中等强度砂岩饱水力学参数变化试验及动态出砂规律

摘要

In order to quantify the effect of water saturation on sand production, a series of core tri-axial testing was performed u-sing 27 cores from a medium strength sandstone reservoir in an oilfield abroad, in which different water saturating time and water saturation at various surrounding pressure were applied for the measurement and analysis of the failure strength, elastic modulus, Poisson ratio, cohesive force and internal friction angle of the rock samples. The variations of rock strength at different water satu-ration conditions were revealed in the experiments. Based on the experimental data, an empirical ultimate strength exponential model was fitted to quantify the relationship of rock strength and water saturating conditions. The results indicate that the failure strength of the rock samples and elastic modulus decrease obviously with water saturating time varying from 0 to 50 d, and can drop to 70% of their initial value under water saturating time of 50 d and surrounding pressure of 40 MPa. With water saturation chan-ging from 0 to 100%, the compressing failure strength, elastic modulus and cohesive force decrease dramatically at the early stage,then slowly to a stable state, the final dimensionless strength ratio of which is about 72%. The empirical model can be further used for the prediction of dynamic sanding behaviors. The increasing water saturation to 0. 85 can cause a reduction of the critical san-ding pressure drawdown (CSPD) by 25%-30%. Meanwhile, a 30% reduction of the reservoir pressure can lead to 10%-11% de-creasing of the CSPD. The increase of water saturation and reduction of reservoir pressure in the Iraq reservoir studied can be the main inducing conditions for sand production in the process of future production.%为了研究含水条件变化对砂岩储层出砂的定量影响规律,对国外某油田N储层中等强度砂岩27块岩心进行不同饱水时间和饱水度条件下的三轴强度破坏试验,测试不同围压三轴破坏强度、弹性模量、泊松比以及内聚力、内摩擦角随饱水时间和饱水度的变化.试验揭示中等强度砂岩力学参数随饱水度的变化规律.根据试验数据,利用极限稳定强度指数模型拟合岩心力学强度参数随饱水时间和饱水度的变化经验关系式,用于该储层的动态出砂临界条件预测.结果表明:在饱水时间为0~50 d内,不同围压下的三轴破坏强度、弹性模量随着饱水时间明显下降,饱水50 d时围压40 MPa下的破坏强度和弹性模量均下降约30%;随着饱水度从0增加到100%,岩心三轴破坏强度、内聚力和弹性模量先降低较快,然后趋于稳定,最终无量纲强度比约为72%;该储层含水饱和度上升至0.85时,储层出砂临界压差下降25%~30%,而地层压力下降30%情况下,出砂临界压差降低10%~11%;未来含水上升和地层压力下降是导致该储层出砂的主要诱发条件.