Abstract
In general, there is stress sensitivity damage in tight reservoirs and fractures. Furthermore, the flow in tight reservoirs is the low-velocity non-Darcy flow. Currently, few researches of pressure analysis for volume fracturing vertical well are conducted simultaneously considering the low-velocity non-Darcy flow and stress sensitivity. In the paper, a novel flow model of a volume fractured vertical well is proposed and solved numerically. Firstly, the threshold pressure gradient, permeability modulus, and experimental data are, respectively, utilized to characterize the low-velocity non-Darcy flow, matrix stress sensitivity, and fracture stress sensitivity. Then, a two-region composite reservoir is established to simulate the vertical well with volume fracturing. After that, the logarithm meshing method is used to discrete the composite reservoir, and the flow model is solved by the method of finite difference and IMPES. Finally, the model verification is conducted, and the effects of the low-velocity non-Darcy flow and stress sensitivity on the pressure and pressure derivative are analyzed. The six flow regimes are identified by the dimensionless pressure and pressure derivative curve. They are, respectively, the fracture linear flow regime, early transition flow regime, radial flow regime, crossflow regime, advanced transition flow regime, and boundary controlling flow regime. The stress sensitivity and threshold pressure gradient have a great effect on the dimensionless pressure and pressure derivative. With the increase of reservoir stress sensitivity, the pressure and pressure derivative are upward at the advanced transition flow and boundary controlling regimes. However, the pressure and pressure derivative are downward at the advanced transition flow and boundary controlling regimes when the fracture sensitivity increases. An increase in the threshold pressure gradient results in a high dimensionless pressure and pressure derivative. This work reveals the effects of low-velocity non-Darcy flow and stress sensitivity on pressure and provides a more accurate reference for reservoir engineers in pressure analysis when developing a tight reservoir by using the volume fracturing vertical well.
Highlights
Volume fracturing breaks up effective permeable reservoirs to form a fracture network, maximizes the contact area between fracture sides and reservoir matrix, minimizes the fluid seepage distance of oil and gas from the matrix in any directions to fractures, and greatly improves the entire reservoir permeability [1]
Many microfractures are developed in the tight reservoir after volume fracturing [9, 10]
Due to the tiny porous and ultralow permeability of tight reservoirs, the flow in tight reservoirs obeys the law of low-velocity nonDarcy flow instead of Darcy flow [15], which is the fact that the flow velocity in a low pressure gradient regime is lower than what is estimated from Darcy’s law [16]
Summary
Volume fracturing breaks up effective permeable reservoirs to form a fracture network, maximizes the contact area between fracture sides and reservoir matrix, minimizes the fluid seepage distance of oil and gas from the matrix in any directions to fractures, and greatly improves the entire reservoir permeability [1]. The reservoir is composed of multiple linear flow regions, which include SRV and un-SRV They do not consider the effect of stress sensitivity and low-velocity non-Darcy flow. The researches mentioned above are all about volume fracturing, but they do not take the effect of the stress sensitivity and low-velocity non-Darcy flow into consideration. By considering the effect of the stress sensitivity and low-velocity non-Darcy flow, Wu et al [26] proposed a pressure analysis model for a multistage fractured horizontal well in the tight reservoir. Few works simultaneously considering the effect of the low-velocity non-Darcy flow and stress sensitivity are conducted for the volume fracturing well.
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