Fluid Production Profiling in Deep-Water Carbonates: Intrawell Chemical Tracers Application in Long Horizontal Drains

Downhole well surveillance is a critical aspect for reservoir management. In fact, information derived from static and dynamic data acquired at wells represent key factors in understanding well and reservoir behavior and optimizing lifetime production. Fluid movement monitoring is one of the purposes of the surveillance activities and it is mainly aimed at highlighting where hydrocarbon is coming from, how much and where water breakthrough is occurring. In recent years, reservoir developments are increasingly characterized by complex wells in very challenging environments, including long horizontal sections, multi-lateral designs, pump-assisted methods, asphaltene problems in production, remote locations, or deep-water scenarios. Conventional downhole monitoring tools, such as wireline or coiled tubing conveyed production logging (PL), are difficult to deploy in these environments, considering the high risk of tool stuck, the limited borehole access and/or the very high operational costs. In that respect, intrawell chemical tracers (ICT) represent a valid and relatively cheap wireless monitoring solution alternative to PL. This paper deals with the complete modeling of tracer concentration behavior under transient conditions for accurate production allocation. The focus is on long horizontal drains intercepting deep-water carbonate reservoirs, where ICT can provide production profiling without risky and costly well interventions. ICT are installed in typical carriers placed at different pre-selected intervals along the well completion. Each carrier is characterized by a unique oil and/or water tracer signature. The tracer concentration is then determined by means of advanced chromatographic and mass spectroscopic analyses carried out on each fluid sample taken at the surface during the transient production period. Finally, tracer concentration behavior over time is interpreted, for each producing intervals, by means of an ad-hoc one-dimensional partial differential equation model with proper initial and boundary conditions, including advection and diffusion physical phenomena. The methodology has been developed and validated using a dozen of tracer campaigns worldwide in different challenging environments. Recently, the reliability of the ICT-based production allocation was demonstrated in several case studies by comparison with the results of parallel PL measurements. The very good match thus obtained has furtherly opened the way to the use of ICT for dynamic monitoring in complex scenarios. The approach is here presented with a case study, where ICT systems have been installed along several long horizontal drains drilled in a deep-water carbonate reservoir. The aim was to avoid the operational risks associated to standard PL, together with cost reduction purposes. At target, the typical completion of the wells consists of a slotted liner with oil and water tracers installed to properly discretize the interval open to flow (approximately 1000 m) in representative sub-intervals. These were chosen after the geosteering operations, and according to the petrophysical characterizations, to properly map the heterogeneities encountered by the drains. For each well, three oil sampling campaigns are performed in transient conditions: pre-acid clean-up, post-acid clean-up, and main flow during normal well production. In addition to the actual length of the drain contributing to production, the interpretation of the two clean-up phases provides important information about stimulation efficiency. The results associated with the main flow (two years after the start of production) are considered to be those that properly characterize the well from a dynamic point of view. This fact has been also demonstrated by the good correlation between production allocation outcomes, log-based flow capacity per interval, image log interpretation results, and nuclear magnetic resonance log responses. Hence, the dynamic characterizations through ICT have been used to calibrate well test interpretations, as critical benchmarks for reservoir modeling, and to further optimize geosteering operations and completion strategies in the subsequent development phases of the field.