otc 24324 geomechanical analysis and critically stressed fractures

OTC 24324

Geomechanical Analysis and

Critically Stressed Fractures

M. Cruz; J. Oliveira; B. Silveira.

Paper # • Paper Title • Presenter’s Name

Slide 2

• Fractured reservoirs show a complex distribution of faults

and fractures which reflects the progressive activity of

several stress fields along geological history.

• However, for the purposes of hydrocarbon production, the

hydraulic conductivity of the fractured medium is strongly

influenced by the current stress field.

• Fractures at the appropriate angle to the current stress

fields to be mechanically active are expected to be hydraulically active (Critically Stressed Fractures).

Theoretical Background

OTC 24324 . Geomechanical Analysis and Critically Stressed Fractures . José Oliveira


Slide 3

• Because fracture permeability is strongly influenced by the

current stress tensor, it is necessary prior determination of

in situ stress in order to identify the population of critically

stressed fractures.

• The determination of in situ stress requires the estimation

of magnitude and spatial orientation of the three principal

stress axis ( Sv, SHmáx and Shmin) based on data from

image profiles, well logs, leak-off tests, hydraulic fracturing,

amongst others.

The Stress Tensor



Slide 4

Main stress axes in sedimentary basins


Vertical stress magnitudes, Shmax orientation and

magnitude, Shmin magnitude and pore pressure are basic

Geomechanical determinations.


Slide 5

Main tensional regimes


a) normal, b) reverse and c) strike-slip


Slide 6

The critically stressed fractures paradigm

Fractures at the appropriate angle to the current stress fields to

be mechanically active are expected to be hydraulically active.


Zoback, 2010


Slide 8

Critically stressed fractures

The discrimination of the critically stressed fractures is a key

factor for a successful well location.


Zoback, 2010


Slide 9

The Stress Tensor in our work


Data included for this study involved wireline logs (Gamma Ray ,

Resistivity, Density and Acoustic), drilling reports, mud weights,

casing depths, formation tops and well surveys, pore pressure

measurements and interpreted images available of 3 wells.

Previous studies with 19 wells in the same region were

considered to understand the geomechanical model.

Hydrostatic pore pressure were used due to results of formation

tests.

The leak of tests and volume of breakout ocurrencies to confirm

the events data are missing and therefore some uncertainties

exist to constrain UCS, SHmax and Shmin.


Slide 10

The Stress Tensor in our work


The area is in normal tensional regime:

Sv > Shmax> Shmin

= Sv Sv = 2.13 SG

SHmax = 1.91 SG

Shmin = 1.69 SG

Sv = 17.37 PPG

SHmax = 15.61 PPG

Shmin = 13.84 PPG

Sv = 19.44 PPG

SHmax = 17.25 PPG

Shmin = 15.07 PPG

Well A

Well B

Well C


Slide 11

Once determined the tensor, it is possible to calculate the

shear stress/normal stress ratio for each of the fracture

surfaces interpreted in the image profiles and, thus,

determine the population of fractures whose spatial

orientation favors a better hydraulic performance based on

theoretical assumptions stated above.

We made this in 3 wells, named A, B and C.

The Critically Stressed Fractures on

the study area



Slide 12

Well A: no critically stressed fractures

Sv = 2.13 SG, SHmax = 1.91 SG, Shmin = 1.69 SG



Slide 13

Well B: two critically stressed fractures

Sv = 17.37 PPG, SHmax = 15.61 PPG, Shmin = 13.84 PPG



Slide 14

Well C: Conjugate pair of critically stressed fractures

Sv = 19.44 PPG, SHmax = 17.25 PPG, Shmin = 15.07 PPG



Slide 15

Conclusions

- The study indicates that the stress tensor is under a

normal stress regime.

- With the calculated stress values, the modeling stress

fitting of the fractures is only possible by using a

coefficient of friction much lower than normally used

for such calculations.

-This fact suggests that the fractures do not play a

relevant role in the dynamics of the analized rocks; it is

unlikely that the spatial attitude of fault planes has not

been addressed by the modeling performed in

fractures.



Slide 16

Thank you !

The authors would like to express their gratitude to

colleagues from OGX, Paulo Ernesto and Dayse Daltro by

criticism, suggestions and incentives.

We also thank the petrophysicist Albano Bastos for fruitful

discussions which preceded the final version of this paper.


otc 24324 geomechanical analysis and critically stressed fractures

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