Surface recorders can be used along with bottomhole recorders to eliminate the guesswork in determining when a well test can be terminated when real-time surface readout is not available.
Typically bottomhole pressure recorders are used to monitor pressure buildup and falloff tests, which leaves no way to know how the test is progressing. The well remains shut-in for a predetermined amount of time, at which point the recorders are recovered and downloaded and a pressure derivative is generated to establish flow regimes.
If surface recorders are used along with bottomhole recorders, downhole pressures can be calculated and reviewed to determine whether test objectives have been met. A decision can then be made to extend the buildup / falloff test or to terminate the test and retrieve bottomhole recorders. Extending the test may be necessary if the pressure derivative is still transitioning out of wellbore storage into radial flow, to confirm a reservoir heterogeneity, or simply to reduce the error bar on pressure extrapolation. Reviewing pressure data prior to making the call to terminate the test is always a good idea.
Conversely, time and money can be saved if tests can be terminated early. When completing and testing multiple zones, rig time can be saved if recorders can be pulled ahead of schedule once test objectives are met. Sometimes tests are compromised by a leaking packer or a leak at the wellhead, by wellbore effects or by interference effects from offset producers or injectors. If this is the case, there is no point continuing the test and having surface recorders may help save valuable time.
Typical Use for Surface Recorders
Surface recorders are frequently used when completing and testing multiple zones and uphole operations (i.e. rig standby) are waiting for the test to end. Also, surface recorders are frequently used to monitor mini-frac tests. Mini-fracs are conducted in very low permeability formations, which makes it difficult to guess when reservoir dominated (i.e. radial flow) has been reached.
Example 1: Flow and Buildup Test
A cased hole DST was conducted on a productive zone prior to suspending the zone below a bridge plug and moving uphole to complete and test the second zone. In addition to using downhole pressure gauges to monitor bottomhole pressures, surface gauges were used to record wellhead tubing and casing pressures. Bottomhole pressures were calculated daily and a pressure derivative plot was generated to determine whether the buildup could be terminated ahead of the planned seven days shut-in. It was observed that the pressure derivative was transitioning into radial flow after approximately four days of shut-in.
A decision was made to terminate the test after 93 hours of shut-in instead of the planned 168 hours, saving three days of rig time. When the pressure recorders were retrieved and downloaded, it was observed that the calculated bottomhole pressures were in good agreement with the measured bottomhole pressures.
Wellhead falloff pressures were monitored during a mini-frac test. Bottomhole pressures were calculated every few days to monitor the progress of the falloff.
As wellbore storage dissipates, the pressure derivative transitions into a negative unit slope, indicative of radial flow. The test was terminated earlier than planned after observing the development of reservoir dominated radial flow. Analysis was then conducted using bottomhole gauge data and the development of radial flow was confirmed.
Issues with Using Surface Recorders
- Freezing Surface Lines: If surface recorders are used, steps must be taken to prevent surface lines from freezing.
- Ambient Temperature Effects: Surface recorders should be isolated/insulated to reduce ambient temperature effects.
- Vacuum at the Wellhead: Surface recorders can only be used if the wellhead will not go on vacuum during the test (i.e. hydrostatic pressure of the fluid column will not exceed the reservoir pressure).
- Fluid Migration: During mini-frac falloffs in higher permeability formations, reservoir fluid (i.e. oil or gas) can enter the wellbore and displace water into the formation. This results in a changing wellbore pressure gradient and cannot be accounted for. Bottomhole pressure cannot be reliably calculated in these cases. Caution should be exercised when working with surface data if conditions for fluid migration exist.
- Changing Fluid Level: Bottomhole pressures cannot be reliably calculated if a fluid level is present in the wellbore. Fluid level will typically be changing, adding complexity. Utilizing acoustic well sounder equipment to collect fluid level data may be considered.
Surface pressure recorders allow tracking the progress of falloff or buildup tests. Tests can be terminated early if objectives have been met or extended as necessary. Monitoring tests at surface works well when bottomhole pressures can be easily calculated, i.e. when the wellbore contains single phase fluid to surface. This is the case with flow and buildup tests on gas wells and with mini-frac tests pumped with water. Surface gauges should be used to monitor the progress of the test only and are not meant as a replacement for bottomhole gauges.
Vadim Milovanov is a Consulting Principal at IHS Markit.
Posted 10 August 2017