Fracture Characterization during Flowback with Two-Phase Flow in Tightand Ultratight Oil Reservoirs

Flowback rate transient analysis (RTA) is a practical tool for characterizing hydraulic fracture (HF)properties. However, the accuracy of the interpreted results from flowback RTA is challenged by thecomplexity in two-phase flow in the hydraulic fracture and matrix system. Accordingly, we present a newsemianalytical method to characterize HF attributes and dynamics using multi-phase flowback data for tightand ultratight (shale) oil wells. The proposed method includes a two-phase diagnostic plot, a fracture RTAapproach for straight-line analysis, and a matrix model capable of characterizing water and oil flow. TheRTA approach is based on fracture infinite acting linear flow (IALF) and boundary dominated flow (BDF)solutions, which treats HF as an open tank with a variable production rate at the well and the contributionof water and oil from matrix within the distance of investigation (DOI). The pressure-dependent fluid andgeomechanical properties, such as permeability and porosity, are considered in the pseudotime defined infracture and matrix to reduce the nonlinearity of the system. We tested the accuracy of the proposed methodagainst numerical results obtained from commercial software and verified its applicability by analyzing theflowback and long-term production data from a field example in Eagle Ford shale. The validation resultsconfirm that our method can closely calculate water and oil influx from matrix as well as the average pressureand saturation in the HF and matrix DOI. The accurate estimation of the initial fracture permeability andpore volume demonstrates the applicability of the proposed method in quantifying HF properties from two-phase flowback data exhibiting fracture IALF and BDF regimes. The analysis results show that the estimatedinitial fracture pore volume shows more accuracy than initial fracture permeability due to the differentcalculation sources in the straight-line analysis. In short, the proposed method is, to our best knowledge, thefirst RTA approach incorporating the two-phase water and oil influx from matrix into the inverse analysisof fracture properties and dynamics using straight-line analysis, instead of history matching.