摘要
煤储层进入饱和气体单相流后,储层有效应力系数的变化规律是正确评估储层渗透性的关键.本文采集不同变质程度的煤岩样品,对不同外界荷载和孔隙压力条件下煤储层的等效有效应力系数进行了研究,结果表明:在孔隙压力一定的条件下,外界荷载对煤储层等效有效应力系数具有负效应,随外界荷载加载,煤储层等效有效应力系数减小,且外界荷载越大负作用越强;在外界荷载一定条件下,随孔隙压力增加,煤储层等效有效应力系数增大,孔隙压力对煤储层等效有效应力系数具有正效应;相对于中阶煤和高阶煤储层,低阶煤储层由于孔隙类型多样,孔隙尺度大,储层的等效有效应力系数在外界荷载和孔隙压力耦合作用下影响程度相对较小.由于煤储层等效有效应力系数在不同外界荷载和孔隙压力下差异较大,煤储层进入饱和气体单相流后,在不考虑解吸收缩效应条件下,煤储层的有效应力变化并非等同于储层孔隙压力的变化,而与煤阶类型、地应力高低和孔隙压力大小密切相关,原位高地应力条件下,由于煤储层的等效有效应力系数较小,随孔隙压力降低,煤储层有效应力变化相对微弱.
Abstract
The variation law of equivalent effective stress coefficient of the coal reservoir at the stage of the saturation single-phase gas flow is essential in evaluating the reservoir permeability.The coal samples with differ-ent degrees of metamorphism were collected and their equivalent effective stress coefficient was studied under dif-ferent applied loading and pore pressure conditions.The results showed that under a constant pore pressure,with the application of the loading,the equivalent effective stress coefficient of the coal decreases.Applied loading has a negative effect on the equivalent effective stress coefficient of coal reservoirs,and which is strong with the increase of applied loading.Whereas,at a certain loading,with the increase of pore pressure,the equivalent effec-tive stress coefficient of coal reservoirs increases.Pore pressure has a positive effect on the equivalent effective stress coefficient of coal reservoirs.Compared with the middle-and high-rank coal,there are much more pore types in the coal.And the its pore size is much larger.Under the coupling effect of applied loading and pore pressure,the equivalent effective stress coefficient of low-rank coal reservoir is less affected compared to the middle-and high-rank coal reservoirs.There is a significant difference in the equivalent effective stress coefficient of coal reservoir under different applied loading and pore pressure.Thus for the coal reservoir at the saturation single-phase gas flow stage,without considering the coalbed methane desorption-induced coal matrix shrinkage effect,the effective stress change of coal reservoir is not equivalent to the change of pore pressure.And it is closely related to the magnitude of geo-stress and pore pressure.Under the high in-situ geo-stress condition,due to the small equivalent effective stress coefficient,the change of effective stress for the coal reservoir is little with the decrease of pore pressure.
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