Evaluation of Optimum Mud Weight Window for Prevention of Wellbore Instability in Niger Delta Wells

Abstract

A Borehole instability problem contributes significantly to increase in non-productive time (NPT) and overall cost of the drilling. These problems can occur in a variety of forms including stuck pipe, loss circulation, hole enlargement, breakout. Borehole problems such as drill pipe sticking, hole collapse, breakout, ,caving and tight holes have been experienced during drilling of wells in an oil field in the Niger Delta. This work analyzed the major cause of wellbore instability in a field in the Niger Delta by using data from two wells drilled and proposes an optimum mud window for cost-effective and safe drilling operations. Geomechanical model was used to evaluate the in-situ stress and induced stresses with Mogi Coulomb and Mohr Coulomb failure criteria to predict the breakout profile and estimate the optimum mud weight to avoid sticking of the drill string. The result shows that the Mogi failure criterion was 1.46sg and 1.39sg while Mohr failure criteria were 1.48sg and 1.42sg for well 1 and well 2 respectively. Based on the results obtained, Mogi coloumb is preferable to Mohr coloumb criteria. Mogi failure criteria give more accurate result to obtain optimum mud weight window for the field considered in the Niger Delta. Therefore, in the prediction of an optimum mud weight window for any well in the Niger Delta, mohr failure criteria should be employed and adopted.

Full text

IOSR Journal of Engineering (IOSRJEN) www.iosrjen.org
ISSN (e): 2250-3021, ISSN (p): 2278-8719
Vol. 10, Issue 10, October 2020, ||Series -I|| PP 61-66
International organization of Scientific Research 61 | Page
Evaluation of Optimum Mud Weight Window for Prevention of
Wellbore Instability in Niger Delta Wells
Nmegbu Godwin Chukwuma, Bright Bariakpoa Kinate and Samuel Nmezi
Frances
Department of Petroleum Engineering, Rivers State University, Nigeria
Received 10 October 2020; Accepted 26 October 2020
Abstract: A Borehole instability problem contributes significantly to increase in non-productive time (NPT)
and overall cost of the drilling. These problems can occur in a variety of forms including stuck pipe, loss
circulation, hole enlargement, breakout. Borehole problems such as drill pipe sticking, hole collapse, breakout,
,caving and tight holes have been experienced during drilling of wells in an oil field in the Niger Delta. This
work analyzed the major cause of wellbore instability in a field in the Niger Delta by using data from two wells
drilled and proposes an optimum mud window for cost-effective and safe drilling operations. Geomechanical
model was used to evaluate the in-situ stress and induced stresses with Mogi Coulomb and Mohr Coulomb
failure criteria to predict the breakout profile and estimate the optimum mud weight to avoid sticking of the drill
string. The result shows that the Mogi failure criterion was 1.46sg and 1.39sg while Mohr failure criteria were
1.48sg and 1.42sg for well 1 and well 2 respectively. Based on the results obtained, Mogi coloumb is preferable
to Mohr coloumb criteria. Mogi failure criteria give more accurate result to obtain optimum mud weight window
for the field considered in the Niger Delta. Therefore, in the prediction of an optimum mud weight window for
any well in the Niger Delta, mohr failure criteria should be employed and adopted.
Keywords: wellbore instability, Mohr Criterion, Mogi Criterion, Geochemical Property, mud weight window
I. INTRODUCTION
Wellbore instability problems are very common in the drilling of oil and gas wells. During drilling
process, the wellbore may collapse or cave-in. Formation rock may fracture resulting to loss of substantial
volume of drilling fluid. This can lead to a number of severe problems during drilling operation like; stuck drill
string and consequent fishing, sidetracking and reaming operations, reduction in fluid hydrostatic pressure due
to decrease in mud column height, drilling string twisting or parting due to excessive torque and drag, or even a
complete loss of wellbore. Generally, the issue of wellbore instability and other similar challenges significantly
contribute to the already high cost of well construction. Wellbore instability is more common and severe in
shale rock formations. Wellbore instability issues in wells drilling results to lose of budget due to unexpected
and unplanned events caused, which translates to huge amount of money wasted during the well delivery
process.
In this research project, wellbore instability issues in an oilfield in the Niger Delta are investigated.
The oilfield, which is designated “S” Field in this research work, contains oil and gas reservoirs and is located in
the Niger Delta sedimentary basin, Nigeria. The field was first explored in 1993 but has not been producing due
to the fact that no well has been successfully drilled and put to production in the field. The non-production has
resulted to loss of the huge revenue that would have been accruing to the economy, as well as loss of jobs. For
almost 25 years no drilling campaign has been carried out since the two wells drilled in the field experienced
instability issues during drilling, including stuck pipe and loss circulation incidents. These problems led to the
plugging and abandonment of the wells and field after costing the company a huge fortune.
Wellbore instability is a function of formation pore pressure, in-situ stresses, and rock strength properties.
Before a well is drilled, the formation is in equilibrium. Once drilling starts, formation rock is removed. The
wellbore becomes subjected to the stresses surrounding it due to removal of the rock. This changes the in-situ
stresses near the wellbore wall, resulting to stress concentration. This stress concentration will lead to a failure
in the wellbore wall. The basic problem therefore, is the accurate prediction of the rock reaction to mechanical
loading during drilling of a borehole.
In order to avoid wellbore failure an appropriate wellbore pressure (mud pressure) need to be altered to
redistribute the stress concentration near the wellbore. Moreover, wellbore orientation with respect to the in-situ
stresses needs to be taken into consideration to prevent the wellbore failure. The easily controllable parameter in
any drilling operation is the drilling mud pressure. The drilling mud pressure can prevent wellbore failure if it
lies between the collapse pressure and formation fracture gradients. It has the added advantage of controlling or
reducing the effect of mechanical wellbore failure (Bourgoyne et al., 1986). Drilling mud performs many

Evaluation of Optimum Mud Weight Window for Prevention of Wellbore Instability in ..
International organization of Scientific Research 62 | Page
functions, such as cooling and lubrication of the drill bit and drill string, cleaning of the drill bit, transport of
drill cuttings to the surface, transmission of hydraulic energy to mud motors and bit through the drill string, and
control of formation pressure. Traditionally, the drilling mud pressure is designed to control the influx of flow of
the formation fluid into the wellbore regardless of the field stresses and the rock strength effects. Normally, in
practice the mud pressure is designed in excess of the formation fluid pressure by 100 to 200 psi (0.3 to 0.5
lb/gal), the exact value depending on the well operator (French & McLean, 1992; Awal et al., 2001).This creates
a positive overbalance between the mud pressure and formation pore pressure which helps to prevent the flow of
the formation fluid into the wellbore. Thus, due to the in-situ stresses, the mud pressure required to sustain the
wellbore should be greater than the pressure required to balance. Therefore, better approaches are sought to
obtain optimum mud pressure based on the accurate evaluation of rock properties, stresses around the wellbore,
and wellbore trajectory to safe and cost-effective delivery of a well.
II. METHODOLOGY
Data used for this investigation were obtained from daily mud reports for determining daily losses and
cutting size, mud logging report for formation lithology, final well report for productive and NPT, well logging
data for borehole breakout zone and stress orientation from two wells drilled in the oil field.
2.1 Geomechanical models for “S” field
From theoretical to experimental aspects, a comprehensive model on the mechanical effects on
wellbore stability in the “S” field was considered in this study. The process of building a geomechanical model
implies the prediction of the elastic and mechanical properties of the formation rock from physical equations
and correlations (Zoback, 2007; Aadnoy,& Looyeh 2019). Then, the magnitudes of three principle stresses
(vertical stress, minimum horizontal stress, and maximum horizontal stress) and pore pressure are calculated.
Hence, pore pressure, rock mechanical properties, and in-situ stresses are considered among the main factors for
building a geomechanical model. In this study, the two wells so far drilled, located 9 km apart are examined.
2.2 Mohr – Coulomb Failure Criteria.
The Mohr-Coulomb model was considered in the analysis of the failure criteria and is presented below
(1)
Where is the shear stress, c is the rock cohesion is the normal stress, and φ is the internal friction angle.
The coefficient of internal friction angle can be formulated as:
(2)
Mohr failure criteria can also be expressed by the maximum and minimum principle stresses, as follows:
(3)
Where are the maximum and minimum principle stresses, respectively. is the unconfined
compressive strength, which is a function of cohesion and internal friction angle and q is the flow factor, which
is related to internal friction angle and can be obtained by:
(4)
(5)
2.3 Mogi-Coulomb criteria
For Proper analysis and comparison, Mogi Coulomb model was also considered and is presented below
(6)
(7)
Mogi also suggested new criteria, defined as:
(8)
Where the octahedral shear stress is expressed as:
(9)

Evaluation of Optimum Mud Weight Window for Prevention of Wellbore Instability in ..
International organization of Scientific Research 63 | Page
After this observation, it became obvious the importance of . Therefore, many 3D failure criteria have been
developed. After performing extensive reviews of rock failure models, Al-Ajmi and Zimmerman (2005)
introduced a 3D failure criterion called the Mogi-Coulomb criterion. This criterion can be formulated as a linear
relation in a similar format to the Mohr-Coulomb criterion as follows:
(10)
Where a and b are material constant and are related to c and φ as follow:
(11)
(12)
III. RESULTS
The minimum mud density is estimated by using a constitutive geomechanical model connected with
two failure criteria (Mohr and Mogi). The input parameters for the geomechanical model are summarized in
Table 1. The recommended mud weight express as specific gravity by using Mohr-Coulomb and Mogi-
Coulomb are listed in Table .2.
Table 1: Input for the geomechanical model (in-situ stresses, pore pressure, and mechanical properties), Case
Study 1.
Young’s
modulus
25.2
Gpa
UCS
22
Mpa
Lal (1999)
Friction angle
22.6
Plumb (1994)
Table 2. The Output of the Geomechanical Models, Case Study 1.
Used
Mohr-
Coulomb
Mogi-
Coulomb
1.44
1.48
1.46
Well-2 is the second well in the “S” field. Its objective was to test the sands penetrated in Well-1,
particularly the zone at which Well-1 failed due to instability. It was also designed to appraise all reservoir sands
indicated in Well-1. The well is located 9.3 kilometers away from Well-1. Instability problems such as caving
and tight spots were observed during drilling the well-2 formation. The lithology description in the Well-2
shows the same description as Well-1. Case 1 procedure was followed to construct the stress profile, predict the
rock strength properties, and optimum mud density for Well-2. Figure 1 shows the in-situ stress and pore
pressure. Rock strength properties are displayed in Figure 2. The input (in-situ stresses, rock strength properties,
and pore pressure) and output parameters are listed in Table .3 and Table 4., respectively.
Parameters
Value
Unit
Source
Depth
2975
m
S
v-
58.21
Mpa
SH
56.2
Mpa
Peng (2007)
S
h
47.5
Mpa
Holbrook et al.
(1993)
P
p
33
Mpa
RFT
Poisson’s ratio
0.21

Evaluation of Optimum Mud Weight Window for Prevention of Wellbore Instability in ..
International organization of Scientific Research 64 | Page
Figure 1. In-situ stresses and pore pressure profile through “S” field Case Study Well-1.
Figure 2.Rock strength parameters, Case Study Well-2
Table3. The Input of the Geomechanical Model, Case Study 2.
Parameters
Value
Unit
Source
Depth
2705
m
Sv
65.72
Mpa
SH
59
Mpa
Peng (2007)
S
h
44
Mpa
Holbrook et al. (1993)
P
p
31.7
Mpa
RFT
Poisson’s ratio
0.26

Evaluation of Optimum Mud Weight Window for Prevention of Wellbore Instability in ..
International organization of Scientific Research 65 | Page
Static Young’s modulus
17.8
Gpa
UCS
29
Mpa
Lal (1999)
Friction angle
22.3
Plumb (1994)
Table 4. The Output of the Geomechanical Model, Case Study 2.
Used
Mohr-Coulomb
Mogi-Coulomb
MW (SG)
1.35
1.42
1.39
Table 5: Input and Output of the Geomechanical Model for the Two Cases.
Parameters
Well-1
Well-2
Depth
2975
2705
Sv
58.61
65.72
S
H
56.2
59
Sh
47.5
44
SH Orientation
45°-50°
45°-50°
Pp
33
31.7
UCS
20
29
Friction angle
22.6
22.3
Poisson’s ratio
0.21
0.26
Young’s Modulus
25.2
17.8
Related problems
Breakout, pack-off and stuck pipe
Tight spots, breakout, pack-off and
stuck pipe
Mogi-Coulomb
1.46
1.39
Mohr-Coulomb
1.48
1.42
Two wells were drilled with mud weights of 1.44sg and 1.35sg, respectively; the results show that the
used mud weights are less than that required to sustain the borehole wall. The Mohr-Coulomb failure criterion
predicts the optimum mud weights to be 1.48sg for well-1 and 1.42sg for well-2, while Mogi-Coulomb predicts
the optimum mud weighs of 1.46sg for well-1 and 1.39sg for well-2. Vernik and Zoback (1992) pointed out that
Mohr-Coulomb failure criterion did not provide realistic results. Recently, Rahimi and Nygaard (2015) stated
that the Mohr-Coulomb prediction showed overestimated value, while the Mogi-Coulomb prediction was more
reliable. In addition, comparing the mud weights of 1.44sg and 1.35sg used in drilling the two wells and the
geomechanical model outputs show that Mogi-Coulomb gives reasonable predictions of 1.46sg for Well-1 and
1.39sg, for Well-2, which are in close agreement with field observation.
IV. CONCLUSION
This research presents a case study in the “S” oil field in Niger Delta. From the results of the geomechanical
analysis of wellbore instability in the two wells drilled in the field, the following conclusions were drawn;
 A geomechanical model was developed using two failure criteria. The analysis of the output of the
geomechanical model shows that the Mogi-Coulomb criterion gives more appropriate results than the
Mohr-Coulomb criterion. Mogi-Coulomb analysis results are in close agreement with field observation.
This is because the Mohr-Coulomb criterion underestimates the rock strength by disregarding the effect of
intermediate principle stress. In contrast, the Mogi-Coulomb criterion gives a more realistic model by
considering the effect of intermediate stress on rock strength.
 Several wellbore collapses, stuck pipe, and shale caving (wellbore failure) were observed in the “S” field.
This wellbore failure was due to the use insufficient mud weight without recourse to appropriate
geomechanical analysis.
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.
Nmegbu Godwin Chukwuma, et. al. "Evaluation of Optimum Mud Weight Window for
Prevention of Wellbore Instability in Niger Delta Wells." IOSR Journal of Engineering
(IOSRJEN), 10(10), 2020, pp. 61-66.