An Experimental Study on Percolation Characteristics of a Single-PhaseGas in a Low-Permeability Volcanic Reservoir Under High Pressure
In this paper, we find that with the decrease in the average pore pressure in the process of gas production, both the slippage effect and the stress sensitivity effect will gradually increase; the increase in the slippage effect is significant, while the increase in the stress sensitivity effect is not. In this paper, the Kalamay volcanic gas reservoir of the Junggar Basin in China was selected as the object of our research. The gas reservoir has typical fractured volcanic reservoirs, and the long-term percolation feature remains unclear. To study the percolation characteristics of single- phase gas under high pressure, the experimental method was designed to simulate these characteristics in the process of gas production by measuring the gas flow in the core and the input and output pressure at both ends. We carried out simulation experiments of single-phase gas flow percolation characteristics under high pressure using 11 pieces of vol- canic rock samples in three wells of the study area. The results show that as the core pore pressure increased, the per- meability of low-permeability cores of the volcanic rock decreased significantly at room temperature. However, this decrease became more gradual, which means that the higher the core pore pressure is, the smaller the permeability variation caused by gas slippage is; when the pore pressure is above 10 MPa, the permeability is nearly constant, slip- page effect significantly reduces in the process of gas percolation, so it can be completely ignored under these forma- tion conditions. As the pore pressure decreases, the slippage effect and stress sensitivity effect will gradually increase; when the pore pressure is less than 10 MPa, the permeability appears to increase significantly, and this is especially true for a pressure of 5 MPa. The main cause of this result is the slippage effect of gas seepage during the depletion of the gas reservoir, when the pore pressure is lower than a certain value. The valid stress changes of the core are not large, and the stress sensitivity is not strong, so the slippage effect plays a major role, which leads to an increase in the gas permeability during the late period of certain flow gas production.
EXPERIMENTAL STUDY ON SEEPAGE PROPERTIES OF NON-DARCY FLOW IN CONFINED BROKEN ROCKS
In the underground engineering, there are two changes that will take place in the structure of the broken rock during the seepage: One is the variation in pore structure caused by water pressure; another is the breaking of the edges and corners caused by the contacting stresses between the solid grains. Thus the flow in broken rock is different from the one in rock-fills packed up naturally, and its permeating properties change greatly with the displacement or the load. In this paper, the MTS815.02 Rock Mechanics Test System together with a seepage instrument is used to test the seepage properties of non-Darcy flow in the broken sandstone, coal gangue and limestone with the grain sizes 5mm―10mm, 10mm―15mm, 15mm―20mm, 20mm―25mm and the mixed grain size by the method of steady permeating and the method of controlling the axial displacements. By the experiments, the relation curves of the pore pressure gradient and the seepage velocity corresponding to the variant axial displacement of the rock samples are obtained; furthermore the rules of the permeating properties varying with porosity are acquired. The experimental results indicate that the coefficient of deviating fromDarcy-flow may be negative. Some conclusions can be drawn as follows: 1) In a general trend, the permeability declines while the absolute value of the coefficient of deviating from Darcy-flow increases with the decreasing of the porosity; 2) The permeability is related with the loading level, the present porosity, the grain size, the construction style of the grains and the initial pore structure; 3) The magnitude of the permeability for the three kinds broken rock is 10-16m2―10-12m2 and the magnitude of deviating coefficient is 1012kg/m4―1015kg/m4; 4) when the breaking of the grains is very serious, the coefficient of deviating from Darcy-flow may be negative very possibly.
PREVENTION OF GAS LEAKAGE FROM UNLINED RESERVOIRS IN ROCK
Liquid petroleum gas (LPG), compressed air etc. can be stored in unlined caverns located below the natural ground water level. The pressure of the ground water must exceed the pressure of the gas etc. Theoretical calculations showed that the ground water above the cavern must also flow in the downwards direction with a hydraulic gradient IO > 1.0 if bubbles of gas shall not be able to move from the cavern upwards through the fractures of the rock and also that IO > 1.0 cannot be fullfilled by means of the natural flow of ground water to the cavern unless it is located unreasonably deep. Water shall instead be infiltrated from an array of boreholes in the rock above the cavern. Thereby inadmissible drainage of the ground water during the construction period can also be prevented. Equations are given for the required pressure and discharge of water from the boreholes. Du gaz liquifie (GPL), de l'air comprime, etc. peuvent etre emmagasines dans des cavites artificielles depourvues de revetement et se trouvant sous le niveau de la nappe phreatique. La pression de l'eau phreatique doit depasser celle du gaz, etc. Des calculs theoriques indiquent que l'eau phreatique au-dessus de la cavite doit s'ecouler vers le bas avec un gradient hydraulique IO > 1,0 pour que des bulles de gaz ne puissent pas monter de la cavite et traverser des fractures dans la roche, et que Io > 1,0 ne peut pas etre obtenu par le courant naturel d'eau phreatique jusqu'a la cavite, a moins de se trouver exagerement en profondeur. Au lieu de cela, de l'eau doit etre infiltree par une rangee de trous de forage dans la roche au-dessus de la cavite. Un drainage inadmissible de l'eau phreatique pendant la periode de construction peut ainsi etre evite. Des equations sont donnees pour la pression requise et le debit d'eau des trous de forage. Flussiges Erdgas (LPG), Druckluft etc. konnen in unausgekleideten Felskavernen unter dem naturlichen Grundwasserspiegel gelagert werden. Der Grundwasserdruck muss dabei hoher sein als der Gasdruck. Theoretische Berechnungen zeigen, dass das Grundwasser oberhalb der Kaverne mit einem hydraulischen Gradienten IO > 1.0 nach unten fliessen muss, um das Aufsteigen von Gasblasen durch Risse im Felsen zu verhindern. Die Berechnungen haben ausserdem gezeigt, dass IO >1.0 nicht durch den naturlichen Grundwasserzufluss zur Kaverne erreicht werden kann, wenn die Kaverne nicht auf unrealistischer Tiefe liegt. Um IO >1.0 zu erreichen, muss Wasser unter Druck aus einem System von Bohrlochern einfiltriert werden. Dadurch wird auch die sonst unvermeidliche Absenkung des Grundwassers wahrend der Bauperiode verhindert. Fur die Berechnung des erforderlichen Wasserdruckes sowie der erforderlichen Wassermenge wurden Gleichungen aufgestellt.