Impact of Irreversible Retention on Tracer Deployments; Constraining Novel Material Deployments

Abstract

Abstract Full field inter well tracer programs have become more and more ubiquitous for effective reservoir surveillance. Novel tracer materials with much higher detectability and lower costs have been actively screened. One of the biggest challenges in deploying novel material types, however, is their elevated irreversible retention to reservoir rocks. Herein we benchmarked traditional inter well tracer chemicals and then the sensitivity of ever-increasing irreversible retention that might be associated with unconventional materials. Using field-scale reservoir simulations with a Langmuir-type tracer irreversible retention model, we rigorously test the limits for tracer irreversible retention in order to have successful inter well tracer test (IWTT). Specifically, we studied the tracer breakthrough peak concentrations as a function of tracer irreversible retention as well as inter well spacing in synthetic waterflood patterns. Custom reservoir simulator functionalities were built to perform the simulations. Additionally, coreflood experiments on common oil field tracers were conducted to acquire independent irreversible retention values and compared to the modeling results. For the reservoir simulations, we first tested the ideal tracer case with no irreversible retention and found perfect agreement with the standard Brigham-Smith model. We then tested for tracer breakthroughs with increasing irreversible retention values and found that the tracer breakthrough peak concentration drops off dramatically. With the consideration that the limit of detection (LOD) of contemporary analytical instruments are at the part per trillion (ppt) level, the simulation results suggested that the tracer irreversible retention should be less than 10 μg/g-rock (mass of adsorbed tracer / mass of rock) in order to have meaningful IWTT with a well spacing of 2000 ft and an injection tracer mass up to 100 kg. Finally, two field tests using fluorobenzoic acid (FBA) based tracers deployed in the highly saline and retentive carbonate reservoirs in Saudi Arabia were compared. The irreversible retention number of the FBA based tracers was estimated to be less than 5 μg/g-rock from the model. Corresponding coreflood experiments for FBA tracers in high temperature and salinity carbonate cores show 0 +/− 10 μg/g-rock irreversible retention number within error ranges, verifying the prediction of our simulation results. This paper broadens the scope of the extensively used Brigham-Smith tracer behavior model by incorporating tracer irreversible retention effects. More accurate design and interpretation of inter well tracer tests may be achieved through the new insights presented. Better waterflood management can then be established because of the reduced uncertainties from the more precise tracer data. In addition, this study set an unambiguous standard for the tolerable irreversible retention limits for any new materials targeting inter well tracing applications.