Downhole Gauges Save Rig Time in Extended Well Tests and Provide Unique Long Term Pressure Data

In the 3rd quarter of 2006, Qatar Shell drilled and tested the second of two vertical appraisal wells in the North Field of Qatar as part of their world scale Pearl Gas To Liquids development. While the primary well objectives were linked to data acquisition, it was also required to suspend the well and convert it to a development producer at a later date.

Well test deliverables included establishing; basic reservoir and fluid properties and well deliverability. These "short term" deliverables had to be balanced with longer term objectives of determining the extent of high permeability layers, depletion within the reservoir and re-establishing the well's productivity on re-entry. The opportunity to drill the well early, a desire to reduce costs and a tight schedule drove the development of a new extended well test methodology and some novel operational procedures.

Originally 8 sequential tests were planned but after optimisation, 5 were conducted with 1 being a commingled test of 2 layers. The approach for each of the tests was identical in that the interval being tested was isolated during the final build-up period. This allowed test data and the test string to be retrieved to surface; the next shallower test to commence while build up data was still being recorded on the interval just suspended. This methodology resulted in significant rig timesavings when compared with a conventional test approach where the rig would have effectively been "tied up" during each build up period.

To achieve this, the downhole test set up comprised of a series of stacked retrievable packers with tail pipes separating successive test intervals. The tailpipe of each packer was equipped with wireline nipple profiles and an external gauge carrier with multiple gauges featuring acoustic telemetry for remote data retrieval. This arrangement offered flexibility in terms of the number of intervals to be tested, the ability to interrogate gauges between tests to download build-up data, re-program sample rates and to modify operations in light of immediate data analyses.

All pre suspension test objectives were achieved on schedule and the well was suspended at the beginning of 2007. In January 2009, the well was re-entered and all of the packers retrieved complete with pressure gauges. The well was then completed and it's pre-suspension production performance restored.

The extended well test method developed resulted in over thirty days rig time being saved when compared to a conventional test approach. Gauge data recovered during the suspension period was limited due to unexpected problems with the gauges. Data recovered however covered a sufficiently broad time span to yield a valuable insight into the impact of production from neighbouring projects. The data was also used to condition the field dynamic model and has proven invaluable in improving understanding of the local geological features and pressure behaviour.