Volume 40: Energy Transitions toward Carbon Neutrality: Part III

A novel dynamic boundary model of pressure propagation for volume fractured horizontal wells in shale oil reservoirs with developed natural fractures Binyu Wang, Linsong Cheng*, Yafei Zhang, Renyi Cao

Abstract

With the international energy demand and the development of exploration and development technology, the development of shale reservoirs has gradually become a hot topic all over the world, and the existing theories of fluid flow can no longer accurately characterize and describe the flow characteristics of shale oil reservoirs. The formation pressure performances in shale reservoirs are quite different with that of conventional reservoirs due to the strong low-velocity nonlinear flow and starting pressure gradients, which will result in dynamic boundary. Few research mentioned the effects of dynamic boundary caused by the flow mechanism in shale oil reservoirs. In this paper, the nonlinear equation of shale oil reservoirs, segmented linearization, division of the flow space into matrix and SRV regions, application of the source function and Newman product method, and creation of finite-length strip source functions are employed to analyze the effects of dynamic boundary. A novel model of unsteady flow in horizontal wells with multi-fractured section volume fracturing taking the development of natural fractures into consideration was established, and the method of solving the pressure of this model by applying the iterative method of immovable point was put forth using the principles of pressure superposition and unsteady flow superposition. The characteristics of the dynamic boundary of pressure propagation in volume fractured horizontal wells were determined, as well as the effects of various reservoir physical parameters on the movement of the dynamic boundary, with the aid of the proposed mathematical model of unsteady flow in volume fractured horizontal wells taking the development of natural fractures into consideration. The analysis demonstrates that, under the nonlinear flow condition, the formation pressure propagation manifests as a dynamic boundary problem. The pressure propagation dynamic boundary expands more quickly in the early period due to the natural fractures in the reservoir, but tends to stabilize in the later period, indicating the existence of a limiting distance for pressure mobilization in the reservoir. The major determinants of formation pressure wave and mobilization are the physical characteristics of the reservoir (permeability, natural fractures). The stronger the flow nonlinearity and the more developed the natural fractures, the greater the degree of effects on pressure propagation.

Keywords Natural fractures, Shale oil reservoirs, Dynamic boundary, Nonlinear flow