粘土断层泥含量对油气区断层活动性的控制作用研究

Clay gouge is common in fault zones at shallow depth and its role in weakening of fault frictional strength increases significantly with its increased content in fault zones. The weakening mechanics of clay gouge is a cause of wellbore instability during drilling operations, potential risk elevations of sheared casing of production wells and hydrocarbon leakage along fault zone. Hense, a characterization method of heterogeneous frictional strength of fault zones affected by clay gouge content, is of great significance to reduce risks in hydrocarbon exploitation, which can contribute to more accurate prediction of fault activation. We select No.234 fault located in Xingshugang structure in Daqing oil field to determine the distribution of clay gouge content on fault surface based on the analyzation of internal structure of fault zones and component of natural fault gouge and its wall rock. With the help of this work, a friction experiment will be conducted on synthetic fault gouge under in situ geological conditions of No.234 fault for purpose of building a quantitative or semi-quantitative characterization method of frictional strength affected by clay gouge content within fault zones. The method will be used to represent frictional strength heterogeneity in 3D space and evaluate fault activity in oil-gas field for identifying fault behavior of stick-slipping or creeping and promoting accuracy in fault activity prediction for mining safety in hydrocarbon recovery operation.

浅埋断层带中粘土断层泥广泛发育，其含量的增加对断层摩擦强度具有显著的弱化作用，易造成油气区过断层井的井壁不稳定，增加剪切套损风险及潜在的油气渗漏风险。如何表征这一因素下的摩擦强度非均质性，实现更精确的断层活动性评价，对降低油气开发风险具有实际意义。本项目以大庆油田杏树岗构造带的234号断层为研究对象，以天然断层泥及其母岩的分析测试和断层带内部结构解剖为基础，明确该断层的粘土断层泥展布特征。开展基于该断层天然断层泥组分及其原位地质条件下的人工断层泥摩擦滑动实验，建立一种粘土断层泥含量影响下的摩擦强度半定量-定量表征模型，实现三维空间内摩擦强度非均质性的表征及其在油气区断层活动性评价中的应用，为判定油气区断层粘滑或蠕滑，提高断层活动性评价精度奠定基础，指导油气安全开采。

断层带中粘土断层泥发育程度是不均匀的，造成了在空间上断层摩擦强度存在显著差异，这给断层稳定性预测造成更大难度。本项目结合野外、岩心分析、实验研究和大量调研，明确了断距、脆性程度、应变特征和岩石强度4方面主控因素对断层泥含量的影响程度，建立了基于断层带内部结构的粘土断层泥含量预测方法。大庆杏树岗油田杏七区萨Ⅱ1-高Ⅰ1层段16条断层的断层泥含量特征显示，断层泥含量在纵向上集成了母岩地层的“分层”特征，不同层系对应的断层泥含量差异显著。结合相似性剪切实验，测定并分析了原位地质条件下的“断层泥含量——摩擦强度”相关性，摩擦强度随断层泥含量从5%增加到100%过程中，呈现出“稳定的高强度——强度下降——稳定的低强度”的变化规律，即断层泥含量越大，断层摩擦强度越低，据此构建了粘土断层泥含量影响下断层带摩擦强度半定量-定量表征模型。实现了上述16断层的摩擦强度定量表征，并结合应力资料，明确了断层稳定性差异主要受断面产状和泥质含量的控制，越接近最优向产状时断层风险性越高，产状相近的条件下泥质含量越高，稳定性越差。判定风险断层为261a、261b、262、263、264、268、281西段和280+N1，该评价结果为提高断层附近剩余油安全注水开发提供了重要依据。