Multi-stage and multi-layer percolation model of tight gas reservoir based on pressure gradient and stress sensitivity

Abstract

Tight reservoir with poor physical property, complex seepage characteristics and strong heterogeneity result in the difficulty in the development, so the study of reservoir horizontal and vertical heterogeneity and the establishment of a multi-layered and multi-stage (different scales) seepage model based on multi-stage seepage experiments in heterogeneous layers is essential for improving oil recovery.Firstly, three kinds of experiments are carried out: Slippage effect, starting pressure gradient and stress sensitivity.Experimental results show: when the reservoir effective permeability is between 0.01×10 -3 μm 2 and 1.0×10 -3 μm 2 and the gas reservoir pressure is higher than 10MPa, the effect of slippage effect on the production is less than 3% which can be ignored; there is no starting pressure gradient in single gas flow in tight gas reservoir; for the gas-water two-phase seepage in high water saturation reservoirs, due to the role of capillary force, the macro performance of the reservoir is the starting pressure phenomenon; compared with the matrix core, the stress sensitivity of the micro-crack core permeability is stronger and stress-sensitive lag is also stronger.Then, considering the starting pressure gradient and the stress sensitivity, the Multi-Layered and Multi-Stage seepage model of tight sandstone gas reservoir is established.Finally, the sensitivity analysis of model parameters was carried out by using the relationship between dimensionless quasi-pressure and dimensionless time.Research shows: the interlayer parameters mainly affect the later stage, the larger the ratio of the formation coefficient of the fracture, the greater the later pressure drop, so the fracturing scale is uniform when the production is in several layers, otherwise the pressure between the layers differentiates significantly, likely to cause a single layer of gas overriding and gas flow; in addition, the permeability modulus of the deformed medium, the elasticity ratio of the medium and the starting pressure gradient in in-layer parameters all affect the transition and the late stage.Therefore, it is very important to maintain the pressure at the later stage of gas reservoir development.