MODELING CHANGING STORAGE DURING A SHUT-IN TEST

Phase redistribution has been observed to occur in oil, gas, and steam -injection wells during a shut-in test. Two empirical approaches-invol ving exponential and error functions-are currently available to analyz e field data. However, these methods do not have any mechanistic basis and may lead to erroneous estimates of reservoir parameters, such as skin. More importantly, these methods cannot be used in a forward mode to predict the reservoir and/or well conditions that would cause chan ging wellbore storage situations. Such predictions are desirable for d eciding on the need for expensive downhole shut-in or flow measurement . Phase redistribution is a consequence of the higher relative velocit y of the gas phase in a well's production string after shut-in at the surface. The resulting segregation of phases caused by buoyancy of the gas phase may cause wellbore pressure buildup to be quite different f rom normally expected response before the onset of the middle-time per iod. Under one scenario, the bubbles may segregate rapidly to the top of the wellbore leading to an increasing storage situation. Conversely , the bubbles may travel slowly without reaching the top to precipitat e a decreasing storage situation. In either event, the commonly used c onstant-storage model cannot be used for test interpretation. A mechan istic approach, based on the physics of migration of a swarm of bubble s, is developed that explains the changing storage phenomenon by incor porating continually decreasing after flow from the reservoir after sh ut-in, the variation of void distribution within the wellbore with dep th and time, and the combined effect of after flow and bubble migratio n on the wellhead and bottom hole pressures (BHP's). The model is used to develop a simulator, and a few cases are examined to illustrate th e simulator's capabilities.