Method for analysing pore pressure in shale formations

1. A method for analyzing pore pressure in a subterranean organic-rich or gas- and/or oil-bearing shale formation comprising:

gathering from a well formed in a subterranean formation (i) resistivity log data and (ii) one or more types of log data indicative of porosity selected from the group consisting of: sonic slowness and bulk density;

storing the resistivity log data and the one or more types of log data indicative of porosity on a non-transitory computer storage medium;

combining the resistivity log data with the one or more types of log data indicative of porosity, via a computer processor to generate a combined log data;

generating a combined log data graph for a defined depth window of the subterranean formation, wherein the combined log data graph provides a more accurate pore pressure as compared to either the resistivity log data or the one or more types of log data indicative of porosity;

calculating an estimated pore pressure for a given depth in the shale formation, wherein the estimated pore pressure is given by

P combined =( a* ( Sv− ( Sv−Ph )*( R 0/ Rn )x+ b* ( Sv− ( Sv−Ph )*( ΔTn/ΔT 0) y ))/( a+b )

where:

a, b are constants which can be varied;

Sv is vertical stress at current depth;

P combined is pore pressure calculated from both resistivity and sonic curves;

Ph is hydrostatic pressure;

R0, ΔT0 is based line (trend) resistivity and sonic interval transit time at current depth;

Rn, ΔTn observed resistivity and sonic interval transit time at current depth; and

x, y are Eaton exponents; and

managing a back-pressure on a drilling mud system to maintain the back-pressure at or above the estimated pore pressure.

2. The method according to claim 1 , wherein the estimated pore pressure is derived using an Eaton formula.

3. The method according to claim 1 , wherein:

a first intermediate value for pore pressure is derived separately from the resistivity log data;

one or more further intermediate values for pore pressure are derived separately from porosity-indicative log data; and

the first intermediate value and the one or more further intermediate values are combined to derive the estimated pore pressure.

4. The method according to claim 3 , wherein the first intermediate value for pore pressure and further intermediate values for pore pressure are combined using a weighted mean, where the weighted mean includes two weighting values (a, b) that may be selected in advance based on rock type.

5. The method according to claim 1 wherein the resistivity log data and the one or more types of log data indicative of porosity are combined using a weighted mean, where the weighted mean includes two weighting values (a, b) that may be selected in advance based on rock type.

6. The method according to claim 1 , wherein the drilling mud system manages the back-pressure by adjusting a mud pressure via one or more pumps.

7. The system according to claim 6 , wherein the drilling mud system manages the back-pressure by adjusting a mud density.

8. The method according to claim 1 , wherein the drilling mud system manages the back-pressure by adjusting a mud density.

9. A system for analyzing pore pressure in a subterranean formation comprising:

a drill string disposed in a well formed in a subterranean formation, the drill string operable to gather resistivity log data and porosity log data during operation;

a non-transitory computer storage medium operable to store the resistivity log data and the porosity log data; and

a computer processor having instructions stored on the non-transitory computer storage medium and executable by the computer processor to:

combine the resistivity log data with the porosity data to generate a combined log data;

generating a combined log data graph for a predefined depth window of the subterranean formation,

calculate an estimated pore pressure for a given depth in a shale formation,

wherein the estimated pore pressure is given by

P combined =( a* ( S v −( S v −P n )*( R 0 /R n )x+ b* ( S v ( S v −P h )*( ΔT n / ΔT 0 ) y ))/( a+b )

where:

a, b, are constants which can be varied;

S v is vertical stress at current depth;

P combined is pore pressure calculated from both resistivity and sonic curves;

P h is hydrostatic pressure;

R 0 , ΔT 0 is based line (trend) resistivity and sonic interval transit time at current depth;

R n , ΔT n is observed resistivity and sonic interval transit time at current depth; and

x, y are Eaton exponents; and

manage a back-pressure on a drilling mud system to maintain the back-pressure at or above the estimated pore pressure.

10. The system according to claim 9 , wherein the porosity log data is selected from the group consisting of: sonic slowness and bulk density.

11. The system according to claim 9 , wherein the estimated pore pressure is derived using an Eaton formula.

12. The system according to claim 9 , wherein:

a first intermediate value for pore pressure is derived separately from the resistivity log data;

one or more further intermediate values for pore pressure are derived separately from porosity-indicative log data; and

the first intermediate value and the one or more further intermediate values are combined to derive the estimated pore pressure.

13. The system according to claim 12 , wherein the first intermediate value for pore pressure and the one or more further intermediate values for pore pressure are combined using a weighted mean, where the weighted mean includes two weighting values (a, b) may be selected in advance based on rock type.

14. The system according to claim 9 , wherein the resistivity log data and the one or more types of log data indicative of porosity are combined using a weighted mean, where the weighted mean includes two weighting values (a, b) may be selected in advance based on rock type.

15. The system according to claim 9 , wherein the drilling mud system manages the back-pressure by adjusting a mud pressure via one or more pumps.