裂缝性低渗砂岩气藏水基钻井液欠平衡钻井储层保护

采用水基钻井液欠平衡钻进裂缝性低渗砂岩气层时,裂缝面和基质孔隙内同时存在指向地层的毛细管力,欠平衡压差并不能完全抵消该力,因而为钻井液侵入提供了驱动力。评价水基钻井液欠平衡钻井是否对裂缝性低渗砂岩储层造成损害的相关研究较少,为此,设计了一套试验装置模拟水基钻井液欠平衡钻井条件,研究了裂缝性低渗砂岩气藏在水基钻井液欠平衡钻井过程中的损害情况及其机理。研究结果表明:水基钻井液欠平衡钻井过程中逆流自吸效应以及伴随的钻井液吸附是造成储层损害的主要原因,但损害程度低,侵入深度浅;如果欠压状态不稳定,钻井液会迅速侵入裂缝,影响其渗流能力,并且这种影响随裂缝宽度的增大而增强。

Underbalanced Water Based Mud to Protect Fractured Tight Sandstone Gas Reservoirs

In underbalanced drilling fractured tight sandstone gas reservoirs using water based mud(WBM),the underbalanced pressure difference can not offset the capillary force existing between the fracture and the matrix,which caused the drilling mud to invade the formation.While few laboratory studies to evaluate the formation damage of the fractured tight sandstone gas reservoir during the WBM underbalanced drilling are available.Therefore this paper presents a set of laboratory apparatus designed to simulate the WBM underbalanced drilling to study the degree and mechanism of formation damage.The study shows that the imbibitions effect and drilling mud absorption are main reasons for formation damage during the WBM underbalanced drilling,while the damage is weak and the invasion depth is shallow.If the underbalanced drilling is instable the drilling fluid will invade the fracture rapidly and affect its flow capacity,and with the increase of fractures' width,the effect is increasing.