Abstract
A large-scale formation evaluation program has been conducted in multiple fields to enhance reservoir description and fluid characterization. Well-logging targets include siliciclastic and carbonaceous formations that pose numerous challenges to petrophysical interpretation and wellsite operations. First, it is difficult to select formation pressure points as permeability prediction by conventional logs is hindered by complex lithology, grain size variations and microporosity. The second challenge is the complex distribution of fluids where hydrocarbons may vary from heavy oil to gas within a single reservoir section. This often necessitates a number of fluid pumpouts or samples that are by far the most time-consuming and expensive element of the logging program.
Nuclear magnetic resonance (NMR) logs noticeably reduce the uncertainties of log interpretation and increase operational efficiency in multiple ways. Formation tester measurements demonstrate that the readily available NMR Free Fluid Index (FFI) predicts fluid mobility with remarkable consistency especially in complex shaly sand intervals. Advanced NMR T1, T2 and diffusivity measurements also helped identify distribution of formation water, oil and gas.
This work presents case studies to demonstrate each of the above benefits with notes on principal limitations and areas of improvement. It also reviews technical lessons from NMR logging performed by different vendors in several evaluation wells in the area from the viewpoint of everyday tasks. Practical guidelines are formulated for logging tool and activation selection, log quality-control, and NMR data processing with standard basic and advanced algorithms followed by recommendations for future reservoir description studies.
