estimation of land surface subsidence induced by hydrocarbon …mjbas.com/data/uploads/4001.pdf ·...

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.




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




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




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




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-




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




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




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.




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




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.




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




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).




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




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




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




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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