Abstract
China is accelerating the exploration and development of tight sandstone and shale reservoirs of the Permian Fengcheng Formation targeting condensates. The formation evaluation is challenged because of the complex mineralogy, low porosity, and unknown formation water salinity. In addition, the operator requires a technique for fluid typing of the condensate reservoir at subsurface conditions for the optimization of the development strategy. This paper introduces a new method combining nuclear magnetic resonance (NMR) T1-T2 and T2-D mapping for formation evaluation and subsurface fluid typing of the condensate reservoir. The NMR relaxation-diffusion (T2-D) mapping successfully addresses the problem of fluid typing, by differentiating gas from oil. The measurements of continuous two-dimensional NMR T1-T2 enable the quantitative estimation of the bound and movable fluid volumes in the pores. The integration of wireline NMR, induced gamma spectroscopy, and the borehole image logs provides insight of the potential of the condensate reservoir in the Permian Fengcheng Formation. In case studies from tight sandstone and shale reservoirs in the Junggar basin, NMR T2-D mappings prove the pore fluid is dominated by gas (at reservoir conditions) in the condensate tight sandstone reservoir, for the first time confirmed by wireline logging. However, the reservoir will produce both gas and oil because of the drop in temperature and pressure. A robust methodology derived from the concept of blind source separation for extracting signals from two-dimensional NMR T1-T2 measurements is proposed. Clay-bound water volume and porosity derived from cluster volumetric analysis of NMR T1-T2 match well with those from spectroscopy and core experiment results. The movable hydrocarbon saturation proves to be effective and robust in sweet zone identification. This paper discusses an integration of NMR T1-T2 logging and T2-D mapping methods from a new-generation wireline NMR tool in the unconventional reservoir. It helps to understand the potential of the reservoirs. The subsurface fluid type is extremely meaningful for the decision of the development strategy. The condensate sweet zones identified by the new method have encouraging gas and oil productions.
