Many shale wells undergo a gradual production decline before reaching a linear flow decline. This gradual decline lasts for several days and often matches a quarter-slope. Some authors attribute this gradual decline to bilinear flow, which occurs as a result of simultaneous linear flow in natural fractures and matrix (Al-Ahmadi and Wattenbarger 2011; Tivayanond et al 2012). In this paper, we show that bilinear flow in natural fractures and matrix is short and, therefore, it cannot explain the sustainable gradual decline observed in shale wells. Instead, we provide two alternative explanations. Bilinear flow caused by low-conductive hydraulic fractures is one explanation. Another one is an artificial shift in data caused by ignoring flowback.
First, we use analytical solutions and show that bilinear flow is short unless matrix permeability is extremely low or natural-fracture spacing is very large. Second, we show that analytical solutions are unrealistic when natural fracture spacing is large. Third, we use reservoir simulation to obtain more realistic results and show that bilinear flow in matrix and natural fractures is not sustainable. In conclusion, we show that ignoring flowblack data results in a fictitious quarter-slope and demonstrate the concept with field examples.
This work shows the importance of including flowback data into rate transient analysis. In addition, it describes a procedure to accurately analyze production data and avoid misleading bilinear flows. Several sensitivity studies are also shown to define the geometrical and petrophysical conditions under which the bilinear flow regime is not expected. Concepts presented in this paper provide engineers with better tools for analyzing and forecasting production from shale gas wells.