Pressure Transient Analysis for Afterflow-Dominated Wells Producing From a Reservoir With a Gas Cap
Authors: Streltsova-Adams T.D. 
Published: December 2007

Summary
A method for analyzing afterflow-dominated transient pressure buildup data from a partially penetrating well completed near a gas cap is developed. The gas cap modifies the middle-time region of pressure curves in a manner such that conventional pressure buildup analysis no longer can be applied. For a damaged well, the method becomes nonunique and requires specification on the well damage to estimate the reservoir transmissibility.

Introduction
Buildup tests in reservoirs with a gas cap exhibit a specific performance: a continuous flattening of the pressure curve at the middle-time region (MTR) with an approach to a constant pressure value at the late time. Such behavior makes the middle-time pressure data so distorted that in a Horner plot the straight-line portion of the MTR cannot be identified. This makes it impossible to determine reservoir and wellbore parameters by the standard procedures, which are based on the MTR slope of pressure data. A method for analyzing transient pressure buildup data in a well of partial completion producing from a gas-cap reservoir is given here. The buildup data discussed are affected by afterflow. The method is based on type curves generated by applying the McKinley afterflow routine to a dimensionless pressure drawdown (buildup) function developed for a well of limited flow entry producing at a constant rate from an oil zone under a gas cap. A working set of type curves is prepared for various well penetration ratios. Information obtained from type-curve application includes wellbore and formation transmissibilities, well damage or stimulation, flow efficiency, and formation productivity. If the well is undamaged, the curve-matching technique gives an unambiguous analysis. But if the well is damaged, the analysis of the pressure buildup data is found to be nonunique: many combinations of damage and formation parameters will produce nearly the same buildup pattern. For a damaged well, an extension is made to give an indication of wellbore permeability relative to the bulk formation permeability - i.e., to determine formation transmissibility and flow efficiency for a given depth of damage. Thus, the analysis of damaged wells is reduced to the problem of assessing a range of the possible formation and wellbore properties consistent with a reasonable depth of damage.

Effect of Gas Cap on Well Pressure Behavior
High oil recoveries with free-gas drive are maintained as long as the gas cone does not break into the producing section of the wellbore. To reduce gas coning, producing wells in reservoirs with a gas cap often are completed such that they are open only in the lower part of the oil zone. As oil is withdrawn, the decrease in the oil volume causes the gas, compressed initially under reservoir pressure, to expand. This maintains a fairly constant pressure on the oil. The period when the pressure at the advancing gas/oil interface is relatively constant is the period of efficient oil production. A gas cap, forming a nearly constant pressure boundary at the top of the oil zone, causes vertical oil-flow components that modify the otherwise radial flow to a wellbore. The performance of a gas-cap-dominated well is affected in such a manner that a conventional transient buildup or drawdown plot shows a continual flattening of the pressure-time curve. The flattened pressure curve, which gradually approaches the horizontal, gives rise to a variety of slopes, none of them yet justified for determination of the formation properties.
JPT
P. 743^