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A method of testing an earth formation can include incrementally opening a choke while drilling into the formation is ceased, thereby reducing pressure in a wellbore, and detecting an influx into the wellbore due to the reducing pressure in the wellbore. Another method of testing an earth formation

A method of testing an earth formation can include incrementally opening a choke while drilling into the formation is ceased, thereby reducing pressure in a wellbore, and detecting an influx into the wellbore due to the reducing pressure in the wellbore. Another method of testing an earth formation can include drilling into the formation, with an annulus between a drill string and a wellbore being pressure isolated from atmosphere, then incrementally opening a choke while drilling is ceased, thereby reducing pressure in the wellbore, detecting an influx into the wellbore due to the reducing pressure in the wellbore, and determining approximate formation pore pressure as pressure in the wellbore when the influx is detected. Drilling fluid may or may not flow through the drill string when the influx is detected. A downhole pressure sensor can be used to verify pressure in the wellbore.

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1. A method of testing an earth formation, the method comprising: drilling a wellbore into the formation;circulating drilling fluid into the wellbore;then ceasing drilling the wellbore and ceasing circulation of the drilling fluid;then incrementally opening a choke while drilling the wellbore is cea

1. A method of testing an earth formation, the method comprising: drilling a wellbore into the formation;circulating drilling fluid into the wellbore;then ceasing drilling the wellbore and ceasing circulation of the drilling fluid;then incrementally opening a choke while drilling the wellbore is ceased and while circulation of the drilling fluid into the wellbore is ceased, thereby reducing pressure in the wellbore; anddetecting an influx into the wellbore due to the reducing pressure in the wellbore. 2. The method of claim 1, further comprising resuming circulation of the drilling fluid into the wellbore; and then verifying the pressure in the wellbore with at least one pressure sensor in the wellbore. 3. A method of testing an earth formation, the method comprising: drilling into the formation, with an annulus between a drill string and a wellbore being pressure isolated from atmosphere;ceasing circulation of drilling fluid into the wellbore;then incrementally opening a choke, thereby reducing pressure in the wellbore;then detecting an influx into the wellbore due to the reduced pressure in the wellbore while circulation is ceased; anddetermining approximate formation pore pressure as pressure in the wellbore when the influx is detected. 4. The method of claim 3, wherein incrementally opening the choke is performed multiple times. 5. The method of claim 4, wherein incrementally opening the choke ceases when the influx is detected. 6. The method of claim 3, further comprising resuming circulation of the drilling fluid into the wellbore; and then verifying the pressure in the wellbore with at least one pressure sensor in the wellbore. 7. The method of claim 3, further comprising verifying the pressure in the wellbore with at least one pressure sensor in the wellbore. 8. The method of claim 3, wherein detecting an influx comprises detecting how fluid flows out of the wellbore. 9. The method of claim 3, wherein determining approximate formation pore pressure comprises summing pressure in the annulus near the surface with hydrostatic pressure in the wellbore. 10. A method of testing an earth formation, the method comprising: drilling into the formation, with an annulus between a drill string and a wellbore being pressure isolated from atmosphere;circulating drilling fluid into the wellbore;then ceasing drilling the wellbore and ceasing circulation of drilling fluid into the wellbore;then incrementally opening a choke while drilling is ceased, thereby reducing pressure in the wellbore;detecting an influx into the wellbore due to the reducing pressure in the wellbore; anddetermining approximate formation pore pressure as pressure in the wellbore when the influx is detected. 11. The method of claim 10, further comprising resuming circulation of the drilling fluid into the wellbore; and then verifying the pressure in the wellbore with at least one pressure sensor in the wellbore.