A flowing material balance equation for two phase flowback analysis
Abstract Flowback data from wells completed in shale gas reservoirs usually show a surprising behavior of immediate two phase flow. This is especially the case in wells with an extended shut in period. This paper seeks to understand the fundamental reasons behind this early time behavior by identifying trends in water/gas field production data. Diagnostic Gas Water Ratio plots of 8 multi fractured horizontal wells from the Muskwa and Otter Park formations show a unique v shaped trend. Based on this early time production signature, we hypothesize that the early gas production is linked to the initial free gas in the complex fracture network of the shale reservoir. We test this hypothesis by numerical simulation of the shut in and flowback processes, and observe a similar v shaped trend caused by a gradual build up of free gas saturation in the fracture network during the shut in period. This conclusion is also backed by extensive imbibition experiments conducted on the corresponding shale samples and by the presence of gas saturated natural fractures. We also develop a simple mathematical model to describe gas and water production during the early hours of flowback. Based on the observations from field and simulated data, the primary drive mechanism is assumed to be the expansion of free gas built up in the fractures during the shut in period. Other drive mechanisms considered include fracture closure and water expansion. The driving forces are modeled by an effective compressibility term analogous to the total compressibility in conventional multi phase flow formulations. This model gives an easy method for the quick interpretation of early time two phase flowback data in a manner similar to conventional well testing methods. It can be applied to estimate the effective fracture volume and a dimensionless fracture parameter, which measures the flow capacity of the fracture network. Finally, we demonstrate the application of this model by analyzing flowback data from 2 fractured horizontal wells completed in the Horn River Basin.