Innovative Solutions for Ultra-Deep Water and Ultra-Shallow Gas and Hydrate Reservoir Wireline Formation Testing

Abstract With the implementation of advanced seismic interpretation techniques and innovative exploration theories, a significant number of high-quality natural gas reservoirs have been discovered in the ultra-deepwater region of the South China Sea, many of which were previously unidentified or overlooked. This paper focuses on the challenges encountered during wireline formation testing (WFT) in the LS36-X block and the solutions implemented throughout the exploration campaign. The water depth in this region exceeds 1,500 meters, and the reservoir is located at a depth of 200–300 meters below the seabed, with reservoir temperatures ranging from 14°C to 18°C. This sets a record as the shallowest known buried gas reservoir in the world. From a geological perspective, the reservoir exhibits characteristics such as unconsolidated formations prone to sand production, along with high shaly sand content in tight reservoir formations. Regarding reservoir fluids, the field features a diverse range of fluid types, including gas, gas-water coexistence, gas hydrates, and ice-gas coexistence. In terms of drilling and completion methods for these types of reservoirs, the challenges include the use of low-temperature, enhanced-viscosity drilling fluids in both riser and riser-less open-water drilling environments. These conditions led to a number of technological difficulties for WFT operations. The challenges included distinguishing between tight formations and sand plugging during slow pressure buildup, penetrating low-temperature viscous mud cakes, resolving pressure build-down issues caused by probe repressurization, and achieving effective packer seals in hydrate zones and sandy formations during pressure testing and sampling. To address sand production issues during sampling, a customized pumping module and filter sizing, based on grain size analysis, were developed. These modifications enabled the use of an extremely low pumping rate, allowing fine sand particles to pass through the filter, thereby preventing sand plugging at the inlet. Implementation of these solutions enabled successful deployment of WFT operations, proving critical for reservoir fluid characterization and dynamic permeability evaluation in the LS36-X field. Through integrated sand control measures and equipment/workflow optimizations, comprehensive pressure measurements and fluid samples were acquired in the gas reservoir. Significantly, formation sampling was successfully achieved across both hydrate-bearing zones and hydrate-gas mixed zone – representing one of the few documented global successes in hydrate layer via WFT.