This document discusses structural interpretation of horizontal and inclined wells from image log and dipmeter analysis. It notes that image logs in deviated wells may be muted if features are approached at an acute angle. It also discusses bias effects in horizontal wells, including under-sampling of shallowly dipping beds. Additionally, it explains that increasing uncertainty occurs as the solid angle between features and the wellbore decreases. Broad folds are commonly seen in horizontal wells but may not be identifiable in vertical wells. Structural benefits of imaging deviated wells include identifying fractured reservoirs and assessing lateral structure.
2. Structure 3/2
Effect upon imaging
• Image log response
may be muted if features
are approached at an
acute angle, images may
appear defocused as
features close to the
well bore are seen.
• Hole damage is common, notably the development of a
low-side furrow and breakout of the sides of the hole
• LWD logs of horizontal wells may not show hole damage
as the image is logged close to the bit and time-dependent
hole damage may not have occurred.
3. Structure 3/3
Severe bias effects in horizontal wells
North
Horizontal well deviated south:
Bedding infrequently identified
Blue set not intersected
Red set not intersected
Horizontal well deviated east:
Bedding infrequently identified
Blue set slightly under-sampled
Red set slightly under-sampled
• Beds may be under-sampled if shallowly dipping and this
can make structural dip difficult to assess.
• Steep features will be appear rarely if striking parallel to the
well deviation direction and commonly if striking
perpendicular. If analysis of a particular fracture set is
important, well design must take fracture strike into account.
4. Structure 3/4
Uncertainty and bias
• Increasing uncertainty
as solid angle
between features and
the well bore
decreases.
• Cut-off of 10 used for
Terzaghi correction
• Assumption that
features are
consistently spaced
may well be wrong!
0
5
10
15
20
25
30
35
0 20 40 60 80
Angle between fracture and borehole (q)
Derived weighting factor
Bias correction factor
(1/cos
q)
q
=
84.2°
Weighting factor = 10
5. Structure 3/5
Bias towards steeper features
• Bedding in horizontal
holes is sampled with
a bias that leads to
over-sampling of
steep surfaces and
under-sampling of
shallow surfaces.
• Local irregularities on
surfaces may be
emphasised by this
effect.
Depth, ft
Dynam ic RAB im ages
0 Gam m a ray, API 100
Static RAB im ages
0
10
20
30
40
RAB data
Many contacts appear steep and
can be interpreted as faulted.
Mean sand-shale contact
inclination was 60°.
6. Structure 3/6
Upright folds
Dips to SE Dips to NW
Crest of fold
Bedding pattern and style repeat
themselves either side of the fold crest
N
Great circle fitted to fold data to find the plunge
250 ft
Broad folds are commonly seen in horizontal wells but
are not identifiable in vertical wells.
8. Structure 3/8
Effect of dip on flow
Dip has an effect on oil cut during immiscible displacement.
Cut is part function of wetting preference, dip and direction of
displacement (i.e. up or down dip). We have observed fold
axes partially controlling ingress of fluids.
For up dip flow larger alpha
accentuates stabilisation in oil-
water system.
9. Structure 3/9
Vertical interval sampled
• Well deviated 85°
87 ft TVD in 1000 ft MD
• Well deviated 60°
500 ft TVD in 1000 ft MD
• Well deviated 30°
866 ft TVD in 1000 ft MD
• Well deviated 5°
996 ft TVD in 1000 ft MD
Highly deviated wells with shallow
structural dip sample very short
sections that may be unrepresentative.
Strata may be sampled several times,
allowing lateral changes to be
assessed.
11. Structure 3/11
Structural benefits of imaging deviated wells
• If oriented optimally relative to fractures, deviated wells can be
very helpful in identifying and describing fractured reservoirs.
• If structural dip is steep, highly deviated wells may yield the best
bedding data.
• Lateral structure may be assessed.
• Broad folds are commonly seen in horizontal well sections. They
may be analysed and their effect upon reservoir production
assessed.
• LWD logs are commonly run for geosteering and geostopping
operations. These data-sets often yield good structural
information.
• If several sidetracks are imaged, geometrical techniques may be
used to correlate structure between wells.
