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Linking the Earth Science and Engineering Disciplines to
Geology
Create Additional Value
a report by
Harvey Goodman
Chevron Fellow, Rock Mechanics and Mechanical Earth Modelling
Various oil and gas operators and service providers have laid the structure is used to predict pre-drill stress using numerical methods.
groundwork that has enabled large geological bodies (geo-bodies) The major value of this approach is that it affords asset planners a
to be defined as engineering materials for oil and gas field design fully linked geological and engineering reference frame against
and performance prediction over the life of the asset. The direct link which well systems and reservoir performance can be measured in
between the earth science and engineering disciplines is defined every phase of project development. This consistent reference frame
using geomechanical principles and numerical modelling techniques. can be adjusted/improved/calibrated as drilling continues, so the
Value is created in the early stages of the project, when well control most efficient well plan can be recognised and implemented early in
is limited, by ensuring the uncertainty in the subsurface model asset development.
during development is accounted for by the well systems design.
Figure 3 shows the increased value derived from good project
As asset production is brought online, technology has been further definition in the early project-planning phase (dark green and yellow
developed to enable field development planners to forecast the shading). When good project definition is achieved in the early
consequences of prolonged production on reservoir recovery. phases, there can still be relatively high value creation even if the
Applications include casing failure risk management within the project is poorly executed (see blue and purple shaded areas).
overburden from reservoir compaction, which also influences surface
facilities performance, affording an opportunity to manage risks to Furthermore, consideration of the full cycle asset development plan
these facilities through production management. from appraisal to abandonment reduces the risk of missed future
opportunities due to well systems design constraints. For example,
The Value of an Integrated Approach reservoir pressure depletion and subsidence can affect borehole
Often, seemingly unrelated events that can plague high-risk, high- stability to the extent that complex well designs are necessary to fully
cost developments can be best understood when the geological exploit the asset. Well placement depends not only on the subsiding
environment, i.e. the geology and geophysics (GG), is described as reservoir section, but also on the reaction of the overlying geological
an engineering material, i.e. the rock mechanical properties that section that must be drilled through to reach the reservoir.
include formation strength and stress throughout the geo-body. A
generic geomechanical earth model is shown in Figure 1. Rock stress For land- based operations, the consequences of well complexity
and strength values (dark blue shading) are calibrated to formation may be more easily addressed, the downside being a poor estimate
properties and offset well performance criteria to the left. of field recovery that can either rob opportunities for outlying
prospects or, in the worst case, cause the asset to be uneconomical.
The formation failure calibration process can be closely linked to
high-end numerical modelling methods available to geomechanics For major capital projects, such as deepwater subsalt fields, the
staff today by utilising high-fidelity measurements to monitor capital outlays are immense, with single wells costing up to US$100
drilling, completion and reservoir performance and production million. For these deepwater projects, fewer wells are required to
operations. Examples include downhole measurement while drilling produce reliably for longer periods of time. Well engineers need to
(MWD) measurements to recognise hole failure with time, sand get it right the first time.
detection devices and various completion systems monitoring
multiphase flow to picture the effects of solids production on The Rock Mechanics of Hole Failure
surface facilities. Boreholes drilled during exploration and development must
accommodate the formation stress that builds up at the
The idea is to ‘test’ the hole failure and sanding predictions, revising borehole wall. The magnitude of these stresses depends on the
the geomechanical predictions as needed using drilling, completion pre-drill formation in situ stress field character and the trajectory of
and production systems monitoring to understand the consequence the well.
to operations. The challenge is first to determine the consequences
of hole failure, sand failure and reservoir compaction on the The assumption that the overburden pressure gradient defines the
drilling/production asset, and second to know when this degree of maximum principal stress magnitude and direction is often made by
instability will cause a change in well systems design. rock mechanics practitioners using single well (1D) data sets.
However, the geomechanical earth model approach provides a
Figure 2 displays a geological model composed of formation tops means of verifying this assumption. The consequence to borehole
within a 3D volume. Knowledge of each formation layer stiffness, or stability behaviour when the assumption is invalid is illustrated below
rheology, and the distribution of that layer across the geological for a deepwater subsalt case.
© TOUCH BRIEFINGS 2009
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