Migration of Natural Gas from Storage Reservoirs.

Abstract

This work has dual tasks: (1) to locate the moving gas-water interface in gas storage, and (2) to study the observation well performance during gas encroachment. Under the first task, a one-dimensional mathematical model describing the time-space distribution of pressure in the aquifer surrounding the gas bubble was derived. The model permits the calculation of pressures occurring at any fixed time and location in the aquifer using assumed positions of the gas-water interface. To locate the position of the gas-water interface at any time, the calculated pressure must match the observed pressure at the given time and fixed location in the aquifer. Satisfactory results have been obtained for adjacent gas-water well-pairs which have good pressure communication between them. Factors that could contribute to poor results as well as the sensitivity of the results to small errors in the data have been studied also. An extension of the above mathematical model permitted the calculation of the formation hydraulic diffusivity in the gas bubble or the aquifer. An important requirement here is the availability of two pressure-monitoring wells in the gas bubble or aquifer which are communicating with each other. The purpose of the second task is to obtain a method which permits the prediction of the depth of the partial gas column at any time in a shut-in observation well as the migrating gas enters it from below. Two theoretical models--one relating to short columns and the other to tall columns--were developed for the laboratory column and field observation well respectively. Data from laboratory experiments and from an actual field experience of an observation well 'going to gas' were used to verify the respective models and were found to give very satisfactory results. The models also permit the calculation of the rate at which natural gas enters the water column.