Fracture identification and evaluation using conventional logs in tight sandstones: A case study in the Ordos Basin, China

Abstract

Fractures are of great significance to tight oil and gas development. Fracture identification using conventional well logs is a feasible way to locate the underground fractures in tight sandstones. However, there are three problems affecting its interpretation accuracy and practical application, namely weak well log responses of fractures, a lack of specific logs for fracture prediction, and relative change omission in log responses. To overcome these problems and improve fracture identification accuracy, a fracture indicating parameter (FIP) method composed of a comprehensive index method (CIM) and a comprehensive fractal method (CFM) is introduced. The CIM tries to handle the first problem by amplifying log responses of fractures. The CFM addresses the third one using fractal dimensions. The flexible weight parameters corresponding to logs in the CIM and CFM make the interpretation possible for wells lacking specific logs. The reconstructed logs in the CIM and CFM try to solve the second problem. It is noted that the FIP method can calculate the probability of fracture development at a certain depth, but cannot show the fracture development degree of a new well compared with other wells. In this study, a formation fracture intensity (FFI) method is also introduced to further evaluate fracture development combined with production data. To test the validity of the FIP and FFI methods, fracture identification experiments are implemented in a tight reservoir in the Ordos Basin. The results are consistent with the data of rock core observation and production, indicating the proposed methods are effective for fracture identification and evaluation.