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Mediterranean Journal of Basic and Applied Sciences (MJBAS)
(Referred International Journal), Volume 2, Issue 3, Pages
01-18, July-September 2018
1 | P a g e ISSN (Online): 2581-5059 Website: www.mjbas.com
Estimation of Land Surface Subsidence Induced by Hydrocarbon
Production in the Niger
Delta, Nigeria, using Time-Lapse Orthometric Leveling Data
Etim D. Uko
1, Dickson A.Famuyibo
2 and Kenneth Okiongbo
3
1Department of Physics, Rivers State University, PMB 5080, Port
Harcourt, Nigeria. 2Science Laboratory Technology Department, Ken
Sarowiwa Polytechnique, Bori, Rivers State, Nigeria. 3Department of
Physics, Niger Delta University, Wilberforce Island, Bayelsa State,
Nigeria.
Article Received: 12 March 2018 Article Accepted: 27 June 2018
Article Published: 17 July2018
1. INTRODUCTION
Land surface subsidence and reservoir compaction due to fluid
withdrawal has created a great interest due to its
relevance in gas, oil and groundwater extraction. One serious
problem associated with petroleum production is
ground subsidence resulting from reservoir compaction (Greetsma,
1973). In Po River delta around Venice, the
subsidence rate measured between 1968 and 1969 had increased
from its low historic rate to 1.7cm/yr in the
industrial area and 1.4cm/yr in the city centre (Brighenti and
Mesini, 1986). Goose Creek field south of Houston, in
1918, subsided more than 0.9m (Pratt and Johnson, 1926; Snider,
1927). The Wilmington field in California (USA)
subsided 10m, Lake Maracaibo fields in Venezuela subsided 3.5m
(Sroka and Ryszard, 2006). The Groningen in
Netherlands showed noticeable subsidence on seafloor at about
24.5cm (Poland and Davies, 1969). The Norwegian
North sea fields (Ekofisk and Eldfisk and Calhall fields)
reservoirs compacted; resulting in current subsidence rate
of 20cm/yr. The Ekofist field also showed formation pressure
decline from the discovery of 7200psi to a potential
abandonment at 3200psi resulting in decrease in porosity from
38% to 33% (Barkved and Kristiansen, 2005).
Low-strength carbonate reservoirs in Northwest Java field,
Indonesia, and fields offshore Sarawak, Malaysia, have
also experienced significant subsidence (Susilo et al., 2003).
The Belridge field in California and neighbouring
diatomite fields subsided and had numerous well failures
(Fredrich et al., 1996). Compaction is the decrease in
volume of a reservoir resulting from pressure reduction and
production of fluids (water, oil and gas). The term
compaction and subsidence describe two distinct processes.
Compaction is a volumetric change in a reservoir while
subsidence is a change of level of a surface. The surface could
be a formation top, the mudline in a submarine area
or a section of the Earth’s surface above the compacting
formation. Land subsidence can lead to flooding over wide
areas, particularly when unfavourable meteorological events of
high-tide, sea storm, and wind blowing in the direction of
the shore take place (Carbognin et al., 1984b; Carbognin et al.,
1984a). These situations could be aggravated by erosion
AB STRAC T
Time-lapse orthometric levelling measurements, acquired in 1988
and 2003 in the south-east Niger Delta basin, are used to estimate
surface
subsidence resulting from hydrocarbon withdrawal. The value of
the subsidence was determined by finding the differences from the
orthometric
heights in the base and the monitor surveys. The elevation
ranges between –30m along river channels and 3m for the base 3D
survey while that for
the monitor survey shows elevation of -27m to 5m. Hydrocarbon
production in reservoir under this area was 89.52stb/day initially
and declined to
13.92stb/day, and the reservoir pore-fluid pressure depletion is
only 674psi, initially at 3833psi but dropped to 3159psi in 15
years. The results from
the analysis show that the rate of land subsidence at each
location of levelling varies from 66.67mm yr-1
to 200.00mm/yr with an average of
86.00mmyr-1
. When comparing the land subsidence trend, hydrocarbon
production and reservoir pressure declines, there is no positive
correlation
between the three phenomena. This is an indication that land
subsidence is localized where the measurements are carried out
mainly in river channels
and slopes caused by erosion, and not on a regional scale. The
results of this work can be used for engineering and environmental
works.
Keywords: Hydrocarbon-production, orthometric-levelling,
subsidence, Niger delta, Nigeria and time-lapse.
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Mediterranean Journal of Basic and Applied Sciences (MJBAS)
(Referred International Journal), Volume 2, Issue 3, Pages
01-18, July-September 2018
2 | P a g e ISSN (Online): 2581-5059 Website: www.mjbas.com
of the river channels leading to a retreat of the shoreline.
Land subsidence is capable of upsetting an area's entire
hydraulic system (Carbognin et al., 1984a; Gambardella and
Mercusa, 1984). Moreover, damage can be done to
buildings, in addition to listing severe cracks due to sudden
facies changes and, therefore, sudden compressibility
changes due to land subsidence and reservoir compaction (Capra
et al., 1991; Cancelli, 1984).
In some fields, the compacting reservoir acts as a support for
enhancing petroleum (Mah and Draup, 2004). The
motion in a subsidence can have devastating effects on
pipelines, roads, and other structures unless they are
designed to accommodate the strain (Gambardella and Mercusa,
1984). The bowl formed by subsidence affects
pipelines, roads and other structures. Lateral movement within
the bowl can generate damage. A fault extending to
the surface can generate step-offsets resulting in damage to
structures crossing the fault (Zodoc and Zinke, 2002).
The Belridge of California and neighbouring diatomite fields
subsided and had numerous well failures, including
loss of air gap between the lower decks and the maximum
water-wave height (Gambardella and Mercusa, 1984).
Prior to production of hydrocarbon from The Groningen gas fields
in The Netherlands had had no recorded seismic
activity. Since 1986, there have been several tremors in these
fields, some causing minor damage to property
(Gambardella and Mercusa, 1984). In the Norwegian fields, chalk
flow like toothpaste. In limestone formation,
sanding is common. Barkved and Kristiansen (2005) report a
strong correlation between overburden faults, casing
failure and borehole breakout at the Valhall Field.
Faults in the overburden also can reactivate because of
differential movement, and the bedding planes may have
differential slippage movement. A compacting formation pulls the
cemented casing along with it, compressing the
axial dimension of the casing. The stress on the casing can
exceed its mechanical strength and cause collapse within
the compacting zone or fail in tension in the overburden.
In coastal regions, vertical movements of the surface may result
in flooding or generate extra costs for securing the
banks. The production of oil in Venezuala, where subsidence
above a number of important oil reservoirs bording
Lake Maracaibo is a constant phenomenon, and huge dykes have
been built to protect the coastal area from
flooding (Greetsma, 1973). In the Houston-Galveston area, land
subsidence induced by large-scale groundwater
withdrawal since 1906 has been up to 3m (Gabrysch and coplin,
1990). The implication of elevation changes in
coastal wetlands can have dramatic impact on the wetland
ecosystem as Reed and Cahoon (1992) suggest that a
slight decrease in elevation can lead to frequent flooding that
can deteriorate, and eventually destroy, vegetation.
Erosions followed by the loss of vegetation will further
accelerate the loss of wetland in these areas (Segall, 1989).
Compaction and subsidence can also lead to changes in porosity
and permeability which have implications for
production performance. Compaction of reservoir rocks represents
a major drive mechanism. Significant volumes
of produced hydrocarbons may be credited to this effect. In the
weak chalks of the North Sea and the diatomites of
California, the rock-drive can be many times greater than
fluid-expansion drive. Formation permeability can
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Mediterranean Journal of Basic and Applied Sciences (MJBAS)
(Referred International Journal), Volume 2, Issue 3, Pages
01-18, July-September 2018
3 | P a g e ISSN (Online): 2581-5059 Website: www.mjbas.com
increase or decrease, because open fractures can close or new
fractures can be generated. Matrix permeability
generally decreases as the pore spaces collapse or grains break.
The chalk failure can lead to improved production.
Faults in the overburden can be reactivated because of
differential movement, and bedding planes may have
differential slippage (White and Tremblay, 1995). Reactivation
of faults might lead to leakage of hydrocarbons and
affect the reservoir drainage patterns. Reactivated faults can
have close relation with light tremors. Prior to
production of hydrocarbon from The Groningen gas fields, this
part of The Netherlands had had no recorded
seismic activity. Since 1986, there have been several tremors,
some causing minor damage to property (Segall,
1989). Zodock and Zinke (2002) recorded numerous
microearthquakes at the Valhall Field during a six-week
monitoring period. They found the microearthquakes consistent
with a normal faulting stress regime.
There is no publication at the time of writing this Paper in
seismic and geomechanics carried out in any part of the
Niger Delta with the main objective of ascertaining ground
surface subsidence that may result from hydrocarbon
withdrawal in the oil-prolific basin. Some of the local
geological studies aimed primarily at establishing the oil and
gas potential of the area (Ejedawe, 19810; Ekweozor and Daukoru,
1994; Doust and Omatsola, 1990). The purpose
of this paper is to use time-lapse orthometric geodetic
levelling method in the study area in an attempt to estimate
ground subsidence over a producing reservoir. The results of
this work can be used for engineering works as
changes in vertical positions (height/elevation) affect
infrastructures and other related activities in the area of
study.
2. GEOLOGY OF STUDY AREA
The study area covers an area of approximately 287.27km2 and it
has bounding coordinates of 496184.787E –
516997.287E and 55127.772N – 68930.318N in Transverse Mercator
Nigerian Mid-belt projection and Minna
datum. The south-east Niger Delta, Fig. 1, where it is located
is part of the sedimentary complex which detailed
geological information has been published by several authors
(Short and Stauble, 1967).
Fig. 1: Map of the Niger delta showing the area of study
The study area comprises mainly the New Calabar, Cawthorne
Channel Rivers, a lot of creeks, and tributaries (Fig.
1). The topography of study area and environs is relatively
below sea-level, ranging from –30m along river
channels. Production in reservoir under the study field peaked
89.52stb/day initially and declined to 13.92stb/day in
15 years of production. The reservoir dimension is thickness is
52m, length 4000m, and width of 617m at the depth
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Mediterranean Journal of Basic and Applied Sciences (MJBAS)
(Referred International Journal), Volume 2, Issue 3, Pages
01-18, July-September 2018
4 | P a g e ISSN (Online): 2581-5059 Website: www.mjbas.com
of 2410mss. Four wells penetrate this reservoir having
penetrated to average datum depth of 2940mss. Its Initial
pressure was 3833psi dropped to 3159psi in 15 years.
3. THEORETICAL BACKGROUND
3.1 Formations involved in subsidence
The formations involved in subsidence resulting from fluid
withdrawal are divided into four parts: the compacting
volume, the overburden, the sideburden and the underburden (Fig.
2). The last two terms refer to materials laterally
connected to the compacting formation and those beneath it and
the sideburden, respectively.
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Earth’s Surface
Subsidence
Overburden
Compaction
Si
de
-B
ur
de
ns
Reservoir Reservoir
Si
de
-B
ur
de
ns
Si
de
-B
ur
de
ns
Si
de
-B
ur
de
ns
Fig. 2: Sketch of the formations involved in subsidence
resulting from fluid withdrawal
The compacting volume may include more than the
hydrocarbon-bearing formation. Aquifers beside or below may
also compact as they drain, and should be modelled as part of
the compacting formation, albeit with different
properties in many cases. The decrease in volume caused by
compacting a buried formation is usually transmitted
to the surface. The subsidence bowl is generally wider than the
compacted area. The amount that it spreads depends
on the material properties of the overburden and the depth of
the compacting formation. In addition, if the
overburden does not expand, the volume of the bowl at surface is
equal to the compaction volume at depth.
A subsidence bowl tends to be approximately symmetric, even if
the compaction in the underlying volume is not.
Because the bowl is a superposition of subsidence resulting from
each compacting element, it tends to average out
the variation. Overburden anisotropy from faults or material
anisotropy can restrict or change the shape of the bowl;
faults can allow slippage, preventing the spread of
subsidence.
The overburden can also expand, although this is a minor effect
for most overburden rocks. However, this volume
change can result in a time-dependent effect as the overlying
rock slowly creeps, first in expansion and later in
compaction. When a formation compacts, the sideburden often does
not, either because it is impermeable,
separated from the compacting formation by a sealing fault and
therefore not experiencing an increase in effective
stress, or simply because it is stronger material. The
overburden weight that had been supported by the compacting
formation can now be supported partially by the sideburden. This
creates what is termed a stress arch over the
compacting formation. The extent and effectiveness of the stress
arch in supporting overburden are functions of the
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Mediterranean Journal of Basic and Applied Sciences (MJBAS)
(Referred International Journal), Volume 2, Issue 3, Pages
01-18, July-September 2018
5 | P a g e ISSN (Online): 2581-5059 Website: www.mjbas.com
material parameters of the over- and sideburden, the lateral
extent of the compacting zone and the amount of
compaction. Although the predominant motion in a subsidence bowl
is vertical, horizontal movements also occur.
The horizontal movement is zero in the middle and at the outer
boundary of the bowl and reaches a maximum,
inward displacement in between.
3.2 CAUSES AND CONSEQUENCES OF LAND SUBSIDENCE DUE TO FLUID
WITHDRAWAL
Subsidence is a sinking of a surface, such as ground level,
relative to a stable reference point. It involves principally
a downward movement/ displacement of surface material caused by
natural or artificial removal of underlying
support. It is the net sum of tectonic activity, isostatic
adjustment, sediment compaction, fluid withdrawal and sea
level rise. Most deltaic areas experience relatively great
subsidence balanced by large input of river-borne
sediments under natural conditions. If the river is channelled,
diverted or damaged, subsidence may be
uncompensated. Surface subsidence and compaction slightly differ
in their connotation. Compaction refers to
thickness reduction of a given formation whereas subsidence
refers to a decrease in elevation of the ground
surface. Compaction is the decrease in volume of a reservoir
resulting from pressure reduction and production of
fluids (water, oil and gas), and sand. Compaction is a
volumetric change in a reservoir while subsidence is a change
of level of a surface. The surface could be a formation top, the
mudline in a submarine area or a section of the
Earth’s surface above the compacting formation. Subsidence
occurs over a much larger area than the areal extent of
the reservoir rock undergoing compaction. The difference between
surface subsidence and compaction at any given
point is determined by depth, mechanical properties of the
overburden and the areal extent of the reservoir.
Evidence of subsidence of geological strata should not be
considered as a general lowering of the land surface. It is
a natural process in areas of unconsolidated sediments because
it reflects the gradual compaction of deeply buried
sediments in response to overburden pressure. Generally
equilibrium exists between sediment supplied to the
surface and subsidence so that the land levels do not
significantly change. Activities by man, however, can create
subsidence at the land surface by reducing the sediment supply
and by accelerating compaction of the sediments.
Large scale flood control and drainage schemes or river
diversion can interrupt sediment supply while extensive
groundwater withdrawals could significantly reduce subsurface
ground water pressure leading to increased vertical
compaction of sediments.
Compaction of a geological formation resulting from pore
pressure decline, and the accompanying subsidence can
pose serious environmental problems. Pumping of ground water is
known to cause surface subsidence in Santa
Clara and San Joaquin Valleys (California), in areas of Mexico
City, Houston-Galveston (Texas), Savannah
(Georgia) and Bangkok (Thailand) [Colazas and Strehle,
1995].
Production of oil and gas can also lead to ground subsidence, in
relatively shallow reservoirs. While this poses
environmental problems, formation compaction provides an
important drive for oil and gas production. There are
cases such as Bolivar Coast fields in Western Venezuela where
80% of the oil production had been due to
compaction of the reservoir rock. Measurement and prediction of
surface subsidence is of interest for en-
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Mediterranean Journal of Basic and Applied Sciences (MJBAS)
(Referred International Journal), Volume 2, Issue 3, Pages
01-18, July-September 2018
6 | P a g e ISSN (Online): 2581-5059 Website: www.mjbas.com
vironmental reasons and also from the standpoint of reservoir
production. Production histories of fields exhibiting
compaction-subsidence phenomena show that incremental subsidence
volumes are roughly equal to incremental
fluid withdrawals except during the initial period, suggesting
that compaction is the principal production
mechanism (Colazas and Strehle, 1995).
Compaction of a geological formation resulting from pore
pressure decline, and the accompanying subsidence can
pose serious environmental problems. Pumping of ground water is
known to cause surface subsidence in Santa
Clara and San Joaquin Valleys (California), in areas of Mexico
City, Houston-Galveston (Texas), Savannah
(Georgia) and Bangkok (Thailand) [Cahoon et al., 1999).
Production of oil and gas can also lead to ground subsidence, in
relatively shallow reservoirs. While this poses
environmental problems, formation compaction provides an
important drive for oil and gas production. There are
cases such as Bolivar Coast fields in Western Venezuela where
80% of the oil production had been due to
compaction of the reservoir rock. Measurement and prediction of
surface subsidence is of interest for en-
vironmental reasons and also from the standpoint of reservoir
production. Production histories of fields exhibiting
compaction-subsidence phenomena show that incremental subsidence
volumes are roughly equal to incremental
fluid withdrawals except during the initial period, suggesting
that compaction is the principal production
mechanism.
Surface subsidence and compaction slightly differ in their
connotation. Compaction refers to thickness reduction
of a given formation whereas subsidence refers to a decrease in
elevation of the ground surface. Compaction is the
decrease in volume of a reservoir resulting from pressure
reduction and production of fluids (water, oil and gas),
and sand. Compaction is a volumetric change in a reservoir while
subsidence is a change of level of a surface. The
surface could be a formation top, the mudline in a submarine
area or a section of the Earth’s surface above the
compacting formation. Subsidence occurs over a much larger area
than the areal extent of the reservoir rock
undergoing compaction. The difference between surface subsidence
and compaction at any given point is
determined by depth, mechanical properties of the overburden and
the areal extent of the reservoir.
Land subsidence is usually caused by the removal of fluids
(water, gas, or oil). The principal lithological and
structural characteristics of the subsiding areas include the
following (Allen et al., 2005): Sediments are
unconsolidated and lack appreciable cementation; Sediment
section is thin; Porosity of the sands is high (20 –
40%).
Sands are interbedded with clays, fine silts and/or siltstones,
and shales; Fluid production is voluminous; Standing
fluid levels in the wells exhibit large drops; In the case of
water-producing areas, aquifers cover large areas and are
shallow and flat-lying; Subsidence rate is cyclic, controlled by
seasonal fluid-level fluctuations; Age of sediments
is Pliocene or younger in the case of water-producing horizons
and Miocene or younger in the case of oil-producing
areas; Producing formations are located at shallow depth, 300 –
1000m (1000 – 3300ft); Overburden is composed
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7 | P a g e ISSN (Online): 2581-5059 Website: www.mjbas.com
of structurally weak sediments; In oil-producing areas, the
reservoir beds have flat or gentle dips at the structure
crest; Tension-type faulting, often with graben central block,
are present; and Reservoir fluid pressures are lowered.
4. MATERIALS AND METHODS
4.1 Field Production and reservoir pressure history
Reservoir hydrocarbon production and pressure histories were
collected (Tables 1 and 2; Figs. 3 and 4).
Table 1: Rock and Reservoir Properties
Variables Symbol Value Unit
Volume of reservoir V 372.67x106 m
3
Average reservoir radius R 2000 m
Reservoir depth of burial D 8327 ftss
Average well datum Z 9670 ftss
Top reservoir Ztop 8251 ftss
Base reservoir Zbase 8402 ftss
Average reservoir thickness H 151/46.02 ft/m
Poisson’s ratio 0.199 -
Young’s modulus E 39.42x10-3
Kgs-2
m-1
Initial reservoir pore fluid pressure Pi 4220 psi
Final reservoir pore fluid pressure Pres 3500 psi
Reduction of pore fluid pressure ΔP 4220 – 3500
=720
psi
Compaction factor Cm 2.26x10-6
(psi)-1
Porosity 25 %
Water saturation Sw 13 %
Rock density ρs 2.65 gcm-3
Time interval between base and monitor
surveys
tb – tm 5400/15 days/years
Table 2: Production history for combined Wells.
Time
(Day)
Oil production
(Mmbbl)
Pressure
(psi)
Oil Production
Rate (Mmbbl/Day)
Pressure
Depletion
Rate
(psi/Day)
250 72146 4203 288.58 16.81
266 71919 4077 4494.94 254.81
432 69798 4002 373.62 21.99
591 67428 3994 424.08 25.12
675 53083 3962 631.94 47.17
815 51065 3909 364.75 27.92
1115 49214 3881 164.05 12.94
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1374 43704 3876 168.74 14.97
1438 36961 3870 577.52 60.47
1778 34255 3906 1007.5 11.49
1928 32049 3868 213.66 25.79
2176 26217 3921 105.71 15.81
2385 26191 3912 125.32 18.72
2801 23810 3944 57.24 9.48
3804 23069 3937 23 3.93
4137 21094 3928 63.35 11.8
4562 19938 3948 46.91 9.29
5704 16073 3947 14.07 3.46
6580 15507 3872 17.7 4.42
6634 12637 3866 234.02 71.59
7265 9670 3833 15.32 6.07
8664 7073 3783 5.06 2.7
9064 5742 3694 14.36 9.24
9440 4389 3694 11.67 9.82
Fig. 3: Oil production-Time cross plot for combined Wells.
4.2 Determination of Surface Subsidence using Time Lapse
Orthometric Heights
The first (baseline) levelling was acquired in 1988. The survey
area covers an approximate area of 200km2. A number of
stations were destroyed by development after 1988 survey. The
second (monitor) was acquired in 2003. The time-lapse data
can be useful to observe the changes induced by hydrocarbon
production or by implementation of any enhanced oil recovery
(EOR) method, if the data is repeatable (Vedanti et al., 2009).
The residual differences in the repeated surveys, which are not
related to the changes in the reservoir affect the applicability
of time-lapse surveys and act as time lapse noise.
Repeatability
errors arose from not being able to achieve replicated the exact
base survey levelling positions due to obstruction around base
positions.
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Fig. 4: Presure-Time cross plot for combined Wells.
In considering the repeatability of the monitor survey, the same
factors as in previous baseline surveys such as traverse
receiver
and source lines and positions were used for the monitor seismic
acquisition. We could not meet these requirements
completely. Most parts of the field have been developed with
wellheads, settlements, pipelines and flow stations. Some of
the
base survey levelling positions could not be replicated because
of these obstructions around settlement in addition to the
facilities. These urbanisation and industrial growth make repeat
levelling surveys highly challenging both technically and
operationally.
To determine changes in orthometric heights in the two epochs,
the heights from 1988 and 2003 surveys are
compared at each measurement point, and the differences in the
heights are used to determine the magnitude of any
vertical land-surface changes. The vertical land-surface
changes, between the 1988 base and 2003 monitor surveys,
were calculated by differencing the orthometric heights of the
levelling determined for the two surveys, and are
presented Table 3, which is used to contour orthometric heights
over baseline and monitor maps.
In order to approximate the repeatability of the monitor survey,
the monitor survey levelling data was acquired as close to base
survey locations as possible by moving close to obstructions.
Baseline 3D and monitor 4D data were overlaid on one another
and gridded adequate spatial data distribution, and to determine
areas where data points are the same (Fig. 5). In so doing,
some
of the monitor points which were not coincident with the base
position but were 10m away from the base locations were also
selected, and we achieved 77.58% repeatability. The 22.42%
repeatability error arose from not being able to replicate the
exact
monitor survey levelling positions due to obstruction around
base positions.
Levelling measurements were made at the geodetic monuments to
determine their ellipsoid heights. Ellipsoid
height is the vertical coordinate relative to a geodetically
defined reference sea-level ellipse. To determine changes
in ellipsoid heights, the heights from 1988 and 2003 surveys are
compared, and the differences in the heights are
used to determine the magnitude of any vertical land-surface
changes. The vertical land-surface changes, between
the 1988 base and 2003 monitor surveys, were calculated by
differencing the ellipsoid heights of the geodetic
monuments determined for the two surveys. Most of the study area
field is now developed with wellheads, settlements,
pipelines and flow stations. Some base survey levelling
positions could not be replicated because of the obstruction
around
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10 | P a g e ISSN (Online): 2581-5059 Website: www.mjbas.com
settlement in addition to facilities. These urbanisation and
industrial growth make repeat levelling surveys highly
challenging
both technically and operationally.
Fig. 5: Map showing the 3D and 4D fused images and the
corresponding seismic data.
5. RESULTS AND DISCUSSION
From Table 3 and Figs. 3 and 4, there is reduction in production
over time can be observed resulting from
corresponding reservoir pressure decline. Since production and
pressure have depleted, reservoir compacts leading
to surface subsidence. The 3D baseline elevation contour map
over baseline map of 1988 is presented in Fig. 6. The
4D monitor contour map over monitor map of 2003 is presented
Fig. 7, while the difference (Base - Monitor)
elevation map over monitor map of 2003 showing the land
subsidence as presented in Fig. 8. The rate of subsidence
contour map is plotted over baseline map of 1988 to show the
rate of land subsidence over time, and is presented in
Fig. 9. The average rate of ground surface subsidence in the
study area is 0.860cm/year. According to Allen et al.,
(1971), land subsidence can be caused by the removal of fluids
(water, gas, or oil) when fluid production is
voluminous, and the standing fluid levels in the wells exhibit
large drops. In our study, voluminous hydrocarbon
had been produced which declined with time thus meeting Allen et
al (1971) requirement. They further said that for
land subsidence to take place, producing formations are located
at shallow depth, 300 – 1000m. In our research, the
reservoir is deeply-seated at a depth of 2410mss. Another
criterion for land subsidence is that the reservoir beds
should have flat or gentle dips at the structure crest (Allen et
al (1971). The reservoir under study is in a steep
complex collapsed-crest roll-over anticline elongated in an E-W
direction, with two crests, separated by a saddle.
Hatchell and Bourne (2005a) and Allen et al (1971) observed in
their works that, in addition to the above and other
factors, reservoir compaction and land surface subsidence take
place when the reservoir pore-fluid pressure is
lowered by 1000s psi. The reservoir pore-fluid pressure
depletion in the Field of study is only 674psi.
Subsidence caused by hydrocarbon production in the study area is
legible due to the depth of the reservoir
(2410mss), and the subsidence affects only the immediate area
and do not affect the field of study on a regional
scale. This conclusion regarding minimal impacts of hydrocarbon
production on land subsidence is based only
orthometric height difference. It is neither on subsurface data
from the producing field nor from any numerical or
analytical models that incorporate the physical changes of the
reservoir formations associated with stress changes.
Surface deformation data can only be explained by a combination
of reservoir compaction.
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Table 3: Orthometric information for Base (3D) and Monitor (4D)
Surveys
Site Eastings Northings
A
(m)
B
(m)
C
(m)
D
(cm/year)
Site Eastings Northings
A
(m)
B
(m)
C
(m)
D
(cm/year)
1 499127.00 64120.00 2 1 1 6.67 22 510014.00 66986.00 2 1 1
6.67
2 499128.00 64170.00 2 1 1 6.67 23 510016.00 67136.00 2 2 0
0
3 499129.00 64270.00 2 1 1 6.67 24 510016.00 67186.00 2 1 1
6.67
4 499130.00 64320.00 2 1 1 6.67 25 510017.00 67236.00 2 1 1
6.67
5 499171.00 67322.00 1 1 0 0 26 510017.00 67286.00 2 1 1
6.67
6 499171.00 67372.00 1 1 0 0 27 513514.00 66943.00 2 2 0 0
7 499171.00 67422.00 2 1 1 6.67 28 513516.00 67043.00 2 2 0
0
8 500562.00 66703.00 -10 -9 -1 -6.67 29 513517.00 67143.00 2 2 0
0
9 500564.00 66804.00 -9 -7 -2 -13.33 30 513518.00 67193.00 2 1 1
6.67
10 500564.00 66854.00 -8 -7 -1 -6.67 31 514193.00 65434.00 -10
-8 -2 -13.33
11 509707.00 64415.00 2 2 0 0 32 514198.00 65484.00 2 1 1
6.67
12 509807.00 64413.00 2 2 0 0 33 514198.00 65534.00 2 2 0 0
13 504405.00 66556.00 -5 -5 0 0 34 514199.00 65584.00 2 2 0
0
14 504410.00 66657.00 -2 -2 0 0 35 499470.00 63566.00 2 1 1
6.67
15 504416.00 67156.00 2 2 0 0 36 499787.00 60616.00 -6 -5 -1
-6.67
16 504417.00 67206.00 2 2 0 0 37 499787.00 60662.00 -6 -6 0
0
17 504417.00 67256.00 2 2 0 0 38 499788.00 60712.00 -7 -6 -1
-6.67
18 504418.00 67306.00 2 2 0 0 39 499814.00 63061.00 2 1 1
6.67
19 504419.00 67356.00 2 2 0 0 40 499815.00 63111.00 2 1 1
6.67
20 509657.00 64415.00 2 1 1 6.67 41 499819.00 63412.00 2 1 1
6.67
21 509707.00 64415.00 2 2 0 0 42 504369.00 63406.00 2 2 0 0
43 504370.00 63456.00 2 2 0 0 64 514135.00 60535.00 2 2 0 0
44 504371.00 63506.00 2 2 0 0 65 514136.00 60585.00 2 2 0 0
45 504371.00 63556.00 2 2 0 0 66 514136.00 60635.00 2 2 0 0
46 507158.00 62421.00 2 2 0 0 67 499465.00 57042.00 2 2 0 0
47 507158.00 62471.00 2 1 1 6.67 68 499515.00 57041.00 2 2 0
0
48 507159.00 62521.00 2 1 1 6.67 69 499564.00 57041.00 2 2 0
0
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49 509234.00 60594.00 2 1 1 6.67 70 499614.00 57040.00 2 2 0
0
50 509234.00 60644.00 2 2 0 0 71 502639.00 59001.00 -10 -9 -1
-6.67
51 509236.00 60745.00 2 2 0 0 72 498402.00 60055.00 1 1 0 0
52 509236.00 60795.00 1 -3 4 26.67 73 498452.00 60054.00 1 1 0
0
53 509237.00 60845.00 2 1 1 6.67 74 498502.00 60054.00 1 1 0
0
54 510245.00 63408.00 2 1 1 6.67 75 498551.00 60053.00 1 1 0
0
55 510295.00 63407.00 2 1 1 6.67 76 507289.00 58944.00 2 2 0
0
56 510345.00 63407.00 2 1 1 6.67 77 507339.00 58944.00 2 2 0
0
57 510395.00 63406.00 2 2 0 0 78 507389.00 58943.00 2 2 0 0
58 510320.00 63382.00 2 2 0 0 79 504105.00 59984.00 -7 -7 0
0
59 510320.00 63432.00 2 2 0 0 80 504203.00 59985.00 -6 -6 0
0
60 512058.00 62257.00 -8 -5 -3 -20.00 81 509541.00 57091.00 -9
-8 -1 -6.67
61 512058.00 62308.00 -2 1 -3 -20.00 82 509542.00 57141.00 2 1 1
6.67
62 512058.00 62360.00 2 1 1 6.67 83 509543.00 57191.00 2 2 0
0
63 514134.00 60485.00 2 2 0 0 84 509543.00 57241.00 2 2 0 0
85 512783.00 58376.00 2 2 0 0 89 510276.00 60082.00 -8 -8 0
0
86 512883.00 58375.00 2 2 0 0 90 510279.00 60133.00 -8 -8 0
0
88 510280.00 60033.00 -8 -8 0 0 91 510942.00 57174.00 -10 -9 -1
-6.67
92 510947.00 57224.00 -10 -10 0 0
93 510944.00 57273.00 -11 -11 0 0
A = Leveling-derived orthometric height (m) for 3D Base survey
(1988); B = Leveling-derived orthometric height (m) for 4D Monitor
survey (2003) after 15 years; C =
Difference (Base – Monitor) – Subsidence after 15 years; D =
Annual subsidence rate (cm/year).
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0 2000 4000 6000 8000
Fig. 6: 3D Baseline Elevation Contour Map over Baseline Map of
1988
0 2000 4000 6000 8000
Fig. 7: 4D Monitor Contour Map over Monitor Map of 2003
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0 2000 4000 6000 8000
Fig. 8: 3D-4D Difference Elevation Map over Monitor Map of
2003
0 2000 4000 6000 8000
Fig. 9: Rate of Subsidence over Baseline Map of 1988
The 3D baseline elevation contour map over baseline map of 1988
is presented in Fig. 6. The 4D monitor contour
map over monitor map of 2003 is presented in Figs. 7, while the
difference (3D-4D) elevation map over monitor
map of 2003 showing the land subsidence as presented in Fig. 8.
The rate of subsidence contour map is plotted over
baseline map of 1988 to show the rate of land subsidence over
time, and is presented in Fig. 9. The average rate of
ground surface subsidence in the study area is 0.860cm/year.
According to Allen et al. (1971), land subsidence can
be caused by the removal of fluids (water, gas, or oil) when
fluid production is voluminous, and the standing fluid
levels in the wells exhibit large drops. In our study,
voluminous hydrocarbon had been produced which declined
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15 | P a g e ISSN (Online): 2581-5059 Website: www.mjbas.com
with time thus meeting Allen et al. (1971) requirement. They
further said that for land subsidence to take place,
producing formations are located at shallow depth, 300 – 1000m.
In our research, the reservoir is deeply-seated at a
depth of 2410mss. Another criterion for land subsidence is that
the reservoir beds should have flat or gentle dips at
the structure crest (Allen et al., 1971). The reservoir under
study is in a steep complex collapsed-crest roll-over
anticline elongated in an E-W direction, with two crests,
separated by a saddle. Hatchell and Bourne (2005a) and
Allen et al. (1971) observed in their works that, in addition to
the above and other factors, reservoir compaction and
land surface subsidence take place when the reservoir pore-fluid
pressure is lowered by 1000s psi. The reservoir
pore-fluid pressure depletion in the Field of study is only
674psi.
Subsidence caused by hydrocarbon production in the study area is
legible due to the depth of the reservoir
(2410mss), and the subsidence affects only the immediate area
and do not affect the field of study on a regional
scale. This conclusion regarding minimal impacts of hydrocarbon
production on land subsidence is based only
orthometric height difference. It is neither on subsurface data
from the producing field nor from any numerical or
analytical models that incorporate the physical changes of the
reservoir formations associated with stress changes.
Surface deformation data can only be explained by a combination
of reservoir compaction.
From Figs. 3 and 4, there is reduction in production over time
at the rate of 9.23mmbbl/day or 3369.21mmbbl/year
resulting from corresponding reservoir pressure decline at the
rate of 0.099psi/day or 36.0psi/year. Since
production and pressure have depleted, reservoir compacts
leading to surface subsidence. Average rate of ground
surface subsidence in the study area is computed to be
0.775cm/year. The horizontal displacement from the centre
of subsidence has been computed to be 0.611cm/year.
Elevations in the area of study are highly variable in the two
epochs of 1988 (Base) and 2003 (Monitor), ranging
between -11m and 2m above mean level as presented in Table 3.
The subsidence in the river basins is much higher
than in the flat land part. This can be explained from
topographic and drainage structures of study area, where the
land part as an alluvial plane has much higher compressibility
than the river-channel part which is heavily under
erosion. The average rate of ground surface subsidence based on
the difference in orthometric heights between
1988 and 2003 is estimated at 0.860cm/yr.
6. CONCLUSION
The rate of land subsidence at each location of levelling
monitoring points varies from 66.67mm yr-1
to
200.00mm/yr, and an average of 86.00mmyr-1
. When comparing the land subsidence trend and the oil
production
and reservoir pressure declines, there is no positive
correlation between the three phenomena. This is an indication
that land subsidence in this Field is localized where the
measurements are located. Localized subsidence associated
with this Field is mainly in river channels and slopes caused by
erosion. Subsidence caused by hydrocarbon
production in the study area is negligible due to the depth of
the reservoir (2410mss), and the subsidence affects
only the immediate area and do not affect the field of study on
a regional scale. This conclusion regarding minimal
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impacts of hydrocarbon production on land subsidence is based
only orthometric height difference, and not on the
reservoir stress changes.
Ground surface subsidence and reservoir compaction caused by
hydrocarbon production in the area of study is
negligible due to the depth of the reservoir (2578.07m) and the
subsidence affects only the immediate area and do
not affect the land on a regional scale. This conclusion
regarding minimal impacts of hydrocarbon production are
based also on subsurface data from the producing reservoir that
incorporate the physical changes of the formations
associated with depletion and the corresponding stress
changes.
The rate of land subsidence at each location of levelling
monitoring points varies from 66.67mm yr-1
to
200.00mm/yr, and an average of 86.00mmyr-1
. When comparing the land subsidence trend and the oil
production
and reservoir pressure declines, there is no positive
correlation between the three phenomena. This is an indication
that land subsidence in this Field is localized where the
measurements are located. Localized subsidence associated
with this Field is mainly in river channels and slopes caused by
erosion. Subsidence caused by hydrocarbon
production in the study area is negligible due to the depth of
the reservoir (2410mss), and the subsidence affects
only the immediate area and do not affect the field of study on
a regional scale. This conclusion regarding minimal
impacts of hydrocarbon production on land subsidence is based
only orthometric height difference, and not on the
reservoir stress changes.
ACKNOWLEDGEMENTS
The authors thank Shell Petroleum Development Company (SPDC) of
Nigeria, Port Harcourt for provision of data.
Our gratitude also goes to Survey & Geomatics, Exploration
and Quantitative Interpretation Departments of SPDC
for software and general assistance.
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