A Comprehensive Approach for the Performance of the Re-Fracturing Horizontal Well in Tight Oil: Coupling Fluid Flow, Geomechanics and Fracture Propagation
Authors: Lei Zhengdong , Wu Shuhong , An Xiaoping , Zhu Shengju , Zhu Zhouyuan , Hu Die , Liu He 
Published: October 2018

Multi-stage fractured horizontal wells have been successfully implemented in tight oil reservoirs. It is often observed that oil production declines rapidly in unconventional reservoirs. Consequently, re-fracturing may be necessary to improve/enhance oil production and ultimate recovery. However, many challenges exist in optimizing re-fracturing treatment designs, due to lack of accurate quantification of depletion-induced pressure and local stress changes around the fractures. We propose a workflow here to study and optimize re-fracturing job to increase reserves utilization level and energy level.

We present a comprehensive approach that integrates discrete-fracture, geomechanics, and multi-well production simulation models. First, the approach couples the simulated 3D reservoir pressure with a geomechanical finite-element model (FEM) to quantify the changes in the magnitude and azimuth of the in-situ stresses from the development process. Second, the altered stress field is utilized as the input for modeling the new fracture system created by the re-fracturing treatment. In addition, the finite element method is used in the calculation in the coupled reservoir flow and geomechanics model. And a unique full 3D unstructured mesh generation method effectively is adopted to simulate the longitudinal propagation of fracture. The synthetic cases of interference between wells due to stresses and fracture design are also investigated in this work. Furthermore, a systematic sensitivity study is performed for the effects of re-fracturing mode, formation pressure and matrix permeability.

The model prediction agrees well with the observed pressure response and microseismic event results. Results show that quantification of stress field changes during reservoir depletion provides new insights for the design and evaluation of re-fracturing treatments to enhance field development. There is a critical time in the well life that protection refracture could help pressurize the formation directly by increasing the pore pressure through the fluid injection, and indirectly by mechanical dilation of existing fractures, respectively. The dynamic changes captured timely in the stress field are invaluable to guide the optimization of re-fracturing mode for reducing the stress shadow effect. The dynamic change in the pressure field can be utilized to optimize the fracturing fluid volume, which can increase the volume of the fracture transformation and supply formation energy. This comprehensive approach is demonstrated to be very practical and effective in enhancing oil recovery, achieving satisfactory re-fracturing results.

The paper presents a novel approach in calculating stress changes and dynamic fracture propagation during the development of tight oil reservoirs. This work provides better understanding on the propagation of new fractures as well as old fractures during re-fracturing process, which serves as a great potential solution to improved oil recovery in tight oil reservoirs.