The procedure of Negative Pressure Test or Inflow Test
A retrievable packer with a circulating valve is used for carrying out an inflow test. A short tailpipe is also used in the BHA below the packer. The conceptual steps for an inflow test, which could vary with the well conditions and service tool operational specifications are as follows:
BHA (Bottom Hole Assembly) for an inflow test includes a retrievable packer, a circulating valve, a safety joint, and a short tailpipe below the packer. BHA should be designed in consultation with the service tool provider.
Scrap the casing around the depth where the packer is planned to be set. The scrapping is recommended to ensure good sealing of the packer element. Packer is normally set 50 ft above the area to be tested for integrity.
Run the packer a few feet deeper and then pick up to set at planned depth as per operating procedure recommended by the service tool provider.
Test the packer by pressurizing the drill pipe annulus up to 1,000 psi to ensure that the packer element is properly sealing.
Line up the cementing unit, pressure test lines, and open ports of the circulating valve as per instructions of the service tool provider and circulate one drill pipe volume to make sure that there is homogeneous fluid in the drill pipe.
Displace the drill pipe with the required volume of lighter fluid to achieve pre-determined differential pressure for carrying out a negative pressure test. Sometimes an air cushion is also used for achieving the required differential pressure.
Close ports on the circulating valve and bleed off trapped pressure.
Monitor well and record flow rates to perform the negative pressure test.
Once the test is completed, open circulating valve ports as per instructions of the service tool provider and reverse out 1.5 times drill string volume until the system gets the original weight homogeneous fluid.
Close circulating valve, unseat packer and reverse out drill string capacity.
Flow check and observe the well dead. Pull out and lay down the retrievable packer assembly to conclude the test.
While carrying out the test, flow back volume could increase initially but shows gradual diminishing returns if no leak path exists. The initial increase in flow back could also be seen due to the thermal expansion of the wellbore fluid. Heat transfer between the fluids in different annuli could take longer to achieve temperature equilibrium. This could take a long time before stable conditions are achieved. Engineers and well-site supervisors commonly use the ‘Horner Plot’ to avoid ambiguity and minimize the time for the inflow test. However, a continuous increase in the flow back volume may be a sign of a leak path from the formation into the well and may require terminating the test and killing the well to regain control.
A suitable test packer is selected based on the well environment. BHA configuration and its components are selected based on specific needs and challenges of the inflow test operation. Commonly used retrievable packers are ‘RTTS’ and ‘Champ Packer’ from Halliburton. The ‘Well Commissioner’ from Schlumberger is also used for special applications for carrying out liner clean-out and liner top inflow tests in one run.