南堡沙河街组硬脆性泥页岩地层漏失压力预测方法

南堡沙河街组硬脆性泥页岩是典型的井壁易失稳地层,钻井过程中,井壁垮塌、漏失现象频发。目前,针对页岩井壁失稳现象,研究主要集中在结构面影响下的井壁垮塌。然而,实际钻井过程中,由于泥页岩层理、裂缝等结构面发育,导致地层漏失明显,严重制约了安全高效钻井。因此,本文基于室内力学实验,考虑页岩结构面发育特征,融合页岩沿基体破裂、沿层理或裂缝破裂及沿裂缝扩展等多类漏失机制,构建了一种新的硬脆性泥页岩地层漏失压力预测方法,明确了地层漏失压分布特征及影响因素。研究结果显示：层理与天然裂缝作为典型弱结构面,是硬脆性泥页岩地层漏失的主要控制因素。相比较均质地层,结构面发育的页岩地层漏失压力明显降低,安全井眼轨迹方位减少,钻井难度增大。此外,受弱结构面影响,难见沿基体的破裂漏失现象。漏失优先出现在天然裂缝面,天然裂缝对漏失影响强于层理面。研究成果可为硬脆性页岩地层钻井设计提供理论基础,对实现页岩地层安全高效钻井具有重要意义。

The research on the prediction method of loss pressure of hard brittle shale in shahejie formation, Nanpu

Hard brittle shale in shahejie formation, Nanpu oilfeild, is the typical formation with wellbore instability. During drilling process, wellbore collapse and drilling leakage frequently occur. Currently, for wellbore instability in shale formation, most of researches focus on wellbore collapse with structural plane. However, in drilling process, due to rich structural planes, like bedding planes and fracture planes, drilling leakage in shale is serious, extremely restricting safe and effective drilling. Therefore, based on rock mechanical test, considering the characteristics of shale structural planes, combining mechanism of leakage from failure along matrix, failure along bedding plane or fracture and failure propagation, a novel model of loss of pressure in shale formation has been established, determining the distribution and influence factors of loss of pressure. Results indicate that bedding plane and natural fracture are typical weak plane, and are the major controlling factor of causing leakage in shale formation. Compared to homogeneity condition, loss pressure in shale with multiply structural planes is hugely smaller. Also, well trajectory with safe drilling is decreased, increasing the difficulty of drilling. Furthermore, under the influence of weak plane, it is unlikely to see the failure along rock matrix. Leakage firstly happens at natural fracture. Besides, natural fractures has larger influence on leakage compared to bedding plane. Outcomes of this paper can offer theoretical basis for drilling design in hard brittle shale formation, and also meaningful for safe and efficient drilling in shale formation.