Download - A comparative study of profile permeability Otosigbo, et. al., · permeability versus air permeability reveals that the general regression R2 is 0.212 while the linear equation is

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FUNAI Journal of Science and Technology

3 (1), 2017, 110-123

A COMPARATIVE STUDY OF PROFILE PERMEABILITY AND AIR

PERMEABILITY IN RESERVOIR CHARACTERIZATION: THE CASE OF GX 2

WELL IN WESTERN NIGER DELTA BASIN

Otosigbo, Gloria Ogochukwu

1 , Nzekwe, Kenneth Emeka

2 , Eluwa, Ndidiamaka Nchedo

1

1. Department Of Physics/Geology/Geophysics, Federal University, Ndufu-Alike, Ikwo,

2. Delta Terratek Laboratories Limited, Ajah, Lagos State

(Received 4 February, 2017; Revised 19 June 2017; Accepted:23 June, 2017)

Abstract

Profile permeability has been compared to air permeability to ascertain the prospect of using it as a cost

effective method determining core permeability in reservoir characterization. The study was carried out

using pressure decay permeameter (PDPK-300™). Plugs were extracted of oil prior to measurements in

the case of permeability while in profile permeability the oil was not extracted from the cores. Plugs

dimensions, Length (L) and diameter (D) were taken to be imputed in the Darcy’s equation. In probe

permeability, the ratio of the external radii, ro to internal radii, ri of the probe tip was used to calculate a

dimensionless geometrical factor, Gf with several limiting factors. During air permeability

measurements, the cylindrical plugs were confined one at a time in Hassler cell while in probe

permeability, the slabbed cores were not confined. In GX well, the results of profile permeability (Kp)

were compared with that of air permeability (Ka) at 27 points. Air permeability (Ka) values ranged from

121md to 2870md while probe permeability ranged from 33.9 to 2190mD. The mean values calculated

were 956 and 1001mD respectively for Ka and Kp. A plot of Ka and Kp(log10) versus Depth (ft) revealed

that major deviations were observed at depth between 7265ft(2421.6m) and 7272ft(2424.0m) and also

between the depth 7253ft(2417.6m) and 7256ft(2418.6), which corresponds to Silty Sandstones (high

permeable zone) and Shaly sand (low permeable zone) intervals respectively. A linear graph of profile

permeability versus air permeability reveals that the general regression R2 is 0.212 while the linear

equation is Y = 0.840X, which is equivalent to Kp = 0.840Ka. The deviation of probe permeability values

from air permeability could be caused by coarse surface of sand grains resulting to imperfect sealing of

the probe tip against the core surfaces, heterogeneity caused by bioturbations and diagenetic features

which were accounted for by close samplings and dimensionless geometrical factor lapses.

Keywords: core analysis, profile perm, geometric factor, Darcy

A comparative study of profile permeability... Otosigbo, et. al.,

FUNAI Journal of Science and Technology, 3(1), 2017 Page 111

1. Introduction

Petrophysical properties of sedimentary rocks

are decisive parameters for the quantitative

and qualitative evaluation of reservoir

rocks.Permeability (K) is one of the most

important quantitative parameter for reservoir

which describes the magnitude of flow

through porous rock. Reservoir engineers are

faced with challenges of the equipments and

methods of obtaining accurate permeability

results. More often, different equipments are

used in combination to obtain dependable

results. Reliable permeability values are a

prerequisite for the assessment and modelling

of hydrocarbon reservoirs (Li et al., 1995;

Branets et al., 2009), their economy and

sustainable production (Davies and Davies,

2001; Dutton et al., 1991). They are also

crucial for hydrological studies (Huysmans et

al., 2008; Todd and Mays, 2005). In core

samples, permeability is determined on core

plugs or slabbed cores using gas or liquid

phase. The experiment could be carried out

under steady state or unsteady state. Steady

state flow is where the external temperature

and pressure remains constant throughout the

experiment. The pressure fall-off or pulse

decay flow could be run under steady state.

The flow could be applied axially, transverse

or radially on the core plugs or slabbed cores.

Each method has its advantages and

limitations. For the purpose of this study, gas

phase under steady state have been chosen for

both probe permeameter and air permeameter

method. Both approach are normally

corrected for Klinkenberg(1941) effect .

Klinkenberg effect is as a result of the

slippage of gases along the pore walls which

gives rise to an apparent dependence of

permeability on pressure because gas does not

adhere to the pore walls as liquid does. Often

times, reservoir engineers use more than one

method in combination to achieve accurate

results. Economy and time are also crucial to

the management in preparation to production

of crude oil. Probe permeameter is a fast, easy,

and non destructive equipment used to

measure the permeability at very closed

interval of 1cm. It has the advantage of

obtaining 3D permeability image where cores

from 3 wells are available with their

geographical coordinates. The air permeability

using core plugs is a time consuming method

but it has the advantage of obtaining oil/water

saturation, pore volume/porosity, and easily

solve the problem of geometry resolution

since plugs have definite shapes. This study

critically evaluated the accuracy and reliability

of the profile permeability method compared

to that of air permeability results. Also, to see

the possibility of using probe permeability



results independently for reservoir

characterization.

1.1. Geology of the Study Area

Study cores were taken from GX2 well in

Western Niger Delta Basin (Fig 1). The well is

situated within the coastal swamps depobelts.

From bottom to top, the Niger Delta consists

of three formations; Akata Formation, Agbada

Formation and Benin Formation. The three

diachronous Formations (Akata, Agbada and

Benin) in the Niger Delta were deposited in

each of the five depobelts shown in Fig. 1.

The depobelts are 30–60 km with the oldest

northward while progradation is 250 km

south-westward over oceanic crust into the

Gulf of Guinea (Stacher, 1995). The

interaction of sagging and rate of sediment

supply caused deposition of each depobelt

(Doust and Omatsola, 1990). These depobelts

are in separate unit and represent a break in

regional dip of the delta and is confined

landward by growth faults and basin-ward by

large counter regional faults or the growth

fault of the adjacent basin-ward belt (Evamy

et al., 1978; Doust and Omatsola, 1990).

These depobelts are characterized by distinct

sedimentation, deformation and petroleum

history (Michele et al., 1999). The Akata

Formation which comprises at least 6500m of

marine clays with silty and sandy interbeds

(Whiteman,1982). The Agbada Formation

(petroleum bearing unit), which is

characterized by paralic to marine coastal and

fluvial-marine deposits mainly composed of

sandstones and shale organized into

coarsening upward off-lap cycles (Weber,

1987).

The Benin Formation consists of continental

and fluvial sands, gravel, and back swamp

deposits (2500 m) thick (Reijers (2011). The

sedimentation in each depobelts was caused

by deposition and sedimentation rate with syn-

sedimentary growth fault upsetting the balance

(Evamy et al., 1978). The local stratigraphy

section of the studied well is shown in fig 2.

Total depth of core ranged from 7250ft

(2416.7m) to 7273ft (2440.3m).

Fig. 1: Location of the studied well, GX2

within Coastal Swamp Depobelts.



Dark grey mudstone to fissile shale with sand pinchout.

Depth(m)

2416.7

2418.4

2419.1

2419.5

2421.6

2424

2425

2440.3

Parallel to symmetrical ripple lamination heterolithics sand = 50%, shale/siltstone = 50% folded along laminars. Also cross

bedded with synsedimentaryfaulting. Presence of slump and load structure at

basal contact with shale.

Ripple laminated silty sand. Syndepositional fault.

Soft sediment deformations contorted bedding(chevron folding?)

Presence of carbon streaks.

Cross laminated heterothics, shale: 65%, sandstone: 35%.

Erosional surface, scored. Angular unconformity.

Parallel laminated. Silty sandstone sometimes contortedbedding. Presence of shale laminars. Burrowed.

Ripple laminated cross bedding heterolithics, sand = 55%,shale = 45% burrowed with syn sedimentary faulting. Presence of

loading structures.

Lithology/Grainsize

Lower Shoreface

Description Environment

. . .. . . .... .. ...

.... ..

. ..... . .. . .

L e g e n d

S a n d

R ip p le la m in a t io n

C r o s s b e d d in g

C o n to r te d b e d d in g

P la n a r la m in a t io n

P a r a lle l la m in a t io n

C o a r s e s a n d

S h a leM u d r o c k

H e te r o lith ic s

B u r r o w s /b io tu r b a t io n s

GX 2 Well Lithology Description

Fig 2: Stratigraphic Section of GX2 Well displaying varying lithologies. Total thickness is

23.6(m)



2. Methodology

The two methods used are Air Permeability

using Gas permeameter and Profile

Permeability using mini-permeameter.

2.1. Air permeability-Steady State Method

Permeability was carried out on 27 plugs (Fig.

3) drilled from the cores horizontally to the

sedimentary bedding surface. Three plugs

were drilled in every 3feet core. These plugs

were extracted of oil and water and dried to

constant weight for three days. The

dimensions of the plugs were taken with

calliper to calculate L and A for Darcy’s

equation. The gas permeameter used is a

steady state device using nitrogen gas. The

plugs were confined under pressure of 400psi

in Hassler’s cell one at a time. The gas flow

was axial. Nitrogen gas known viscosity is

injected into the core plug while mounted in a

steel chamber. Inlet pressure was measured

directly at the input sample face using

pressure transducers calibrated to measure

high, medium and low permeability. The

apparatus used was dependent upon the

pressure drop measured across the sample.

Exit pressure was measured at the pressure

transducers. Flow rate was measured directly

by flowing soap film through a graduated

burette. The principal is based on Darcy’s

equation for laminar flow. The average of

three measured consecutive flow rates was

used to calculate the final reported

permeability value.

Fig 3: Plug sample for Air permeability

Permeability, K is calculated in accordance

with Darcy’s Law (1856) as:

Where,

K: permeability in millidarcys,

Q: flow rate in cm3/s

η: viscosity of nitrogen in centipoises

L : the length of sample in cm

A: sample cross-sectional area in cm2

ΔP: pressure difference between the injection

and outflow in PA.

2.2. Probe (Profile) Permeability-Pulse

Decay Method

Profile permeability was run using Pressure

Decay Profile Permeameter (PDPK 300TM)

on the cores slabbed perpendicular to their

bedding surfaces (Jones, 1992). The cores

PA

LQK

https://en.wikipedia.org/wiki/Viscosity



were not extracted of hydrocarbon. The cores

were marked with meter rule at 30 equal

points interval in a 3-feet or 1 metre. The

slabbed core sample was placed onto a core

rack. The probe assembly and the core rack

were moved to a desired measurement

location (Fig 4). The probe was lowered

pneumatically, and its rubber tip was sealed

against the sample. The valve was opened and

gas was flowed through the sample at a

constant measured pressure of about 24.7psi.

The computer system connected to the

permeameter arrangement reads out the flow

rate, temperature and pressures while it

decayed and stabilized. The permeability

reading at stabilization was taken against the

depth and recorded. The procedure was

repeated point to point to obtain a profile cm-

cm. In profile permeability, probe tip was

placed on the slabbed core and the region of

the gas flow was hemispherical. The

geometrical factor (half space solution) was

modelled using the internal and external radii

of the probe tip. Since, it is almost impractical

to obtain a true hemispherical geometry (API,

1998). Goggin, et al.(1988) performed

numerical calculations to produce

dimensionless geometrical flow factors that

account for the lack of a hemispherical cavity

on the surface of a small, finite sample, and a

robe seal of small, finite interior and exterior

radii.

Fig 4: Probe Permeameter in use:

Photographed at Delta Terratek Limited

Laboratory, Lagos.

Fig 5. Schematic Diagram of Steady State

Probe Permeameter ( After API, 1998)

These dimensionless factors are displayed in

Fig 5 as a function of the ratio of the outer

radius, ro, of the probe seal to its inner radius,

ri. The value for Go for true hemispherical is

2π. This is applicable with little error on

samples where depth is at least 4 times the



Fig 6: Dimensionless Geometric Factor for

Measurements with Probe Permeameter. After

Goggin et al. (1988)

radii of the interior of probe seal and where

the lateral boundary is at least 4ri from the

axis of the probe. Geometrical factor Gf is

given by the equation:

iof rGG

Where G0 is obtained from Goggin et al

(1988) (fig 6) to account for hemispherical

and ri is the interior diameter of the probe. Gf

is substituted for L/A in Darcy’s Equation

above.

The probe permeability Kp, is given by

Where:

Q = Flow rate in cm3/s

η = Viscosity of Fluid(Nitrogen gas) in

centipoise

ΔP = Change in pressure

A total of 240 points were run and

permeability values calculated automatically

by the software attached to the system were

recorded against depth.

3. Result and Discussion

The results of the 240 points run for probe

permeability were plotted against the depth in

feet as shown in Fg 6. High permeability

readings were observed at the mid section of

the log around 7258ft depth. This region

consists mainly of silty sand From 7265ft, the

permeability values were lowered, probably

owing to increasing clay contents. This

decreasing permeability is an indication of

decreasing grain sizes as a result of reduced

porosity . Air permeability results were

compared with that of profile permeability at

27 points and the result is shown in Table 1.

The values of air permeability ranged from

121md to 2870md while probe permeability

ranged from 33.9 to 2248mD.

The general results have been grouped into

three ranges of permeability for the dominant

lithology types in the study area as shown

below.

Quality Range Lithology

Type

High

permeability

1000 –

2870mD

silty Sand

Medium

Permeability

200 -

<1000mD

Heterolithics

Low

permeability

<200 –

39mD

sandy Shale

P

rGQK io

p



Probe permeability( Kp) values were higher

than that of air permeability(Ka) at some,

while air permeability are higher at some other

regions. It was observed from the Table 1, Fig

7 and Fig 8 that the values of probe

permeability were mostly higher than that of

air permeability at silty sandstone zones. This

is because of the reasons discussed later on.

Fig. 6: Depth versus profile permeability Log

run at Delta Terratek Laboratory

The values of air permeability were mostly

higher than that of probe permeability at sandy

Shale zones which is low permeable zones.

The cross plots of permeability values from

probe method and gas permeameter shows fair

correlation of both at medium permeable

layers (Fig 7). The regression, R2 = 0.212,

while the linear equation is Y = 0.840X,

which is equivalent to Kp = 0.840Ka or Ka =

1.25Kp or Ka = Kp125%. The air

permeability (Ka) is higher than probe by

25%.

The graph of air and probe permeability

versus depth confirms the deviation at depth

between 7265ft(2421.6m) and 7272ft(2424m)

and also between the depth 7253ft(2417.6m)

and 7256ft(2418.3m)(Fig 8). These depth

ranges were silty Sandstones: high permeable

zone and sandy Shale: low permeable zone

respectively. The graph of depth versus

permeability deviation also revealed the major

deviations at the above depth intervals. For

permeability <100mD, there was significant

deviation of probe from that of air

permeability result (Fig 8). The results affirms

Filomena (2014) conclusion that profile

permeability results show apparent aberration

from air permeability results for low

permeable rocks (<10mD). The large

discrepancy of the probe results from that of

air at the sandy zone could be as a result of

rough surface which eventually affected the

sealing quality of the probe. A leak tightness

and suboptimal probe tip sealing apparently



lead to higher permeability values for probe

than that of air. On the other hand, preferential

flow in Hassler’s cell mainly contributes to the

overall air permeability of the sample. A

technical explanation for underestimation of

probe minipermeameter recordings at the

Shale region could be that a strong contact

pressure of the minipermeameter probe may

slightly force the sealing rubber towards the

inner part of the probe tip (Filomena et. al.,

2014). This would also narrow the inflow

/outflow tube diameter to a certain degree.

The effect of a reduced in- or outflow

diameter then results in lower permeability.

Another reason for low values of probe

permeameter is as a result punctual

preferential flow which takes care of

heterogeneity in rocks. Non homogeneity is

caused by diagenetic minerals (e.g. sideritic

clasts), sedimentary structures types,

bioturbations and sedimentary structures (Fig

2). Natural rocks are not isotropic.

Table 1: Comparison of Profile Permeability and Air Permeability at depth.

Depth ft Probe

permeability(mD)

Air

permeability(mD)@

400psi

Permeability

Deviation(mD) lithology

7251.65 251 288 37 Shaly Sand

7252.75 112 174 62 Shaly Sand

7253.70 33.9 865 831.1 Shaly Sand

7255.35 1304 121 1183 Shaly Sand

7255.55 1551 1138 413 Silty Sand

7256.60 1567 1449 118 Silty Sand

7256.90 1081 1896 815 Silty Sand

7257.20 1094 532 562 Silty Sand

7260.30 2244 1656 588 Silty Sand

7260.55 1603 2870 1267 Silty Sand

7260.90 2190 2644 454 Silty Sand

7261.15 1771 2493 722 Silty Sand

7262.30 1494 1468 26 Silty Sand

7263.50 992 1465 473 Silty Sand

7265.40 553 571 18 Heterolithics

7265.70 1195 856 339 Heterolithics

7266.30 650 277 373 Heterolithics

7266.75 540 545 5 Heterolithics



7266.45 591 432 159 Heterolithics

7266.90 1270 616 654 Heterolithics

7268.30 537 269 268 Heterolithics

7268.50 831 562 269 Heterolithics

7270.15 935 1129 194 Heterolithics

7270.30 1157 259 898 Heterolithics

7270.80 454 262 192 Heterolithics

7271.10 668 247 421 Heterolithics

7271.75 350 715 365 Heterolithics

Fig 7: Cross plots of Air Permeability and Probe permeability showing equation and Regression,

R2



1

10

100

1000

10000

7250.00 7255.00 7260.00 7265.00 7270.00 7275.00

Perm.Kair(mD)

Profileperm

CrossplotofAirpermeabilityandProbePermeabilityVersusDepth

Depth(ft)

B

A

Fig 8: Air Permeability/Probe permeability versus Depth showing depth ranges of major

deviations

Fig 9: Graph of Depth versus Permeability Deviation showing zones of major and minor

deviations and corresponding lithologies


FUNAI Journal of Science and Technology, 3(1), 2017 1

The efficiency of probe method is very

sensitive to a lot of factors which sometimes

are impractical to control (API, 1998). For

example, the tip seal should be laterally

confined and essentially flat on the bottom (in

the unstressed condition) because it affects the

sensitivity of the geometric factor. The

dimensionless geometrical factor could cause

overestimation or underestimation of probe

permeability results. Flat smooth surface of

the slabbed core is needed for perfect sealing

of the probe, but it is hard to guarantee.

Erroneous permeability values are obtained

when the surface of the core is not dry because

of severe relative permeability of fluids. The

presence of oil in the core during probe

permeability measurements gives an effective

permeability but could not be relied upon

because the slabbed surface could cause

evaporation of oil especially after a long time

and repeated measurements could vary

considerably. Repeated measurements of air

permeability on plug samples gives virtually

same result because of oil extraction.

Therefore air permeability results are

standardized and preferable to profile

permeability because the results are constant

over the years.

Summary and Conclusions

Probe permeameter has the advantage of

providing closely spaced, non-destructive

permeability data, which are mostly suitable to

get 3-D permeability when up to 3 wells are

known with their geographical locations. The

close spacing of results do account for

heterogeneity of reservoir rocks. Sometimes,

its results are overestimated than air

permeability which could be caused by several

factors such as imperfect sealing of probe tip

against the slabbed surface, incomplete flat

surface of the slabbed cores and lapses of

dimensionless geometrical factor. Also,

because slabbed cores were not cleaned of oil,

effective permeability values were obtained.

But, then, the values could vary considerably

with time when the surfaces are exposed and

fluids continue to evaporate. It is difficult to

mimic the reservoir pressure condition for

probe permeability in the laboratory unlike

gas driven permeability where plugs could be

confined to actual pressure of the reservoir

during analysis. The Air permeability method

using Hassler’s cell is more accurate and

consistent than probe (profile) permeability

with the following reasons. The plugs were

extracted of hydrocarbon and dried to stable

weight and this ensures consistent results any

time irrespective climate. Secondly, length (L)

and area(A) were defined by the cylindrical



plugs which readily adapt to Darcy’s equation

. Also, the method makes it easier to calculate

pore volume, porosity and oil/water saturation.

Major deviations of Profile permeability

results were observed in low permeable rocks

(heterolithics rock or shaly rocks), therefore

accuracy was poor in shaly rocks(Filomena,

2014) and method should not be used in low

permeable rock such as shales and muddy

sandstones. Irrespective of speed and close

data range of permeability and 3D gain using

probe permeameter (PDPK 300Tm), profile

permeability results should not be used

independently in reservoir evaluation.

Filomena et al (2014) analysis using probe

meter (PDPK 400Tm) reveal that air

permeability results were higher than that of

probe by 37% while the present analysis with

probe meter(PDPK 300Tm) reveal that air

permeability results were higher than probe by

25%. There is quite a similarity in the results.

Probe permeability results are fairly accurate

but results should be calibrated with air

permeability results.

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Download - A comparative study of profile permeability Otosigbo, et. al., · permeability versus air permeability reveals that the general regression R2 is 0.212 while the linear equation is

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