From injection to insight: Tracing efficiency in surfactant huff and puff

CLAUDIO RAMOS, Technical Consultant, and RICHARD ABOWD, Reservoir Consultant, Tracerco 

Surfactant huff and puff has become an increasingly important method for operators looking to improve recovery in unconventional reservoirs. However, as with many enhanced oil recovery techniques, its impact is difficult to measure. Operators face several fundamental questions: where does the injected fluid go, how much is recovered, and does the addition of surfactant truly make a difference to production? 

Traditional production data don’t always provide the answers to these questions. Flowrates and water cuts may hint at trends, but they do not reveal the underlying fluid pathways, mixing behavior or the degree of communication between wells. To address this gap in understanding for a recent huff and puff project, Tracerco applied tracer diagnostics, enabling direct measurement of fluid recovery and interwell communication. 

The following article outlines the design, execution and outcomes of this Tracerco-led field trial. It shows how diagnostic tools can be integrated with huff and puff cycles to give operators a clearer understanding of fluid migration and surfactant effectiveness. This ultimately supports better decision-making in unconventional, enhanced oil recovery. 

TRACER DIAGNOSTICS IN UNCONVENTIONAL EOR 

Tracers have been used in oil and gas fields for decades to understand reservoir communication, flow paths and sweep efficiency. In conventional fields, they have often been applied to waterfloods or polymer floods, to detect crossflow between injectors and producers. However, in unconventional plays, their use has historically been less common, partly due to the cyclical nature of huff and puff and the complexity of fractured reservoirs. 

Fig. 1. Simplified schematic of the Huff-and-Puff cycle, showing tracer application during injection, soaking and production phases.

In principle, the tracer approach is straightforward. A chemical marker, detectable at extremely low concentrations, is added to the injected fluid. Samples are then collected during production and analyzed for the tracer. By plotting tracer concentration against time, engineers can observe when the injected fluid first emerges, how quickly it returns and in what proportions, relative to the water and hydrocarbons produced. 

For huff and puff projects, tracers offer particular value. The cyclic injection, soak and production phases create opportunities to observe first-in last-out behavior, mixing between injection stages, and the extent of recovery from different parts of the cycle. By applying tracers continuously across an injection period, it is possible to differentiate between early and late-injected fluids, revealing both the efficiency of recovery and the effects of surfactant treatments, Fig. 1. 

CASE STUDY: SURFACTANT HUFF AND PUFF WITH TRACER INTEGRATION 

Project design. The trial was conducted with a group of four wells. Wells A and B were treated with surfactant, while wells C and D were left untreated, providing a control group for comparison. Approximately 20,000 bbl of water were injected during the test, with tracers applied throughout the cycle. 

Fig. 2. Tracer data confirm no migration of injected surfactant beyond the target zone. Offset wells showed negligible-to-no tracer detection, indicating isolated flow behavior and minimal inter-well communication.

To enable differentiation between early and late injection, tracers were changed midway through, so that the first half of the injection cycle carried one tracer signature and the second half carried another. This approach allowed engineers to compare the recovery of fluids injected early versus late, as well as to track mixing behavior over time. 

Sampling was not limited to the treated wells. A broader set of offset wells (E through L) were monitored to detect potential communication and leak-off. By including these in the sampling regime, the project aimed to verify whether injected fluids remained contained within the targeted zone or migrated laterally, Fig 2. 

MONITORING AND DATA COLLECTION 

Samples were taken at high frequency throughout the production phase. The sampling design aimed to capture both early breakthroughs—which might suggest preferential pathways or channeling—and longer-term trends that reflected mixing and overall recovery. 

Tracer concentration curves were then plotted, with concentration shown on the vertical axis and time on the horizontal axis. Distinct patterns emerged that illustrated the dynamics of recovery across the wells. 

KEY FINDINGS 

The tracer data provided several significant insights:  

• Containment of injection. Recovery was largely confined to the injection wells, with offset wells showing negligible tracer return (typically close to 0.1% or less). This confirmed that the injection was well isolated, with minimal leak-off. 
• Last-in, first-out behavior. Tracer returns showed that fluids injected later in the cycle were preferentially recovered during the early stages of production, while those injected earlier emerged more gradually. This pattern is consistent with a last-in, first-out trend. 
• Mixing over time. After roughly a month of production, tracer curves showed convergence, with both first and second injection signatures being recovered in equal proportions. This demonstrated reservoir mixing between injection slugs and the establishment of equilibrium. 
  

  Fig. 3. Each well showed recovery of only its assigned tracer, verifying containment of injected fluids and no inter-well communication.
• Differences between surfactant and non-surfactant wells. Wells A and B, which received surfactant, showed lower overall water recovery compared, to wells C and D. For example, recovery from the first half of injection averaged around 11% to 15% across all wells, but the surfactant-treated wells consistently reported reduced water return, compared to untreated ones. This was interpreted as evidence that surfactant was mobilizing bound water and oil, reducing the volume of free water being recovered. 
• Quantitative recovery data. Across the four wells, the percentage of tracer recovery aligned proportionally with water production. For instance, in well B, the first half of injection returned around 8%, while the second half returned 12%, closely tracking water production volumes. In well C, recovery was higher, at 14% for the first half and 22% for the second half. These differences provided a measurable basis for comparing surfactant and non-surfactant performance, Fig 3. 

Taken together, the data showed that tracers could directly confirm containment of injected fluids, quantify the proportion of fluid recovered from different stages of injection and reveal differences attributable to surfactant application. Importantly, the tracer evidence provided a more robust assessment of surfactant effectiveness than production data alone, which can be influenced by geological variability and other operational factors. 

OPERATIONAL INSIGHTS FROM TRACER RESULTS 

Injection timing. Tracer results highlighted the significance of when fluids are injected. Late-cycle injection was recovered preferentially in early production, while early-cycle injection tended to remain in the reservoir longer. This suggests that timing surfactant application later in the cycle could improve its efficiency by targeting the most readily recoverable fluid fraction. 

Soak time. The project also investigated the impact of soak duration. Short soak times were found to limit recovery, while excessively long soaks risked equilibrium being reached, reducing the retrievability of injected fluids. Tracers provided direct evidence of these dynamics by showing how quickly injected fluids began to reappear after production commenced. 

Injection volume. A key lesson was the diminishing return of excessive injection volumes. With 20,000 bbls injected, it became clear that not all water could be recovered, even over extended production. Tracer data showed that beyond a certain point, injected fluid became unrecoverable, trapped, or mixed within the reservoir. This insight can guide operators in designing more efficient injection volumes. 

Surfactant effectiveness. One of the most important outcomes was the ability to measure surfactant impact indirectly. Wells treated with surfactant recovered less free water, suggesting that the additive was working, as intended, by releasing water bound to rock surfaces or otherwise immobile. By contrasting these results with untreated wells, the tracer analysis provided a clear demonstration of surfactant action. 

Field management lessons. Beyond individual parameters, the tracer trial underscored broader operational lessons. These included the value of maintaining injection containment, the importance of monitoring for early breakthrough or channeling and the potential to apply tracer diagnostics to marginal or late-life fields. In each case, tracers offered a practical means of reducing uncertainty and optimizing decision-making. 

LOOKING AHEAD 

The trial demonstrated that tracers are not a replacement for production data but rather a complementary tool. They do not provide a direct measure of oil recovery, but they can confirm containment, quantify fluid recovery and highlight differences caused by additives, such as surfactants. In doing so, they provide operators with a diagnostic tool that supports more informed choices in designing and evaluating huff and puff projects. 

Tracerco, which has applied tracer diagnostics in a range of EOR contexts worldwide, believes there are opportunities to expand the use of tracers. Bio-surfactant trials, hydrocarbon-phase tracers and multilayer reservoir applications all stand to potentially benefit from the methodology demonstrated here. The ability to isolate and measure recovery from different reservoir sections, or to validate the effectiveness of novel additives, could be valuable in both unconventional and mature fields. 

Ultimately, tracer diagnostics offer a path from injection to insight. By providing direct evidence of how fluids move and are recovered in the reservoir, they enable operators to reduce uncertainty, improve economic outcomes and make more confident decisions about enhanced oil recovery strategies. As a result, projects can deliver greater oil reserves and achieve higher recovery factors, delivering measurable, long-term gains from every project. 

CLAUDIO RAMOS is a technical consultant for Tracerco. Before that, he was a region engineering advisor at ProTechnics, a division of Core Laboratories, for over 10 years, from November 2013 to February 2024. Prior to that, Mr. Ramos was senior district engineer at Basic Energy Services for about four years. He began his career at SLB as a district technical engineer from June 2005 to January 2009. Mr. Ramos earned a BS degree in chemical engineering from University of Oklahoma in 2005. 

RICHARD ABOWD is a reservoir consultant at Tracerco. He has worked for the company for nearly 11 years in that capacity, based in Houston. Mr. Abowd earned a BS degree in petroleum engineering from University of Houston in 2014.