Multistage hydraulic fracture stimulation is commonly used with horizontal well technology as one of key technologies for the economical and effective development of unconventional reservoirs such as tight oil. The field shows that the technology of fracturing, shut in, flowback and production (FSFP) has achieved a positive effect on increasing production, and the lower flowback rate does not necessarily have a negative impact on the production increase, on the contrary, the lower the flowback rate, the initial daily production will increase, which is contrary to conventional wisdom. However, there are few standardized and effective numerical modeling approaches available for guidance. In the traditional simulation method, the pressure difference between the oil phase and the water phase in the fractured medium is considered to be capillary pressure, which cannot accurately characterize the influence of the compressibility of the fluid on oil and water pressure, especially in the period of flowback and production which is followed by shut in process. In this work, a novel numerical model of fluid flow for flowback and production process in tight oil reservoirs is proposed considering fluid compression effect in fracture media. In the model, the multi-scaled fracture system of tight oil reservoirs is described as a set of subdomains and handled by a multi-continuum conceptual model. To model the effect of fluid compression, the fractures of tight reservoir are classified and divided into two elements according to the different compressibility of tight oil and fracturing fluid. Then, the analytical solution of 1-D single phase compressible fluid flow is used to verify the correctness and reliability of the model, and it is found that the effect of fluid compressibility cannot be ignored when the compression-viscosity coefficient is greater than 3. Finally, several examples are presented to demonstrate the applicability of the proposed method. Effects of the compressibility of fluid are investigated. The results show that the pressure in different phases is not only a capillary pressure difference, but also related to the compressibility of fluid. The higher the compression factor, the greater the fluid pressure during the flowback and production period, thereby rending it more conductive to production. The model in this paper can describe the fluids flow in pores and fractures in a more detailed and accurate manner, and has certain reference value for understanding the relationship between fracturing fluid flowback and production rate, and enhancing tight oil recovery.

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Effect of Compressibility of Fluid on Pressure Difference Between Oil and Water in Fractured Media

  • Zhixue Zheng,
  • Yuan Di,
  • Shuiqing Hu,
  • Yu-Shu Wu

摘要

Multistage hydraulic fracture stimulation is commonly used with horizontal well technology as one of key technologies for the economical and effective development of unconventional reservoirs such as tight oil. The field shows that the technology of fracturing, shut in, flowback and production (FSFP) has achieved a positive effect on increasing production, and the lower flowback rate does not necessarily have a negative impact on the production increase, on the contrary, the lower the flowback rate, the initial daily production will increase, which is contrary to conventional wisdom. However, there are few standardized and effective numerical modeling approaches available for guidance. In the traditional simulation method, the pressure difference between the oil phase and the water phase in the fractured medium is considered to be capillary pressure, which cannot accurately characterize the influence of the compressibility of the fluid on oil and water pressure, especially in the period of flowback and production which is followed by shut in process. In this work, a novel numerical model of fluid flow for flowback and production process in tight oil reservoirs is proposed considering fluid compression effect in fracture media. In the model, the multi-scaled fracture system of tight oil reservoirs is described as a set of subdomains and handled by a multi-continuum conceptual model. To model the effect of fluid compression, the fractures of tight reservoir are classified and divided into two elements according to the different compressibility of tight oil and fracturing fluid. Then, the analytical solution of 1-D single phase compressible fluid flow is used to verify the correctness and reliability of the model, and it is found that the effect of fluid compressibility cannot be ignored when the compression-viscosity coefficient is greater than 3. Finally, several examples are presented to demonstrate the applicability of the proposed method. Effects of the compressibility of fluid are investigated. The results show that the pressure in different phases is not only a capillary pressure difference, but also related to the compressibility of fluid. The higher the compression factor, the greater the fluid pressure during the flowback and production period, thereby rending it more conductive to production. The model in this paper can describe the fluids flow in pores and fractures in a more detailed and accurate manner, and has certain reference value for understanding the relationship between fracturing fluid flowback and production rate, and enhancing tight oil recovery.