Far-field strain analysis for fiber optic monitoring of hydraulic fracturing in a deep naturally fractured shale reservoir

Deep shale reservoirs (burial depth greater than 4 000 m) in the southern Sichuan Basin (hereafter referred to as southern Sichuan) are characterized by developed natural fractures and complex in-situ stress distribution. Casing deformation and other problems in the process of hydraulic fracturing restrict the efficient development of shale gas resources severely. To monitor the far-field strain during the fracturing of natural fractures in deep shale reservoirs, this paper applies the offset-well DAS strain monitoring technology to monitor the zipper fracturing process of two wells on the Platform A in the Luzhou Block. And then, the monitoring results are analyzed and discussed on the basis of the fracturing design and reservoir characteristics. And the following results are obtained. First, the 300–350 m monitoring well spacing and the pumping-type inside-casing optical fiber layout can provide a lot of favorable data for the assessment of far-field fracture communication and evolution, but less information for the main stimulated regions of a fractured well. Second, during the fracturing of different wells on the same platform, the response of far-field strain may be quite large. The fracturing parameter design of high-risk wells usually play a key role in the communication of far-field fractures, while low-risk wells mainly show a following role. Once a fracture communication region is formed, it is very likely to evolve into the common fracture communication region of two fractured wells. Third, the response region of far-field strain and the current fracturing stage may differ in location by 200–300 m. The strain communication region between different stages of the same well may be formed in advance, and it may be 400–500 m from the current fracturing stage. Fourth, the far-field fractures formed by fracturing the natural fractures in deep shale reservoirs present completely new morphological characteristics, including universal inclined fractures and horizontal bedding-parallel fractures near the flexural section. In conclusion, the DAS strain monitoring technology provides a new method for analyzing the evolution of far-field strain during the fracturing of natural fractures in deep shale reservoirs. And combined with geology-engineering integrated fracturing design scheme and casing deformation mechanism and prevention research, it is expected to assist the large-scale benefit development of deep shale gas in the southern Sichuan. © 2024 Natural Gas Industry Journal Agency. All rights reserved.