Shallow Water - Trouble in the Deep - Lumina Geophysical

te deepwaterdrilling problem of shallow-water flows
(SWF) has proven to be particularly challenging. It is
neither easilyidentified nor avoidedand has the potential
to cause total well failure when not handled properly.
During the past 10 years of drilling in the deepwater Gulf
of Mexico,offshoreWestAfricaand other basins, SWFhas
been one of the most expensive hazards for deepwater
operators, causing significant cost over-runs and respudding
of numerous wells. Shallow-water flow events
also have been the cause of template failures involving
multiple wells, such as the Ursa template failure in the
Gulf of Mexico,and may be the single most costly and
dangerous hazard in the deepwater exploration and production
business.
The nature of SWF
Shallow-water flows have been observed in water depths from
I,500ft to 7,OOOft(457m to 2,135m) and up to 4,OOOft
,(1,220m) below the seafloor in areas where rapid sedimentation
has caused sands to retain abnormally high
porosities at or near the critical porosity for the material.
Critical porosity is that at which a rock begins acting like a
load-bearing solid. Shallow-water flow sands exist at low
effectivestresses and are close to incipient failure. This results
in a situation where even the agitation of the drillbit can cause
the formation to begin collapsing into the well during
DEEPWATER. JULY 2003

Drilling-
Shallow Water Flows
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Co
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a
Sel
Z
3
4
5
6
7
8
9
10
.1 .2
.3
Pressure (kpsl)
~ .S -6 .7 -S .9
Owrburdm
(100 fi \\'D)
E1fectM Stress
(100 Ii \\'D)
LEGEND
100 It waler depth
4000 It waler depth
Effective Slmt$
(4000 ft \\'Di
Overourdm
(-toaG
ft WO)
Figure 1: A comparison of shallow water and deepwater pressure
environments shows the differences between the effective stress/
overburden and pore pressure for water depths of 100h (30.Sm)
and 4,OOOft(1,220m).
penetration or soon after. Some SWFevents have been delayed
by several hours to several days. Because it "excavates" the
formation, SWF has resulted in failure of the shallow portion
of wells long after the section had been cased.
To understand SWF, it is useful to understand the basis
of the old truism that "compaction begins at the seafloor."
This empirical observation is based on the physical
phenomenon that compaction, which includes consolidation,
reduction of porosity and strengthening of the
sediments, is because of the grain-to-grain forces in the
sediments. This is the effective stress or the difference
beh'l'een the overburden and the pore fluid pressures.
In cases of rapid sedimentation, pore fluids may not
escape, as the load increases. The fluids bear some of the
weight of the overlying solids and become overpressured
beyond the normal hydrostatic pressure of the overlying
waters. The effective stress on the sediments is abnormally
low and the porosity is preserved.
Consider a comparison of stress conditions in a shallowwater
case vs. a deepwater case (Figure 1). The effective
stress on the formation at the mudline is zero, and it increases
with a gradient of about 0.535 psi/ft if hydrostatic communication
is maintained with the water column. In
shallow water, the effective stress becomes non-zero at 100ft
(30.5m) below sea level where the total overburden stress
from the water co'umn is relatively smalL At a water depth
of 4,OOOft,however, the overburden stress is larger
because of the higher water column. However, as long as the
pore fluids are in pressure communication, the pore
pressure will be increased an equal amount. The
effective stress remains unchanged between shallow
and deep water at the same depth below the mud line. As a
result, the consolidation of the sands is the same for equivalent
depths below the mudline. Thus, not only are the sediments in
the deepwater case less compacted, but they also are at
relativelyhigher pore pressures for the same depth below sea
level compared with their shallow water equivalent.
In addition to the low compaction state for deepwater
sediments, the severity of SWF may be exacerbated by the
presence of structural hyper-pressuring, also known as the
centroid effect. This concept suggests a sand body positioned
on a structure or slope will develop a pressure gradient that is
e hydrostatic, even though the gradient in the surrounding
sediments is non-hydrostatic. For the shallow burial conditions
in which SWF events occur, the amount of structural hyperpressuring
required to cause a seal failure is not great.
Solutions
The industry has chosen to address SWF in two primary
ways:
... pre-drill detection and avoidance, and
... detectionand mitigationwhiledrilling.
These two efforts have occurred in parallel, often with no
interaction between the efforts. The pre-drill prediction
effort has been focused on seismic prediction and loggingwhile-drilling/pressure-while-drilling
(PWD) analysis, while
the mitigation effort has been addressed by new drilling
techniques, new mud circulation approaches (especially
dual-gradient drilling) and new mud chemical treatments to
prevent hole collapse.
Pre-drill prediction
and avoidance
The most common approach to SWF prediction is tied to
analog wells and analysis of seismic reflection character on
data processed with standard techniques. Well locations
have been chosen to avoid or minimize exposure to
suspected SWF zones. Avoidance relies on "pattern recognition"
and has not always been successful. Of course,
seismic time-to-depth conversion is important for predicting
the depths of these zones.
Another key pre-drill tool is seismic velocity analysis for
pressure prediction. Departures of these velocities from
"normal" trends have proven useful for quantifying the degree
of overpressure and allowing accurate casing program design.
The bases for these techniques date to the late 1960s and have
been recently applied with better accuracy and understanding.
The geophysical community has been making new
advances in the pre-drill detection of SWF. Research at the
JULY 2003.
DEEPWATER

Shallow
Drilling -
Water Flows
Universityof Oklahomasponsoredbya
consortium ofcompanies
led by ConocoPhillips
and jointly fundedby the U.S.
MineralsManagementService
demonstrated the usefulness
of seismic data for detecting
the low shear wavevelocities
and abnormally high VpNs
ratios that are diagnostic of
SWF sands. The basic concepts
and feasibilityof these
methods have been demonstrated
and extended with
further analysis showing the
distinct signatures of SWF
sands in a deepwater area with
known SWF events (Figures
2 and 3).
Detection
and mitigation
while drilling
The overpressures ofSWFsand sections can be detected during
drilling in two ways. First, the most basic form of detection is
observation of the wellhead with remotely operated vehicle
video while drilling of the tophole section of the well. This
section usually is drilledwith seawater instead of drilling muds
and without mechanical pressure control, so SWF events are
readily observed as dramatic flows of sandy slurries from the
well. The more precise method of detection is through the use
of PWD technology. This has proven helpful in real-time
detection and quantification of the overpressures so weighted
fluids may be introduced rapidly for flow control.
Mitigation of SWF is accomplished in two ways. Some
operators chose to "topset" the suspected SWF zone and
then set another string of casing immediately below the
zone after drilling through the zone with weighted mud to
counterbalance the overpressured pore fluids. This method
protects the wellbore from the influx of pore fluids and
unconsolidated sands while the SWF section is drilled, then
protects the weak formation from the heavier weighted
muds needed to drill the deeper portions of the well. This
~~, ... 'o.!_~_!'L~'~~_~~!'.!'U2' ~..~'~._~._"'2..~..111 ... 7" ". 7.. "" 7911813621"2 " ... ... ... .29 .., ... 972
7CO
...
Figure 2: When estimating Vp/Vs ratios for shallow-waler Row detection, abnormally high Vp/Vs ratios
or blue and purple anomalies between 600 ms and 700 ms indicate possible sand boc/ies responsible for
shallow-water Row.
approach has proven effective but is expensive because of
the requirement for the extra casing string.
An alternative is to use dual-gradient techniques. The
simplest of these is riserless drilling using weighted drilling
fluids. In the technique known as "pump and dump," the
weighted drilling fluids are not returned to the surface but
are allowed to vent from the wellbore at the seafloor. The
dual-gradient is because the normal pressure build-up
through the water column and the higher gradient of
pressure increase because of the weighted drilling fluids
from the seafloor downward. This technique reduces the
pressure of the drilling fluids on the formation and avoids
fracturing and fluid loss into the SWFzones while being able
to counterbalance overpressured pore fluids.
This technique has proven effective,but it has some drawbacks.
Other systems are under development to mimic
riserless drilling by providing mud lift from the sea floor.
"Closed" dual-gradient systems are being developed to return
the drilling fluids to the surface, which has several advantages:
... expensive muds can be recovered;
a wider variety of muds can be used; and
cuttings can be returned and examined on the rig.
/n..the technique known as (pump and dump,"
tkg weighted drilling fluids are not returned to the surface but are allowed
tq,pe,gt.frorn the wellbore at the seafloor.
DEEPWATER. JULY 2003

Drilling -
Shallow Water Flows
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y
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about the nature of the phenomenon
and how to cope with it. New and
emerging techniques are availabJe for
predicting the existence of the sands and
the degree of overpressure in them as
well as for drilling them safely. The
skilJs and knowledge of several diverse
disciplines are needed in this effort.
With communication and a common
understanding of the problem by the
geoscience and engineering disciplines,
progress has been made in mitigating
the hazard and drilling efficient
deepwater wells. ~
.~
~1
common data point locations on the line
Figure3: In analyzing the deviation in the estimated Vp/Vs ratio from background Vp/Vs
from a seismic inversion, purple and red zones have abnormally high Vp/Vs, possibly
indicating sand bodies responsible for shallow water flows.
Conclusion
While SWFsands have been a major ha7,ardto deepwater
drilling and development, experience has taught much
About the authors: Alan R. Huffman
(huffman@fusiongeo.com) is president
and chief executive officer of Fusion
Petroleum Technologies Inc. in The Woodlands,
Texas. Robert J. Bruce is a fusion
resenlOir associate at Fusion Petroleum Technologies Inc.
John P. Castagna is a professor of geophysics at the
Universityof Oklahoma.
s I
103 W. Boyd St.
Norman, OK 73069 F u
o~
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www.fusiongeo.com