Pressure Distribution in a Layered Reservoir with Gas-Cap and Bottom Water

Abstract

Oil production from a layered reservoir with a top gas cap and bottom water acting simultaneously poses serious challenges of rate and pressure maintenance. To achieve clean oil production both rates and pressures regimes have to be chosen carefully according to available to avert production of unwanted external fluids. Furthermore, well tests analyses of pressure data would require that flow from each layer is adequately quantified and delineated. For layers with crossflow interface isolating each layer through a test analysis is additional challenge. If the layers contain oil of different properties, well completion strategy has to be specially crafted to achieve optimal individual layer production performance.

It is with a view to addressing these challenges that this study becomes absolutely necessary. In this study, dimensionless pressure and dimensionless pressure derivatives are derived for each layer of a two layered reservoir, both drained through one vertical wellbore. The difference in flow behavior of the different layers is normalized, for crossflow layers, through a dimensionless time frame. The normalization enabled the crossflow layer to be treated as one enlarged reservoir and was utilized to discriminate flow from layers, no matter the choice of well completion and disparity in layer fluid properties. Flow times considered is elaborate and ranged from very early to early and late time, large enough for at least one of the external boundaries to be felt in a test period. Because the external boundaries impose a steady state, the emergence of steady state is considered as end of flow of clean oil in our computations. The characteristic signatures of the log-log plot of dimensionless pressures and pressure derivatives for early time and late flow periods were then used to characterize the reservoir system.

It is revealed that time for clean oil production is longer for larger and thicker layers for constant production rate history. Furthermore, a flattening and a collapse to zero trends are observed on dimensionless pressure and dimensionless pressure derivative plots, respectively, when the effects of the top and/or bottom boundaries are felt. When a permeable interface is felt, similar trends are observed but there is cessation shortly afterwards depending on the degree of interlayer crossflow. Furthermore, it was noticed that perforation location does not seriously affect well productivities, especially at early times. Finally, only fluid ratios is recommended as adequate to reveal which of the external fluids accidentally reaches the wellbore during oil production, since each of the external fluids is capable of manifesting steady-state behavior.