The lower the permeability and gas pressure are, the more prominent gas slip effect would be [10]. The influence 17-AAG manufacturer factors of gas slip effect include permeability, pore pressure, and water saturation. Gas slip effect would be prominent when the permeability is less than 0.1 �� 10?3��m2 and the pore pressure is a low value, while the specific boundaries of water saturation are not clear [11]. Gas slip factor is related to the pore structure [12].Slippage effect affects gas production. In laboratory, gas permeability is usually measured at a succession of pressures to obtain the Klinkenberg permeability by correcting because of Slippage effect. Laboratory working is increased. Equation (1) suggests that gas permeability is equal to Klinkenberg permeability when b/pm = 0.
Some researchers indicated that gas slip effect can be prevented by increasing pore pressure of high permeable core samples, but the study about tight sandstone is rare, and the results of tight sandstone are very different. Until Now there is no terminology to describe this phenomenon. In the past, some researchers exerted a big backpressure by rule of thumb to reduce Klinkenberg effect, which increases the pressure-bearing demand of experimental cardholder.When gas permeability was close to Klinkenberg permeability by improving mean pressure to cause b/pm to approach to zero, we define the pore pressure or backpressure at the outlet of the core sample as limit pressure.
If we know the limit pressure, we can measure permeability by exerting a backpressure which is equal to or a little greater than limit pressure to mitigate slippage effect on experimental results, such as the effect of gas velocity on gas permeability due to fine migration [13].The impetus for this work was a concern that finding the relation between limit pressure of eliminating gas slippage effect and pore structure parameters can help obtain the limit pressure of specific pore structure rock.2. Experimental Samples and Procedures2.1. Core SamplesIn this study, the tight sandstone core samples, from Permian in Upper Paleozoic in Ordos basin, involve four permeability levels (<0.1 �� 10?3��m2, (0.1~0.3) �� 10?3��m2, (0.3~1) �� 10?3��m2, and >1 �� 10?3��m2). Nitrogen is regarded as displacing medium. The schematic diagram of the experimental apparatus is shown in Figure 1.
It mainly consists of a high pressure core holder, a high pressure nitrogen cylinder, a high pressure pump, a backpressure regulator (BPR), and a gas flowmeter.Figure Cilengitide 1Schematic diagram of the experimental apparatus.2.2. Procedure(1) Seven samples of four permeability levels (<0.1 �� 10?3��m2, (0.1~0.3) �� 10?3��m2, (0.3~1) �� 10?3��m2, and >1 �� 10?3��m2) are selected in the experiments. SS-1 is an outcrop sample that is different from others. Before conducting the porosity and permeability test, the core samples in this work are dried for more than 48 hours at 60��C.