Fluid Typing With T1 NMR: Incorporating T1 and T2 Measurements forImproved Interpretation in Tight Gas Sands and Unconventional Reservoirs

摘要

Have you ever tried to make a positive distinctionbetween oil, gas and water using the conventional NMRT2 analysis? Were you successful in yourinterpretation? It was difficult, wasn't it? Now add thecomplexity that occurs in a low porosity, lowpermeability or unconventional reservoirs. This makesNMR interpretation a very difficult issue for the enduser in these types of reservoirs. The recent applicationof NMR T1 logging techniques in tight sands hasintroduced a more robust method to identify fluid typesin the reservoir.The problem starts in the physics of the measurement.In T2 space, the methane response is often masked bythe water response due to its high diffusivity. In orderto separate gas from water in the T2 spectrum,traditional analysis methods would be to acquiremultiple T2 measurements taken at different wait times.This is taking measurements at different levels ofpolarization. The differences in the T2 spectrumbetween these different wait time measurements arethen interpreted to determine the fluid type, hopefully.In tight low permeability reservoirs, the NMR signalsare small; and the resulting differences betweenmultiple wait time measurements often do not provide aconfident interpretation of reservoir fluid type. Hence,the application of recording the T1 measurement offersnew insight into this issue.T1 measurements made during the polarization or waittime of a conventional T2 measurement CMPG cycle. Ittakes advantage of the fact that T1 is insensitive todiffusion and that the gas signal is clearly separate intime from the water signal in the T1 spectrum. In verylow permeability reservoirs, invasion, or imbibition, ofborehole fluids is very shallow. In many reservoirs, thedepth of investigation of the tool is greater than thedrilling fluid invasion. Therefore reservoir fluid typecan readily be identified independently and inconjunction with standard formation analysis.A systematic approach of modeling T1/T2 fluidresponse and is used to integrate T1 and T2 used in thetight sand examples from the Rocky Mountain region ofthe US in both gas and oil reservoirs.