Interpretation of Well Test Data in Gas-condensate Fields
Authors: Lewis James O. 
Published: June 2014

The operation of gas-condensate fields is comparatively new and involvesphysical principles with which the industry was not previously familiar. It hasbeen necessary to devise methods for testing gas-condensate wells and tocorrelate laboratory and field data. In this paper new methods are advanced forinterpreting well G.P.M. t tests whereby dilution by dry gas can be estimatedand information supplied on revaporization of condensates by dry gas.Inaccuracies in well test data are discussed. Changes in flow rates can causechanges in the G.P.M. of the effluent and it may take some time for a well tobecome stabilized after an important change in flow rate. Retrograde tests asusually conducted cannot be directly compared with field G.P.M. data, becausethe retrograde tests usually include solution gas in the condensates, which arereported on the cumulative basis whereas field data (G.P.M.) usually reportpentanes plus on a rate or instantaneous basis.

Introduction

Though long known to physicists, the phenomena of retrograde condensationbecame known to the oil and gas industry only in recent years, when deeperdrilling disclosed that the behavior of vapors under changing pressures wasdifferent in high-pressure reservoirs than under the low-pressure operationswith which the industry was familiar. These deep, high-pressure reservoirs inwhich condensation and vaporization behaved inversely to the accustomed mannerbecame known as "gasdistillate," or preferably "gas-condensate"fields. Partly to prevent precipitation and retention of liquids in thereservoir, but still more to prevent the waste of residue gas, the operationknown as "recycling," or preferably "cycling," was instituted, in which the residue gas is reinjected into the reservoir, thus maintainingpressure and driving the wet gas to the producing wells.

For lack of field experience to serve as a guide, the industry had to assumethat theory and laboratory experiments could accurately forecast retrogradebehaviors in operating these gas-condensate fields, but it is obvious thatthese assumptions should be checked at the first opportunities against actualfield experience. To make such checks, it is necessary to accumulate a largequantity of reliable field data, but experience has shown that it is difficultto obtain reliable field data and that the laboratory data as usually presentedare not in directly comparable form. The problems of correlation involve thecomplex phase behavior on the one hand, and a knowledge of field data and avisualization of underground conditions on the other hand. This paper is anapproach from field experience.

The purposes of this article are to discuss the sources of uncertainty in thedata and to present methods of analyzing, interpreting and correlatinglaboratory and field data. Data mostly from lower Gulf Coast fields arepresented, from which some conclusions can be derived, but better data frommore sources than those available to the writer will be required beforesatisfactory answers to many questions can be obtained. This paper is presentedto stimulate the analysis and publication of data from other fields, thereexamination of field and laboratory methods, and the taking of more completeand reliable field tests. For these purposes, the writer presents somespeculations that otherwise would not be appropriate.

T.P. 2053