Pressure-Transient Behavior of Injection/Falloff Tests

Pressure-transient tests can provide useful information for characterization and evaluation of reservoirs under secondary and tertiary processes if they are properly designed and analyzed. In this work, we present the dimensionless pressure and pressure-derivative behavior of gas injection and falloff periods following water injection with skin and outer-boundary effects. To generate the bottom-hole flowing pressure, Laplace transform finite difference (LTFD) method with quasi-stationary assumption is used. The solution method decouples the saturation and pressure equations that describe slightly compressible three-phase flow. The pressure equation is solved using LTFD method, whereas the saturation equation is solved separately using the upwind scheme finite volume method, which is Riemann solver-free. As expected, skin effect is significant in the early times for gas injection period following water-oil displacement, but it becomes insignificant as time increases. For gas injection period following both favorable and unfavorable water-oil mobility ratio displacement with positive skin, the pressure drop starts decreasing after sometime as the front approaches the skin zone boundary, which results in negative derivative values. The effect of outer-boundary condition is also significant for gas injection period following unfavorable water-oil displacement. The pressure drop starts decreasing as the constant-pressure outer-boundary starts being felt. This leads to an increase in injectivity. However, the no-flow boundary condition exhibits a resistance in flow that leads to a reduction in injectivity.