Transient analysis to locate and characterize plugs in gas wells
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1996 Society of Petroleum Engineers, Inc. We propose to use pressure transient analysis to indicate the location of a flow restriction in a gas well and to characterize such a blockage. We have developed a transient simulator for a gas well with a single partial blockage at any location in the flowline. The early-time wellhead pressure drop is influenced by the volume reduction due to the plug and its location while the late-time steady flow wellhead pressure drops give an indication of the severity of the plug. Thus, the early-time and late pressure response data can be used for blockage detection. Our study found that the thickness, the length, the location, and the roughness of the plug strongly influence the wellhead pressure response. Of these factors, the extent of reduction in channel diameter (due to plug thickness) has the most effect. In addition, we found that the flow rate at which the test is run has a pronounced effect on both early and late time wellhead pressure transients. We also noted the influence of reservoir pressure on gas density and velocity, which in turn, have some effect on the wellbore storage and the frictional pressure drop. Our study showed that wellbore deviation did not have any appreciable effect on wellhead pressure drop. Geothermal gradient was also observed to have negligible effect on wellhead response. In addition, we found that for most scenarios, the transient pressure response characteristics for off-shore production are similar to those for on-shore wells. Thus, the analysis presented in this report is suitable for off-shore wells as well.
