Modeling and Simulation of Gas Migration During a Well Testing for Gas Well

Abstract During well-testing operations, hydrocarbons is expected to enter the well from the reservoir. In gas wells, this influx is primarily gas, which migrates upward. As the gas rises, it expands and displaces the mud, reducing well pressure. Excessive pressure reduction can lead to an underbalanced condition, triggering further influx and potentially causing a hazardous situation. In order to reduce cost and improve efficiency, one may try to retrieve multiple samples at different depths in a single well-testing operation trip, which may then trigger multiple gas bubbles entering the well. This study models the migration velocity of multiple gas bubbles and their expansion due to temperature and pressure changes during the migration. As a function of time, the depth and volume of gas bubbles in the well are calculated. An iterative method of calculating the bubbles’ depth and volume is used to determine the pressure profile along the well. The pressure history at any depth of interest can then be obtained. By comparing this with pore pressure, the time at which the well becomes underbalanced can be determined, which is called time-to-underbalanced. This information helps guide well testing operations, indicating how long they can be safely conducted and when to take preventive action. The study simulates gas migration during well testing, providing pressure and temperature histories at key depths, including the casing shoe and influx depths. It also calculates the "pit-gain" history at the surface, representing the total volume history of the bubbles during migration. Generally, well pressure decreases as gas bubbles rise, especially when a gas bubble approaches the wellhead, where rapid expansion causes a swift increase in total influx volume and a corresponding drop in well pressure. This suggests that operators should intervene before bubbles reach the wellhead. The calculated time-to-underbalanced is a valuable indicator of when the well will become underbalanced. If the well remains balanced even when all bubbles reach the wellhead, it suggests the well will not become underbalanced. The total influx volume stabilizes once the bubble front reaches the surface, but the bubble end has not yet surfaced. The study also explores scenarios where well pressure initially decreases and then "abnormally" increases. This study provides insights into well pressure behavior during sampling with gas influx. It introduces a time-to-underbalanced indicator, informing engineers of the likelihood and timing of the well becoming underbalanced. This knowledge empowers operators to conduct operations confidently, minimizing potential gas kicks and safeguarding lives and assets.