First Application of LWD NMR and Quadrupole Sonic Measurements Derived Formation Evaluation in a Carbonate Reservoir in Sichuan Basin, China

Abstract

Abstract Natural gas production in the Sichuan Basin has been increasing in recent years with double-digit growth. Exploration and development of complex carbonate reservoirs ensures production growth. A major challenge is that the target Triassic Feixianguan Formation is characterized by extremely high sulfur. Use of radioactive source measurements such as density and neutron, are prohibited, for the sake of safety, giving rise to uncertainty in the interpretation of lithology and porosity. It is necessary to introduce source-less measurements as substitutions for the formation evaluation. This paper discusses a novel application of logging-while-drilling (LWD) nuclear magnetic resonance (NMR) and quadrupole sonic measurements for formation evaluation of this carbonate reservoir. The LWD NMR offers simultaneous measurement of T1 and T2, and the joint inversion of T1-T2 data provides real-time answers of the porosity, permeability, and pore structure of the reservoir. The LWD quadrupole sonic tool not only acquires formation compressional, shear, and Stoneley slowness, but also provides unique insight into high angle fracture identification. The integration of NMR and sonic measurements reveals the potential of the carbonate reservoir in the Triassic Feixianguan Formation. A case study is presented from a carbonate reservoir of Triassic Feixianguan Formation in the PetroChina SouthWest Oil and Gas Company. The lithologies of Feixianguan Formation are composed of dolomite and calcite. Mineralogy is successfully estimated by integration of NMR porosity and formation compressional slowness. The NM-derived effective porosity of the dolomite reservoir ranges from 2-20%, and the permeability ranges from 0.01-1000 mD. The well is drilled using oil-based mud, so the NMR free fluid T2 cutoff could be higher than the default 100 ms. Factor analysis based on the NMR T2 data indicates the free fluid T2 cutoff to be 210 ms in the dolomite reservoir. The pore structure of the dolomite reservoir is dominated by macro pores. The integration of NMR free fluid volume, crossover of compressional and shear slowness, and fracture analysis based on Stoneley data proves effective and robust in potential zone identification. This paper discusses integration of LWD NMR T1-T2 logging and quadrupole sonic measurements in the carbonate formation evaluation. It helps operator to understand the potential of the reservoirs. The gas production of the well that adopts the technology breaks the gas field production record. The workflow can also be applied to other high-sulfur reservoirs in China.