Study on the effect of cemented natural fractures on hydraulic fracture propagation in volcanic reservoirs

Hydraulic fracturing has been widely used in tight reservoir to generate fractures to improve the conductivity and productivity. However, due to combined effects of volcanism, tectonic movement, and water dissolution in volcanic reservoir, characteristics and cementation strength of natural fractures are complex, and the propagation laws of hydraulic fracturing are complex. In order to study the influence of typical fracture characteristics of volcanic rocks on hydraulic fracturing and provide technical means for optimizing the favorable area of fracturing, numerical research was carried out. Considering the mechanical differences between natural fractures (NFs) and rock matrix, the cross verification is carried out based on indoor hydraulic fracturing tests and numerical simulation to confirm the reliability of the numerical method. In addition, the characteristics of typical NFs in volcanic reservoirs are obtained based on outcrop observation and core analysis, and hydraulic fracturing models are established by using stochastic modeling method. The results reveal that hydraulic fracture (HF)'s propagation rate will increase and initiation pressure will decrease with the decrease of cementation strength ratio. When the cementation strength of NFs decrease to 0.1, the propagation direction of HFs is also affected by the cementation strength and morphology of NFs significantly in addition to the maximum principal stress, and fracture networks will appear after fracturing. Therefore, the angle between the NF and the direction of the maximum principal stress is smaller, and the cementation strength is lower, the hydraulic fracturing effect is better under the same pumping pressure, which has important guidance for the selection of fracturing sweet spot and development of volcanic reservoirs.