Early kick detection in an oil and gas well

1. A method of detecting a gas influx event in a borehole fluid during a drilling operation, comprising:

generating acoustic pulses over a time period using an acoustic transducer disposed in a borehole;

obtaining measurements of acoustic velocity of the acoustic pulses through borehole fluid over the time period at an acoustic sensor disposed in a borehole;

obtaining measurements of temperature of the borehole fluid over the time period at a temperature sensor disposed in the borehole;

determining a difference between a curve derived from the measurements of acoustic velocity and a curve derived from the measurements of temperature over the time period; and

detecting the gas influx event when the acoustic velocity changes over the time period and the difference between the curve derived from the measurements of acoustic velocity and the curve derived from measurements of temperature is substantially constant during the time period.

2. The method of claim 1 , wherein obtaining the measurements of acoustic velocity of the borehole fluid further comprises:

generating the acoustic pulses in the borehole fluid at a downhole location;

reflecting the generated acoustic pulses from at least two reflecting surfaces of a stepped reflector in the fluid to provide at least two reflected pulses; and

determining the acoustic velocity of the borehole fluid from a difference in arrival time of the at least two reflected pulses.

3. The method of claim 1 , further comprising determining a difference between a logarithmic plot of the acoustic velocity over the time period to a logarithmic plot of the temperature versus time over the time period.

4. The method of claim 1 , wherein the acoustic sensor and the temperature sensor are disposed in an annulus between a drill string and a borehole wall.

5. The method of claim 1 , further comprising disposing the temperature sensor at one of: (i) in a drill bit; (ii) proximate the drill bit; and (iii) proximate a bottomhole assembly.

6. The method of claim 1 , further comprising detecting the gas influx event when a change in the acoustic velocity exceeds a selected acoustic velocity threshold and a change in the temperature exceeds a selected temperature threshold.

7. The method of claim 6 , wherein the change in the acoustic velocity exceeds the selected acoustic velocity threshold and the change in the temperature exceeds the selected temperature threshold at substantially a same time.

8. The method of claim 6 , further comprising comparing a first derivative of the acoustic velocity to a selected threshold value and comparing a first derivative of the temperature to a selected threshold value.

9. An apparatus for detecting a gas influx event in a borehole fluid during a drilling operation, comprising:

an acoustic transducer disposed in the borehole configured to generate acoustic pulses in the borehole fluid over a time period and to obtain a measurement of an acoustic velocity of the acoustic pulses in the borehole fluid over the time period;

a temperature sensor configured to obtain nail measurements of a temperature of the borehole fluid over the time period; and

a processor configured to:

determine a difference between a curve derived from the measurements of acoustic velocity and a curve derived from the measurements of temperature over the time period, and

detect the gas influx event when the acoustic velocity changes over the time period and the difference between the curve derived from the measurements of acoustic velocity and the curve derived from measurements of temperature is substantially constant during the time period.

10. The apparatus of claim 9 , wherein the acoustic transducer generates the acoustic pulses in the borehole fluid toward a stepped reflector configured to provide at least two reflections of the generated acoustic pulse to the acoustic transducer.

11. The apparatus of claim 9 , wherein the processor is further configured to determine a difference between a logarithmic plot of the acoustic velocity over the time period to a logarithmic plot of the temperature over the time period.

12. The apparatus of claim 9 , the acoustic transducer and the temperature sensor are disposed in an annulus between the drill string and borehole wall.

13. The apparatus of claim 9 , wherein the temperature sensor is disposed at one of: (i) in a drill bit; (ii) proximate the drill bit; and (iii) proximate a bottomhole assembly.

14. The apparatus of claim 9 , wherein the processor is further configured to detect the gas influx event when a change in the acoustic velocity exceeds a selected acoustic velocity threshold and a change in the temperature exceeds a selected temperature threshold.

15. The apparatus of claim 14 , wherein the processor is further configured to detect the gas influx event when the change in the acoustic velocity exceeds the selected acoustic velocity threshold and a change in the temperature exceeds the selected temperature threshold at substantially a same time.

16. The apparatus of claim 14 , wherein the processor is further configured to compare a first derivative of the acoustic velocity to a selected threshold value and compare a first derivative of the temperature to a selected threshold value.

17. A non-transitory computer-readable medium having a set of instruction stored thereon that when read by a processor enable the processor to perform a method of detecting a gas influx event in a borehole fluid during a drilling operation, the method comprising:

receiving measurements of an acoustic velocity of a borehole fluid from an acoustic transducer disposed in a borehole resulting from a reflection of acoustic pulses generated by the transducer in the borehole fluid over a time period;

receiving measurements of temperature of the borehole fluid over the time period from a temperature sensor disposed in the borehole;

determining a difference between a curve derived from the measurements of acoustic velocity and a curve derived from the measurements of temperature over the time period; and

detecting the gas influx event when the acoustic velocity changes over the time period and the difference between the curve derived from the measurements of acoustic velocity and the curve derived from measurements of temperature is substantially constant during the time period.