注水开发引起泥岩层套管损坏机理研究

Mechanism study on casing failure of mudstone layers caused by waterflooding development

张志全 游园 何永明 柏笑寒 莫非

ZHANG Zhiquan YOU Yuan HE Yongming Bai Xiaohan MO Fei

长江大学石油工程学院，湖北荆州　434023 大港油田公司第三采油厂，河北沧州　061023

Petroleum Engineering College of Yangtze University, Jingzhou 434023, China No.3 Oil Production Plant of Dagang Oilfield Company, Cangzhou 061023, China

油田注水开发以后，由于泥岩层吸水软化，改变了作用在套管上的非对称外挤蠕变载荷分布而导致套管出现径向缩径变形的损坏现象。通过建立套管—水泥环—围岩的三维有限元数学模型，定量分析了注水开发引起泥岩蠕变对套管的影响，给出了注水开发引起泥岩层套管受蠕变载荷作用而损坏的机理。研究表明：蠕变载荷从0上升到最大的稳定值时间与泥岩的蠕变特性有关，一般约需400 d；当弹性系数由10 GPa降低到6 GPa时，套管所受径向载荷由61.95 MPa增大到68.36 MPa；泥岩吸水软化后，随着泥岩弹性模量的降低，套管外壁的附加载荷会明显增大，即泥岩在较高的注水压力下吸水软化，加剧蠕变载荷分布的非均匀性并加大蠕变载荷值，超过套管的抗外挤能力使套管柱发生径向缩径变形而损坏。

After waterflooding, mudstone layers are softened by water injection, as a result, asymmetry load distribution of external creep deformation is changed. So casing failure is caused by the diameter reduction and deformation at radial direction. Through the establishment of the “casing-cement mantle-surround rock” 3D finite-element mathematical model, the effect of mudstone creep deformation on casing resulted from waterflooding development is analyzed quantitatively. The mechanism of casing failure of mudstone layers caused by waterflooding development is presented. Study shows that the time of creep load increasing from 0 to the maximum is related to the creep characteristics of mudstone, about 400 d are needed generally. When the elastic coefficient decreases from 10 GPa to 6 GPa, the load on casing at radial direction will be increased to 68.36 MPa from 61.95 MPa. When the mudstone gets softened by water injection, the additional load on casing will witness an obvious increase with the decrease of elastic modulus. That is to say, as the mudstone gets softened under high water injection pressure, the heterogeneity of creep load distribution is intensified and the creep load exceeds the resistance to external pressure. As a result, the casing string becomes deformed at radial direction.