Use of Tracers To investigate Drilling-Fluid invasion and Oil Flushing During Coring

Abstract

Summary This work develops a method in which chemical tracers in the drilling fluid help determine mud filtrate invasion and the degree of oil flushing during coring of steamed and unsteamed heavy-oil formations. Salts of iodide and bromide were added to the drilling fluid while Well T03 was cored through the Lombardi and Aurignac zones at San Ardo field in California. Vertical core plugs, taken from the periphery to the center of the retrieved whole core, were analyzed for tracer concentration. Tracer analyses indicated minimal filtrate invasion in the not-yet-steamflooded Lombardi zone and complete filtrate invasion in the steamflooded Aurignac zone. Tracer and oil saturation analyses showed the Lombardi zone to be uniform from top to bottom with an average oil saturation of 42.5% and an average porosity of 31.1%. interpretation of tracer and oil saturation data permitted the construction of a layered model for the Aurignac zone. The layers ranged from an average oil saturation of 8% in the steamflooded layer to 37% in the bottom layer. The data showed that significant oil flushing (6%) occurred only in cores taken from the hot-waterflooded layer just below the steam zone. Vertical core-plug porosities and saturations, as determined by a unique calculating scheme, were compared with conventional and Elkins-corrected values. The comparison indicated that misapplication of the Elkins method in high-temperature formations may result in significant errors.