盆地内渗透性地层超压形成机制及钻前压力预测

The Overpressuring Mechanisms in Aquifers and Pressure Prediction in Basins

在世界范围内,沉积盆地高超压区的压力预测和安全钻井是长久未能解决的难题。在钻井过程中可能引起各种钻井工程事故的主要原因是渗透砂岩层内的高压,而现在通用的压力预测和监测方法却建立在低渗透的泥质岩内超压产生及对其岩性、物性变化影响的基础上。本文讨论了沉积盆地内异常高压分布的特征,着重探讨了在渗透性砂岩层中异常流体压力产生的机制,划分出自源、邻源和他源3种成因类型,并讨论了各类超压对钻井工程的危害性和可预测性。他源高压是预测性最差而危害性最高的超压类型,以开启断裂或先前钻井造成的原先过剩压力不同地层间的连通而在较浅处形成的高超压为最。分析了平衡深度法为基础的压力预测方法存在的缺陷后,笔者提出一些在高超压盆地内进行压力预测和监测的思路。

The prediction and testing of formation pressure before and during drilling have not been well completed in highly overpressured areas all over the world. In such areas, serious drilling damages may result from the high overpressures existing in permeable sandy formations, but, unfortunately, the prediction methods commonly used are based on the lithological or physical changes measured within the low permeable shale neighbours! It is believed that high pore pressures must be generated in low permeable shales, which in turn transmit into the sandy formations. The discussion in this paper shows that the high pressures in sandy formations may be generated by other mechanisms and the role of the shales may be only a seal. The pressuring mechanisms in sandy formations may be classified into three types: the self-source type, neighbour-source type and allo-source type. The damage and predictability of the overpressures are discussed. Generally, the allo-source type high pressures may easily cause serious problems but are too difficult to be predicted. After analysing the defects of currently-used methods, some ideas of the prediction and testing of pressures are proposed.