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There are many ways to evaluate production. One is to evaluate stimulation performance to ensure that treatments are going where they were intended to go. Another is to evaluate production to verify the contribution of each stage of the completion and to gauge the effectiveness of the stimulation. The recent growth of the hydraulic fracture stimulation market spurred by unconventional gas development has cast a light on all aspects of well construction and production that can benefit from the use of tracer technology.
There are two main categories of tracers – radioactive and chemical – but there are several applications for each.
There are two general methodologies for detecting tracers – in situ, using logging instruments, and at the surface, principally by taking samples from produced fluid. Usually, the former method is used in detecting radioactive tracers, and the latter method is used for detecting chemical tracers.
To locate a material downhole, simply tag it
Tracers can be used for a wide variety of applications. Perhaps the most common application is the use of tracers to “tag” fluids, solids, or slurries that have been spotted in a well. This includes cement, gravel pack slurries, and stimulation services. An early use of radioactive tracers was to tag tool joints, downhole hardware, or perforating charges so they could be located later using a gamma ray logging tool. This practice still is in use today.
Typical radioisotopes for tracer use include Iridium, Scandium, and Antimony. These are gamma ray emitters, and because the gamma signatures of these elements are different, they can be detected and discriminated by a gamma spectroscopy device.
Because a logging instrument is required to map the distribution of the tracer elements in the completion, the precise depth of tracer placement is known. The three isotopes listed have half lives measured in days. Others, like Cobalt and Cesium, have half lives of several years and are used for marking items that require long-term monitoring. Usually, radioactive tracers are used to tag different stages of a job, whether it is a cementing operation or hydraulic fracture treatment. A subsequent logging pass can tell engineers exactly which zones took the tagged material. Common applications include primary or squeeze cement integrity profiling, perforation effectiveness evaluation, frac height containment, frac stage tracking, matrix acidization effectiveness, and conformance control polymer treatment evaluation.
An application of radioisotopes is ProTechnics ZeroWash tracer that uses precise injection of customized ceramic particles containing embedded isotopes into discrete segments of a completion treatment. The isotope cannot be washed off the particle by subsequent treatments or flow periods. A typical use of the material is to trace proppant placement in a hydraulic fracture treatment.
The company also uses radioactive Tritium, a beta emitter with a 12.3-year half life, for an invasion profiling application. By doping water-based drilling mud with Tritium before conventional coring operations are conducted, the retrieved cores can be analyzed to determine the degree to which they have been invaded by mud filtrate. This provides valuable insight as to fluid mobility within the formation as well as the degree of invasion that must be taken into account when interpreting logging services. A similar technique using a chemical flag is provided when drilling with oil-based muds.
Chemicals offer distinct advantages
Recently, the tracer service industry has expanded its scope through the addition of chemical tracers. Chemical tracers offer versatile applications without the impediment of accountability and disposal regulations that accompany radioactive tracer use. There are more unique chemical tracers than radioactive ones, which gives the engineer more tools to analyze treatments and injection processes. Perhaps the biggest advantage of chemical tracers is the fact that detection is achieved by catching samples of produced fluid at the surface – no downhole logging instruments are required.
Today, perhaps the number one application for chemical tracers is to analyze post-frac treatment fluids during the cleanup phase. Generally, two techniques are used. Treatment fluids are tagged with uniquely identifiable chemicals when they are pumped. Then flowback fluid is sampled and analyzed to help engineers determine cleanup efficiency, impediments to flow, proper operation of downhole flow control devices, and zonal contribution.
Plotted over time, chemical tracer analysis can show which stages are contributing to total flow and by how much. This information is vital when production from different zones is commingled.
Many subsea completions accept production from satellite wells via a subsea manifold. By tagging different wells with chemical tracers, it is easy to determine each well’s contribution by sampling commingled flow at the surface production facility. If desired, individual stages of a multistage completion can be uniquely tagged so the lateral production zone can be analyzed from heel to toe without the necessity of intervention for a production logging run.
With more than 30 patented chemical tracers, Norway’s RESMAN leads the pack. Tracerco (UK) claims 20 formulations, and ProTechnics has 14. Whereas ProTechnics deploys liquid tracers for field injection scheme profiling, the two European companies use a deployment technique with an automatic triggering feature.
At Tracerco and RESMAN, chemicals are impregnated into plastic-like polymer material that can be trimmed to fit nearly any completion hardware. The polymer substrate resists erosion from high-rate flow streams and remains in place, dormant until it is triggered by its specific triggering medium – oil or water. For example, to identify water breakthrough in a long horizontal or multistage completion, chemical inserts that are triggered by water are placed at each level of the completion. As soon as even 1% water cut is detected, the inserts begin to release their unique tracer chemistry into the flowstream where it can be detected at the surface. Release rates are influenced by well temperature. By knowing which chemical is at each level, it is easy to determine where the water is originating.
A reverse analysis can be made by deploying inserts that are triggered by oil. In this scenario, frac fluid flowback will not trigger the chemicals, but as soon as crude oil starts to flow, it is detected.
According to RESMAN, the oil-actuated system is designed to last 10 to 15 years with extension to 20 years made possible by adding more tracer material. The water-actuated system has a shorter life – about five years. Tracerco said its material was tested and found to be 90% consumed in 145 days. ProTechnics products continue to show up as long as the treated fluid is produced on flowback. Often, both oil-actuated and water-actuated tracers are deployed together. ProTechnics and Tracerco claim the ability to detect traces in the parts-per-trillion level.
In today’s world of long, multizone, horizontal completions, knowing where produced fluids are coming from and when they arrive at surface is critical to understanding reservoir and completion behavior.