关键词: 致密砂岩, 孔隙结构, 核磁共振, 动态渗吸, 裂缝, 基质

Abstract: To clarify the dynamic imbibition characteristics of water flooding in fractured tight reservoirs and solve the problems of low water flooding recovery, this paper studies the characteristics of microscopic pore structure of Chang 6 oil layer of Yanchang Formation in the Jiyuan oilfield by means of high-pressure mercury injection, nuclear magnetic resonance T₂ spectrum, scanning electron microscope and cast thin section analysis, establishes the classification and evaluation criteria for three types of reservoirs, and carries out dynamic imbibition experiments on representative cores based on NMR online scanning. This paper also simulates the dynamic imbibition process between the fracture and the matrix during water flooding, quantitatively characterizes the producing degree of crude oil in pores with different pore sizes at the microscopic scale, and evaluates the degree of influence of eight reservoir physical parameters on the dynamic imbibition efficiency. The experimental results show that the pore structure of the target reservoirs can be divided into three types. With the pore structure deterioration of reservoirs, the pore types gradually become simplified, and the reservoiring performance and percolation capacity continue to decrease, resulting in the continuous decline of dynamic imbibition efficiency. The dynamic imbibition process of type I and II reservoirs can be divided into three stages:(1)under the action of displacement, the recovery percent of macropores rises rapidly; (2)under the action of imbibition, the recovery percent of tiny pores rises slowly; (3)the stage of dynamic imbibition equilibrium. However, the type III reservoirs only experienced the first two stages in the experiment. With the pore structure deterioration of reservoirs, the producing proportion of tiny pores increases, and the imbibition effect is obvious. Although the contribution to the total core recovery increases, the total recovery rate is low. Permeability, movable crude oil saturation, pore radius, movable crude oil porosity, clay mineral content and wettability are the main factors affecting the dynamic imbibition efficiency, and their degree of influence on the imbibition efficiency successively gradually decreases. Sorting coefficient and porosity are the secondary factors that affect the dynamic imbibition efficiency, and their degree of impact on the imbibition efficiency is relatively small.

Key words: tight sandstone, pore structure, nuclear magnetic resonance, dynamic imbibition, fracture, matrix