Shallow Water - Trouble in the Deep - Lumina Geophysical

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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

;S

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

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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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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~

405-364-8663 (phone)

405-321-7571 (fax)

25231 Grogan's Mill Road, Suite 175

The Woodlands, TX 77380

281-363-4903 (phone)

281-363-4657 (fax)

www.fusiongeo.com

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