砂岩气藏水基欠平衡钻井逆流自吸效应实验研究

LABORATORY STUDY ON BACKWASH IMBIBITION GENERATED DURING WBM UNDERBALANCED DRILLING IN SANDSTONE GAS RESERVOIRS

采用水基钻井液欠平衡钻开砂岩气层时,欠平衡压差往往并不能完全抵消岩石的自吸毛细管力,从而产生逆流自吸效应。这种自吸效应可能对砂岩气藏水基欠平衡钻井的储层保护效果产生影响,但目前与之有关的实验研究还很缺乏。为此,设计了一套实验装置,模拟水基欠平衡条件,研究了砂岩基块孔隙逆流自吸规律及其影响因素。结果表明,水基欠平衡钻井过程的逆流自吸效应引起岩石气相渗透率降低,削弱欠平衡钻井的储层保护效果;欠平衡压差越小,逆流自吸作用越强,过小的欠压值不能起到欠平衡钻井保护储层的作用;随着逆流自吸时间延长、围压增加和黏土矿物膨胀,逆流自吸导致气相渗透率下降的程度加剧;逆流自吸引起的气相渗透率下降速率可以表示成时间的幂函数。

When the sandstone gas reservoir is drilled open by the underbalanced drilling using water based mud (WBM), the underbalance pressure normally can hardly offset the imbibition capillary force of the rock, which may result in backwash imbibition. The backwash imbibition may compromise the reservoir protection effects of WBM underbalanced drilling. However, research on this subject has seldom been done up till now. This paper presents a set of laboratory apparatus designed to simulate the WBM underbalanced drilling conditions to study the regularity and control factors of backwash imbibition occurred during underbalanced drilling in sandstone gas reservoirs. The study shows that the backwash imbibition leads to gas permeability decrease, resulting in weakening of reservoir protection effects of underbalanced drilling; the smaller the underbalance pressure difference, the stronger the backwash imbibition; extremely small underbalance pressure will lead to poor formation protection; and with the increase in imbibition time, confining pressure and the expansion of clay minerals, the decrease rate of gas permeability is aggravated. The gas permeability decrease rate can be expressed as power function of time.