Research on the characteristics of fiber optic signals for neighboring wells with hydraulic fracture propagation at different inclination angles

Considering that the current fiber optic signal interpretation models are based on vertical fractures, and the influence of natural fractures and bedding planes makes hydraulic fractures tilted during its propagation process. What’s more, its optical fiber monitoring signals will also differ from those produced by vertical fractures, rendering existing optical fiber signal interpretation models inapplicable. Therefore, a three-dimensional inclined fracture propagation model is established in this paper, considering the tensile and shear mechanical behaviors during crack extension, and solved by the finite element method. The model in this paper simulates the fiber optic signal characteristics under vertical crack extension conditions has good agreement with the discontinuous displacement method of Liu and Wu, which verifies the correctness of the numerical model in this paper.After that, we build the geometry model of inclined fractures, the effects of different inclination angles, tilt directions, fracture heights, fiber-optic monitoring distances, and in-situ stress states on fiber-optic strains and strain rates are investigated during the propagation of hydraulic fractures with inclination angles. Some conclusions can be drawn from the simulation results:red and blue bands appear on strain rate waterfall plots monitored by fiber optics in neighboring wells during inclined hydraulic fracture propagation. Symmetrical red and blue strain ellipses and strain rate bands appear when the hydraulic fracture inclination is at 30° to 55°. As the inclined hydraulic fracture continues to propagate, when the inclined fracture hits the fiber, multiple heartshaped zones appear on the strain rate waterfall map. The fiber-optic-monitored strain and strain-rate waterfall maps are also able to reflect the tilt direction of the hydraulic fracture. The width of the red bands of the strain rate waterfall plot increases with increasing hydraulic fracture height, which provides us with a way of interpreting the height of fractures based on the optical fiber monitoring. For hydraulic fractures of the same morphology and size, the optical fiber monitoring signals under different initial stress states exhibit significant differences, among which, the optical fiber monitoring signals under the normal fault stress state have the most distinct band characteristics. Under the simulation conditions of this paper, with each 100 m increase in fiber-optic monitoring distance, the intensity of the fiber-optic-monitored strain and strain-rate signals decreases by one order of magnitude. The research in this paper is of great significance in guiding the interpretation of hydraulic fracture morphology and size through neighboring well fiber-optic signals and guiding the placement of fiber optics in neighboring wells.