The procedure of Negative Pressure Test or Inflow Test
A retrievable packer with a circulating valve is used for 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. Scrapping is recommended to ensure good sealing of the packer element. The packer is usually set 50 ft above the area to be tested for integrity.
Run the packer a few feet deeper and then pick it up to set it at the planned depth per the operating procedure recommended by the service tool provider.
Test the packer by pressurizing the drill pipe annulus up to 1,000 psi to ensure the packer element is properly sealing.
Line up the cementing unit, pressure test lines, and open ports of the circulating valve as per the service tool provider's instructions, and circulate one drill pipe volume to ensure homogeneous fluid in the drill pipe.
Displace the drill pipe with the required volume of lighter fluid to achieve pre-determined differential pressure for a negative pressure test. An air cushion is sometimes used to achieve 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 the circulating valve ports as per the service tool provider's instructions and reverse out 1.5 times the drill string volume until the system has homogeneous fluid of the original weight.
Close the circulating valve, unseat the packer, and reverse out the drill string capacity.
Flow check and observe that the well is 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 show 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 signal 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. The BHA configuration and its components are selected based on the specific needs and challenges of the inflow test operation. Commonly used retrievable packers are Halliburton’s RTTS and Champ Packer. The Well Commissioner from Schlumberger is also used for special applications to conduct liner clean-out and top inflow tests in one run.