Improved Surfactant-Alternating-Gas Foam Process to Control Gravity Override

Abstract

Gravity override in foam mobility-control processes results from the competition between lateral pressure gradient and gravity. Therefore, overcoming gravity override with continuous foam injection requires raising injection-well pressure, possibly risking fracturing the formation. Foam processes employing surfactant-alternating-gas (SAG) injection at a fixed injection-well pressure, however, can overcome gravity override without excessive injection pressures. This advantage applies even in radial flow, where declining flow rates at increasing distance from the well make gravity effects especially intractable. This advantage does not apply to processes with continuous foam injection or SAG injection at fixed injection rate. Moreover, this advantage of constant- pressure SAG foam processes appears to be relatively insensitive to the details of foam behavior. In such a process, injection rate increases as foam dries out near the well and injectivity increases; as a result, most of the fixed pressure drop between wells is focused on the displacement front, with maximum suppression of gravity effects. We illustrate these effects with numerical simulations of foam displacements in 2D homogeneous rectangular alid cylindrical reservoirs. Simulations of SAG foam processes are exceptionally sensitive to numerical artifacts. Earlier work in this area was affected by these artifacts. Some of the artifacts, and ways to control them, are discussed. P. 491