Long-Term Post-Frac Performance Analysis Based on Flowback Analysis Using Chemical Frac-Tracers

Abstract

Abstract Chemical frac tracing is used to evaluate flowback and cleanup efficiencies for eight different wells in a field located Central America. The technique utilizes a family of unique, environmentally-friendly, fracturing fluid compatible chemical tracers to quantify segment-by-segment recovery for individual fracturing treatments and stage-by-stage recovery for multi-stage fracturing treatments. Each well was traced with a number of different tracers during hydraulic fracturing. Upon flowback, samples were collected and analyzed for tracer concentrations. With the use of the mass balance method, total flowback and flowback efficiency for each stage were calculated. Production for each well was closely monitored for three years. This paper presents in-depth details relating short-term total flowback and fluid stage-by-stage flowback efficiencies to the long-term post-frac performance. Introduction Chemical Frac Tracers. Chemical frac tracers, CFT's, are used to precisely calculate flowback and hence flowback efficiency and to evaluate fracture cleanup. Various chemical tracers with unique chemical characteristics are mixed at a known concentration and injected according to a predetermined design throughout individual frac fluid segments, such as the pad and the proppant laden fluid stages. These chemical tracers do not react with each other, the formation or the tubular. They do not degrade with temperature or time, do not self-concentrate, and do not react with frac fluids. These tracers are detectable at low concentrations of 50 ppt (parts per trillion). They are also environmentally safe to handle, pump downhole and to dispose of. They are soluble in water, and unlike polymers, do not concentrate upon leakoff. The CFT's are injected into the treatment fluid segments at a concentration of 1 ppm with a positive-displacement peristaltic pump on the low-pressure side of a frac pump. At these extremely low concentrations, the CFT chemicals have been thoroughly proven to have no effect on the stability or performance of the fracturing fluids into which they are injected. Upon commencing flowback, samples of the flowback fluid are collected every 10 min to 12 hr via a sampling valve at the surface prior to the flowback fluid entering a dedicated flowback tank (which is used to measure the cumulative fluid flowback volume recovered at the time each flowback sample is collected). Samples are collected for a minimum of 48 hr and for a realistic maximum of 30 days. The collected samples are then analyzed using a sophisticated analytical procedure which is capable of concurrently quantifying all of the CFTs down to the 50 ppt (parts per trillion) level. These analysis results are then normalized and/or plotted vs. elapsed flowback time. The mass of each CFT recovered in a given time period together with the total recovered fluid flowback volume for that same time period are used to calculate fluid flowback efficiencies by employing the mass balance principle. The resulting segment-by-segment fluid recovery profiles are then used to help characterize the effectiveness of cleanup and to make recommendations regarding potential improvements in treatment fluid cleanup that might be obtained by changes in the treatment fluid chemistry (e.g., type of crosslinker, polymer loadings, breaker loadings, gel stabilizer loadings, etc.), as well as proppant scheduling schemes, flowback procedures, etc. Background. Although fluid flowback is an important part of a fracture treatment, it has been overshadowed by proppant flowback in recent years. The detrimental effects of reduced fracture conductivity as a result of poor flowback and cleanup are well documented. Most research has focused on the effects of using improper flowback procedures on well performance.1,2,3,4 The associated effects are proppant movement into the wellbore, proppant crushing at or near the wellbore, and fracture plugging.