Dynamic dual-laterolog responses: model and field applications in the Bohai Gulf of China

The process of mud filtrate invading into an oil/gas reservoir is complicated and time-dependent. Filtration causes dual-laterolog responses to deviate from the true-formation resistivity. The magnitude of deviation depends on the time when the well is logged. A model of dynamic dual-laterolog responses is suggested by considering the invasion time effect and is solved numerically. The model can give the variation of dual-laterolog readings with time. When saline-water mud filtrate invades a reservoir, dual-laterolog responses decrease with time. Laterolog response is affected by invasion time, saturation, permeability, porosity, and oil-viscosity. Generally, if a formation is invaded by saline filtrate for a week, the deep laterolog reading will be less by a factor of two or three than true formation resistivity. This decrease in measured resistivity influences not only the quantitative calculation of saturation, but also the determination of an oil/gas reservoir. Field application in the Bohai Gulf of China indicates that logging values deviate significantly from the true formation resistivity if the well is not logged repeatedly and quickly, so that the time effect ought to be taken into account. The present study suggests a reasonable method to determine true formation resistivity and to evaluate a reservoir for dynamic mud filtrate invasion by history matching field data.