone exceeds the thread’s limit capacity, integrity tests considering both internal and external loads must be conducted. This paper introduces an innovative evaluation and selection method for premium threads that accounts for downhole load conditions and time-dependent effects.