Fracture network types revealed by well test curves for shale reservoirs in the Sichuan Basin, China

Abstract

Pressure buildup testing can be used to analyze fracture network characteristics and conduct quantitative interpretation of relevant parameters for shale gas wells, thus providing bases for assessing the well productivity and formulating proper development strategies. This study establishes a new well test interpretation model for fractured horizontal wells based on seepage mechanisms of shale reservoirs and proposes a method for identifying fracturing patterns based on the characteristic slopes of pressure buildup curves and curve combination patterns. The pressure buildup curve patterns are identified to represent three types of shale reservoirs in the Sichuan Basin, namely the moderately deep shale reservoirs with high pressure, deep shale reservoirs with ultra-high pressure, and moderately deep shale reservoirs with normal pressure. Based on this, the relationship between the typical pressure buildup curve patterns and the fracture network types are put forward. Fracturing effects of three types of shale gas reservoir are compared and analyzed. The results show that typical flow patterns of shale reservoirs include bilinear flow in primary and secondary fractures, linear flow in secondary fractures, bilinear flow in secondary fractures and matrix, and linear flow in matrix. The fracture network characteristics can be determined using the characteristic slopes of pressure buildup curves and curve combinations. The linear flow in early secondary fractures is increasingly distinct with an increase in primary fracture conductivity. Moreover, the bilinear flow in secondary fractures and matrix and the subsequent linear flow in the matrix occur as the propping and density of secondary fractures increase. The increase in the burial depth, in-situ stress, and stress difference corresponds to a decrease in the propping of primary fractures that expand along different directions in the shale gas wells in the Sichuan Basin. Four pressure buildup curve patterns exist in the Sichuan Basin and its periphery. The pattern of pressure buildup curves of shale reservoirs in the Yongchuan area can be described as 1/2 → 1/4, indicating limited stimulated reservoir volume, poorly propped secondary fractures, and the forming of primary fractures that extend only to certain directions. The pressure buildup curves of shale reservoirs in the main block of the Fuling area show a pattern of 1/4 → 1/2 or 1/2, indicating greater stimulated reservoir volume, well propped secondary fractures, and the forming of complex fracture networks. The pattern of pressure buildup curves of shale reservoirs in the Pingqiao area is 1/2 → 1/4 → 1/2, indicating a fracturing effect somewhere between that of the Fuling and Yongchuan areas. For reservoirs with normal pressure, it is difficult to determine fracture network characteristics from pressure buildup curves due to insufficient formation energy and limited liquid drainage.