In a previous paper (SPE 17163), a v-shape Gas-Water-Ratio (GWR) was observed and discussed from eight wells completed in the Horn River Basin, based on which the flowback data were divided into Early Gas Production (EGP) and Late Gas Production (LGP). The negative sloping GWR observed during EGP was assumed to be a ‘fracture cleanup’ period with negligible influx from the matrix system and a closed-tank model was developed for the fracture system to analyze flowback data. However, the effective fracture network is actually an open system which allows fluid influx from the surrounding matrix blocks. In this paper, we extend the previous model by considering the effect of gas influx during EGP.

Analyzing flowback data requires challenging mathematical procedures, which in most cases are still insufficient to capture the complexity of the two-phase flow in the fracture network. Material Balance Equation (MBE) is a simple but convincing tool in that it eliminates the assumptions lying in the flow equations. This paper seeks to use the MBE technique to analyze early-time two-phase flowback data in shale gas reservoirs. Diagnostic plots from field production data show an immediate gas breakthrough and a negative GWR trend at the beginning of flowback, which indicate the presence of initial gas in the fractures before opening the wells. Based on this observation, we develop a model to describe gas and water production during the early hours of flowback. The early-time two-phase production comes from an ‘effective fracture system’ initially saturated with gas and water. The model considers four drive mechanisms including gas expansion, water expansion, fracture closure and gas influx from matrix. Further, we propose a procedure to analyze field data using different forms of MBEs. It allows the estimation of the effective fracture volume which is difficult to measure in field cases. Finally, we demonstrate the application of the proposed material balance model by analyzing flowback data of one well completed in the Horn River Basin.

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