Numerical Modeling of Waterflood Induced Fractures in Fractured Tight Reservoirs: Field Cases in as oilfield, China
Authors: Cai Dingning , Cheng Shiqing , Bai Wenpeng , Wei Cao , Guo Xinyang , Wang Yang 
Published: May 2024

Waterflood-induced fracture (WIF) is often found in tight reservoirs with the water injection operation, which can significantly exacerbate reservoir heterogeneity and result in unidirectional advancement of injected water. Accurate characterization of dynamic propagation behavior of WIFs is crucial during development plan design, reservoir numerical simulation, and stimulation measure selection. However, currently-used reservoir simulation software tends to overlook dynamic propagation behavior of WIFs, and simplify the WIFs into a time-independent fracture with a specified direction and fixed length. In response to this issue, we established a damage-based finite element model for WIF, considering the stress sensitivity effect of matrix and the interaction effect of natural fractures and matrix.

A coupled hydro-mechanical-damage (HMD) model is established. We define strain-related damage variables to facilitate the calculation of fracture and matrix evolution in a unified form. The presence of filler content in natural fractures under initial conditions is regarded as a filled joint element with a certain thickness. The stiffness of fractures is derived with reference to the Goodman joint, which is used to calculate the normal/shear displacement of natural fractures. The porosity and permeability are related to stress and strain, and dynamically change during the simulation process. The coupling model is solved using a finite-element numerical simulator to obtain the deformation and pressure change of the reservoir during the water injection process. Finally, a case study of China's AS Oilfield is conducted using the proposed method to discuss the pressure response characteristics, mechanical characteristics of natural fractures and WIF extension trajectories, under two working schemes of single well injection-stewing-production and one injection well and two production wells.

The results show that the fracture characteristics during water injection period can be summarized in three forms: generation of WIFs, activation of natural fractures, and communication of natural fractures. WIFs appeared first in the injection well, extended along the direction of the maximum horizontal principal stress, and appear to be locally deflected when the natural fractures are around. With formation pressure increasing, the natural fractures near the injection well gradually open up, while the distant ones appear to close. When WIFs communicate natural fractures, the width of which is significantly increased. The results also show that the WIFs show a better effect of enhancing the water supply capacity of the reservoir, which helps to replenish the formation pressure. The closer the leading edge of the WIFs is to the production well, the better the effect of replenishment of energy is. The established model furnishes a visual representation and offers a quantitative analysis of the fracture evolution process, presenting an analytical idea for time-dependent WIF research.