Completion and pressure gauges

Missing downhole shut-in tool

A downhole shut-in valve is recommended not only to minimize wellbore storage, but also to minimize wellbore phase redistribution, reduce vibrations from surface facilities, even reduce the risk of sand production for high permeability reservoirs, etc… It will guarantee the best quality well test data.

Not enough gauges in the tubing

More pressure gauges should be placed in various gauge carriers at specific distances in the tubing. Multiple gauges need to be placed strategically in the wellbore to monitor the fluid density over time and quality check the data. It is also good for redundancy, imagine if all your gauges failed for example due to temperature…

Incorrect positions of the pressure gauges

The location of the gauges are critical and could be a major risk: the entire well test could be misleading and not representative of the reservoir performance. The deepest gauges need to be placed as close as possible to the perforations and real-time gauges like Metrol could be clamped on the OD of the tubing.

Sampling rate not adapted for pressure transient analysis (PTA)

Sometimes the gauge sampling rate is not properly set up. This will have an impact on the interpretation plots. This also applies for production wells, with other problems like the compression algorithm.

DST Equipments for E&A Well Testing

Well test design: 2 PBUs, one after clean-up and another one after main flow

Opportunity to save time

The short 1-2 hour PBU following a very short initial flow could be removed; experience showed that it doesn’t lead to representative data. Also, complex deliverability tests are not recommended in exploration and appraisal stage. A simple flow-after-flow test could be performed at the start of the main flow, once the well is cleaned.

2 PBUs please!

The well test sequence should have 2 important PBU tests, one after clean-up and another one after main flow.

A well test design study will help to make sure the 2 PBU tests are long enough to achieve the subsurface objectives and use the deconvolution technology.
The two PBUs can also help you detect any condensate drop out (or gas breakout) and understand its impact on productivity.

Standard Exploration and Appraisal Well Test

Operations

Stay in control during the well clean-up

During clean-up, start the well at very low choke size, then gradually and slowly increase the choke, taking 30 min to 1 hour per choke size, if needed. Make sure you understand the well behaviour and stay in control.

Optimize the test with access to real-time pressure data

Access to real-time data during the operations will help to take real-time decisions and optimize the test (reduce flaring and shorten the test operations while achieving the objectives with the best quality data). It will also improve safety.

Choke Manifold from the Well Test Simulator

Well test interpretation

Quality check the data

• The data should be quality checked: the different gauges should be compared and all the PBU data should be shown on the same derivative plot.

• It is not recommended to use the pseudo-time for the derivative. This could add some additional features in the derivative that can be mistakenly taken as flow regimes.

Compare the data from different gauges

Find more than 1 solution that can match all the data

• Use more than one reservoir model. Well test analysis is a non-unique process so various reservoir configurations should be investigated.

• The radial composite model shouldn’t be used to characterize the reservoir in exploration and appraisal (unless acidification or multiphase in the reservoir). The radial composite model defines two circular regions around the well of different mobility (KH/μ) and/or storativity (ϕCt H) values. As a result, it does not well represent a geological feature.

• Efforts should be made to have a good quality match of the conventional and deconvolved derivative data. Other plots, such as the superposition plot and the production history plot should also be matched. Several reservoir configurations should be investigated and well matched using all the plots. Then the solutions should be integrated with other sources of subsurface information (seismic, geology, etc…)

• A wrong selection of the radial flow stabilization will result in uncertainty in the initial pressure, skin and permeability values.

Extract more value

• The minimum connected volume, i.e. how much hydrocarbon the test has proven, should be included in the well test results.

• Deconvolution should be the driving tool in well test interpretation. Deconvolution helps to refine initial pressure, provide a better understanding of the test and investigate further away in the reservoir. It could provide evidence for larger connected volume.

Matching all the plots in well test analysis