SYSTEM AND METHOD FOR DETERMINING OIL, WATER AND GAS SATURATIONS FOR LOW-FIELD GRADIENT NMR LOGGING TOOLS

摘要

A well logging system and method are disclosed for detecting the presence andestimating the quantity of gaseous and liquid hydrocarbons in the nearwellbore zone. The system uses a gradient-based, multiple-frequency NMRlogging tool to extract signal components characteristic for each type ofhydrocarbons. To this end, new data acquisition methods are proposed in whichmeasurements at different frequencies are interleaved to obtain, in an singlelogging pass, multiple data streams corresponding to different recovery timesand/or diffusivity for the same spot in the formation (225, 245, 265). Theresultant data streams are processed to determine mineralogy-independent waterand hydrocarbon saturations and porosity estimates. Gas and oil saturationsare used to obtain accurate estimates of the water content, permeability andother parameters of interest. In another aspect, a novel diffusion-enhanceddata acquisition sequence is disclosed for use with low field gradient tools(108).