气藏型储气库循环载荷作用下出砂特征试验研究

Experimental Study on Sand Production Characteristics of Gas Reservoir-Type Gas Storage under Cyclic Loading

  • 摘要: 目前储气库建库防砂完井设计多以常规油气藏出砂理论为依据,储气库循环注采特点下砂岩储层出砂规律与出砂机理研究未成体系,缺乏对建库方案的针对性支撑。利用新疆呼图壁储气库地区红砂岩露头岩样进行了常规工况与储气库循环加载工况下的出砂模拟试验、单轴压缩与循环加卸载力学试验,探讨了不同工况下的出砂规律、出砂特征及储层孔眼破坏形态,揭示了储气库循环载荷作用下的出砂机理。研究发现,常规工况下,出砂现象主要出现在稳定生产前段,出砂速率呈现“增加−平稳−衰减至零”的过程,约持续30 min,此后岩心内部结构趋于稳定;循环工况下,累计出砂量大幅增加,约为常规工况的2~3倍,且出现持续出砂现象,即使生产压力低于临界出砂压力,随着循环次数增加,岩石因疲劳损伤仍会产砂,风险持续存在。常规工况下产出砂的粒度中值(D50)为162~193 μm,循环工况下为120~125 μm,显著低于前者。常规工况下,砂粒团块大、胶结物保存较好;循环工况下砂粒破碎更彻底、胶结物脱离更明显,颗粒间错动加剧导致细粒砂占比升高。循环工况下,孔眼破坏严重,形成对称V形“狗耳”状剪切破碎区,破碎面积为431.17~496.59 mm2,为常规工况(162.39~194.58 mm2)的2.2~3.0倍。孔眼腔体体积、直径及延伸高度均显著扩大,岩石在循环加载进入破坏阶段后出现宏观裂缝并伴随离散砂脱落,表明破坏阶段后期是储气库出砂的主要发生时机。疲劳损伤导致岩石骨架松动、胶结断裂,多轮次摩擦错动进一步加剧颗粒破碎与细粒化。储气库防砂策略应纳入岩石在循环载荷下的强度衰减和破坏特征,以提升长期运行安全性。研究结果可为储气库的建库设计与防砂措施优化提供理论支撑。

     

    Abstract: The current design of sand control completion for gas storage construction is mostly based on sand production theories for conventional oil and gas reservoirs. However, the patterns and mechanisms of sand production in reservoir sandstones under the cyclic injection and production characteristics of gas storage have not been systematically studied, resulting in insufficient targeted support for gas storage construction design. To address this issue, sand production simulation tests, uniaxial compression tests, and cyclic loading and unloading mechanical tests were carried out using red sandstone outcrop samples from the Hutubi gas storage in Xinjiang, under both conventional conditions and cyclic loading conditions representative of gas storage operations. The sand production patterns, sand production characteristics, and perforation damage morphologies under different conditions were investigated, and the sand production mechanism under cyclic loading in gas storage was revealed. The results showed that under conventional conditions, sand production occurs mainly in the early stage of stable production, with the sand production rate following a pattern of “increase, stabilization, and decay to zero” over approximately 30 minutes, after which the internal structure of the core stabilizes. Under cyclic loading conditions, the cumulative sand production was significantly increased, approximately 2–3 times that under conventional conditions, and continuous sand production is observed. Even when the production pressure is lower than the critical sand production pressure, sand is still generated due to rock fatigue damage as the number of cycles increases, indicating persistent risk. The median grain size (D50) of the produced sand under conventional conditions is 162–193 μm, while under cyclic conditions, it is 120–125 μm, significantly lower than the former. Under conventional conditions, sand grains form larger aggregates with relatively intact cementation; under cyclic conditions, sand grains are more thoroughly fragmented; cement detachment is more pronounced, and increased intergranular displacement leads to a higher proportion of fine particles. Severe perforation damage is caused under cyclic loading, characterized by the formation of symmetrical V-shaped “dog-ear” shear failure zones, with a failure area of 431.17–496.59 mm2, which is 2.2–3.0 times that under conventional conditions (162.39–194.58 mm2). The cavity volume, diameter, and extension height of the perforations are all significantly enlarged. After the rock enters the failure stage under cyclic loading, macroscopic fractures are developed, accompanied by discrete sand shedding, indicating that the late failure stage is the primary period of sand production in gas storage. Fatigue damage loosens the rock framework and fractures the cement, while multiple cycles of frictional displacement further exacerbate particle fragmentation and fining. The strength degradation and failure characteristics of rock under cyclic loading should be incorporated into sand control strategies for gas storage to enhance long-term operational safety. Important theoretical support for gas storage construction design and the optimization of sand control measures can be provided by these findings.

     

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