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Tracer testing is a powerful tool for reservoir surveillance. Conducting and analyzing an interwell tracer test (IWTT) before starting an enhanced-oil-recovery (EOR) flood provides specific information required to balance the pattern's swept pore volume (PV) and to optimize chemical or solvent use. Tracer testing also plays an important role in EOR by establishing a baseline for estimating incremental volume swept and recovery factor. A key limitation to tracer testing is the time required to obtain a complete interpretation of the test results. Conventional interpretation methods require observing exponential decline in the tracer concentration from all producers; depending on the pattern size and processing rate, this can take many months or years. Meanwhile, the EOR flood is delayed, thereby eroding its value and reducing the remaining oil target. We have developed a method for analyzing tracer data from relatively early times that is based, in part, on a new way to extrapolate the tracer data, providing timely information to optimize flood performance. The extrapolation can be performed soon after the peak tracer concentration is observed in the well with the slowest response. We illustrate the extrapolation procedure with a synthetic, yet realistic, data set showing that the new methods result in a good approximation at much earlier times. We then compare 'early time analysis' swept PVs with the (correct) swept PVs using the complete tracer data set. We also discuss how early-time approximations can thus provide the opportunity to optimize chemical-flood performance and to enable field startup of EOR floods in a timely fashion. [ABSTRACT FROM AUTHOR]

Enhanced oil recovery (EOR) by surfactant flooding is the key to unlocking the next billion barrels of oil for Minas, one of the world's largest waterflood fields. An interwell tracer test (ITT-1) was performed before a surfactant field trial (SFT) to ensure well injectivity, demonstrate pattern confinement, quantitatively describe interwell connectivity and sweep efficiency, and provide sufficient data for reservoir evaluation. The tracer test was designed by numerical simulation. The test started in November 2009 and was terminated in February 2010. Analytical interpretation based on moment analysis and numerical reservoir simulations was conducted to evaluate ITT-1 results. Interpretation of the test results indicated various operational and reservoir properties that would have likely led to failure of the surfactant pilot. Hydraulic control of the SFT pattern was not achieved; in fact, less than 20% of one tracer was recovered. Many small-scale heterogeneities were identified that led to a lower-than-expected reservoir volume contacted. Unexpected communication between the target sand and the underlying sands outside the pattern also contributed to low tracer recovery and low swept volume. The tracer test was history matched, and additional features were incorporated in the reservoir model, and a new tracer design (ITT-2) was optimized to correct low sweep efficiency and poor hydraulic control. New information from ITT-2 will be used to further optimize operating conditions for SFTs. Failure to conduct the tracer tests would have likely revealed these unfavorable reservoir and operational conditions during the SFT. Had oil recovery been poor (because of low swept volume), it would have erroneously been attributed to a poor SFT rather than to the true causes. ITT-1 is considered successful because it allowed us to redesign injection/hydraulic control during the relatively inexpensive tracer test and thus evaluate the surfactant trial without bias. [ABSTRACT FROM AUTHOR]