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Well Test - Questions and Answers
Well test information is second only to production data in importance for the prudent management of oil or gas reservoirs. As such, well testing is an integral part of the overall production and depletion strategy of a reservoir. The lowest costs and the most benefit are realized when an appropriate number of high quality tests are run throughout the producing life of the reservoir.
General Information
Q: What is the required format for submissions?
A: The results of all well tests conducted must be submitted to the EUB, in electronic format, as per section 11.120 of the Oil and Gas Conservation Regulations, and in Appendix A: WTC PAS File Submission Formats, Business Rules, and Implications for Noncompliance.
Q: What are acceptable test standards?
A: The manner in which tests/surveys are conducted and reported is always critical to the value of the data obtained. These are defined for the ERCB in Directive 40, Appendix A: WTC PAS File Submission Formats, Business Rules, and Implications for Noncompliance.
Q: What are the deadlines for submission?
A: Data must be submitted based on the following schedules:
- All pressure and deliverability tests must be submitted within 90 days of completing the fieldwork, including reporting of volumes and methods produced during cleanup and testing.
- All DSTs must be submitted within 30 days of the well’s finished drilling date, including misruns.
- All gas and fluid analysis must be submitted within 45 days of the completion of the test.
- All volumes produced, whether flared, vented or collected (in-line) must also be reported through the Petroleum Registry of Alberta.
Initial Pressure Tests
Q: Why is an initial pressure test required?
A: An accurate initial pressure is probably the most important pressure taken in a well. It determines the initial pool pressure in exploratory wells, it helps delineate pools in development wells, and it can show the drainage and recovery efficiency in infill wells. Without this pressure, subsequent pressures may be of limited value.
Q: When are initial pressure testing requirements submitted?
A: Initial subsurface pressures are required on productive oil and gas wells:
- Gas Wells - on all productive wells, within the first three months of production (one well per pool per section),
- Oil Wells - on all productive exploratory, discovery, development or step-out wells; prior to any sales or production, other than test production (one well per pool per quarter section).
Drill Stem Tests
Q: When do Drill Stem Tests (DSTs) need to be submitted?
A: All DSTs conducted, including misruns, must be submitted to the EUB in DST.PAS electronic format, within 30 days of the well’s finished drilling date.
Fluid Analysis
Q: What Fluid Analyses need to be submitted?
A: All fluid analyses conducted on samples gathered at a well which are representative of the formation (not mixed stream), must be submitted.
Further, gas and/or fluid analyses are required in conjunction with the following tests:
- DSTs conducted on wells outside of existing pools, as per the current EUB pool order
- fluid analysis is required if fluid is recovered during the test
- gas analysis is required if gas to surface during the test
- All deliverability tests require gas analysis for the fluid analysis correlation and must therefore be submitted with an AOF test on all wells drilled outside of existing pools, as per the current EUB pool order.
Annual Surveys
Q: What's the deadline for Annual Pressure Surveys?
A: Annual pressure surveys must be conducted by year end (December 31) for oil and gas pools, as specified in the annual survey schedules:
- Survey 25 per cent of the producing well count in oil pools, based on quarter section spacing (e.g., approximately one survey per pool per productive section).
- Survey 25 per cent of the producing well count in gas pools, based on one section spacing.
Deliverability Tests
Q: What's a deliverability test?
A: A deliverability test is a test to predict the absolute open flow potential (AOFP) of a well, and its deliverability potential under various pipeline backpressures. A deliverability relationship is needed because a gas well may not be producing at capacity. A gas well’s deliverability is a function of wellbore configuration and gathering system back pressure, and requires a stabilized flow rate. A stabilized rate is required to be a calculated value, based on the time to pseudo steady state. This calculation corrects the actual extended test rate to a lower estimated stabilized rate. Higher permeability reservoirs will have very little correction to stabilize, where lower permeability reservoirs will have a large correction. Although the time to pseudo steady state varies with the well geometry and reservoir shape, one can assume a well in the center of a one-section drainage area for a gas well; or if the data or mapping suggests a different drainage area, adjustments must be made as indicated in section 5 of EUB Directive 034: Gas Well Testing Theory and Practice.
Q: What are the main types of deliverability tests?
A: The main types of deliverability tests used today are:
- Flow After Flow Test: requires a static reservoir pressure and stabilization of three to four flow rates. This test provides good radius of investigation, but often results in a lengthy test, resulting in excessive flaring of gas. For this reason, this test is best for use in high permeability reservoirs that stabilize quickly otherwise serious consideration should be given to testing in-line.
- Isochronal Test: requires a static reservoir pressure, a flow period of fixed duration, followed by shut-in until pressure stabilizes again. This sequence of flow and build-up to stabilized pressure is repeated with only the final or extended flow rate required to stabilize. This test is still quite lengthy, and again best suited to high permeability reservoirs. Modified Isochronal Test: requires a static reservoir pressure, then flow and shut-in periods of equal duration. This method was developed for testing tight reservoirs, but is often used today on high volume, tubing restricted and/or partially penetrated wells with fair to good permeability.
- Single Point Test: requires a stabilized rate and flowing pressure measured before the well is shut in and built up to a stabilized reservoir pressure. This test is widely used for deliverability tests where the turbulence factor is known; usually for subsequent tests on a well, for initial tests in a relatively mature pool, or where deliverability may be poor or flow conditions are predetermined by pipeline or plant restrictions. A build-up test conducted with any type of deliverability test, will provide information on current reservoir pressure, permeability, formation flow capacity, apparent skin and reserves should depletion be detected. A multi-rate deliverability test will indicate the effect of pressure loss due to turbulence.