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REFERENCES Document Repository
CATEGORY Visualizations
KEYWORDS Stress modelling, Geo-resource production= impact, Production - seismicity interaction
CITATION Please acknowledge use of this applicatio=
n in your work:
IS-EPOS. (2019). Fracture Network Models - Mechanica=
l Stresses [Web application/Source code]. Retrieved=
from https://tcs.ah-epos.eu/
About the Service
This service interactively plots stress patterns associated with the hyd= raulic fracturing of a shale gas well. The data is based on th= e physical and geological conditions at the Preese Hall Episode site.The ap= plication allows you to produce Coulomb, Normal or Shear stress maps for a = specific source mechanism weighting and key parameter.Also calculated is th= e lateral respect distance, that is the minimum distance that fracking shou= ld occur from a pre-existing critical fault in order not to reactivate it.T= he modelling which is used to create the plot is explained in the reference= document - Westwood, R. F., Toon, S. M. & Cassidy, N. J., 2017. A sens= itivity analysis of the effect of pumping parameters on hydraulic fracture = networks and local stresses during Shale Gas operations.
License Terms
This work is licensed under a Cre= ative Commons Attribution-ShareAlike 4.0 International License.For f= ull details of the license see https://c= reativecommons.org/licenses/by-sa/4.0/legalcode.
Figure 1. The options available for = the service.
Inputs
Grid Resolution: This is the resolution of the grid used for th=
e stress map. Options are:
- 50 m =E2=80=93 this produces a stress map 4=
km x 4 km with 50 m spacing; or
- 20 m =E2=80=93 this produces a stress map 1=
km x 1 km with 20 m spacing.
Source Mechanism Weightings: Here you can specify the source me=
chanism used to generate the fractures as a ratio between inflation, revers=
e and strike-slip. Values should be between 0 and 1 and must total 1.
Key Parameter: The stress calculation is based on this parameter=
. Options are:
- dP =E2=80=93 This is the pressure differenc=
e between pore pressure and normal pressure on the fractures at injection.<=
br>
- Flow Rate - Fluid is injected into the well=
at this rate.
- Pump Time - This is the length of time that=
the fluid is pumped into the well.
Parameter Value: The va=
lue of the key parameter you selected in MPa for dP, m3/s for flow rate and=
minutes for pump time.
Monte Carlo iteration: The simulati=
ons were run for 50 Monte Carlo iterations, generating a new discrete natur=
al fracture network each time, so the results are each slightly different. =
Enter the iteration you wish to visualise here.
Stress type: Select whether you wish to visualise Normal, Shear or Coulomb stress.
=
Lateral distance threshold: Enter the threshold value in MPa t=
o use to define the stress required to reactive a critical fault. Values ty=
pically range from 0.001-0.1 MPa.
=E2=80=9CFreed (2005) uses aftershock studies to define triggering at 0.1 M=
Pa to 0.3 MPa or less. Kilbet al. (2002) state that the optimal triggering =
threshold is 0.1 MPa, but find correlations with seismicity rate change for=
value between 0.001 MPa and 0.5 MPa. The results of Shapiro etal. (2006) a=
lso indicate that triggering occurs as low as 0.001 - 0.1 MPa.=E2=80=9D Wes=
twood et al.(under review)
Plot Fractures: Tick this box if you wish to plot the fractures.
Outputs= p>
Three output files are generated:= a jpg of the stress map, a text file containing additional information and= a .mat file containing the stress plot data in Matlab format.
JPG Stressmap: The algorithms described by Okada [28] are used = to calculate failure on optimally orientated strike slip faults. The calcul= ation is performed at each point of a discretized cube. A 2D stress map is = obtained by calculating the maximum and minimum stresses over depth, result= ing in a map like the one provided in Figure 2. The maximum and minimum str= esses are indicated by the black and grey lines respectively at 0.00001 MPa= (50 m) and 0.000001 MPa (20 m) intervals.
Text file The text file (see Figure 3 for an example) contains = additional information about the calculation inputs.The first section of th= e text file states the key parameter and parameter value used for the calcu= lation and the Monte Carlo iteration.The second and third blocks contain th= e geological parameters and regional stresses, respectively,used in the cal= culation.The final block states the threshold value used for the lateral re= spect distance and the mean,median, maximum and minimum lateral respect dis= tances over all 50 Monte Carlo iterations.
Figure 2. Example t= ext file output.
MAT file: The= stress plot data saved in a MAT file
Figure 3. An exampl= e stress map at 50 m resolution.