Suo, Yu (University of New South Wales) | Chen, Zhixi (University of New South Wales) | Rahman, Sheik (University of New South Wales) | Xu, Wenjun (University of New South Wales and Southwest Petroleum University)
Hydraulic fracturing is a significant way to improve the productivity of the unconventional reservoir with low permeability and porosity. Current hydraulic fracturing simulation models are mostly based on poro-elastic theory. However, for rocks such as shale, the viscoelastic feature has been observed in both field investigations and laboratory experiments. This paper presents a 3D numerical model for fracture propagation in viscoelastic shale gas formations using ABAQUS platform. The cohesive elements based on damage mechanics were adopted to simulate the initiation and propagation of hydraulic fractures. The model was used to investigate formation properties and treatment parameters on fracture geometry, especially the fracture behaviour when entering into the barrier formations. It is found that higher treatment pressure is required to initiate and propagate the hydraulic fracture and the fracture is wider but shorter in poroviscoelastic formation comparing to poro-elastic formation. The higher differential in-situ stress, tensile strength and Young modulus in barrier formations and lower fracturing fluid injection rate and lower fracturing fluid viscosity have positive effect on the controlling of fracture vertical growth and restricting hydraulic fracture within the pay zone. Results of this study will provide the industry a better understanding of hydraulic fracture behaviour in shale gas formations.
Xu, Wenjun (Southwest Petroleum University) | Zhao, Jinzhou (Southwest Petroleum University) | Li, Yongming (Southwest Petroleum University) | Rahman, Sheik S (University of New South Wales) | Fu, Dongyu (Southwest Petroleum University) | Chen, Xiyu (Southwest Petroleum University)
Complex fracture network makes it possible for commercial exploition of shale gas by means of hydraulic fracturing. It was believed that the interaction between hydraulic fracture (HF) and natural fracture (NF) had a significant impact on HF complexity. In this paper, a new numerical model has been developed to investigate HF/NF intersection under different geological and engineering parameters. Displacement discontinuity method (DDM) and finite volume method (FVM) are used to numerically model and solve the problem of coupled rock deformation, fluid flow, interface slipping, and opening associated with HF propagation and its interaction with NF. In addition, the model also considers the effects of fracture fluid leak-off. Based on the model, sensitivity analyses of key influence parameters are implemented. The numerical model results provide detailed quantitative information on fracture-geometry evolution, interfacial stress distribution and injection-pressure history. The simulation results show that the HF tends to cross the NF under the conditions of high principal stress difference, high intersection angle, high interfacial friction, high injection rate, high fracturing fluid viscosity and low initial conductivity of the NF. Moreover, the morphology of HF is significantly affected by two engineering parameters, the injection rate and the viscosity of the fracturing fluid. The effect of these two engineering parameters on the morphology of HF can be expressed as the product of them. The same value of the product results in the same HF morphology at the times of same injected-fluid volumes. In addition, the injection pressure curves can also help determine whether a crossing HF is developed when a HF interacts with a NF. The numerical model provides an effective approach for quantitatively analyzing the development of various types of HF/NF interaction behavior. It allows us to gain a better insight to the performance of hydraulic fracturing treatments in naturally fractured reservoirs.
Sensitivity kernels are used in travel-time tomography and waveform inversion for its obvious advantages in matching wave-motion theory rather than ray theory. However, there still remain some concerns regarding its accuracy and when the small-perturbation theory will break down. We investigate these questions using numerical simulations. Sensitivity kernels calculated in the background model and based on the linear scattering theory are used to predict the traveltime delay caused by the velocity perturbations. On the other hand, the traveltime differences between the background and the perturbed velocity models are directly calculated from the synthetic seismograms generated by the finite-difference method. The predicted traveltime delays are compared to these direct measurements and the results are used to judge the accuracy of the linear theory. Velocity models with perturbations of different scales or different perturbation values are used to conduct the tests. Our results show that extending the scale or increasing the amplitude of the velocity perturbations or both can affect the precision of the traveltime sensitivity kernel. These factors also complicate waveforms of synthetic seismograms as well as the shape of the sensitivity kernels in the perturbed velocity model. Nevertheless, within a large range of velocity perturbations, the sensitivity kernels based on linear theory still give reasonably accurate traveltime delay, indicating the linearization plus iteration method is still effective under reasonably large velocity perturbations.