Formation testing while drilling: robust and accurate pore pressure measurement
Herein are described methods and systems for characterizing noise and noise level in pressure measurements in a downhole pressure testing operation in formation testing while drilling. The pressure measurements are acquired using a formation testing tool disposed on a drill string. The methods include drilling a borehole, extending the one or more probes into an inner surface of the borehole, performing a pressure testing operation with a first mud pump rate, wherein the pressure testing operation comprises a drawdown period and a build up period, decreasing the first mud pump rate to a second mud pump rate while measuring pressure during the build up period, decreasing the second mud pump rate to a third mud pump rate while measuring pressure during the build up period, and calculating a noise associated with pore pressure measurements during the first pressure testing operation.
1 . A method comprising:
conveying a formation testing tool into a borehole, wherein the formation testing tool comprises:
one or more probes that extend into a formation;
a probe fluid passageway to connect a fluid from the formation through the one or more probes;
a probe pressure sensor disposed on the probe fluid passageway;
a formation testing tool fluid passageway fluidly connected to the probe fluid passageway; and
a tool pressure sensor disposed on the formation testing tool fluid passageway;
moving the formation testing tool to a depth within the borehole;
extending the one or more probes into an inner surface of the borehole;
performing a first pressure testing operation comprising pore pressure measurements using a first mud pump rate, wherein the first pressure testing operation comprises a drawdown period and a build up period;
decreasing the first mud pump rate to a second mud pump rate while measuring pressure during the build up period;
decreasing the second mud pump rate to a third mud pump rate while measuring pressure during the build up period; and
calculating a noise associated with pore pressure measurements during the first pressure testing operation.
2 . The method of claim 1 , wherein the first mud pump rate is from about 5 gallons per minute to 500 gallons per minute.
3 . The method of claim 1 , wherein decreasing the first mud pump rate to the second mud pump rate is performed after the build up period.
4 . The method of claim 1 , wherein the first mud pump rate is slowed down from 5% to 90% to a second mud pump rate.
5 . The method of claim 1 , wherein the second mud pump rate is slowed down from 5% to 90% to a third mud pump rate.
6 . The method of claim 1 , further comprising performing a second pressure testing operation comprising a drawdown period and a build up period and a first mud pump rate, a second mud pump rate, and a third mud pump rate.
7 . The method of claim 6 , wherein the first mud pump rate for the second pressure testing operation is different from the first mud pump rate of the first pressure testing operation.
8 . The method of claim 6 , wherein the second mud pump rate for the second pressure testing operation is different from the second mud pump rate of the first pressure testing operation.
9 . The method of claim 6 , wherein the third mud pump rate for the second pressure testing operation is different from the third mud pump rate of the first pressure testing operation.
10 . The method of claim 1 , wherein calculating the noise is performed after pressure measurements in at least two different sampling zones.
11 . A method comprising:
quantifying a noise in a pressure measurement in a formation testing while drilling comprising:
drilling a borehole using a formation testing tool disposed on a drill string, wherein the formation testing tool disposed on the drill string comprises:
one or more probes that extend into a formation;
a probe fluid passageway to connect a fluid from the formation through the one or more probes;
a probe pressure sensor disposed on the probe fluid passageway;
a formation testing tool fluid passageway fluidly connected to the probe fluid passageway; and
a tool pressure sensor disposed on the formation testing tool fluid passageway;
stopping the drilling at a depth within the borehole;
extending the one or more probes into an inner surface of the borehole;
performing a first pressure testing operation comprising pore pressure measurements with a first mud pump rate, wherein the first pressure testing operation comprises a drawdown period and a build up period;
decreasing the first mud pump rate to a second mud pump rate while measuring pressure during the build up period;
decreasing the second mud pump rate to a third mud pump rate while measuring pressure during the build up period; and
calculating a noise associated with pore pressure measurements during the first pressure testing operation.
12 . The method of claim 11 , wherein the first mud pump rate is from about 5 gallons per minute to 500 gallons per minute.
13 . The method of claim 11 , wherein the first mud pump rate is from about 100 gallons per minute to 300 gallons per minute.
14 . The method of claim 11 , wherein a pulser is stopped during the build up period.
15 . The method of claim 11 , wherein the first mud pump rate is slowed down from 5% to 90% to a second mud pump rate.
16 . The method of claim 11 , wherein the second mud pump rate is slowed down from 5% to 90% to a third mud pump rate.
17 . The method of claim 11 , further comprising performing a second pressure testing operation comprising a drawdown period and a build up period and a first mud pump rate, a second mud pump rate, and a third mud pump rate.
18 . The method of claim 17 , wherein the first mud pump rate for the second pressure testing operation is different from the first mud pump rate of the first pressure testing operation.
19 . The method of claim 17 , wherein the second mud pump rate for the second pressure testing operation is different from the second mud pump rate of the first pressure testing operation.
20 . The method of claim 17 , wherein the third mud pump rate for the second pressure testing operation is different from the third mud pump rate of the first pressure testing operation.