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.
Southern Sichuan BasinDeep shale gasNatural fractureHydraulic fracturingCasing deformationDASFar-field strainFracture communication
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