Three groups of cores with different imbibition thickness were connected in series to perform physical simulation experiments of depletion. The effect of imbibition later on gas recovery can be measured and the phase trapping by imbibition layer are verified.
Depletion experimental test. Core sample preparation: Three groups of cores with similar permeability but different imbibition thickness is used for the depletion experiment (Fig. 5). The core of group C is dry samples with length of 100cm and permeability of 0.22mD. The core of group A is also dry samples with length of 100cm and permeability of 0.27mD. The core length of group B is 20cm, the permeability is 0.17mD, which is put between group A and group C act as the imbibition layer with different imbibition thickness. The imbibition layer is established by spontaneous imbibition, and the imbibition thickness is 0cm, 8.8cm and 12.8cm respectively with the imbibition water saturation of 75%.
The experimental process is as following: (1) Gas is injected into the three groups of cores synchronously and slowly through both the inlet and outlet until the pressure is stable at 20.5MPa to simulate the original formation pressure of the gas reservoir. (2) The outlet is connected with a mass flow meter to control the gas flow rate according to the designed gas rate and simulate the depletion process of gas reservoir. (3) The pressures measured every 20cm both on group A and group C. The pressures are measured both the inlet (left side of group C) and outlet (right side of group A), and between the three core holders. The pressure profile can be built at any time from the beginning of the depletion to the end of the test.
Analysis of experimental results of depletion. The cores of group A and C keep unchange, and the cores of group B with different imbibition thickness are successively replaced to conduct three depletion experiments. The pressure drop profile during the depletion test is shown in Figure 6.
There is no imbibition layer in the first depletion experiment, that is, the core of group B is dry sample, and its permeability was close to that of the cores of groups A and C. The overall pressure profile decreased uniformly (Fig. 6a). At the end of experiment, the average residual pressure of the core of group C is 1.60MPa. Assuming that the gas compression factor is constant, the estimated gas recovery of group C is 92.2%.
As for the second depletion experiment, the imbibition thickness is 8.8cm in core of group B. Although the thickness of the imbibition layer is not so large, the lower-permeability effect is obvious (Fig.6b). At 90min, the average core pressure of group C is maintained at 20.13MPa, even though he pressure drop of group B with imbibition layer is as high as 17.65MPa and the pressure gradient is 0.88MPa/cm. With the prolongation of time, the gas flow gradually breaks through the core of Group B, and the water saturation in the core of group B is gradually reduced, and the seepage capacity is gradually increased. At 180min, although the pressure drop at both ends of group B is still as high as 17.79MPa, the pressure of group C had decreased 0.85MPa, indicating that the reserves in group C began to be recovered. At the end of the experiment(4371min), the pressure drop at both ends of group B has decreased to 2.69MPa, indicating that its seepage capacity has been greatly improved, but the pressure gradient at both ends of group B is still up to 0.13MPa/cm, indicating that its seepage resistance is still very high. In contrast, the pressure gradient at both ends of group B was only 0.016MPa/cm in the first experiment without imbibition layer. Due to the existence of imbibition layer, the average residual pressure of group C was 3.3MPa at the end of the experiment, and the gas recovery is 83.5%, which is 8.7% lower than that in the first experiment.
The third experiment also contain imbibition layer with the imbibition thickness increased to 12.8cm. The pressure drop at both ends of the core in group B is also obvious, and the barrier ability is stronger (Fig.6c). At 180min, although the pressure drop at both ends of group B is as high as 19.67MPa with the pressure gradient of 0.98MPa/cm, the core pressure of group C remains at the original pressure, indicating that the reserves in group C have not been recovered yet. At the end of experiment(3968min), pressure drop at both ends of group B is still as high as 10.95MPa with pressure gradient of 0.55MPa /cm. The residual pressure in group C is as high as 11.75MPa, and the gas recovery decreased to 42.7%, which is 49.5% lower than that without imbibition layer.
The comparative analysis of the three experiments shows that the imbibition layer has high water saturation and strong separation ability even the imbibition thickness is not so high. Of course, the greater the imbibition thickness, the more obvious the sealing effect, the greater the proportion of trapped gas and the lower the gas recovery.