Abstract This paper compares flowback efficiencies using polymer concentration and frac fluid tracer methods. Results are presented for the flowback efficiency of each frac fluid segment using non-radioactive chemical frac tracers injected in a well as well as the results for the total flowback efficiencies using polymer concentration and frac fluid trace analysis methods. Two wells were fraced and traced with various chemical frac tracers. Upon commencing flowback, samples of produced aqueous solution were collected according to a pre-designed sampling schedule that lasted for 72 hours. Samples were analyzed for tracer, polymer, calcium, potassium, sodium, and chlorine concentrations. With the use of the mass balance technique, the total flowback volume and flowback efficiency for each fluid segment were calculated using the tracer method. In addition, total flowback and flowback efficiency were calculated using both polymer concentration and tracer methods. To better evaluate and compare the results of polymer concentration and frac fluid tracer analyses, dynamic fluid leakoff tests were conducted in a laboratory environment using both low and high permeability core samples. Detailed laboratory and field results are presented along with comparison of flowback results from both polymer concentration and frac fluid tracer methods.
Introduction Chemical Frac Tracers Chemical frac tracers (CFTs) were originally developed in an effort to bolster the level of understanding regarding the dynamics of hydraulic fracture placement, subsequent fluid flowback and proppant bed cleanup. Borrowing from many years of experience with interwell tracing where non-radioactive chemical tracers have been successfully used to evaluate interwell communications, several families of these chemical compounds were identified that could potentially be placed in each segment of the frac fluid so as to more directly measure the flowback efficiency of each fluid segment. Armed with this flowback profile data together with the treatment pressure history of the frac treatment, it was believed that much could potentially be learned both about the dynamics of segmented fluid placement as well as segmented fluid flowback and cleanup. Given the established formation/fracture damage potential for conventional proppant transport fluids, those fluid segments not adequately recovered following the treatment could, in principle, detrimentally affect flow capacity of the propped fracture.
Chemical frac tracers were designed to be placed in chemically-differentiated and/or proppant-differentiated fluid segments of the fracturing fluid so as to assess the cleanup of the fracture as a function of segment fluid chemistry and/or fracture geometry. In so doing, it was believed that the sufficiency or insufficiency of addition rates for key frac fluid additives such as polymers, breakers and gel stabilizers could be assessed. It was also believed that the relative cleanup of individual frac treatment segments in a multiple stage completion procedure could be monitored. It was further hoped that inferences could be made from these data regarding lateral placement effectiveness of proppants and vertical communication between zones. Furthermore, the tracer analysis results could be used to assess the amount of each injected segment recovered and hence to calculate flowback efficiency.
Background Fluid flowback can be either of a fracture-tip or near-wellbore type. If flowback is of the near-wellbore type, it indicates extensive near-wellbore leakoff due to a highly permeable zone around the wellbore. This causes the entire pad fluid segment to leakoff near the wellbore and, therefore, the pad fluid is first to be recovered. In a low permeability formation, pad flows to the fracture tip due to low permeability and/or damaged permeability around the wellbore resulting in minimal leak-off near the wellbore. Once the well is subjected to flowback under this condition, what is injected first flows back last, if fluids are formulated properly. If some segments of gelled frac fluid are not broken effectively before the well is subjected to flowback, the early injected fluids could potentially finger through the late injected unbroken fluids and flowback first.