This work shows that effective well damage determination from pressure buildup tests in complex, developed reservoirs may be best achieved through early time data analysis. Extensive analysis was done using field data from the Prudhoe Bay reservoir in Alaska. Conventional Horner analysis is largely ineffective in this reservoir because of distortion of the late time data by boundary effects. The early time data are affected by partial completion of a large net pay zone. Five different analysis techniques were used to analyze the pressure buildup data. Results from these methods were correlated with production histories and scale damage removal treatments on thirty-two Prudhoe Bay wells. A radial implicit simulator was also used to history match the pressure behavior in several buildup tests to verify pressure behavior in several buildup tests to verify the analytic results.
It was found that the McKinley Early Time Analysis Method, modified for partial completion, provided the most reliable estimate of well damage at Prudhoe Bay. The method can provide a qualitative indication of well damage in the presence of a constant-pressure boundary (the gas cap at Prudhoe Bay). The more reliable estimates of well damage allowed us to plan a more effective well stimulation treatment program.
Since oil production began at Prudhoe Bay in 1977, over 1500 pressure buildup tests have been conducted on more than 500 Prudhoe wells. Analysis of these tests can provide valuable reservoir information which is critical to an effective reservoir surveillance program. The objective of this study was to recommend an effective, practical method for engineers in the field to analyze Prudhoe Bay pressure buildup tests. The recommended method should provide a reliable estimate of well damage to adequately evaluate costly well stimulation treatments. Because of complex reservoir boundary effects at Prudhoe, conventional semilog analysis methods were not applicable.
Five methods of pressure buildup analysis were examined in this study:
. Horner semilog analysis with a skin correction for partial completion
Modified Horner Analysis which searches for two semilog lines on the buildup plot
McKinley early time analysis method including partial completion
The approach taken to evaluate these methods was to examine individual analysis results on a number of wells and correlate them with production history trends and acid stimulation results. For example, high damage factors should result in low productivity indices and vice versa. Formation kh values should be reasonable and approximately satisfy the steady-state equation for well deliverability. Finally, use of a RZ implicit well test simulator was employed in a few cases to further verify results.
At Prudhoe Bay, several reservoir and wellbore complexities combine to restrict the use of the conventional Horner method. Gas cap pressure support and water leg pressure support can influence the late-time well test data by creating constant pressure boundaries. Interbedded shales create zones of varying reservoir thickness which complicate the analysis. Well drainage boundaries move and change shape based on well production histories and those of adjacent wells. Large tubing sizes and increasing well gas/oil ratios cause high wellbore storage coefficients and restrict the use of some data. A large pay interval is only partially completed to minimize pay interval is only partially completed to minimize gas coning. Multiphase gas/oil flow is dominant.