Pre-salt carbonates in the Pre-Caspian Basin have prolific hydrocarbon-bearing formations onshore and offshore. Recent exploration wells more than 5000 m deep in the region have had limited success, mostly due to challenging conditions for well testing. This paper summarizes key elements of one of the first successful drill stem testing (DST) campaigns in Ansagan Field.
The holistic DST design for Ansagan field addressed perforating methods, brine selection, packer and DST equipments material selection for hostile environment with high-pressure, high-H2S fluids, and corrosive acid to be used for stimulation. The surface equipment design had to reliably control the well, separate fluids, measure flow rates, and dispose hydrocarbons with minimum environmental impact. Surface flow rate measurement combined of a multiphase flowmeter and three-phase separator. Downhole samples were collected with a single-phase reservoir fluid sampler.
Calcium & zinc bromide brine with 1.77-SG density proved effective. The specially modified completion packer design was in the well up to 6 months and was successfully retrieved after the test.
Although perforating in these deep high pressure wells was usually performed on wireline, accurate shock prediction and design adjustments managed the risk for using tubing-conveyed perforating (TCP) to perforate large intervals in underbalance conditions allowing to achieve lower skin, and flow the well immediately.
The DST string enabled performing production logging, downhole fluid sampling, and acid stimulation with coiled tubing. Fluid sampling was performed on wireline to preserve gas condensate components at reservoir conditions. The highly accurate memory gauges, real-time operations, and reservoir engineer's support made it possible to acquire accurate data to characterize the fracture distribution, which is vital information in fractured carbonates.
In summary, three wells have been successfully tested with a total of 16 zones at Ansagan field, achieving all well test objectives.