深层膏质泥岩蠕变特性及非线性蠕变模型研究

泥岩作为油气储层常见复杂介质，在深部高温、高压环境下蠕变力学特性异常复杂，是引起套管损伤破坏的重要因素。以深部油气藏工程含膏质泥岩为研究对象，首先开展了高温为130 ℃、高围压为30 MPa、不同偏应力水平下多试样、单级加载三轴蠕变试验。该泥岩呈显著的时效力学特性，蠕变应力阀值较低，应力和持时对蠕变特性存在显著影响，低应力、长时间作用下泥岩亦呈现明显的稳态蠕变和加速蠕变破坏，且加速蠕变起始时间随应力增加呈指数降低关系。其次探讨了泥岩加速蠕变和强度损伤特性，提出了蠕变损伤起始时间概念，认为泥岩内部蠕变损伤变量可用指数函数予以描述。最后，基于蠕变损伤变量和起始时间，构建考虑泥岩加速蠕变的非线性改进Burgers蠕变模型，对蠕变参数开展辨识和分析，所选模型可准确地对深层膏质泥岩蠕变特性进行描述。研究成果旨在为深部泥岩油气藏套管工程长期稳定分析及安全评价提供可靠的依据。

Creep behavior and its nonlinear creep model of deep gypsum mudstone

Mudstone is widely used as the host rock medium in oil and gas reservoir. The creep behavior is extremely complex and directly influences the failure of casing under high temperature and high confining pressure. This study focuses on the gypsum mudstone in deep oil and gas reservoir engineering. Firstly, under 130 ℃ temperature and 30 MPa confining pressure, triaxial creep tests are performed under different deviatoric stress on multiple samples with a single-stage loading method. The mechanical behaviors of mudstone exhibit significant time-dependent and the creep stress threshold is small. The stress and duration has significant effects on creep behavior of mudstone. The steady creep and accelerated creep failure can be obtained under a long duration even in a small deviatoric stress. The start time of accelerated creep can be described as an exponential function of deviatoric stress; and the start time decreases with an increase in deviatoric stress. Secondly, the accelerated creep and strength damage characteristics of mudstone are discussed in detailed. The threshold of start time of creep damage is proposed. It is considered that the internal creep damage variable of mudstone can be described as an exponential function. Finally, based on the creep damage variable and the start time, the improved Burgers creep model is established. The model is nonlinear and can describe the accelerated creep stage. Then the creep parameters are identified and analyzed. The model can accurately describe the creep characteristics of the deep gypsum mudstone. The research results provide dependable reference for the safety evaluation and long term stability analysis of deep oil and gas reservoir engineering.