Numerical Simulation Study on Creep Characteristics and Fracture Closure of Deep Shale

ABSTRACT: Shale oil and gas resources are currently an important replacement resource for oil and gas in China and have huge potential development. During the mining process, the reservoir shale has creep characteristics due to its own physical properties and mineral composition, which causes the artificial fracturing network to produce a closure effect, which affects the production capacity of the reservoir. This work mainly studies the creep characteristics of reservoir shale and its influence on reservoir productivity. The shale in Chang-7 block of Yanchang formation in the Ordos Basin is selected as the research object. The triaxial creep experiment is used to determine that the shale creep law conforms to the power order. The Abaqus finite element analysis software is used to simulate the creep of the shale sample under the creep time of 1 year, 5 years, and 10 years to observe the changes of reservoir fractures under the action, evaluate the degree of fracture closure, and analyze the effect of creep on reservoir conductivity. The research results show that shale reservoir creep will cause proppant embedment and fracture closure, non-linear reduction of reservoir conductivity, and the growth rate of production gradually slowed down by the effect of time. 1. INTRODUCTION Shale oil and gas resources are currently an important replacement resource for oil and gas in China and have huge development potential. Shale oil and gas resource in China reserves rank first in the world, and shale oil and gas production capacity ranks second in the world, with an annual output exceeding 20 billion m3, which has become the backbone of domestic natural gas production. The creep of shale reservoir refers to the phenomenon that the strain of the shale reservoir rock gradually increases with time under the action of stress. Creep is the slow deformation and will also cause the object to occur when it is less than the elastic limit of the object. Deformation, the creep of shale reservoirs mainly undergoes three deformation stages: initial creep, steady-state creep, and accelerated creep. Among them, the creep strain of the rock in the steady-state creep process increases uniformly with time. This phenomenon that the creep strain gradually increases with time is called time-dependent. Previous research and actual production experience have shown that the permeability of shale reservoirs is very low, and artificial reconstruction of fractured reservoirs is an effective way to increase productivity. After fracturing the shale reservoir, a large number of fractures are generated in the reservoir. The proppants are injected, and the main and branch fractures are interconnected to create conditions for the migration of shale oil and gas (Dragon, 1981), so as to achieve the expected productivity level. However, due to the time-effect characteristics of shale reservoirs, in the long production cycle of the reservoir, the fractures of the reservoir begin to creep and close over time, and the intricate fractures of the reservoir fracturing promote the shale time-effect characteristics, the fractures gradually tend to close under the action of creep. The proppant injected in advance may be embedded in the fracture walls under squeezing, blocking the channels, and obstructing the flow of shale oil and gas. In addition, the capillary force of the small fractures will lead to formation conductivity. The flow capacity is further reduced, and these effects have played a non-negligible hindrance to the formation productivity. Therefore, analyzing and studying the time-effect characteristics and mechanism of shale reservoirs is of great significance to the interpretation and prediction of possible problems in the production process and to increase the productivity of shale reservoirs.