Detection of Abnormal Formation Pressures using Drilling Parameters

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056
Volume: 04 Issue: 06 | June -2017 www.irjet.net p-ISSN: 2395-0072
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Detection of Abnormal Formation Pressures Using Drilling Parameters
Ali Ibrahim Mohammed Ameen1, Prof . Sanjay R. Joshi2
1ME 2nd year, Department of Petroleum Engineering, Maharashtra Institute of Technology (MIT), Pune, India.
2Professor, Department of Petroleum Engineering, Maharashtra Institute of Technology (MIT), Pune, India
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Abstract - The predication ofabnormalformationpressure
is a very important factor in designing suchwellsand ithelpto
avoid many problems through drilling process such as lost
circulation which is caused by using excessive mud weight,
pipe sticking in hydraulic fracturing operation, blowout and
other problems. In wellbore, abnormal Formation pressure
could be caused from formation fluid pressure, if it become
above or below the hydrostatic pressure. The pressures which
be over the hydrostatic pressure referred to an abnormally
high pressure or superpressures. While pressuresthatisbelow
the hydrostatic pressure maybereferredtoanabnormallylow
pressures. The current study deals with the estimation of
overpressures in selected wells in Southern Iraq. Data used in
this study had been obtained fromthemudlogfor WestQurna,
well no.15. Predication of overpressures using drilling
parameters has been achieved by; Raw penetration rate, d-
exponent, dc-exponent and sigmalog.
Key Words: Abnormal pressure, Rate of penetration, d-
exponent, dc-exponent, Sigmalog, Drilling parameters.
1. INTRODUCTION
In the present-day, drilling and completion operations, cost
are the key factors becoming minimum as much as possible
in a maximum well control. In ordertosuccessfullycomplete
a well, proper well planning and drilling operations are
necessary, to minimize the danger of blowouts, stuck pipes,
loss circulation, lost hole, and casing setting problems [1].
Many factors can cause abnormal formationpressure,thatis,
pressure other than hydrostatic. In some area,a collectionof
these factors are prevails. For example, understanding the
importance of petro-physical and geochemical parameters
and their relationship to the stratigraphic, structural, and
tectonic history of a given area or basin, are very necessary
to put the possibility causes of abnormal formation
pressures in appropriate perspective. Because conditions
can vary widely, distinct attention must be taken not to
suppose that the cause of abnormal formation pressure
determined through the experience in a well known area is
necessarily the cause of a similar conditionina nearby basin,
which may not yet have been adequatelytestedbydrilling [2].
2. Description of the Study
The aim of this study is predication abnormal formation
pressure through different formation with a different
lithology of each formation using drilling parameters such
as; rate of penetration (ROP), d and dc-exponent, sigmalog,.
This calculation depend upon data collected with figures
explain the relation of the parameters with depth.
2.1. Pressure Concepts
There are a different type of pressure occur during the
drilling of any well, as explain below [3]:
 Hydrostatic pressure: is equal to the vertical height
of a column of water extending from the surface to
the interesting formation.
 Abnormal formation pressure: is a variation of
interstitial fluid pressure from the hydrostatic
pressure of the subsurface fluid.
 The average total overburden (lithostatic)pressure
gradient: resulting from the combined pressure of
the rocks (grain-to-grain or rock matrix stress) and
their interstitial fluids.
 Fracture pressure: is the pressureinthewellboreat
which a formation will crack.
2.2. Drilling Parameters
2.2.1. Raw Penetration Rate
The rate of drilling is a function of weight on bit, rotary
speed, bit type and size,, hydraulics, drilling fluid, and
formation characteristics. Under controlled conditions of
constant bit weight, rotary speed, bit type, and hydraulics,
the drilling rate in shales decreases uniformly with depth.
This is due to compaction increase in shales with depth.
However, in pressure transition zones and overpressures,
the penetration rate increase. Slower penetration rate is
often spotted in the pressure barrier (cap rock) overlying
this pressure transition. Also any other main lithological
variation in the shales (silty, limey shales, mudstone,.....,etc)
is reflected in penetration rate variations.
Penetration rate should be plotted in (5) to (10) feet
increment in slow-drilling formations or (30) to (50) feet
increments in fast-drilling intervals. Today, drilling rate
recorders are available which automatically plot feet per
hour depth. Regardless of how the rate of penetration is
recorded, one should establish a normal drilling rate trend
while drilling shales in normal-pressure environments for
compaction with faster-drilling overpressured shales [2].
Complication can arise due to bit drilling, which may
disguise any change in penetration rate due to overpressure
[4].

2.2.2.NormalizedRateofPenetration(d-exponent)
The d-exponent was developed to consider changes in the
more significant drilling variables to normalize penetration
rate. It is derived from the fundamental drilling equation
presented by Bingham (1965), which relates penetration
rate to weight on bit, rotary speed, bit size and formation
drilability:
ROP = K ( W / Dh ) Nᵉ (1)
This equation are simplified by assuming (1) the drilability
is relatively constant ( K=1); (2) the rotary speed varies
linearly with penetration rate (e=1). Technical accuracy of
these assumption is questionable. However, as longasKand
N do not vary over too wide range, the results are
proportional. Manipulation for field unites conversion, the
above equation becomes [4]:
ROP = [ ( 12W ) / ( 106 Dh ) ] 6 N) (2)
From thus, the d-exponent can be calculated by:
d = [ log ( ) / log ( ) ] (3)
2.2.3. Normalized Rate of Penetration (dc-
exponent)
The plot of dc-exponent versus depth is similar to that of d-
exponent versus depth. Since the d-exponent is affected by
variations of mud weight, an adjustment has been
introduced to normalize the d-exponent for the effective
mud weight. Mathematically, this cannot be justified, but a
plot of dc-exponent versus depth gives a better pictorial
presentation than d-exponent.Thedc-exponentiscalculated
from the equation[5]:
dc = d . (4)
2.2.4. Sigmalog
At present, drilling parameters are recorded interpreted,
and processed mainly for detection and evaluation of
overpressure at the rig site. To obtain thisinformation, AGIP
(Azienda Generale Italiana Petroli) applies the sigma log
method. The basic equation for sigma log calculation is as
follows [6]:
= (5)
Equation (5) called total raw rock strength equation and is
not corrected for depth, it is in fact considered only accurate
at 7000 m. If the depth is grater or less than 7000m, a
correction must be made, thus:
= + 0.028 ( 7 - D/1000 ) (6)
is simply with a correction for depth that goes to
zero at 7000 m. If all conditions were perfect and thedrilling
fluid used had exactly the same density as the pore fluid
column, then it would be sufficient to plot against
depth. This would give a straight line with a slopthatreflects
increasing compaction and thus rock strength. However, as
all bore holes are drilled in over-or-near balanced
conditions, depth without taking in to account over balance
will result in a curve difficult to interpret. The curve is
corrected for overbalance by the following equation, which
enables one to plot :
= F (7)
Where:
F = 1 + ( ) (8)
The two unknown parameters are thus ΔP and n, where:
ΔP = MW - GPn ) × (9)
The second unknown is n, which is representthefunctionof
the time required to equalize the differential pressure that
exists between the cutting and mud weight. This is
dependent on the lithology and the porosity of the cuttings
themselves, and is therefore reflected in the value of .
- If > 1 (high bit weights and slow drilling):
n = ( 4 - ) (10)
- If < 1 (low bit weight and fast drilling) [1]:
n = (11)
The value of ( ) , plotted versus depth. As a general
trend, this value is always increase with depth through
shales, indicating the presence of normally pressured and
compacted formation. On the contrary, when the curve of
sigma log contains points deviating to the left from the
reference trend line as depth increases, it indicates the
presence of an over pressured zone[6].

3. West Qurna Oil Field (WQ - 15)
It is existed about 70 km NorthwestofBasra cityinsouthern
Iraq. West Qurna is one of the biggest oil fields in Iraq. This
deep well was the fifteenth wells drilledbythe IraqiNational
Oil Company in West Qurna field of Southern Iraq. West
Qurna 15 was the first well that has been drilled near the
crest of West Qurna structure. It is extend from Upper Fars
formation at surface to final depth at Najmah formation at
4400 m. This field contains a certain reserves predestined at
18 billion barrels and reserves potential is estimated at 40
billion barrels. Now, the production of the field is about
120,000 bbl/day, but it can reach to 1 million bbl /day. It is
one of the oils light desired globally, the bottom hole
pressure is around 7200 psi and the number of oil wells is
247 wells, while the number of water injection wells is 64
wells [3].
3.1. Lithology of (WQ - 15)
The formation of this field and its composition which are
covered in this study according to availability of data are
explain through the table below [7]:
Table-1: Formation, depth range and lithology of (WQ-
15).
Formation name Depth rang m Lithology
Tanuma 2170 Black fissile shale,
macro crystalline,
argillaceous,
detritus limestone
Khasib 2218 Chalky,
oligsteginal
limestones
Mishrif 2270 Organic detritus
limestones, beds of
algal, coral-reef
limestone and
limonitic fresh
water limestone
Rumaila 2432 Oligsteginal
limestones, beds of
dolomite,
dolomitic
limestones
Ahmadi 2513 Gray shale,
Limestone
Mauddud 2648 Dolomitize
organic, detritus
limestone
Nahr Umr 2803 Black shales,
grained
sandstones, amber,
pyrite
Shuaiba 2991 Shaly limestone
Zubair 3081 Sandstone,
siltstone,
alternating shale
Ratawi 3406 Slightly pyritic,
shales, beds of
buff, pseudo
oolitic, detritus
limestones
Yamama 3529 Argillaceous
limestones, oolitic
shoal limestones
Sulaiy 2857 Marly limestone,
oolitic limestone
Gotnia 4120 Calcareous shales
and salt
Najmah 4400 Shale with streaks
of limestone
3.2. Pressures Distribution in (WQ-15) Formation
The formation with an abnormal pressure in (WQ-15), are
explain below[7]:
- Shuaiba Formation (Subnormal Pressure):Considerasone
of a critical formations with big problems like of the loss of
drilling fluid circulation (total loss), as well asstuck pipeand
hole failed problems.
- Yamama Formation (Abnormal Pressure): contain oil and
gas in two carbonate units each one with basal reservoir
oolitic limestones overlain by lime mudstone seals.
- Sulaiy Formation (Abnormal Pressure): The problem of
this formation is the appearance of gas with high pressure
that causes the abnormal high pressure with flow of gas
inside the well.
- Gotnia Formation (Abnormal Pressure): Numerous states
of flowing saltwater and gas kicks have been occurred.
- Najmah Formation (Abnormal Pressure):theflowofgasor
salt bed (Abnormality high pressure) problems.
4. Calculation and Results with Discussion
4.1. Rate of Penetration
According to availability of data, raw penetration had been
drawn versus depth to give allusion of the occurrence of
overpressure zone as shown in figure (1) and table (2),
which show that the rate of penetration increased slightlyin
Yamama and Sulaiy formation. While at nearly (4200 m)
depth, we note a high rate of penetration, due to the effect of
Gotnia salt formation which it is denser than compared with
other rocks. Penetration rate in other formation is lower
due to two reasons:
1. The bit show dulling tendencies with depth which
naturally decreases penetration rate throughout the bit life.
2. With an increase in depth, a constant mud density will
result an increase differential pressure across the borehole,
if the pore pressure remains normal.

Fig- 1: Rate of penetration versus depth relationship.
4.2. Normalized Penetration Rate (d-exponent):
Because of the fact that many variables effect upon
penetration rate, so d-exponent was usedtonormalizethese
variables. Equation (3) was used to calculate d-exponent
values from drilling data which includes (penetration rate,
rotary speed, bit diameter, weight on bit, and mud weight).
Example of this calculation shown as below:
- At depth 4025 m
ROP (ft/hr) = 0.677, WOB (1000 Ib) = 18, N (RPM) = 70, Dh
(in) = 8.375
- d = [ log ( ) / log ( ) ]
- d = 2.38752
Table (2), shows these calculation at selected depths. D-
exponent had been plotted versus depth in figure (2). This
plot shows an increase in d-exponent values in normal
pressure zones, while at high pressure zones (Yamama,
Sulaiy, and Gotnia formations), d-exponent values will
decrease. This decreasing in d-exponent may be due to the
effeteness of the lithology of this formations that make a
change in the shale section, such as ( silty shales,mudstones,
limey, marls, etc.).
Table-2: Calculated values of d and dc-exponent at
selected depth.
4.3. Normalized Penetration Rate (dc-exponent):
Since d-exponent method is influenced by mud weight
variations, dc-exponent method will normalize d-exponent
values for the variations of mud weight by using equation
(4).
- For example, at depth, D = 4025 (m), d-exponent =
2.3875269, Mwn = 1.13 (gm/cc), Mw = 2.09 (gm/cc)
- dc = 2.3875269 *
- dc = 1.2908638
The resulted values are tabulated in table (2), and plotted
versus depth in figure (3). This plot shows an increase in dc-
exponent values in normal pressure zones, while at
overpressure zones, dc-exponentvalueswill decreasedueto
the increase of the rate of penetration.

Fig-2: d-exponent versus depth relationship.
Fig-3: dc-exponent versus depth relationship.
4.4. Sigmalog Method:
Interpretation of sigmalog method is based on computing
rock strength values( o)fromdrillingdata whichincludes
(penetration rate, rotary speed, bit diameter, weight on bit,
and mud weight). We started with equation (5) to find the
total rock strength (raw sigmalog) ( t). Which is not
corrected for depth, thereforeequation(6)usedtocorrected
this values in a selected depth.
Equation (7) used to find the value of Sigma log ( o). But
in this equation there is a one parameter called the
overbalance correction (F) which it foundfromequation(8).
The value of Δp) differential pressure will be found from
equation (9). And the value of (n) which represent the time
required to equalize the differential pressure by using
equation (10). Example of this calculation shown as below:
- At depth, D = 4050 (m)
= ( 180.5 × 70 0.25 ) / ( 8.375 × 0.633 0.25 )
= 1.6427634 psi
- Corrected rock strength
= 1.6427634 + 0.028 × ( 7 - 4050 / 1000 )
= 1.7253634 psi
- From Eq (10), find the value of n-function:
n = ( 1 / 640 ) × [ 4 - ( 0.75 / 1.7253634 ) ]
n = 0.0055708
- Eq (9), used to find the value of differential pressure Δp):
Δp = [ 2. 9 × .433 ) - ( 1.13 × 0.433 )] × ( 4050 / 10)
Δp = 168.35 4 psi
- To find the values of overbalance correction (F), used Eq
(8)
F= 1 + [ ]
F = 0.6044462
- Finally, the values of Sigma log found from Eq (7)
= 0.6044462 × 1.7253634
= 1.0428893 psi
The value of ( o) and it calculation are tabulated in table
(3). The plot of ( o) versus depth had been created in
figure (4). Sigmalog plot is similar to d-exponent and dc-
exponent plots, where ( o) values increase with depth,
showing normally pressured zones, while in abnormally

pressured zones ( o) values decrease from the reference
normal trend line.
For the raise of sigmalog plot, shifts of the normal
compaction trends will be created by drawing the best line
through data points for each segment, this is shown infigure
(4). The shifts of sigmalog plot may be due to many causes:
1. Geological cause, such as faults, ....etc.
2. Drilling causes, such as improper bit selection, ..... etc.
Table-3: Calculated values of sigmalog at selected depth
.
Fig-4: Sigmalog versus depth relationship.
5. Conclusions
From figure (1),rate of penetration (ROP) increasedslightly
in Shuaiba formation because, this formation is consider as
an subnormal pressure which is represent a one of critical
formation with differentproblemssuchaslosscirculation,or
stuck pipe. Again a slights increase appear in Yamama and
Sulaiy formation, due to presence of gas with high pressure.
But a high rate occur on Gotnia formation because the affect
of salt which it is denser than the other rocks. In Najmah
formation, ROP will started to decrease but still there is a
sign of abnormal pressure because the effeteness of gas and
salt formation.
In a normalized penetration rate (d-exponent and dc-
exponent) and sigmalog method which showninfigures(2,3
and 4). The values of this techniques decreases in a high
pressure zone (Yamama, Sulaiy, Gotnia and Najmah
formation) due to increase (ROP). And due to the effeteness
of the lithology of this formation that make a change in the
shale section.

NOMENCLATURE
ROP - Rate of penetration (ft/hr)
K - Formation drilability
Dh - Bit diameter (in)
d - Exponent for normalizing penetration rate
N - Rotary speed (RPM)
W - Weight on bit (Ib)
dc - Corrected d-exponent
MWN - Normal mud weight gradient (gm/cc)
MWA (MW) - Mud weight (gm/cc)
- Total rock strength or raw sigmalog (psi)
- Corrected total rock strength (psi)
D - Depth (m)
- Rock strength (psi)
F - Overbalance correction
ΔP - Differential pressure (psi)
GPn - Normal formation pressure gradient for the
area (psi)
ACKNOWLEDGEMENT
A special thanks to may department (Department of
Petroleum Engineering) and professor (Dr. Pradeep B.
Jadhav) Head of the Department to his support.
Thanks and appreciation to my guidance professor (Sanjay
R. Joshi) who has given me his whole potential in giving me
a way to obtain the goal as well as his encouragement to
maintain progress in course.
REFERENCES
[1]. Bhagwan Sahay, Walter H. Fertl, "Origin and Evaluation
of Formation Pressures", Allied Publishers Private Limited,
New Delhi 1988.
[2]. Walter H. Fertl, George V. Chilingarian, and Herman H.
Rieke, "Abnormal Formation Pressures: Implications to
Exploration, Drilling, and Production of Oil and Gas
Resources", Elsevier Scientific Publishing Company,
Amsterdam, Oxford, New York 1976.
[3]. Ahmed Kareem Hassan, " A Study of Abnormal
Formation Pressures Distribution and Their Effect on
Drilling Operation in Middle & South Iraqi Oil Fields",
Petroleum Engg. Dept. Baghdad University, 2016.
[4]. J. R. Jorden and O. J. Shirley, "Application of Drilling
Performance Data to Over-Pressure Detection", JPT, Nov.
1966, pp. (1387-1394).
[5]. Bill Rehm and Ray McClendon, "Measurement of
Formation Pressure from Drilling Data",SPENo.3601,1971.
[6]. P. Belloti and D. Giacca, "AGIP Technology; in Deep
Drilling, Pressure Evaluation and Drilling Performance",
AGIP, Milan, Italy, 1978.
[7]. Saad Z. Jassim and Jeremy C. Goff, "Geology of Iraq",First
Edition, 2006.
BIOGRAPHIES
Ali Ibrahim Mohammed Ameen is a
second year master of engineering (ME),
student in Department of Petroleum
Engineering at Maharashtra Institute of
Technology (MIT), Pune, Maharashtra,
India.
Prof. Sanjay R. Joshi, professor in
Department of PetroleumEngineeringat
Maharashtra Institute of Technology
(MIT), Pune, Maharashtra, India.

Detection of Abnormal Formation Pressures using Drilling Parameters

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