The in situ Rock Stress Field

The contemporary state of rock stress in a reservoir is the result of complex geological and tectonic processes; it is further to a great extent influenced by petroleum production works.

The in situ reservoir stresses control the initiation, direction and reopening of fractures as well as the propagation pressures and direction of induced hydraulic fractures. The stress field has a number of implications for hydrocarbon production: wellbore failure, casing shear, subsidence/compaction, sand production, stress dependent deformation and permeability of matrix rock.

Our clients are:

RWE-DEA, Hamburg
BEB, Hannover
Wintershall AG, Kassel/Barnstorf
Phillips Petroleum, Norway
BGR, Hannover
Mobil Oil, Celle
Dr. R. Braun, Celle
Preussag, Lingen
Amberg Measuring Technique, Switzerland
NAGRA, Switzerland
Geotechnisches Institut, Switzerland

Core-Based Measurements to Estimate in situ Rock Stress

DCM utilizes two different methods of core-based analyses to determine the orientation and the magnitudes of the in situ rock stress. These two methods are:

• Anelastic Strain Recovery  (ASR)

• Wave Velocity Anisotropy (WVA)

Both methods have been successfully applied not only at the KTB-drill site (Germany) but also at different drilling locations of geothermal, gas and oil exploration works in Europe. A further development of the evaluation of the WVA-data allows a reliable interpretation of the 3D-state of in situ rock stress.

 

The Core Investigation Strategy

(1) A core is taken from its in situ environment (deviatoric stress regime). Due to stress relief strain recovery starts (expansion of the core).

(2) By means of a multi-dilatometer the Anelastic Strain Recovery (ASR-method) is measured.

(3) Tension crack formation takes place during strain relaxation of core sample; acoustic emissions are detected.

(4) Ultra-sonic P-waves are used for analyzing the tension crack orientation, which is related to the in situ stress field. (Wave Velocity Anisotropy, WVA-method).

(5) In dependence upon confining pressure the crack closing pressure (Pc) is determined.

 

The orientation of rock stress around a borehole is shown in the diagram on the right side :

A very good conformity in stress orientation is achieved by analysis of core samples of one wellbore. The results of two different methods (ASR and WVA) show a good correlation.

 

Core Orientation

The orientation of the core samples has to be known, either by oriented coring, or by the evaluation of digital scanning data of the core surface in comparison with downhole log data (BHTV, FMS, FMI). A palaeomagnetic method can also be helpful for determining the core orientation.

We made very good experience using the digital scanning images for determining the core orientation. This service is offered by GEOlogik (Dipl.Ing. Dipl.Geol. J. Geßner):

The information about the in situ state of stress provide valuable data for optimizing location for future exploration and exploitation work, e.g.

hydraulic fracturing applications, horizontal drilling, wellbore stability, water flood strategy, establishing the well pattern

 

A comprehensive investigation about the 3D-state of in situ stress around a salt dome and the pay zone, respectively, where carried out in the most southern part of the North Sea. We determined the state of rock stress  and organized the wellbore stability analysis for the extended reach well Dieksand 2.

 

 

Please click an image to see the fullsize picture.

Strategy of the core investigations (after A. ZANG)

 

 

 

Orientation of rock stress around a borehole

CoreScan of GEOlogik