A Field Case Study of Managed Pressure Drilling in Offshore Ultra High-Pressure High-Temperature Exploration Well in the South China Sea

Abstract

Summary The Y basin in the South China Sea has 15 trillion m3 of natural gas and represents typical ultra high-pressure high-temperature (ultra-HPHT) subsurface conditions, with the maximum bottomhole pressure (BHP) of 142 MPa (equivalent density = 2.38 sg), the highest bottomhole temperature (BHT) of 249°C at a depth of 5638 m, and an extremely narrow drilling mud density window. Therefore, there are several kinds of technical challenges during drilling operations. In recent years, managed pressure drilling (MPD) has been successfully applied in the basin with reduction of risks and well cost. Presented research focuses on a successful field case study of MPD in the Y basin. The case study consisted of two parts: MPD operational designs and MPD operational procedures. MPD operational designs included two steps: the precise calculation of drilling fluid equivalent circulating density (ECD) and the optimization of operational parameters. Four models were used in calculating drilling fluid ECD: wellbore heat transfer model, drilling fluid equivalent static density (ESD) model, drilling fluid rheological property model, and incorporating the effects of cuttings in the pressure loss model. The second step included two key operational parameters: mud weight (MW) and surface backpressure (SBP). MPD operational procedures were performed in three steps: roles and responsibilities of key personnel during MPD operations, wellsite training program and emergency procedures. Emergency procedures included unexpected drilling events, downhole equipment failures, and surface equipment failures. Taking X-1 Well as an example from the field case study, the following important observations were made. First, the MPD equipment was installed after remodeling of the drilling platform, installation of the wellhead assembly, and pressure testing of the MPD equipment. Next, the MW was set to 2.15 sg and the SBP was maintained at 525 psi while drilling the 8⅜ in. hole section on the basis of the operational design. In addition, the possible risks and corresponding preventive measures were taken into account and the MPD casedhole fingerprinting exercises were carried out. Finally, the MPD was successfully applied to X gas field featuring offshore ultra-HPHT. In addition, the dynamic formation integrity test (DFIT) and stripping out of hole with SBP were conducted. The well was successfully drilled in the narrow drilling mud density window of 0.05 sg without incidents and a sharp drop in the nonproductive time (NPT) was observed. Also, the casing structure was optimized from 7 to 5 strings with a significant reduction in the well cost and drilling time.