Use of Tracers To Investigate Drilling Fluid Invasion and Oil Flushing While Coring

Abstract

A procedure is described by which oil flushing during coring of steamed and unsteamed heavy oil formations can be estimated. Chemical tracers in the drilling mud are used to indicate if filtrate invasion is occurring. Core saturation analyses are coupled with data from high velocity corefloods and temperature-dependent residual oil saturation measurements to estimate oil flushing during coring. Introduction Accurate oil saturation data are essential for reserve determination when considering steamflooding heavy oil reservoirs. The accuracy of these data are even more critical for implementing post-steamflood techniques, since project economics are very sensitive to oil saturation in a partially depleted reservoir. The most common sources of oil saturation data are core and log analyses. This paper addresses whether core-derived oil saturations are representative of in-situ oil saturations in unsteamed and steamflooded heavy oil formations. Two well-known processes can occur to cause oil flushing during core cutting and retrieval:Mud filtrate invasion due to overbalance pressure at the drill bit during coring operations.Fluid expansion due to a drop in pore pressure during the core retrieval. Expansion and fluid loss during core retrieval is assumed to be negligible for most heavy, "dead oil" reservoirs. Significant reductions in oil saturation can result from flushing by filtrate invasion while coring. Jenks et al. provided information indicating that overbalance pressure is the prime factor in oil stripping during coring operations. This paper, therefore, focuses entirely on oil flushing due to filtrate invasion at the drill bit. Filtrate invasion can be quantified by adding a tracer to the drilling mud and subsequent whole core analysis. Lovelace and Armstrong 2 identified many tracers which had been used in filtrate invasion studies. In particular, they used tritiated water to analyze some of their whole core samples for tracer in inner and outer concentric sections. Water soluble salts of iodide and bromide were used in this study due to their ease in handling and accuracy of analysis. This study extends the work of Lovelace and Armstrong by sampling each whole core at the center, middle radius, and outer radius in order to construct tracer concentration profiles within the whole core.