## 3 Results and Discussion
### Effect of Different Aquifer Sizes  
Nearly all gas reservoirs are surrounded by water-bearing rocks called aquifers [11]. The aquifer size has an important effect on water invasion and gas production in gas reservoirs [6; 10]. The core 3 is used to study the effects of water invasion with different aquifer sizes. The gas production rate and original saturation pressure are 1000 mL/min and 30 MPa, respectively. The experiments of water invasion without aquifer and with 3 times, 7 times and infinite aquifer are performed, and the results are illustrated in Figure 4. From the result of the Figure 4a, it can be observed that the bottom-hole pressure decreases linearly with recovery factor when there is no aquifer. The pressure drop increases gradually with the increase of recovery factor, but the pressure drop is only 1.43 MPa until the production ends, shown in Figure 4b. This suggests that gas seepage resistance is very small without the edge-and-bottom aquifer and the recovery factor is high which can reach 83.26%.  
Figure 4: Bottom hole pressure and producing pressure drop versus recovery factor for different aquifers. (**a**) Bottom hole pressure; (**b**) producing pressure drop.  
Figure 3: Water invasion versus recovery factor for different water invasion energies.  
When there exists the edge-and-bottom aquifer, bottom-hole pressure changes with the increase of recovery factor, and there is a concave change in Figure 4a. This is because the early water invasion delays the decline of bottom-hole pressure. At the same recovery factor, the larger the aquifer is, the larger the bottom-hole pressure is. This shows that the early water invasion with the edge-and-bottom aquifer is the gas supply of energy, and the larger the aquifer is, the greater the energy supply is. In the late water invasion, the seepage resistance increases and bottom-hole pressure drops rapidly due to the existence of two phase flow. Thus, in the production and development of gas reservoirs, we can reversely infer the strength of edge-and-bottom aquifer according to the change of bottom-hole pressure so as to prevent the risk of water invasion. Moreover, there exists an obvious inflection point between pressure drop of gas reservoirs with edge-and-bottom aquifer. The bottom-hole pressure drops slowly before the point and the pressure drop increases slowly. However, the bottom-hole pressure drops sharply after the point and the pressure drop rises sharply. The larger the aquifer is, the lower the recovery factor is [20]. This is because it belongs to single-phase flow in the early water invasion, and the gas-water interface is relatively stable. With an increase of water invasion, the single-phase flow gradually shifts to the two-phase flow. Consequently, the seepage resistance and pressure dropdown increases and the recovery factor reduces.  
From the result of Figure 5, water invasion gradually increases with the increase of aquifer size [4; 11]. However, when the aquifer size increases to a certain value, the growth of water invasion changes slowly. With the increase of aquifer size, the recovery factor increases then decreases, and the recovery factor reaches the maximum with the 3 times aquifer. It shows that when the permeability is 0.1 mD, the small aquifer provides positive energy to gas reservoirs. Less water occupies the storage space of gas reservoirs, which displaces partial gas and improves the recovery factor. With the increase of aquifer size, water invasion energy increases. Water displaces gas and some gas will be trapped through the breaker, flow around and water lock. When water invasion is mainly governed by trapped gas, the recovery factor reduces.