Copyright 2008, AADE

This paper was prepared for presentation at the 2008 AADE Fluids Conference and Exhibition held at the Wyndam Greenspoint Hotel, Houston, Texas, April 8-9, 2008. This conference was sponsored by
the Houston Chapter of the American Association of Drilling Engineers. The information presented in this paper does not reflect any position, claim or endorsement made or implied by the American
Association of Drilling Engineers, their officers or members. Questions concerning the content of this paper should be directed to the individuals listed as authors of this work.


Abstract
Drilling in the clastic over pressured sediments,
mainly sand and shale, is subject to the compaction
disequilibrium phenomenon. This phenomenon is the
main catalyst for subsurface geopressure
compartmentalization due to sediments stratifications
and subsurface structural setting. As a result,
geopressure transgression and regression take place in
the open borehole section. Keeping the Mud Weight
(Equivalent Circulating Density) in balance with the
transgressive and regressive formation pressure is a
challenge.
Strong pressure transgression can push the
formation pressure envelopes to the fracture limit.
Increasing the Mud Weight (MW) to overcome this
excess pressure can cause loss of circulation (LOC).
Pressure transgression can be detected ahead of the
drill bit from seismic velocity, mud logs and LWD
measurements.
Strong pressure regression can cause a substantial
LOC in sand beds. As a result, it creates thick mud cake
on the sand face which leads to drilling hurdles such as
hole bridging, torque and drag. Contrary to
transgression, regression is hard to predict during
operation, therefore causing more drilling abandonment
and side tracks.
Salt interface in several of the sub salt prospects is
characterized by a sharp drop in pore pressure.
Therefore, loss of circulation is common during
penetration of the zone between the base of salt and the
underlying sediments. Side tracks and drilling bypasses
drastically increased the drilling costs of several wild
cats in the frontier deep water subsalt plays.
Case histories of several Gulf of Mexico wildcats
exhibited causes of LOC due to geopressure
compartmentalization, especially in HPHT and sub salt
environments:
Use of overbalance MW exceeds the
drilling tolerance window (FP-PP)
Drilling the interface between salt and
sediments in the sub salt prospects without
advance watchfulness.



Application of MW profile template of a well
drilled on the crest of a structure to a down
dip offset well.
Drilling a wet zone using the same pressure
profile of pay section.
Subsurface geopressure risk assessments, prior to
drilling, can point to zones of instability along the
proposed well trajectory in advance.

Introduction
Loss of Circulation occurs for several reasons. Some of
these are related to the drilling process, such as bad shoe
and incompetent cement job. Others are due to naturally
fractured formations in hard rocks and limestone. Stratified
clastic sediments (sand,mud,shale,silt) where the majority
of oil and gas exploration takes place, are the subject of
this paper.
Lost circulation from a hydraulic stand point, means the
mud column equivalent circulation density (ECD) pressure
exceeds the clastic sediment formation pressure.
Permeable (sands and coarse silt) formations are good
catalysts to vacate mud from the annulus, under these
circumstances. In the case where the ECD exceeds the
fracture pressure in the low permeable beds (shale and
clay stone), LOC can take place. Therefore, the optimum
mud weight (Fig. 1) to be used should stay in the drilling
tolerance window (fracture pressure pore pressure).



AADE-08-DF-HO-22
Loss of Circulation:
Causes and Consequences in Geopressured Systems
Selim S. Shaker, Geopressure Analysis Services (G.A.S)
Unconfined Hydrodynami c
Confined Geopressured
Pressure
PS
H PP
Mud Weight
Drilling Tolerance window
FP
Figure 1: Pressure plot shows the drilling tolerance window
2 Selim S. Shaker AADE-08-DF-HO-22
This paper sheds light on the geological settings that
cause the substantial changes in pore pressure subsurface
profile that leads to LOC and the costly consequences to
the drilling operation.

Geopressure Transgression and Regression:
Deposit of additional sediments in a structurally relaxed
(extensional) basin leads to an increase of the principal
stress (Terzaghi, K. 1943) and consequently results in a
higher transgressive pore pressure profile. This
transgressive pressure (PT) profile is usually
represented by a cascade path, as long as the basin
subsidence accommodates the volume of sediment input
with the absence of structural failure (Fig. 2).
















Conversely, in the case of structural failure and/or when
PP reaches the limit of the fracture pressure (FP),
pressure regression (PR) takes place. The common
regression phenomenon (Fig. 3) is usually a result of the
presence of communication paths through faults and salt
interfaces between the deep, highly pressured reservoir
and the shallower, lesser pressured reservoir.




















Fracture pressure (FP) represents the high perimeter
where impermeable beds yield to hydraulic fracturing by
reaching the matrix minimum stress. The difference
between the FP and PP usually represents the Drilling
Tolerance Window. The drilling mud pressure ideally
stays within the limit of this window. The tolerance
window (FP-PP) varies in magnitude depending on the
PP envelopes shift and the associated FP values. It
tends to be narrow (less tolerant) in the normally
pressured, shallow young deposits and the deeper, high
pressure / temperature environment (Fig. 1). As a result,
shallow water flow (SWF), hydraulic fracturing, loss of
circulation, and flow-kill-breakdown cycles are dominant
drilling problems in this narrow window. On the other
hand, a wider window (more tolerant) prevails in a large
portion of the geopressured (<0.8 PG) section. This
leads to relatively fewer drilling problems. Hydrocarbon
accumulation favors this wider window zone.
The size and direction of the pressure envelopes shift
across the interface, from the seal (shale) to the
reservoir (sand), are responsible for shaping the PP and
FP profile with depth. In case of a large trangressive
shift, the bore hole can suffer a hard kick, especially if a
hydrocarbon-bearing reservoir is encountered. Mud
weight management at this interface is highly
recommended. If substantial overbalanced mud weight
is used to hold the well bore walls intact (stable) in the
seals, a possible thick mud cake build up forms, facing
the reservoir sandy beds below. A larger borehole forms
facing the seals and, conversely, a tight, smaller
borehole forms facing the reservoirs due to LOC.
The process of healing the LOC sometimes leads to the
phenomenon of Flow Kill- Break down. Bypass and
side track is the ultimate solution to such a treatment.
This increases drilling costs and might change the well
bore trajectory and miss the initial objectives.

In case of pressure regression due to structural failure
(mainly fault cuts) and hydraulic fracturing, PP in the
reservoir drops to a lower PP envelope. A substantial
LOC takes place if mud weight tracks the predicted
pressure in the shale above the reservoir. This leads to
thick mud cake and consequently to hole bridging,
sticking pipes and excessive torque.
Most of the commercially discovered fields in the deep
water of the Gulf of Mexico are characterized by large
transgressive geopressure shifts (>1000 psi). This shift
takes place at the interface between the cap shale (seal)
and the targeted reservoir compartment. Popeye (Green
Canyon 72 #A-1), Fuji (Green Canyon 506 #1), Mickey
(Mississippi Canyon 211#1), Ursa (Mississippi Canyon
809 #1) and South Rampowell (Viosca Knoll 1001#1)
fields show PT shifts usually exceeding 1000 psi.




Pressure Envelopes
M
P
P
P
P
P Seal
Reservoir
Type
Rock
AA
Figure 2: PP profile in a Transgressive system
M
P
P
P
P
P
Transgression Regression
AA
P
P
P
Figure 3: PP profile in a Regressive system
AADE-08-DF-HO-22 Lost of Circulation: Causes and Consequences in Geopressured System 3
West Cameron Block 96, Well #1:
This well (OCS-G-15055 #1) was completed as dry and
abandoned (D&A) by Kerr McGee in April 1998. A
substantial increase in MW from 10.8#(5897 psi) to 16#
(9226 psi) was applied to cap the PP increase in the
process of penetrating the transition zone (Fig. 4)
between the normally and abnormally geopressured
systems, subsequently, the pore pressure shows a
transgression envelope at 11,200 feet and MW was
increased to16.7#(11900 psi). Using both velocity and
resistivity to predict PP, a geopressure regression at
12,500 feet was noticed. At that depth, pressure breach
was most likely a result of the presence of a fault cut at
this level. Due to the relatively overbalanced mud
weight, and the proximity between the mud pressure and
fracture pressures, especially deeper than 14,000 feet,
several drilling obstacles occurred.
Borehole size was enlarged facing the shale beds and
conversely, it was very tight in several sections facing
the sand beds due to excessive LOC. This led to several
borehole bridges with high torque on the drill pipes and
high tension spots on the wire line tools. The well was
P&A.

.
















East Break 645 #1
This is a good example that exhibits the Flow - Kill -
Breakdown phenomenon (Fig. 5) in the deep
geopressured system rather than the shallow water flow
case. The mud weight of 11.4 ppg was in balance with
the top shale section where the background gas was
minimal from 9700 to 10220 feet (mud log). The pore
pressure prediction plots from the seismic velocity shows
this relationship where the MW is 0.5 ppg over the
predicted pore pressure in this section.
The nearest well with known pore pressure
measurement in the same section is KMG 689#1
(1 mile to the south). In this well, the shale above the
reservoir was drilled with 11.8#mud. Penetrating the
shale/sand interface the well flowed back and the mud
was cut to 11.5#. Killing the flow, 12.0#MW was used
and increased to 12.8 at the bottom of reservoir. Using
the same MW template to drill EB 645#1 led to several
trials to heal the hole and drill ahead. The Centroid effect
(Shaker, S. 2004) was a factor in the Plugging and
Abandonment of EB 645#1 due to the structural
difference between the two prospects. Frequent
circulation loss can be the result of geopressure
compartmentalization and/or mechanical failure.























Salt Toes and Fold Belts
The new deeper exploration fairway is associated with
the creeping salt toe at the Sigsbee Escarpment
(Chowdhuerry, A. and Lopaz-Mora, S. 2004). As a
result, the lateral stress has generated compressional
fold/fault structural plays in the Wilcox (equivalent
sediment) below the salt. The fault plane in this
structural setting usually yields high sealing capacity.
Perdido, Walker Ridge and the Mississippi fold belts are
the new, exciting and promising exploration plays
needed to rejuvenate domestic energy needs. Based on
the released data from these frontier wells, a
geopressure model (Fig. 6) has been proposed and
might explain some of the trapping mechanism and
drilling challenges facing the industry (Shaker,S. and
Smith, M. 2002).






10000
11000
12000
13000
14000
15000
0 6000 12000 18000
Pressure (psi)
D
e
p
t
h

(
f
t
)
P r e 1.2 P t e 1 MW psi FP
Hydro Litho CSG
T O G
P.Transgression
P. Regression
SP. Tens. Caliper
Figure 4: Drilling challenges in WC 96 #1
Gr ant EB 654#1
Por e Pr essur e Pr edict ion f r om Seismic
4000
6000
8000
10000
12000
14000
16000
7 9 11 13 15 17 19
Pr essur e (EMW)
D
e
p
t
h

(
f
t
)
OB ppg PP ppg MW ppg
LOT ppg Calc LOT ( Frac)
SaltBase
Gr ant EB 654#1
Por e Pr essur e Pr edict ion f r om Seismic
4000
6000
8000
10000
12000
14000
16000
7 9 11 13 15 17 19
Pr essur e (EMW)
D
e
p
t
h

(
f
t
)
OB ppg PP ppg MW ppg
LOT ppg Calc LOT ( Frac)
SaltBase
Proximity of MW to
PPP (EB 645#1)
Pressure
Transgression
Figure 5: PP plot of EB 645#1 shows the cause of
Flow-Kill-Breakdown phenomenon
4 Selim S. Shaker AADE-08-DF-HO-22























In addition to the salt buoyancy effect, on the sediment
below and above the salt, rafted sediment blocks
embedded in the salt mass and gouges (furrows filled
with transported and crumbled sediments) at the base of
the salt impact the subsurface geopressure profile. If
these older rafted blocks are cased with impermeable
layers, PP will show a high gradient. In the case of
plowing the older sediment underneath the salt toe, the
shear stress will substantially reduce the PP in this thin
rubble layer underneath the moving salt (Fig.6). Sub salt
gouges represent a drilling difficulty and hazard in
frontier exploration plays. In addition, the salt buoyancy
will accelerate and decelerate pore pressure above and
below the salt respectively.

Atlantis Field
Atlantis field represents an exploration success of
testing a prospect below the salt toe. This probably will
not be the general case in the frontier lower Tertiary
fairway where the targeted traps are located down-dip
from the tip of the toe.
The geopressure plot of the discovery well #1 ST#2
in Green Canyon Block 699 (Fig.7) exhibits the
relationship between the principal stress, overburden
and fracture pressure. Note, the fracture pressure is in
the proximity of the calculated overburden above the
salt, whereas principal stress far exceeds the
overburden and fracture pressure in the sub-salt. This
leads to a wide window of retention capacity and the
presence of a thick column of oil especially between
17800 and 18500.
On the other hand, the reduction of PS in the sub-salt
section led to a moderate PG of +/- 0.61 psi/ft at depth
+/- 18400 (MDT measurements). This can be attributed
to the thick salt layer above (+/- 7000) and the water
depth of the mud line (-4495). Mud weight was
increased to 10.5 ppg and an extra casing point was set
in the middle of the salt due to the presence of rafted
sediments within the salt body. Moreover, the possible
presence of the interface salt-sediment gouge, causing a
sharp drop in the PP, was responsible for sidetracking
the original hole and multiple drilled bypasses.
The presence of several wet sandy rich sections
below the pay zone, which started at 18500 to TD
(19500 TVD) and concurred with an increase of MW to
12.4 ppg (overbalanced), led to thick mud cake, stuck
pipes, plug back and side track. BP is planning to put
Atlantis field on line the next year. The projected daily
production is estimated to be 250 Mbblod and 180
MMcfgd.





















Jack Prospect
The J ack prospect is a part of the emerging
Wilcox-equivalent salt toe belt at the Sigsbee
Escarpment in the Deep Water of the Gulf of Mexico. It
is located in the Walker Ridge (WR) Blocks 758, 759,
and 678. Applying the same geopressured model
designed for the salt toe ( fig.6) can explain the drilling
hurdles of WR 759 #1 ST 00BP00 (OCS-G-17016),
which jacked up the drilling cost to over one hundred
million dollars. These challenges were:
Lost return at the base of the salt (at depth 19653).
Pump LCM sweep of 13.5#
inability to stop losses
Side track had to be performed
Set Casing at 13507 at the salt mass, possibly due to
presence of rafted block sediment



OB
SS
Gouge
SB SB
Geopressured
Normally Pressured
FP
P S
PP
Lower Terti ary
Fold Belt s New
Discoveri es
Rafted sediments
P
S
Mud Line
Figure 6: Geopressure model of the salt toe at the
Sigsbee Escarpment
Rafted Sediments Salt
PS
FP
(RC)
Modified after Shaker and Smith,2002
PS
Salt
FP
Rafted
Sediments
(RC)
MDT
Gouge
Rafted Sediments Salt
PS
FP
(RC)
Modified after Shaker and Smith,2002
PS
Salt
FP
Rafted
Sediments
(RC)
MDT
Rafted Sediments Salt
PS
FP
(RC)
Modified after Shaker and Smith,2002
PS
Salt
FP
Rafted
Sediments
(RC)
Rafted Sediments Salt
PS
FP
(RC)
Modified after Shaker and Smith,2002 Modified after Shaker and Smith,2002
PS
Salt
FP
Rafted
Sediments
(RC)
MDT
Gouge
Figure 7: PP plot of GC 699#1 shows the large drop in
PP below the salt
AADE-08-DF-HO-22 Lost of Circulation: Causes and Consequences in Geopressured System 5

Conclusions
Geopressure compartmentalization is a double edged
sword. It is the main catalyst for hydrocarbon
entrapments, and yet it causes PP shift which creates
drilling problems. Most of the troubles spots commence
at the interface zones between the seals and reservoir
type rocks. Pressure transgression and regression take
place at the interface zone due to the geological setting
and state of stresses in the basin.
In the transgressive zone, a larger hole can be
developed in the seal and a smaller one facing the
reservoir due to LOC. The overbalanced mud results in
loss of mud and a relatively thick mud cake. This leads
to sticking pipes, borehole bridging, high torque and
erroneous logging measurements.
Drilling the regressive zones, with overbalanced mud,
usually leads to LOC and creates a bore hole bridging
(tight), sticking pipes and excessive torques facing
reservoirs.
Pre-drilling pore - fracture pressure prediction is
essential to dependable wildcat drilling prognoses.
During drilling, it is important to administer the MW
program, especially the ECD, to keep the bore-hole
stable in the shale zones and free of bridges facing the
sand zones.
Sub-salt prospects represent a serious challenge
especially in the Deep Water frontier areas around the
World. The salt-sediment interface zone needs to be
modeled and thoroughly probed before the drill bit
strikes it.
On site drilling surprises can be minimized in advance
by forecasting depth to top of geopressure (TOG),
pressure gradient changes in shale beds with depth,
pressure envelopes shift (transgression-regression) in
sand beds, fracture matrix coefficient, and expected
hydrocarbon density and height. Therefore, pore
pressure prediction using pre-stacked velocities, in
addition to geopressure basin modeling from the offset
wells are vital for pre-spud well planning. It is essential to
use all the geological building blocks to estimate the
pressure differential between the seals and reservoirs
expressed in PSI and PPG MWE.

Nomenclature
H =Hydrostatic pressure
PP=Pore pressure
FP=Fracture pressure
PS=Principal stress
OB=Overburden
SB=Salt buoyancy
RC=Retention capacity
PPP=Predicted pore pressure
MPP=Measured pore pressure
PPG MWE=Pound per gallon mud weight equivalent


References
1.Shaker, S. and Smith, M. Pore pressure prediction in the
challenging supra/subsalt exploration plays in deep water,
GOM,Extended abstract, AAPG Convention, 2002
2.Terzaghi, K. Theoretical Soil Mechanics, J ohn Wiley and Sons,
Inc., New York, 1943
3.Shaker, S. Geopressure compartmentalization in Keathley
Canyon, deepwater of GOM GCAGS transaction, 2001
4.Shaker, S. Trapping vs. breaching seals in salt basins: A case
history of Macaroni and Mt. Massive, Aluger Basin,
GOM,GCAGS Annual Convention, San Antonio, TX, 2004
5.Chowdhurry, A. and Lopez-Mora, S. Regional geology of deep
water salt architecture, offshore GOM,GCAGS and
GCSSEPM, 2004