Experimental study of gas flow characteristics in micro-/nano-pores in tight and shale reservoirs using microtubes under high pressure and low pressure gradients

Due to abundant micro-/nanoscale pores and throats, gas flow behavior in tight/shale reservoirs always showed strongly nonlinearity which deviated from the conventional Darcy’s law. As micro-/nanoscale flow experiment was a direct approach to investigate flow characteristic, in this paper, we improved the microscale flow experiment which was widely used in the field of MEMS to adapt for high-pressure conditions. By using microtubes with inner diameter ranging from 0.2 to 5 µm, we investigated the low velocity nonlinear flow characteristic of nitrogen especially under high pressures. In addition, we used experimental results to evaluate the accuracy of three typical flow models which were widely used in gas apparent permeability determination. The results indicated that gas flow behavior in microtube under high outlet pressures was opposite to that under low outlet pressures. When the outlet pressure was low, slippage effect had a great influence on the flow. With the increase in the pressure gradient, the slippage effect became weakened. And the slippage effect would disappear with the increase in the outlet pressure as well as tube inner diameter. The minimum outlet pressure for eliminating the slippage effect of nitrogen for the 0.2 µm tube was 10 MPa which decreased with the tube diameter. However, under high outlet pressures, the flow resistance increased in the region with smaller pressure gradients and varied inversely with the pressure gradient. This phenomenon became more significant with the increase in the outlet pressure and the decrease in the tube inner diameter. From the comparison of 3 typical flow models against our experimental results, we can find different models will match with the data measured under different mean pressures. And the applicability of those models also varied with the tube inner diameter. Thus, it was crucial to select an appropriate model according to different reservoir conditions.