致密砂岩气藏多尺度效应及生产机理

MULTI-SCALE EFFECT IN TIGHT SANDSTONE GAS RESERVOIR AND PRODUCTION MECHANISM

致密砂岩储层具有裂缝发育、基块致密的特点,通常需要水力压裂才能做到经济开采。储层储渗空间具有强烈的尺度性,按空间尺度分为致密基块孔喉尺度、天然裂缝尺度、水力裂缝尺度以及宏观气井尺度。致密砂岩气的产出是经过致密基块—天然裂缝—水力裂缝—井筒的跨尺度、多种传质等复杂过程,气体流动包括扩散和渗流,并分别用Fick定律和达西定律来进行描述。文章指出了气体在储层不同空间尺度中的流动具有时间尺度性,在裂缝中流动较快,在孔隙和喉道中流动较慢,而微孔中扩散更慢。无量纲导流能力关联了气体不同尺度下的传递过程,只有水力裂缝尺度和天然裂缝尺度的无量纲导流能力合理搭配,渗流能力达到一致,致密气才能经济高效采出。生产实例表明,致密砂岩气藏压裂后气井早期高产,一段时间后供气明显不足,压力急剧降低,稳产困难,表明致密砂岩气藏生产过程非常复杂。

Tight sandstone reservoirs are characterized by highly-developed fractures and tight matrix and hydraulic fracturing stimulation is usually needed to economically develop them. The reservoir spaces display strong scale effects, which are divided into tight matrix pore and throat scale, natural fracture scale, hydraulic fracture scale and macro gas well scale according to the space size. Production of tight sand gas is a complex process with multiple scales and multiple mass transfers through matrix, natural fractures, hydraulic fractures and well bore. Gas flows in the forms of diffusion and percolation, which can be described with Fick law and Darcy law respectively. The gas flowing through different reservoir space scales show different time scale effects, i.e. percolation of gas is relatively rapid in fractures and slower in pores and throats, while diffusion of gas in micropores is much slower. Dimensionless flow conductivity correlates the transfer processes of gas at different scales with each other. Only when the dimensionless flow conductivity and percolation capacity of hydraulic fractures appropriately match with that of nature fractures, economic and highly efficient recovery of tight sand gas can be realized. Case study shows that the initial deliverability of gas wells is high after fracturing stimulation of tight sandstone gas reservoirs, but the formation pressure declines steeply after the gas wells are produced for some time and it is difficult to stabilize gas production, indicating that the production process of tight sand gas is very complex.