Experimental Study on Revaporization Mechanism of Huff-n-Puff Gas Injection to Enhance Condensate Recovery in Shale Gas Condensate Reservoirs
Authors: Meng Xingbang , Sheng James J. 
Published: March 2016

Abstract
Once a shale gas condensate reservoir is produced, the reservoir pressure falls below the dew-point pressure, and the condensate liquid will be formed in the pore space; the condensate can then accumulate near the wellbore. This condensate blockage would reduce the gas relative permeability and decrease the gas production. The condensate is formed by the heavy components in reservoir fluid, and these heavy componenets are very valuable economically in the industry. Therefore, operators are seeking ways to maximize condensate recovery from gas-condensate reservoirs.
Huff-n-puff gas injection is an effective approach for enhancing condensate recovery in shale gas condensate reservoirs, as shown by our earlier papers (Sheng 2015; Meng, 2015). This paper considers the mechanism of the huff-n-puff method applied to shale gas condensate reservoirs by using gas chromatography (GC). In the immiscible gas flooding method, the mechanism is the pressure difference between reservoir pressure and well flowing pressure. Compared with the gas flooding method, huff-n-puff is more effective because of the quick pressure buildup near the wellbore. There exist two mechanisms of the huff-n-puff method (Sheng 2015). The first one is the pressure difference, as in the huff process the pressure near the wellbore builds up quickly. In the puff process, the pressure is depleted to a lower value so the condensate is recovered by the pressure difference. This is similar to the gas flooding method. Another mechanism is revaporization. In the huff-n-puff gas injection, when the pressure is increased to a value that is higher than the dew-point pressure, part of the condensate, which contains heavy components, is revaporized to gas. Thus, these heavy components flow out, mixed with gas when the well is opened during the puff period. There have been no experimental studies on this huff-n-puff EOR mechanism in shale gas reservoirs so far.
In this experimental study, a binary gas condensate mixture was used to investigate the dominant mechanism. The core was saturated with a gas condensate mixture at 2200 psi to simulate the initial reservoir condition. Then, the pressure was depleted to 1500 psi, which was lower than the dew point pressure. During the depletion, the produced gas was collected in a vacuumed gas sample bag. After depletion, the huff-n-puff method was applied. After every cycle of huff-n-puff, the produced gas was collected. GC was used to analyze the compositions of the different gas samples. Also, a CT scanner was used to determine the condensate saturation in the core. From the GC analysis, by comparing the gas sample after primary depletion with the gas sample after the first cycle, it was found that the heavy component-butane increased significantly. This means that most of the heavy components of condensate were revaporized and flew out with the dry gas. This proves the revaporization mechanism of the huff-n-puff gas injection.
Our experiment results show that huff-n-puff was an effective way to enhance the condensate recovery, and revaporization was the main mechanism of huff-n-puff. When the pressure was increased in the huff process, the heavy components were revaporized and flowed out with gas in the puff process. Though in the gas flooding method, the reservoir pressure was also increased, but the near-wellbore pressure was not increased very much in the shale gas reservoirs; thus, heavy components would still be formed near the wellbore. However, in the huff-n-puff method, because of the same well, the pressure near the wellbore would be higher than the dew point pressure in the beginning of production process. Therefore, the heavy component would be recovered with gas. Our GC results visually showed the revaporization mechanism of huff-n-puff in the shale gas condensate.