Chemical frac tracing is used to evaluate flowback and cleanup efficiencies. The technique utilizes a family of unique, environmentally-friendly, fracturing fluid compatible chemical tracers to quantify segment-by-segment recovery for individual fracturing treatments and stage-by-stage recovery for multi-stage fracturing treatments. These chemical tracers with their unique chemical characteristics are mixed at a known concentration into frac fluid stages as the frac fluid is pumped downhole. Upon flowback, samples are collected and analyzed for tracer concentration. With the use of the mass balance method the flowback efficiency for each stage is calculated. These precise flowback calculations yield a more accurate assessment of cleanup efficiency. This paper presents several case histories in which the technique was implemented. Results and fracture flowback prognoses are presented. The results are also used to assess post-frac performance as a function of flowback efficiency.
Chemical frac tracers, CFT's, are used to precisely calculate flowback and hence flowback efficiency and to evaluate fracture cleanup. Various chemical tracers with unique chemical characteristics are mixed at a known concentration and injected according to a pre-determined design throughout the frac fluid stages, such as the pad and the propping laden fluid stages. The characteristics of these tracers are unique. They do not react with each other, the formation or the tubular. They do not degrade with temperature or time, do not self-concentrate, and do not react with frac fluids. These tracers are detectable at low concentrations of 50 ppt (parts per trillion). They are also environmentally safe to handle, pump downhole and dispose. They are soluble in water, and unlike polymers, do not concentrate upon leakoff.
Upon flowback, samples are collected and analyzed for tracer concentration. With the use of the mass balance method, flowback of each frac fluid stage is calculated, and hence flowback efficiency for each stage of frac fluid injected. The precise flowback calculation for each frac fluid stage yields the fracture cleanup efficiency. Chemical frac tracing can also be used as a means to understand vertical communication between zones. Under this scenario, one chemical tracer is injected into each zone. Each zone is flowed back individually after a period of shut-in allowing the flow of fluid within the formation. Samples are collected and analyzed for tracer concentrations. If zones are communicating vertically under static conditions, the tracer from one zone can flow into other zones and hence will be detected accordingly.
