储层原生水对页岩气赋存状态与流动能力的影响

Influence of reservoir primary water on shale gas occurrence and flow capacity

储层原生水对页岩气吸附规律和流动能力的影响情况目前尚不清楚,为此选用四川盆地长宁—威远地区下志留统龙马溪组页岩样品,基于吸附平衡法制取不同含水饱和度的实验样品,开展了高压等温吸附实验,探讨不同含水饱和度下页岩等温吸附作用及机理,并运用自主研发的稳态流动实验装置测试了不同含水饱和度下页岩气的流动能力。研究结果表明:(1)页岩储层微米—纳米级孔隙中原生水的存在降低了页岩的吸附能力,含水饱和度为40%时,模拟计算的总含气量比常规认识下的计算结果下降了18%;(2)页岩视渗透率是压力的函数,低压下努森扩散作用使得页岩视渗透率随压力降低而明显上升,当平均压力为5 MPa、含水饱和度达到50%时,与不含水相比页岩视渗透率下降约70%;(3)当含水饱和度低于临界含水饱和度时,水主要以不可动水赋存在微孔和介孔中,对页岩气流动能力影响较小,而大于临界含水饱和度时,水的赋存位置发生改变,导致页岩气流动能力大幅度降低。结论认为,准确认识页岩储层原始含水饱和度及临界含水饱和度,有助于准确计算页岩气储量、合理预测气井产量。

In this paper, shale samples of Lower Silurian Longmaxi Fm, taken from the Changning–Weiyuan area in the Sichuan Basin, were selected to figure out the influence of reservoir primary water on the adsorption laws and the flow capacity of shale gas. Experimental samples with different water saturations were prepared using the adsorption equilibrium method. Then, high-pressure isothermal adsorption experiments were carried out, and the isothermal adsorption effects and mechanisms of shale under different water saturations were discussed. Finally, the flow capacity of shale gas under different water saturations was tested using the independently developed steady-state flow test device. And the following research results were obtained. First, the presence of primary water in micron–nanometer pores of shale reservoirs reduces the adsorption capacity of shale. When the water saturation is 40%, the simulated total gas content is 18% lower than that in the conventional calculation result. Second, the apparent shale permeability is a function of pressure. Due to the effect of Knudsen diffusion, the apparent shale permeability increases significantly with the decrease of pressure under low pressure. When the average pressure is 5 MPa and the water saturation reaches 50%, the apparent shale permeability is about 70% lower than that of a dry sample. Third, when the water saturation is lower than the critical value, water is mainly presented as non-movable water in micropores and mesopores, and it has less effect on the flow capacity of shale gas. When the water saturation is greater than the critical value, the lodging point of water is changed, resulting in significant reduction of the shale gas flow capacity. It is concluded that an accurate understanding of the original water saturation and critical water saturation of shale reservoirs helps to calculate shale gas reserves accurately and predict gas well production rate rationally.