Subsurface monitoring and surveillance using inter-well gas tracers

Abstract

Tracer technology has gained considerable attention recently as an effective tool in the reservoir monitoring and surveillance toolkit, particularly in improved oil recovery (IOR) operations. Gas flow paths within the reservoir can be quite different from liquid (oil and water) flow paths. This is primarily due to gravity override, and differences in density and relative permeability between the gas and liquid phases.

An Inter Well Gas Tracer Test (IWGTT) is a key monitoring and surveillance tool for any IOR projects. IWGTT should be designed and implemented to track the flow behavior of gas phase. The test generally entails injecting a small amount of unique perflouro-hydrocarbon tracers into the gas phase injectant stream. IWGTT have been conducted on a limited number of fields across the globe, and sample results of some will be presented.

The sampling frequency of the tracers from the producers should be designed carefully to collect the necessary data that will provide insights about the connectivity between the injectors and producers well pairs, gas breakthrough times (“time of flight”), and possible interwell fluid saturations. Different fit-for-purpose unique tracers can be deployed in the subject injector(s) stream and their elution can be monitored in the corresponding up-dip producer(s).

In addition to reservoir connectivity and breakthrough times between injector and producer pairs, an IWGTT helps in optimizing water altering gas (WAG) operations and production strategies for gas injection projects, improve sweep efficiency and ultimately enhance oil recovery. It can also be used to identify source of inadvertent gas leakage into shallow aquifers or soil gas, and help in the planning and placement of future wells.

This paper reviews the workflow and necessary logistics for the successful deployment of an interwell gas tracer test. It will provide the best practices for designing, sampling, analyzing and interpreting a gas tracer deployment. The paper also highlights the benefits of gas tracer data and their usefulness in understanding well interconnectivity and dynamic fluid flow in the reservoir. The results can be used to refine the reservoir simulation model and fine-tune its parameters. This effort should lead to better reservoir description and an improved dynamic simulation model. The challenges associated with IWGTT will also be shared.