The testing of gas wells falls into two general classifications depending upon the status (Shut-in or Flowing) of the well prior to each rate of flow. If rates of flow are imposed in succession without allowing a shut-in period between the flow periods, the test is a multipoint type of test. If the well is test in between the various flow periods, the test is isochronal. A multipoint test is sometimes referred to as a three-, four-, or five point test, depending upon the number of rates of flow used in the test. A one-point test usually means that the well was started from shut-in at a predetermined rate of flow and was allowed to produce for an extended period of time. Deliverability tests are usually one-point tests run under specified conditions or tests in which the results are corrected to correspond to a specific set of conditions.
Multipoint Test (Flow After Flow «FAF») – Natural Gas wells
Starting with a shut-in well, a series of flow rates-usually in increasing sequence-are imposed on the well at fixed time intervals. The objective of the test is to determine the open-flow potential of a well or to determine the exponent (slope) to be used with a one-point test. The exponent from the original multipoint test may be used with subsequent one-point tests to determine the open-flow potential for a well as the shut-in pressure decreases with production and time.
One-Point Test – Natural Gas wells:
Starting from shut-in, the well is opened to flow with producing pressures, temperatures, and rates of flow measured at specified time intervals. The duration of flow is one to three days or longer.
The one-point test is intended to include deliverability testing and tests to determine the maximum allowable or the contractual maximum quantity for a given well. The test, usually with production into a gathering system, extends over a period of 72 hours, with the production during the last 24 hours period being taken as the test rate. Regardless of the information required by the regulatory body or the gas sales contract , the engineers should use the one-point test as an opportunity to gain useful performance information on the well.
Isochronal Test – Natural Gas wells:
The Isochronal test consists of a series of one-point tests, each starting with the well shut-in and with the shut-in pressure constant or nearly constant with time. Producing pressures, temperatures and rates of flow are measured and recorded at specified time intervals after the well is opened to flow. The Isochornal Test is a series of one point tests, each of which starts with well shut-in and the shut-in pressure stabilized or built up to 95% of the stabilized pressure. After flow is started in the well, flowing pressures, temperatures, and instantaneous flow rates are measured at specified time intervals.
Convinient time intervals are 0.5, 1, 2, 3, 6, 24 hr, etc. The isochronal test usually consists of 2 to 4 one point tests with a minimum duration of 3 hours for each rate of flow. Occasionally, one of the series may be extended to 24 hours or more. Each shut-in pressure should be at least 95% of the highest shut-in pressure observed on the well during the series of one-point tests.
Comments on Isochronal Testing:
The advantage of isochronal testing gas wells is that it provides a method
for eliminating the complicated pressure gradients in the reservoir that so often confuse the results of multipoint tests. It thereby permits the determination of the true value of the exponent «n» for the performance curves.
The results of an isochronal test are good measures of well performance over a period of years.
PREPARATION OF A NATURAL GAS WELL FOR TESTING
The wellbore should be cleaned of liquids by flowing at a high rate to a pipeline for a period of 24 hours. If the well does not have a pipeline connection, it may be necessary to produce the well to the atmosphere for a short time if such action is considered safe. Extra precautions should be tanken on new wells to remove drilling mud, solids, and stimulation fluids from the wellbore. If the well has a low capacity to produce gas, extra care should be taken in swabbing the fluids from the wellbore during completion. The time and expense involved in cleaning the well can be kept to a minimum by installing acarefully sized tubing string in the well during completion. The well should be shut in for an appropriate period of time equalize the reservoir pressure around the wellbore. Even with the largest capacity wells, the period between completion and cleanup and connection to the pipeline should be used as the shut-in period.
During the shut-in period, gas measurement equipment should be prepared for use. If the gas is to be measured by an orifice meter, the meter should be calibrated, the diameters and condition of the run and plate should be verified and recorded, and the differential pen should be zeroed in accordance with good meter practice. If a separator is used, the flow rate should be controlled by a production choke, and pressure should be maintained on the spearator by the critical flow prover or a back-pressure regulator when an orifice meter is used. If a separator is not used, the rate of flow can be controlled at the wellhead by the critical flow prover.
- The tester should understand the purpose of testing and know what information will be recorded.
- All surface pressures at the wellhead should be measured by dead weight gauge.
- The well should be conditioned for testing by removing liquids from the wellbore prior to measuring the shut-in pressure.
- Multipoint tests should be run in increasing rate of flow sequence, and each flow period should last the same period of time. During each flow rate, wellhead flowing pressures and flow rate data should be recorded after 15-min period to permit a determination of the degree of stabilization.
- The rate of liquid production should be observed at frequent intervals during testing to ensure good liquid-gas ratio information.
- Multipoint tests on deep, large-capacity wells in reservoirs with high temperature should be preceded by a preflow test at a high rate to ensure temperature stabilization.
DEFINITONS OF TERMS
- Open-flow potential is the flow rate that would be obtained if the bottom-hole pressure opposite the sand face were reduced to zero pressure. The open-flow potential is independent of well equipment. ( the time dependency aspect should be kept in mind).
- Deliverability is the flowrate from a well against a specified pressure (usually a working pressure at the wellhead) after a specified period of time following a specified shut-in period.
- Official tests are tests required by regulatory agencies for alloweable purposes. The tester should obtain the test and calculation procedures from the appropriate regulatory agency before testing a well for allowable purposes. No attempt will be made outline official testing.