Using transient stresses to monitor poroelastic and stress conditions in CO 2 reservoirs

Size: px

Start display at page:

Download "Using transient stresses to monitor poroelastic and stress conditions in CO 2 reservoirs"

Moses Roberts
5 years ago
Views:

1 Using transient stresses to monitor poroelastic and stress conditions in CO 2 reservoirs Andrew A. Delorey and Paul A. Johnson July 6,

2 Stress, Pore Pressure, and Poroelastic Behavior Induced seismicity occurs when effective normal stress is reduced by increasing pore pressure, releasing preexisting stress in the rock. Coupling between pore pressure and stress in the rock matrix is fundamental for understanding the induced seismicity risk and reservoir integrity. 2

3 Stress, Pore Pressure, and Poroelastic Behavior In a reservoir environment pore pressures are perturbed and we want to know what stress is transmitted to the rock matrix. We want some predictive capability to know when faults become critically stressed and induced seismicity initiate or increase. We use naturally occurring transient stresses to reveal stress conditions, and coupling between pore pressure and stress. 3

4 Transient Stresses in the Earth The Earth has a response to seismic waves. van der Elst et al.,

5 Transient Stresses in the Earth The Earth has a response to Earth tides. Thomas et al., [2012] 5

6 Poroelastic Parameters In an undrained isotropic system (triaxial system, σ 2 =σ 3 [Lockner and Stanchits, 2002]) dp = Bdσ m + ηdσ d B = Relates changes in pore pressure to changes in normal stress (Skempton s Coefficient B). η = Relates changes in pore pressure to changes in deviatoric stress which contains shear stress (related to Skempton s Coefficient A). 6

7 Poroelastic Parameters Hart [2000] Lockner and Stanchits [2002] 7

8 Earth Tides SPOTL, Agnew 8

9 What can we learn? Amplitude of tidal modulation of seismicity may infer effective normal stress and pore pressure [Dieterich 2007 and Thomas et al., 2012]. R(ϕ) = r * exp[ (C * cosϕ) / (a * σ n )] Phase of correlation may infer the nucleation time of earthquakes [Beeler 2003]. 9

10 What can we learn? Increasing tidal modulation of seismicity may forecast a large earthquake [Tanaka 2010, 2012]. 10

11 Tidal Stresses Tectonic environment: Parkfield, California Demonstrates both fluid dominated and friction dominated environment Reservoir environment: Oklahoma 11

12 Parkfield, California 12

13 Poroelastic Systems Undrained Anisotropic, B typically ~1 C= τ + μ(1 - B) σ n Undrained Isotropic σ n > σ m C= τ + μ( σ n - B σ m ) μ=0.5 μ=0.5 13

14 Phase correlation τ < σ m < σ n σ m, σ n are in phase τ is out of phase with σ m, σ n 14

15 Tidal Influence on Seismicity 15

16 Stress Shadows 16

17 Upper Versus Middle Crust Upper Crust Peak seismicity correlated with Normal Stress Middle Crust Peak seismicity correlated with Shear Stress LFEs [Thomas et al., 2012] 17

18 Upper Versus Middle Crust Middle Crust Seismicity (LFEs) correlated with Shear Stress τ > μ(1 - B) σ B must be highly anisotropic B must be close to 1 Anisotropic C= τ+ μ(1 - B) σ Thomas et al., [2012] 18

19 Upper Versus Middle Crust Upper Crust Seismicity correlated with Normal Stress μ( σ - B σ m ) > τ μ(1 - B) σ > τ??? B cannot be highly anisotropic Isotropic C= τ+ μ( σ - B σ m ) 19

20 Oklahoma Earthquakes not necessarily on the same fault or have the same fault orientation. On the fault plane for the 2011 Prague earthquake tidal normal and shear stress are in phase so no distinction can be made. Leonard Geophysical Laboratory 20

21 Tidal Triggering in Oklahoma 17 Months prior to 2011 M5.6 Prague Earthquake Correlation increases as Prague earthquake is approached. 21

22 Conclusions The Earth responds to transient stresses in both tectonic and reservoir environments. A time history of Earth s response to transient stresses may reveal evolving stress and porepressure conditions. Tidal stresses can be used to infer poroelastic behavior and stress conditions. 22

23 Transient Stresses in the Earth The Earth has a response to Earth tides and atmospheric pressure. Rojstaczer and Riley [1990] 23

24 Poroelastic Parameters Lockner and Stanchits [2002] 24

Laboratory triggering of stick-slip events by oscillatory loading in the presence of pore fluid with implications for physics of tectonic tremor

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi:10.1029/2012jb009452, 2012 Laboratory triggering of stick-slip events by oscillatory loading in the presence of pore fluid with implications for physics