Interpretation of well tests in a tight gas reservoir poses some specific problems. Often times stabilized flow conditions are not obtained, drawdowns may be very high and pressure buildup is dominated by wellbore storage effects.
In this paper typical examples are presented. The interpretive tools used include conventional analysis techniques, type-curve matching and numerical simulation, It is shown that if the test is designed according to anticipated conditions and if care is taken to ensure that deviations from the implied assumptions are minimized and that the correct time span of data is taken, then meaningful and consistent interpretations result.
In recent years, resource development activity in deep low permeability carbonate gas reservoirs has intensified. The reserves appear fairly attractive, however, development is very expensive, therefore, the economic potential is extremely sensitive to the productivity of the wells. A sound field development plan necessitates accurate determination of some factors upon which well productivity depends: reservoir permeability, wellbore damage, stimulation potential etc.
In determining the reservoir and wellbore properties well testing is an invaluable technique. The interpretation of test data from low permeability gas well tests has in the past been difficult and confusing and has led to a notion that "tight" gas wells behave fundamentally different than others. As a result of its apparent peculiar nature, tight gas well test data often has been regarded as uninterpretable by conventional analytical techniques. Numerical simulation with complex multi zoned or layered reservoir models has become a common method of interpretation.
In the past few years there has appeared in the petroleum engineering literature a great number of new analytical solutions whereby the model employed accounts for additional complicating factors that were previously handled using simplifying assumptions.
Specifically these solutions, commonly referred to as "type curves", describe the early time behavior due to such effects as wellbore storage, skin, and fracture flow. These early time solutions have significantly augmented the standard interpretation techniques.
Re-examination of past well tests under this new light has shown that tight gas well behavior is not so puzzling after all and that standard analytical solutions can be used to accurately interpret the data. The new solution techniques offer such insight into the transient response behavior that tests should now be custom designed to optimize the information they provide. This report, by way of three examples well test interpretations illustrates how the test data of a tight gas yell can be routinely interpreted using standard analysis techniques.
The principle objective of measuring bottomhole flowing pressure during the flow and shut-in periods of a gas well test is to measure pressure transients introduced by the flow rate changes, and from these transients make estimates of reservoir permeability thickness. Deviation of measured pressure from predicted transient response is also used to estimate reservoir boundary conditions (i.e. skin, wellbore storage etc.).
The analytical methods of making such estimates are based on (analytical) solutions of the diffusivity equation.
