Abstract:

For tight sandstone reservoirs experiencing production decline after a period of development, refracturing is a feasible solution to reactivate existing fractures, initiate new fractures, and ultimately enhance production. Refracturing requires consideration of not only operational parameters such as slurry rate, fluid volume, and sand volume, but also whether to adopt fracture reactivation along original fractures or infill perforation completion techniques. Traditional evaluation methods for fracturing operations based on microseismic monitoring results mainly assess fracture dimensions and stimulated reservoir volume by measuring the geometric distribution of microseismic event point clouds. However, these methods cannot quantitatively evaluate the complexity of fracture networks under different operational parameters and the development and extent of new fractures generated by refracturing under different completion techniques. Therefore, a method was proposed to evaluate refracturing effectiveness using continuous fracture networks and apparent stress attribute maps constructed from microseismic events. This method utilized the spatiotemporal distribution characteristics of microseismic events (including temporal sequence and spatial distribution), and connected event points using defined geometric connection criteria (such as the shortest path principle) to build hydraulic fracture networks. The branch index attribute of the continuous fracture networks was used to quantitatively analyze the complexity of the hydraulic fracture networks. The apparent stress attribute values were calculated based on the energy, seismic moment, and shear modulus of the microseismic events. Lower apparent stress values indicated reactivation of existing fractures during refracturing, while higher values indicated that refracturing generated a large number of new fractures in the reservoir. This pattern could be used to evaluate the development of new fractures created by refracturing. The proposed method was applied to evaluate the refracturing effectiveness of a horizontal well in a tight sandstone reservoir in the Huaqing oilfield. The results showed that when refracturing was performed with higher slurry rates and larger fluid volumes than the initial frac (slurry rate ≤ 3 m³/min and fluid volume per stage ranging from 200~350 m³ for initial fracturing, while slurry rate ranging from 6~8 m³/min and fluid volume per stage ranging from 1 850~2 300 m³ for refracturing), the application of original fracture reactivation technology in horizontal wells of tight sandstone reservoirs enabled the formation of more new fractures and more complex hydraulic fracture networks compared to infill perforation.

Key words: refracturing, microseismic monitoring, discrete fracture network, continuous fracture network, branch index, apparent stress