Continuous Characterization of Multiple Fluids in a North Sea Gas Condensate Reservoir by Integrating Downhole NMR with Downhole Sampling

摘要

Gas condensate reservoirs sometimes display compositional grading. The variation in fluid properties can be high in some cases, with gas condensate at the top of the reservoir and light oil near the bottom. The economic value of the field is strongly influenced by the fluid type. Accurate understanding of reservoir fluid properties is therefore a key requirement for successful reservoir management. In many cases, compositional grading is not detected using traditional logging methods. Therefore, fluid variations can be a surprise at the later stage of reservoir appraisal or even at the early stage of reservoir production. In such situations the standard method for identifying fluid contacts relies on the increased density-neutron separation seen in gas zones. This effect is not reliable in gas condensate reservoirs because any small change in curve separation at a fluid contact can be easily perturbed by invasion and clay content. Another traditional method of using pressure versus depth plots can suffer from the lack of sensitivity to detect small changes in fluid gradients. In this paper we present how Nuclear Magnetic Resonance (NMR) coupled with downhole fluid analysis during sampling can be used to provide robust resolution of the fluid variations even in difficult situations. The method is demonstrated in a recent North Sea well, where continuous diffusivity measurements correctly identified the presence of hydrocarbon fluids compositional grading.