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Permeability analysis of

Deep-water reservoir facies using core

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Scapa formation, Witch Ground Graben,

North Sea

 

 

Muhammad
Kamil Iqram Abu Hassan

9475949

 

 

Dr. Ian Kane

(Supervisor)

The
University of Manchester

Table of Contents

 

1.       Introduction                                                                                      1-2

 

2.       Project Timeline                                                                                2

 

3.       Methodology                                                                                     3

 

4.       Mini-permeameter                                                                          3

          4.1    Procedure                                                                                 3-4

          4.2    Principle                                                                                   5-6

          4.3    Industrial
use                                                                           6

          4.4    Analysis                                                                                     6-7

 

5.       Results                                                                                                8-9

 

6.       Future anticipated results                                                             

          6.1    Beddings
(Facies) interpretations                                       9-10

          6.2    Facies
Associations                                                                11

 

7.       Further planning                                                                               11

8.       Reference                                                                                           12

1. Introduction

Many
oil and gas companies have moved their focus to Deepwater exploration due to
depletion in production produced from the
shallow and near-shore region. Deepwater oil
and gas drilling are prosperous; however, they are exposed to various
challenges. The main one is predicting the reservoir quality distribution. They
are often poorly understood due to lack of well
data available. Extreme water depth of about, 300meters to even greater than
1500meters for ultra-Deepwater, leads to difficulty in collecting well data
and high exploration cost (Skogdalen, Utne and
Vinnem, 2011). Deepwater
reservoir heterogeneity has to significant impact on the interpretation of reservoir due to the presence
of layers of thin beddings of high and low permeability which are difficult to
be identified. According to Van Oort 1988, in the North Sea, this has resulted in unreliable
prediction of key factors, which results in under-estimation or over-estimation
of the recoverable reserve.

Scapa
Sandstone lies on the Lower Cretaceous of the ancient abyssal fans in Witch
Ground Graben, North Sea (Riley, Harker and Green, 1992). It is located
112miles Northeast of Aberdeen, Scotland (Figure 1). There were many previous
discoveries around this area but still high in prospect. Made up of discrete
reservoir sandstone unit within marls, limestones and conglomerate mean it has high potential to become a good
reservoir. Discoveries of Scapa Formation were made by accident when testing
out wells such as Britannia and Highlander around this area for deeper Jurassic
target (Department of Energy Climate Change, 2015). Production still continues to this present day for some of these
wells.

Figure
1: Location of Witch Ground Graben, North Sea (McGann et al.)

Abyssal
fans or also known as submarine fans represents the largest type of sediment
deposits. They are formed by turbidity current which is underwater landslide
which pulls sediments from the edge of continental shelf down the continental
slope associated with geological activity assisted by gravity. Current carrying
a dense slurry of sands and muds will
eventually slow down and deposits its content forming abyssal fans. Slurry current continue to flow until it reaches the ocean
bed. Continuous deposition of the turbidity current formed series of graded heterogenous
sediments of sand, silt and mud known as turbidites. The submarine fan can be divided into distinct ‘architectural
elements’ as a product of different processes and sub-environment of
deposition. Different features are formed such as canyons, channels, lobes and
overbank. Canyon and channels which provide sediment transport pathway and link
onshore with Deepwater basin are highly erosive. The whole process of gravity-driven deposition feeds information on
factors affecting sedimentary.

 

The
aim of this project is to:

1)
Study and understand the significance of obtaining mini-permeameter
measurements in interpreting reservoir.

2)
Correlate petrophysical data with lithology of core
sample and sedimentological analysis to investigate reservoir quality.

2)
Distinguish mini-permeameter with conventional core plug permeameter.

3)
Construct predictive depositional environment model using interpreted facies
distribution and sedimentological processes.

4)
Study the relationship present between sedimentological processes respective to
age with reservoir quality.

 

 

2. Project timeline

Figure 2: Project time presented in a Gantt Chart

 

 

3. Methodology

To
achieve the aim, well data need to be collected, correlated and interpreted by
these methods:

·       
Accessing
core samples from 2 wells, 14/20b-H1 and 14/20b at British Geological Survey (BGS)
in Keyworth.

·       
Examine
core sample for lithological features and core log sample from both wells.

·       
Obtain
and analyze sets of permeability reading for both wells between intervals of
core plug data using mini-permeameter.

·       
Interpret
correlation between petrophysical data obtained from both mini-permeameter and
conventional core plug with lithological and sedimentological analysis of core
sample.

·       
Classify
sample into distinct beddings and facies association based on sedimentological
approach.

 

4. Mini-permeameter (literature review)

Aside from the conventional
Hassler-sleeve core plug measurement, mini-permeameter has been a significant
use in the petroleum industry since the late
1960s, according to Eiipe and
Weber, 1971. Mini-permeameter provides quick, cheap and non-destructive
measurements of permeability, which is the opposite when comparing with the
conventional core plug measurement. These characteristics allow a large amount of data, at smaller intervals, to
be collected. Dense sampling grid of permeability measurements will help
produce a better characterization of core permeability, and directly improve
reservoir quality interpretation. This is the main reason mini-permeameter is used in the oil industry, to identify
significant structures and facies. Based on Hurst and Rosvoll, 1990, is it
agreed that wide intervals of conventional core plug permeability data are
insufficient to accurately assess the sedimentary
structure of the reservoir.

 

4.1 Procedure

A basic mini-permeameter (Figure
3) consists of a probe (red arrow) and a
handle which connects to flexible tubing (Temco.Inc MP-204 instructions).
Semi-soft rubber is connected to the end of the probe
for better sealing when pressed against the sample to be analyzed. A more
accurate measurement is achieved by connecting probe to two separate fluid
connections, hydrogen gas in this case and pressure transduces in order to avoid flow in the pressure monitoring line. Presence
of flow in pressure monitoring line will cause pressure drop due to friction.
Gas is injected into the sample to
measure the gas flowrate and then
converted into permeability value. Hydrogen gas is used as it is cheap and easy
to flow due to its low viscosity. Gas flowrate
and pressure need to be made sure have reached steady-state before permeability
reading is recorded (figure 4). A module in the mini-permeameter will convert RS-232 signal data into RS-485, direct
to connected external computer so that data-acquisition module can be
addressed individually.

Figure 3: Temco.Inc MP204 permeameter

 

 

Figure 4
: Green box indicates flowrate and pressure are at steady-state on Temco
mini-permeameter software

 

4.2 Principle

Figure 5: Sketch of flow geometry for an unconfined core
plug sample (Temco. Inc. MP-204 instructions)

 

Gas is injected through a small volume of porous space beneath the inside
diameter of the tip as the probe is pressed against core sample, then gas will
flow in all direction around the outer diameter of the probe tip (Figure 5). For
probe tip, isothermal, steady-state gas flow, modified Darcy equation is
applied:

Where:

Ka           =             air or gas Permeability, md
(millidarcys)

µ             =             Viscosity, centipoise(cp), of gas, at its averaging flowing

Qb           =             volumetric flow rate, standard
cubic centimeters per second (scc/sec),

referenced to Pb

               =             Flow Rate (scc/min) ÷
60

Pb           =             standard Reference Pressure for mass flow meters, atmospheres absolute

(default value
= 1.00)

P1           =             upstream pressure (pressure at
tip), atmospheres absolute

                              (Flow Pressure, psia) ÷ 14.696

P2           =             downstream pressure (pressure at
tip), atmospheres absolute

                              (Atmospheric Pressure, psia) ÷ 14.696

a             =             internal radius of probe tip seal, centimeters (cm)

                              (tip
Inside Diameter {in.} ÷2) x 2.54

Go           =             Geometrical shape Factor
(function of bd, Rd, and Ld), dimensionless

bd           =             b/a (used to mind Go),
dimensionless

b             =             external radius of probe tip seal,
centimeters

Rd           =             Rcore/a, dimensionless

Rcore       =             core radius, centimeters

Ld            =             Lcore/a, dimensionless

Lcore        =             length of core, centimeters

Tref         =             reference temperature of mass flow
meters, K (default value = 294)

                              K
(= 21oC), or =530R (= 70oF)

               =             (Reference temperature, oC) + 273.1

Tact         =             actual flowing temperature of Gas,
K or R

               =             (flowing Temperature, oC) + 273.1

 

 

Assuming that geometry of core
sample is a right cylinder and core tip
is pressed at the centre. Modified Darcy’s
formula is quite similar to the original Darcy’s law linear-flow equation,
except for the presence of Geometrical factor, Go. The Geometrical factor is a function of outer
diameter (O.D.) and inner diameter (I.D.)
of the probe tip and also influenced by
size and length of the sample. The
software on the computer helps us run the
calculation, using input data such as which tip size used, rock geometry and
atmospheric pressure.

 

4.3 Industrial use

Although mini-permeameter is capable of producing a high degree of accuracy, there are few
requirements proposed so that mini-permeameter can be labeled as industry standard
(Halvorsen et al., 1990):

1) The probe must be pressed against core sample at a consistent force
and orientation to ensure a proper seal
around the probe tip.

2) Maintain non-destructive
measurements. Vary probe geometry and applied force in different situations or
samples to optimally measure the permeability.

3) Do not inflict any damage
on the core sample i.e. make holes or damage the edges

4) Use the same level of accuracy to measure flowrates in mini-permeameter as in
conventional permeameter i.e. use equivalent measuring instrument.

 

4.4 Analysis

Mini-permeameter measurements are
best used as a comparison with other
information collected from the reservoir, such as wireline logs. Core data can
be correlated with wireline log measurements for reservoir characterization and
volumetric estimation. According to (Jensen, 1990) even if mini-permeameter
produces less accurate sets of data compared to conventional permeameter, high
sampling density (Figure 6) will give better correlation with other data such
as wireline logs. Although average permeability can be predicted from wireline
logs, however mini-permeameter measurements help
to better predict permeabilities in laminated reservoirs. Logs unable to
resolve thin (1-10cm) beddings. Hence, high
dense sampling of mini-permeameter helps to define heterogeneity present along
the depth of core sample.

Figure 6: Comparison of to demonstrate
higher sampling density of mini-permeameter when comparing with conventional
permeameter

As mentioned earlier, main challenges
in offshore drilling in defining reservoir quality distribution is lack of well
data available. Mini-permeameter allows a huge
volume of cheap permeability measurements to be collected, at a shorter period
of time. This will greatly help to interpret
reservoir zonation and Net-to-gross value estimation. However, mini-permeameter
should not replace higher accurate data collection method, such as conventional
permeameter, for cost-cutting. Instead, mini-permeameter measurements should be
taken as a solution to increase the amount
of data to produce a more reliable reservoir interpretation. Other than that,
its non-destructive nature is advantageous as the same sample can be reused for other core analysis in future.

 

 

 

 

 

 

 

 

 

5. Results

Using obtained
mini-permeameter datasets, correlation can be made with lithological and sedimentological
analysis of core sample and petrophysical data collected from conventional core
plug sample. Justification is made for every variation observed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Variation of lithology can be
seen throughout core sample in Figure 7. Overall porosity and permeability for
the top section of this core sample are higher than bottom section. This is due to
greater grain size which allows a high
volume of pore space. However, there are dips of low permeability and porosity
at the top section mostly due to heavy calcite cementation (green arrow) and
intrusion of the low permeable thin dark
layer (blue arrows). Dense permeability sampling of mini-permeameter can easily
recognize these thin layers of low permeability. Finely lamination layers (black
arrows) can also be seen where a sudden
change in the value of porosity and
permeability occurs. This is due to the very
low permeability of these thin lamination layers, which act as baffles which
prevent the further process of
cementation vertically. These black lamination layers can be seen more frequent
at the bottom section which causes drop
the significant drop in permeability and porosity value. This feature is
usually flaky and tends to break or fracture horizontally. Mini-permeameter
measured the horizontal permeability, hence explains permeability reading of more
than 0.1mD core sections with this feature. Other than that, areas of that are
heavily cemented have low permeability and permeability as this process fill in
spaces between pores and blocks flow pathway.

 

6. Future anticipated results (Literature review)

According to a recent study from (Porten et al., 2016), the most favourable
reservoir properties are a combination of
low detrital clay content, coarse-grained sand and made up of mechanically
strong grains to withstand mechanical deformation. Deposits with rich detrital
clay have weak reservoir properties as
these clay particles fill up the pores
reflecting the low porosity value. These detrital clay particles also will block
the pathway of fluid movement, hence lowering the permeability measurement. Ductile
detrital clay undergoes plastic deformation upon mechanically compacted. Different
for mechanically strong quartzose and feldspathic sandstones, they re-arrange themselves
into a more stable arrangement. However, for quartzose, precipitation rate of quartz
cementation increases exponentially as temperature rises relative to burial depth. Quartz cement will start
filling up pore spaces between grain, hence reducing reservoir properties of the
deposits.

 

6.1 Facies (bedding)
interpretations

Results obtained from the correlation of mini-permeameter permeability
measurements, core observation and other petrophysical data is analyzed to
classify wells section into different types of beddings or facies based on
sedimentological analysis approach on the core
sample. Facies classification helps provide information for future detailed
analysis such as sequence stratigraphic study (Miall, 2016) as each distinctive
type of sedimentary deposit reflects unique depositional processes (Lowe, 1982).
Sequence stratigraphic study is a 4D analysis which relates facies type with
the petrophysical characteristic of the reservoir relative to time.

Main facies division and
characterization (Figure 8) (Porten, et al. 2016):

1) High-density turbidites (HDTs)

Fine- to medium-grained with very
low detrital clay content. Usually characterized as thickly bedded intervals (Lowe, 1982) with the highest porosity and permeability measurements.

2) Low-density turbidites (LDTs)

Silty- to -very-fine-grained and
contain similar or more detrital clay content, when comparing with high-density
turbidites. Commonly grouped up at the lower end of the porosity-permeability dataset.

3) Hybrid Event Bed (Proximal)

Higher detrital clay content and
finer grain size grain size compared to HDTs. Intermediate-high porosities and
a wide range of permeabilities. Variation
in permeabilities is due to the different
distribution of detrital clay in deposits.

4) Hybrid Event Bed (Distal)

Most clay-rich and finest-grained
compared to other bed types. Hence, it has low-to-intermediate
porosity-permeability measurements.

5) Debrite

Debrite deposits made up of fine-
to medium-grained, coarser compared to low-density turbidites. However, it
contains higher detrital clay contain, hence resulting same low
porosity-permeability dataset.

6) Hemipelagite

Easily distinguished by
alternating greenish grey and red due to oxidizing and reducing process of Fe3+
and Fe2+. Low deposition energy, hence consist
of the fine-grained particle. Highly
calcareous and clay-rich facies.

Figure 9: Different
types of facies sediments and with respective characterization. A schematic trends if porosity and permeability
relative to grain size and detrital clay content. (Porten et al., 2016). Transition from high-density turbidites to hybrid-event
beddings.

 

6.2 Facies
associations

Several beddings or facies that are related to each other are
grouped into different facies association to represent the respective depositional environment.
Classifying them into different facies helps understand the vertical and lateral variation of deposits throughout
core sample. This will be beneficial to predict potential areas that exhibit
good reservoir quality. Main deep-water facies association are; basin plain,
canyon, proximal fan, distributary channel and distal fan.

 

 

7. Further planning

Apart from collecting and
recording plenty amount of reservoir well data, produce a reliable reservoir
interpretation for the studied area,
needs high knowledge in the petrophysical,
lithological and sedimentological field. I plan on learning on this by asking
individuals that have more experience in these fields and referring articles of previous
studies that are related to reservoir analysis. For a better presentation of data and results, I also need to learn on
how to use graphic design software such as CorelDraw to transfer data collected
on paper into a digital image.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8. Reference

 

– Porten, K. W., Kane, I. A., Warchol, M. J. & Southern,
S. J., 2016. A sedimentoloical
process-based approach to depositional reservoir quality of deep-marine
sandstones: An example from the Springar
Formation, Northwestern Voring basin, Norweigian
Sea. Journal of Sedimentary Research, Volume 86, pp. 1269-1286.

– Department of Energy and Climate Change, 2015. Lower
Cretaceous deep-water sand plays UK Central Graben

– Lowe, D. R., 1982. Sediment gravity flows II: Depositional
models with special reference to the deposits of high density turbidity currents. Journal of sedimentary petroleogy, 52(1), pp. 279-297

– V AN OORT, B. (1988)
Lessons learned in North Sea oil field developments. Journal of Canadian
Petroleum Technology, 27 , 123-132

– McGann, G., Green, S., Harker, S. and Romani, R. (1991).
The Scapa Field, Block 14/19, UK North Sea. Geological Society, London,
Memoirs, 14(1), pp.369-376

– Skogdalen, J., Utne, I. and Vinnem,
J. (2011). Developing safety indicators for preventing offshore oil and gas
deepwater drilling blowouts. Safety Science, 49(8-9), pp.1187-1199

– EIJPE, R., and WEBER, K..J. (1971) Mini-permeameters for consolidated and unconsolidated sands. American
Association of Petroleum Geologists Bulletin, 55, 307-309

– HURST, A. and ROSVOLL, K.J.
(1990) Permeability variations in sandstones and their relationship to
sedimentary structures. 2nd International Reservoir Characterisation Technical Conference, Dallas (in press)

– Riley, L., Harker, S. and Green, S.
(1992). LOWER CRETACEOUS PALYNOLOGY AND SANDSTONE DISTRIBUTION IN THE SCAPA
FIELD, U.K. NORTH SEA. Journal of Petroleum Geology, 15(1), pp.97-110.

– Halvorsen, C., and Hurst, A., (1990). Principles, Practice
and Applications of Laboratory Minipermeameter.
Statoil

– JENSEN, J.L. (1990) A model for small-scale permeability
measurements with applications to reservoir description. SPE 20265 JOURNEL, A.G. and HUIJBREGTS, Geostatistics.
Academic Press, London, 600p. C.(1978)
Mining

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