Passive seismic analysis for reservoir
monitoring
September 24, 2010
Capo Caccia, Sardinia, Italy
D. Gei, L. Eisner, P. Suhadolc
Outline
•
Hydraulic stimulation of reservoirs
•
Passive seismic monitoring
•
Surface star array data: some examples
•
Focal mechanism inversion of microseismic events
•
P-wave traveltime inversion for VTI media
Hydraulic stimulation of reservoirs
Hydraulic stimulation is a technique to induce fractures in hydrocarbon and
geothermal reservoirs.
• It is injection of fluids under high pressure in order to overcome minimum stress
and open a hydraulic fractures, either by opening existing fractures or producing
new ones.
• It increases the permeability of the rock from microdarcy to millidarcy range.
• The fluid injected into the formation is typically composed of brine (95%),
additives, proppant (e.g. resin-coated sand, ceramic materials).
• The stimulated volume can extend several hundred meters around the well. The
dimensions, extent, and geometry of the induced fractures are controlled by
pump rate, pressure, and viscosity of the fracturing fluid.
• Reservoir hydraulic stimulations usually induce (significant) microseismic
activity.
Perforation shots
Perforation shots serve to connect wellbore and formation through opening in casing
Heel
Low permeability,
hydrocarbon-rich formation
Toe
Stage 2
Stage 1
Perforation shots
(picture after API Guidance Document HF1, Hydraulic Fracturing Operations – Well Construction and Integrity Guidelines,
First Edition, October 2009)
Hydraulic stimulation
Fluid injection
Stage 2
Stage 1
Microseismic events
(picture after API Guidance Document HF1, Hydraulic Fracturing Operations – Well Construction and Integrity Guidelines, Fisrt Edition,
October 2009)
Passive seismic for reservoir monitoring
Passive seismic monitoring of reservoirs consists in “listening” to the
subsoil during oilfield operations (e.g. production, hydraulic stimulation,
CO2 injection).
Location of events and clustering
Map fracture system
Cap rock integrity
Fault mapping (reservoir compartmentalization)
Focal mechanisms
Anisotropy analysis (P and S waves)
Fracture characterization
Fracture orientation
Fracture density and aspect ratio
Microseismic signals can be recorded by downhole sensors or
surface star arrays of receivers.
Monitoring well
Treatment well
Depth (kft)
~300 m
Microseisms
Treatment wells
Northing (kft)
(from Warpinski, 2009)
Vertical array of 3C geophones (8-12 receivers)
in a monitoring well.
Easting (kft)
Hundreds of receivers disposed in a
star shaped array
Hmax
Treatment well
(deviated)
(picture from www.microseismic.com)
Data example
Microseismic event 1
Perforation shot
Microseismic event 1
Lines 1 - 10
Processing performed
Bandpass freq filter
(2,7,60,70) Hz
Agc for visualization
Microseismic event 1
Lines 1 - 10
Processing performed
Bandpass freq filter
(2,7,60,70) Hz
Agc for visualization
Polarity flip
Polarity flip
Polarity flip
Polarity flip
Microseismic event 1: frequency analysis of the seismic signals
Data from line 10 (1C)
Raw data
Time window width: 0.032 s
Line 10
Line 10
Polarity flip  source location
Red line: frequency peak of the spectrum for each seismic trace
Perforation shot
Perforation shot
Well head
Receivers
Direct arrivals from the perforation shot
Perforation shot
Surface waves from the well head
Direct waves from the well head
Focal mechanisms
Focal mechanisms: event 1
OK
Focal mechanism: oblique dip-slip fault
Focal mechanisms: event 6
Focal mechanism: strike slip fault
Vertical Transverse Isotropy (polar anisotropy)
Anisotropic material: properties (e.g. seismic velocities) depend on
direction. Vertical transverse isotropy can be related to fine layering in
sedimentary basins or to shales.
5 independent elasticity constants (c11, c33, c44, c66, c13)
Thomsen parameters (weak anisotropy)
P-wave traveltime inversion for homogeneous
VTI media
, , VP0
picked arrival time
origin time
one-way vertical traveltime
P-wave velocity // symmetry axis
offset (horizontal projection of
source-receiver distance)
normal moveout velocity
Anellipticity (Alkhalifah and
Tsvankin, 1995)
*
•
•
•
Experimental traveltimes
Computed traveltimes (t0=-0.005 =0.1 =-0.1 )
Computed traveltimes (t0=0.007 =0.2 =0.3 )
Computed traveltimes (t0=0 =0.1 =0.22 )
P-wave traveltime inversion of
perforation shot data
Perforation shot
P-wave velocity profile
P-wave traveltime inversion of perforation shot data
Traveltimes from
experimental data
(layered anisotropic ?
medium)
, 
Traveltimes from synthetic
data (ray tracing - isotropic
layered medium)
Effective velocity for
traveltime inversion
, 
P-wave traveltime inversion from experimental data
Time (s)
Picked arrival times
Time residuals
Experimental data
Inversion results: vti
t0 = -0.244 s
= 0.2734, 0.1172
RMS4.0 ms
Experimental and theoretical
traveltimes - Line 1
P-wave traveltime inversion from synthetic data
Time (s)
Picked arrival times
Time residuals
Synthetic data
Inversion results: VTI
t0 = -0.001 s
= 0.1217, 0.0148
RMS 1.1 ms
Synthetic and theoretical
traveltimes - Line 1
P-wave traveltime inversion from synthetic data
Time (s)
Picked arrival times
Time residuals
Synthetic data
Inversion results: VTI
t0 = -0.001 s
= 0.1217, 0.0148
RMS 1.1 ms
Experimental data
Inversion results: VTI
t0 = -0.244 s
= 0.2734, 0.1172
RMS4.0 ms
Synthetic and theoretical
traveltimes - Line 1
Conclusions
This dataset is characterized by non-unique focal mechanism
The reservoir and/or the overburden are affected by polar anisotropy
Bibliography
Alkhalifah, T., and I. Tsvankin, 1995, Velocity analysis for transversely isotropic media: Geophysics, 60,
1550-1566.
API Guidance Document HF1, Hydraulic Fracturing Operations – Well Construction and Integrity
Guidelines, First Edition, October 2009 (http://www.gwpc.org/elibrary/documents/general/APi%20Hydraulic%20Fracturing%20Guidance%20Document.pdf)
Fischer, T., Hainzl, S., Eisner, L., Shapiro, S.A. and Le Calvez, J., 2008a, Microseismic signatures of
hydraulic fracture growth in sediment formations: observations and modeling. Jour. Geoph. Res., 113,
B02307, doi:10.1029/2007JB005070.
Grechka, V., 2009, Applications of seismic anisotropy in the oil and gas industry, EAGE Publications bv.
Jupe, A.J., Jones, R.H., Wilson, S.A., and Cowles, J.F., 2003, Microseismic monitoring of
geomechanical reservoir processes and fracture-dominated fluid flow, Fracture and In-Situ Stress
Characterization of Hydrocarbon Reservoirs, Geological Society, London, Special Publications.2003,
Ameed, M.S. (Ed); 209: 77-86.
Maver, K.G., Boivineau, A.S., Rinck, U., Barzaghi, L., and Ferulano, F., Real time and continuous
reservoir monitoring using microseismicity recorded in a live well, First Break, 27, 57-61.
Thomsen, L., 1986, Weak elastic anisotropy, Geophysics, 51, 1954–1966.
Warpinski, N., 2009, Microseismic Monitoring: inside and out, JPT, November 2009, 80-85.
Acknowledgments
We are grateful to Microseismic Inc. for supporting and providing us
with the dataset. We thank Vladimir Grechka for providing us with the
P-wave traveltime inversion code.