1 General
Test objectives can vary from a single downhole sample to an extensive long duration reservoir limit test. This will form part of the well programme.
Stimulation and artificial lift are two methods of getting the well to flow; special provision may be required for these.
The purpose of a production test is to record the bottom-hole and wellhead pressures at a number of production rates, and simultaneously record the corresponding gas, condensate or oil rates. The main objectives of a test (including fluid sampling) of the reservoir zone are the determination of the following:
1.Static reservoir pressure and temperature.
2.The inflow performance characteristics of the well (i.e. drawdown as a unction of rate and average reservoir pressure).
3.The composition and P.V.T behaviour of the reservoir fluid.
4.Reservoir pressure build-up (to allow for well test analyses and skin determination) which in turn may lead to consideration of stimulation.
2 Test results
The results obtained from production testing can be used for a variety of purposes including the following:
1.The evaluation of an exploration programme - in particular the decision whether or not to continue.
2.Deciding whether to develop a discovery, to drill additional appraisal wells or to discontinue the drilling completely.
3.The provision of input data for the planning of a field development programme and/or for the design of production facilities and well completions. Well test data acquisition is a fundamental step in field development planning.
3 Test methods
Various types of testing schemes are available, and the selection of a particular method depends on the following factors:
* The type of well to be tested (i.e. oil and/or gas well).
* The amount of time that can realistically be allocated to the test.
* The use to be made of the test result.
3.1 Period 1: Clean-up (partial)
·To establish good communication between the wellbore and reservoir.
·To gain a first impression of well capacity and reservoir fluid composition.
3.2 Period 2: Initial build-up
·To measure the initial reservoir pressure.
·To restore reservoir equilibrium before embarking on the main flow period.
3.3 Period 3: Main flow period (eventually at stabilized conditions)
·To measure the productivity index or better still, establish one point on the inflow performance curve,
Surface samples for laboratory analysis to be taken during the final part of this period.
·To allow determination of skin factor, reservoir transmissibility and boundary effects. Only if flow is stable.
3.4 Period 4: Main build-up period
·To allow determination of skin factor, reservoir transmissibility and boundary effects.
·Measurement of the reservoir pressure may be desired if the initial build-up did not provide a reliable value.
·At the end of this period a gradient survey may render the gradient of the reservoir fluid at reservoir conditions.
3.5 Period 5: Additional flow period at different rates
·To take P.V.T samples (downhole), for oil wells only. To take (additional) surface samples.
3.6 Period 6 to n: Additional flow periods at different rates
·To determine the flow rate dependency of skin factor, GOR, BS & W, sand production etc.
·To further define the inflow performance relationship. Although of interest, period 6 to n are generally not performed when testing oil wells. For gas wells however they are always recommended to assess non-Darcy flow effects (rate dependent skin).
The test design should obviously be related to the test objectives which are shown in bold characters in the preceding text. Although these test objectives are considered to be important during the exploration/appraisal phase of a reservoir, a different set of objectives is conceivable (e.g. additional flow period to determine stable H2S results, reservoir depletion, etc.).