Applied Rock Mechanics (ARM) LLC
PROPOSAL TO DOCUMENT AND MODEL
THE STRESS CONFIGURATION
WITHIN AND AROUND
AN
OIL OR GAS FIELD
submitted by
Applied Rock Mechanics (ARM) LLC
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Applied Rock Mechanics (ARM) LLC
PROPOSAL TO DOCUMENT AND MODEL THE STRESS
CONFIGURATION WITHIN AND AROUND AN OIL OR GAS
FIELD
BACKGROUND
This proposal outlines a generic study on an oil or gas field, which is intended to characterize the
stress regime(s) within and around the field using information from wells and measurements
made on core samples, first a brief description of the first stage is needed before proceedings to
the second stage. The second stage involves feeding these local measurements into a finite
element model and thereby extending an understanding of the stress configuration across the
entire field. Inevitably, the scope of the well data, the availability of core samples and the
sample measurements will vary from field to field. Hence the specific components of each field
study will not necessarily be the same. However, each study will share a common philosophy,
namely: to maximise available data, to augment these data where feasible by new measurements
on samples, to feed all reliable information into a finite element model that conforms to the
structural character of the field and, finally, to manipulate the model until it is substantially in
agreement with the well data.
This proposal can, therefore, be adapted for application to any adequately documented oil or gas
field(s).
BASIC PROCEDURE
The proposed study will be undertaken in two stages, described here as Task 1 and Task 2. The
tasks are defined as follows:
Task 1: Establish the regional and local stress regimes in as much detail and accuracy as existing
well data and geological data permit.
Task 2: Undertake finite element modeling of the field area(s) so as to extend mapping of stress
trajectory across the entire study area.
Task 1 must be completed before the finite element modeling Task 2 begins, although most of
the necessary finite element gridding can be prepared before Task 1 is completed. Task 1
requires data acquired during drilling, well logging and subsurface mapping, as specified below.
Task 2 requires information on geological structure and access to reservoir simulation grids and
software, as noted below.
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DATA REQUIRED FOR TASK 1 – REGIONAL AND LOCAL STRESS REGIMES
1a) Downhole petrophysical logs in digital form required for each well
Dipmeter logs (e.g. SHDT logs, also HEXDIP) and/or
Bore hole Image logs (e.g. FMI, CBIL)
Gamma Ray log for entire well
Density log (FDC-CNL or similar) for entire well, if available
Density log: DEN in SG
Sonic log: DT in usec/ft
All the above logs should be delivered as ASCII standard digital files in .las or .txt format.
Specific items for which log digits are required are listed in the Appendix.
1b) Paper prints of logs required for each well
Paper prints of all logs listed above are needed for checking that the well log digits are accurately
recorded.
2) Descriptive material required for each well
Final well completion reports, and/or other documents containing information on:
Daily drilling events
Drill stem tests
Mud weights
Stratigraphic units (tops)
Bit sizes
Geological sample descriptions (cores and cuttings)
Leak-off tests (LOTs)
Repeat formation tests (RFTs, FMTs)
Geological Summary log
3) Rock Mechanics material parameters
The rock mechanics material parameters are determined from triaxial testing of plugs. Normally
plugs from the reservoir section are tested, but in specific situations it may also be necessary to
work on plugs taken in shales above or within the reservoir section.
A standard triaxial test suite consists of three triaxial tests run at different confining stresses, one
unloading test where the axial load is reduced while the radial confining stress is kept constant at
a high level and, finally, a K0 test, i.e. a compaction test where the sample is loaded axially
while it is prevented from expanding laterally. The confining stresses are varied around the
stress level at the depth of the core, determined from LOTs. These tests provide the following
material properties:
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Strength data:
Uniaxial Compressive Strength (UCS or C0)
Failure angle (β)
Elastic moduli
Young’s Modulus (E)
Shear Modulus (G)
Bulk Modulus (K)
Poisson’s ratio (ν)
Compressibilities
Bulk compressibility (Cb)
Pore compressibility (Cp)
If good correlations between log values (especially sonic) and the mechanical properties
measured on core samples derived from the logged interval is obtained, then these logs may be
used for estimating the rock mechanics parameters over unsampled intervals. If core data is not
available, it may be possible to use drilling data to estimate rock mechanics strength such as:
Rate of penetration, ROP in m/h or ft/h
Weigth on bit, WOB in ton or kilo-lb
4) Maps required for the study area
Regional structural maps showing contoured horizon, elevations and faults are needed/will be
analysed. Local structural maps relevant to the areas in which the wells were drilled will also be
taken in consideration. It is particularly important to have detailed information about faults that
transect or pass close to the wells that are being analyzed since these may locally affect stress
trajectories.
TASK 1 DELIVERABLES
Stress Orientations
The caliper arm extension records logged by dipmeter tool will be examined to look for intervals
with ovalisation. Each such interval will then be assessed in terms of its properties to determine
if it has developed as a consequence of spalling induced by amplification of anisotropic far field
stresses. If so, the long axes of the ovalised intervals will be aligned with SHmin, the smaller
horizontal stress.
Digitised dipmeter logs will be analysed using the most recent version of PFAS software
(Planning and Field Application Software) developed by ITC AS of Tonsberg, Norway.
Annotated graphic logs showing the evidence for ovalised zones, their orientation and their
dimensions will be prepared for each well at a client specified vertical scale. A written summary
will be provided altogether with the graphic log.
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The graphic log will incorporate the interpretation of the prevailing horizontal stress orientation
at the well across the reservoir section (log coverage permitting) and will identify any anomalous
stress orientations and attempt to account for them.
Stress Magnitudes
The vertical stress, SV, will be determined for each well by integrating the density logs so as to
obtain a continuous profile of overburden load versus depth. PFAS software will be used to
make necessary adjustments and generate the profiles.
The smaller horizontal stress, SHmin, will also be determined from analysis of leak-off tests and
compared with the results from dip meters analysis. Where possible, fracture closure pressures
(= SHmin ) will be interpreted from the pressure/time records. Otherwise estimates will be made
from leak-off pressures. Graphic plots illustrating the interpretation of the leak-off tests will be
prepared, as well as short summaries of the interpretation.
The larger horizontal principal stress, SHmax, cannot presently be measured downhole. It can be
calculated from hydraulic fracture and extended leak-off test, if certain assumptions are made,
and this will be done where feasible. Where leak-off tests have been run in inclined or
horizontal wells with different trajectories in a specific geographic area, it may be possible to
identify the magnitudes of both SHmin and SHmax, as well as the direction, from the inversion of
the geometry (i.e. inclination and direction of well) based on data from several wells using the
PFAS software. Likewise, borehole failure can be simulated numerically and the magnitudes
inferred by iterating the variable parameters. Numerical simulations will be undertaken using
the Mohr-Coulomb fracture criterion, the Drucker-Prager or Extended Von Mises fracture
criterion and the Modified Strain Energy Criterion, and the most appropriate simulations
selected to estimate the magnitude of SHmax. Diagrams portraying a variety of numerical
simulations of borehole failure will be prepared and will be accompanied by discussions of what
can be concluded from them with respect to estimating SHmax magnitudes. An interpretation of
the degree of horizontal stress anisotropy (SHmax/SHmin) will be included. Stress magnitude
profiles including pore pressure profiles will be prepared for each well.
Geomechanical Measurements
All the measurements made on core samples will be recorded digitally and transmitted in digital
form to clients. Plots and graphic logs will be prepared to record the progress of laboratory tests
tables. The geomechanical measurements and log derived estimates will then be incorporated
into a Geomechanical Profile for each well in the study. In addition, the Geomechanical Profile
will portray pore pressures, principal stress magnitudes and formation temperatures.
Maps
Maps will be prepared to portray horizontal stress orientations and their spatial relationship if
any, to nearby geological structures. The maps will be accompanied by interpretative summaries
pointing out relationships between stress orientations, the trends of nearby faults and the
resulting implications. Points to consider when developing finite element models will be
emphasized.
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DATA REQUIRED FOR TASK 2 - FINITE ELEMENT MODELING
•
Stress regime(s) interpretation (product of Task 1)
•
Measurements of geomechanical properties
Laboratory tests of strengths of reservoir and other rocks contained within the sequences logged
by caliper logs and/or dipmeters (if available). The data are listed in Task 1, part 3.
•
Geological Maps
Regional structural maps showing contoured horizon elevations and faults.
•
Reservoir Simulation Grid
ECLIPSE or other simulation grids of the reservoir including faults in order to convert to Finite
Element mesh format. This will be executed employing GeoToolTM software developed by FEM
Engineering AS. Ideally, the simulation grid will include the strata immediately above and
below the reservoir section.
•
Pore pressure
Pore pressure profiles across the entire reservoir field and depletion rate information.
•
Fluid connectivity
Information bearing on directional flow of fluids within the reservoir.
TASK 2 - DELIVERABLES
Maps showing simulated stress trajectories, stress magnitudes and strain in and around the
reservoir(s) will be prepared. Full-scale 3D multi-layered finite element modeling allows
simulation of these parameters in areas immediately adjacent to fault zones, which intersect a
reservoir. The fault zones can be simulated as mechanically stiffer bodies than the surrounding
rocks or as mechanically softer features or even as open fractures. The effects of any producing
wells will also be taken into consideration simultaneously with the analysis.
A finite element model for each field will be constructed using existing simulation grids such as
those provided by ECLIPSE software. The modeled area will be extended so that the effects of
far field in-situ stresses can be adequately simulated. A series of iterations will be run to adjust
the finite element model so that it respects, as much as possible, the measured stress orientations
and magnitudes, and so that it properly characterizes fault blocks and the fault zones that
separate them. We will configure the fault zones so that they can contain several different
materials that will vary in the softness and hardness compared to the adjacent rocks. The
geomechanical parameters of each simulation will be selected and adjusted to conform to rock
mechanical measurements made on core samples from the area being studied, assuming that
such measurements have been made. If not, appropriate values will be selected from
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measurements made on rocks in adjacent areas, or will be taken from the scientific literature.
Reservoir pore pressures will also be addressed so as to assess effective stress levels.
The resulting maps can be expected to document stress trajectories with high level of detail, and
we anticipate that they will conform to stress orientations inferred from well bore breakouts and
to stress magnitudes estimated from leak-off tests (Task 1). In faulted and fractured reservoirs,
provided adequate supporting data from logs and cores and structural maps is available, it should
be possible to discriminate between open and closed fracture systems, and to identify those
fracture networks that are likely to best contribute to fluid flow within a reservoir.
In
unfractured reservoirs, the SHmax orientations will define the stress-controlled preferred fluid
flow axes, all other factors being equal. Such information can be of great value when planning
scenarios for optimum field drainage. It should also be emphasized that a map which projects
horizontal stress axes and magnitudes across a field represents a significant tool for planning the
well paths of inclined production wells. It will allow the drilling engineers to select trajectories
that will minimise bore hole stability problems.
All these factors plus the results of the finite element modeling will be addressed in the final
technical report. The main deliverables of the report will include stress and deformation/strain
maps across the studied field and adjacent areas.
SCHEDULING OF TASKS 1 AND 2
Tasks 1 and 2 can be scheduled almost concurrently, since much of the finite element gridding
can be done at the same time as the stress regime analysis. For a program involving stress
analysis of 50 to 60 wells, a complete assignment involving Tasks 1 and 2 could be completed in
6-7 months, depending on the complexity of the assignment..
RELEVANT PREVIOUS WORK BY THE PROPONENTS
Stress mapping at field scale is described and discussed in:
Bell, J. S., 1990. Investigating stress regimes in sedimentary basins using information from oil
industry wireline logs and drilling records. In Hurst, A., Lovell, M. A. and Morton, A. C.,
(Editors) Geological Applications of Wireline Logs. Geological Society of London, Special
Publication No. 48, p. 305-325.
Bell, J. S., 1990. The stress regime of the Scotian Shelf off shore eastern Canada to 6 kilometers
depth and implications for rock mechanics and hydrocarbon migration. In Maury, V. and
Fourmaintraux, D., (Editors) Rock at Great Depth, vol 3, Balkema, Rotterdam, p. 1243-1265.
Sigma H Consultants, 1994.
In-situ stresses and inferred fracture propagation directions,
Tanjung Field, S. E. Kalimantan. Exclusive study prepared for Pertamina - Bow Valley
(Tanjung) Ltd.
Stress deflection by faults is described and discussed in:
Bell, J. S., Caillet, G., and Adams, J., 1992. Attempts to detect open fractures and non-sealing
faults with dipmeter logs. In Hurst, A., Griffiths, C. M. and Worthington, P. F., (Editors)
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Geological Applications of Wireline Logs II. Geological Society of London, Special Publication
No. 65, p. 211-220.
Caillet, G. et Bell, J. S., 1992. Contraintes at Failles. Découvrir (Publication d’Elf-Aquitaine),
No. 7, Mars 1992, p. 17-21.
Bell, J. S. and Caillet, G., 1994. A Reinterpretation of the Stress Regime of the Aquitaine Basin,
Southwestern France, and Implications for Hydrocarbon Recovery. In “Hydrocarbon and
Petroleum Geolgy of France” (Editor) A. Mascle, European Association of Petroleum
Geologists, Special Publication 4, Springer-Verlag, p. 209-219.
Sigma H Consultants Ltd., 1998. Snorre - A Re-evaluation of the Stress Field for Finite Element
Modelling. Exclusive report for Saga Petroleum ASA, 129 pages.
Finite element modelling of stress trajectories and stress magnitudes is described in:
Pourjavad, M., Bell, J. S., and Bratli, R. K., 1998. Stress trajectories in the Neighbourhood of
Fault Zones. Paper 47211, SPE/ISRM Eurock ‘98 Conference, Trondheim, Norway. FEM
Engineering AS, 1998.
Finite Element Modelling of stress, Haltenbanken (Kristin field). Exclusive study for Saga
Petroleum ASA.
FEM Engineering AS, 1998. Finite Element Modelling of stress mapping, North Sea (Snorre
field). Exclusive study for Saga Petroleum ASA.
Finite element modelling of stresses for well bore stability is described in :
Pourjavad , M., Bratli, R.K. and Pedersen K., 1998. Finite Element Modelling as a new design
tool for CTD branch design . 6th International Conference on COILED TUBING AND WELL
INTERVENTION, February 9-11, 1998.
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APPENDIX
Details of log data required for stress regime analysis.
If digital logs are not available, hard copies of the required logs can be digitised..
Dipmeter logs (e.g. SHDT logs, also HEXDIP) and/or
Bore hole Image logs (e.g. FMI, CBIL)
Depth: DEPT in metres
Caliper extension records: C13 in inches, C24 in inches
Caliper 1 azimuth: P1AZ in degrees
Relative bearing: RB in degrees
Hole Azimuth: HAZI in degrees
Deviation: DEVI in degrees
Resistivity: DB1, DB2 etc. in ohms
Gamma Ray log for entire well
Depth: DEPT in metres
Gamma Ray Log: GR in GAPI
Density log (FDC-CNL or similar) for entire well, if available
Depth: DEPT in metres
Density log: DEN in SG
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