Planning and Interpretation of Offshore-Field Tracer Tests Using Accurate and Refined Tracer Simulations

Abstract

Tracers are chemical compounds or radioactive isotopes used to label injection water and gas in petroleum reservoirs. In inter-well tracer tests (IWTTs), well connections and fluid patterns can be established when the tracer appears in production wells. Tracer simulation is an invaluable tool to ease the planning or interpreting results of IWTT. In this paper we introduce a novel tracer simulation methodology which is based on a decoupling from the underlying reservoir simulation. The methodology ensures accurate and very fast tracer simulation; with CPU times in the order of a few percent of ordinary reservoir simulation. The method is based on solving the tracer equations on a grid separated from the underlying reservoir simulation. The separate tracer grid can be refined, with significant improvements in the accuracy of the tracer solution, without affecting the speed of the reservoir simulation. Accurate refinement as well as localized tracer grid refinement is introduced and investigated. Local refinement is compared to global refinement, and we demonstrate that local refinement can be a valid strategy in many cases. We demonstrate the methodology on a real reservoir case, and show that predicted breakthrough times are significantly affected by the accuracy of the solution. The method improves accuracy by overcoming numerical diffusion through separate tracer-grid refinement. In our specific real case, the breakthrough time was underestimated by about one year in the solution obtained using standard methods. This was corrected using the new methodology. The large difference in simulated breakthrough time between the improved and corrected solution has important consequences for the planning and interpretation of IWTT. IntroductionTracer testing in petroleum reservoirs is a mature technology, where fluid movement is tracked by tagging injection fluids by inert chemical or radioactive compounds (for a review see Zemel, 1994). One application of tracers is the inter-well tracer test (IWTT), which is used to obtain information on well-to-well communication, heterogeneity and fluid dynamics (Dugstad, 2007). One of the appealing features of IWTT is that at the first tracer breakthrough in a producer, immediate and unambiguous information on the injector -producer communication is given. In cases with several sources of injection fluids, tracers yield unique information on the origin of fluids. During IWTT, injected fluids are labeled using unique radioactive or chemical tracer compounds, which are then subsequently used to trace the fluids as they move through the reservoir. Many compounds may a priori be candidates for tracer application. However, compounds are subject to harsh temperature and pressure conditions, biodegradation, scaling reactions, ion exchange, adsorption to the solid phase etc. Modeling of these effects would require a detailed description of the chemical and physical conditions in the reservoir, conditions that are generally not known. Rather than attempting to model such effects, it is custom...

Tracer monitoring of enhanced oil recovery projects

“…This is a serious disadvantage if lessons-learned from a first tracer simulation are to be applied in subsequent tracer simulations. The simple solution to this is to store solutions to the reservoir fluid flow problem for each reservoir simulation time-step, and perform the tracer simulation based on the stored results [45,46]. Multiple and successive tracer simulations can then be performed based on stored results from a single reservoir simulation.…”

“…To improve on this situation, one can use successively finer time-steps and a separately refined tracer-grid. The method described in [46] allows for successive refinement of the tracer time and space discretization, at the cost of a single reservoir simulation.…”

Practical methodology for interwell tracer applications

“…This error generates a difference between the real production history (solid blue line) and the production history obtained through the simulation (dotted black line). Adjusting the tracer production history, helps to identify potential simulation model errors [58]- [59].…”