Exploration tools and methods (cơ sở kỹ THUẬT dầu KHÍ SLIDE TIẾNG ANH)

Reservoir rock 3/29/21 Introduction to Petroleum Engineering 9... In the context of traps, therefore, the task of our geophysical methods is to reveal: • the dip of the reservoir rock; •

Vietnam National University - Ho Chi Minh City

University of Technology

Course

Introduction to Petroleum Engineering

Presenter: Tr n Nguy n Thi n Tâm ầ ễ ệ

Falcuty: Geology & Petroleum Engineering

Department: Drilling - Production

Email: trantam2512@hcmut.edu.vn

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Chapter 3

Exploration

Tools and Methods

Learning outcomes

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The objective of any exploration venture is to find new volumes

of hydrocarbons at a low cost and in a short period of time

The petroleum geoscientist uses awide range of tools to help explore and produce petroleum

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The elements of the task

Our first objective in exploration is to identify the geological situations where accumulations of petroleum are possible From our geological studies we know that these situations are

characterized by five features: a source rock, a reservoir rock, a

migration path, a trap, and a seal

Source rock

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• the conditions of deposition;

• the temperature/burial history; and

• the original organic content

Geophysical methods can make some contribution to all these

factors except the last.

Reservoir rock

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Reservoir rock

In the context of the reservoir rock, then, the task of our geophysical methods is to reveal:

• the type of rock;

• its thickness, extent and volume;

• the conditions of deposition and the shape (these often define the type of body and the direction of preferred permeability );

• the present porosity; and

• the present permeability

Again geophysical methods can make some contribution to all these factors except the last Occasionally, geophysical methods can also give some indication of the saturant, particularly if it is gas

Migration path

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Migration path

This may be a permeable rock (such as the silty rock of Figure),

or a permeable zone of fracture Although geophysical methods allow no measure of permeability, they can sometimes indicate the likelihood of such permeable paths However, the problem is complicated by the fact that we are asking for a permeable path

at the appropriate time in the past; it need not be permeable now

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In structural traps such as this the reservoir rock itself may be widespread, and the search is for vertical closure; this may be supplied by four-way dip (a dome) or by a combination of dip and faulting

In stratigraphic traps, however, the reservoir is naturally limited

in some way, and the search is for indications of these limits Examples are the uncomformity trap

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The reef

The sand-filled channel

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The sand bar

An extensive suite of traps

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An extensive suite of traps

An extensive suite of traps

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In the context of traps, therefore, the task of our geophysical methods is to reveal:

• the dip of the reservoir rock;

• the presence of trapping faults; and/or

• a three-dimensional picture of the reservoir body; by which its shape, and so its probable stratigraphic origin, can be determined

The great successes of geophysical methods in the past have been in the search for structural traps To a smaller extent, and with much less certainty, geophysics is now contributing to the search for stratigraphic traps.

This may be an impermeable cap-rock (such as a thick layer of salt, or an unfractured shale, or a dense and unfractured limestone) Alternatively, the seal may be a fault, in which sealing minerals have been precipitated from compaction water escaping

up the fault In stratigraphic traps it may be a lateral transition - a facies change - from a permeable reservoir rock to an impermeable sealing rock

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In the context of seals, therefore, the task of our geophysical methods is to reveal:

in structural traps:

• the nature of the rock above the trap;

• the risk of fracturing in that rock; and

• the risk that such a system of fractures vents, directly or indirectly, to the surface;

in fault traps:

• the likelihood that the fault is chemically sealed; or

• the risk that the fault vents, directly or indirectly, to the surface;

in stratigraphic traps, in addition to the above:

• the likelihood that any critical unconformity is sealed; and

• the likelihood that lateral facies changes represent a seal

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The elements of the task

We shall find that geophysical methods can make some

contribution to all of these factors, but never with the certainty

we would wish.

Exploration Tools

The Tools: Models, Sections and Maps

A key operation in finding oil is the mental visualization of the

geology, and of its development through time, from an array of

data

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Exploration Tools

a) Vertical sections:

displaying the

intersection of the entire

sequence of rock layers

with a vertical plane

These are valuable in

revealing structure, layer

thickness, fault planes,

Exploration Tools

b) Contour maps: each

specific to one selected

interface between layers

- displaying the

variations in depth to

that one interface A

contour map is valuable

as a complete description

of the structure on a

single interface; contour

maps are structure maps

Exploration Tools

c) Interval maps: each specific

to one selected layer -

displaying some property of

that layer Two important such

properties are thickness (an

isopach map) and porosity (a

porosity map); clearly, if we

multiply the two together (a

porosity-thickness map) and

then multiply by the area of the

petroleum accumulation, we

have a first estimate of reserves

in place Isopach maps are also

useful in displaying features

whose recognition hinges on

variations of thickness: fans,

pinch-outs, and tilting during

deposition;

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shape: reefs, bars,

beaches, deltas, river

channels and fans

Geophysical methods

There are various geophysical surveying methods that are

routinely applied in the search for potential hydrocarbon

accumulations Geophysical methods respond to variations in physical properties of the earth’s subsurface including its

rocks, fluids and voids They locate boundaries across which changes in properties occur These changes give rise to an

anomaly relative to a background value; this anomaly is the

target which the methods are trying to detect.

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Geophysical methods

Gravity surveys

The gravity method measures small variations of the earth’s

gravity field caused by density variations in geological

structures The measuring tool is a sophisticated form of spring

balance designed to be responsive over a wide range of values Fluctuations in the gravity field give rise to changes in the spring length which are measured (relative to a base station value) at various stations along the profile of a 2D network

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Magnetic surveys

The magnetic method detects changes in the earth’s magnetic

field caused by variations in the magnetic properties of rocks

In particular, basement and igneous rocks are relatively highly magnetic

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Magnetic surveys

If they are located close

to the surface they give

rise to anomalies with a

short wavelength and

high amplitude (Figure)

The method is airborne

(plane or satellite) which

permits rapid surveying

and mapping with good

areal coverage Like the

gravity technique this

survey is often employed

at the beginning of an

exploration venture

Seismic exploration

Introduction

Principles of seismic surveying

Seismic data acquisition

Borehole seismic surveying

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Advances in seismic surveying techniques and the development

of more sophisticated seismic processing algorithms over the last few decades have changed the way fields are developed and managed From being a predominantly exploration focused tool,

seismic surveying has progressed to become one of the most

cost effective methods for optimising field production In many

cases, seismic data have allowed operators to extend the life of

‘mature’ fields by many years.

Seismic surveys involve generating sound waves which

propagate through the earth’s rocks down to reservoir targets

The waves are reflected to the surface, where they are registered

in receivers, recorded and stored for processing The resulting

data make up an acoustic image of the subsurface which is

interpreted by geophysicists and geologists.

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Seismic surveying is used in

• exploration for delineating structural and stratigraphic traps

• field appraisal and development for estimating reserves and

drawing up FDPs

• production for reservoir surveillance such as observing the

movement of reservoir fluids in response to production

Seismic acquisition techniques vary depending on the

environment (onshore or offshore) and the purpose of the survey In an exploration area a seismic survey may consist of a

loose grid of 2D lines In contrast, in an area undergoing appraisal, a 3D seismic survey will be shot In some mature fields

a permanent 3D acquisition network might be installed on the seabed for regular (6–12 months) reservoir surveillance, called

ocean bottom stations (OBS) or ocean bottom cables (OBC).

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Principles of seismic surveying

The principles of seismic reflection surveying are set out below

Principles of seismic surveying

Sound waves are generated at the surface (onshore) or under water (offshore) and travel through the earth’s subsurface The

waves are reflected back to the surface at the interface between two rock units where there is an appreciable change in

‘acoustic impedance’ (AI) across that interface AI is the

product of the density of the rock formation and the velocity of

the wave through that particular rock (seismic velocity).

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Principles of seismic surveying

‘Convolution’ is the process by which a wave is modified as a

result of passing through a filter The earth can be thought of as a filter which acts to alter the waveform characteristics of the

down-going wave (amplitude, phase, frequency) In schematic

form (Figure) the earth can be represented either as an AI log in depth or as a series of spikes, called a reflection coefficient log or reflectivity series represented in the time domain When the wave passes through the rocks its shape changes to produce a wiggle trace that is a function of the original source wavelet and the earth’s properties

Principles of seismic surveying

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Principles of seismic surveying

When a seismic wave hits

an interface at normal

incidence (Figure a), part

of the energy is reflected

back to the surface and

part of the energy is

transmitted

Principles of seismic surveying

In the case of oblique

incidence the angle of the

incident wave equals the

angle of the reflected wave

as shown in Figure b Again

part of the energy is

transmitted to the

following layer, but this

time with a changed angle

of propagation

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Principles of seismic surveying

A special case is shown in

Figure c where an abrupt

discontinuity, for example

the edge of a tilted fault

block, gives rise to

‘diffractions’, radial

scattering of the incident

seismic energy Such

artefacts can impede

interpretation of the

seismic data but can be

removed or suppressed

during processing (as

outlined later in this

Seismic data acquisition

Seismic sources generate acoustic waves by the sudden release of energy There are various types of sources and they differ in

• the amount of energy released: this determines the specific

depth of penetration of the wave

• the frequencies generated: this determines the specific

‘vertical resolution’, or ability to identify closely spaced

reflectors as two separate events

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Seismic data acquisition

A complete product line for seismic acquisition in any environment:

1 Emission of controlled acoustic energy from a seismic

source

• Compressed air guns (marine)

or

• Seismic vibrator or explosive source (land)

2 Seismic energy is transmitted to the earth and reflected from

the geological boundaries (layers).

3 The reflected energy is detected by geophones (land) or

hydrophones (marine).

4 Seismic acquisition systems record and process the data.

Marine seismic acquisition

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Land seismic acquisition

Marine data acquisition

In general, marine data acquisition is simpler and faster than

land acquisition since in all but the most heavily developed

offshore areas there are few obstacles, leading to routine and rapid data gathering

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Marine data acquisition

In standard marine acquisition, a purpose-built boat (fig 2.2) is

used to tow one or more energy sources and one or more cables

containing (pressure sensitive) receivers to record the

reflections from the underlying rocks At present, the source is

nearly always an array of air guns tuned to give an energy pulse

of short duration with directivity characteristics that concentrate the energy vertically downwards

Marine data acquisition

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Marine data acquisition

Marine data acquisition

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Marine data acquisition

Marine data acquisition

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Marine data acquisition

Marine data acquisition

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Marine data acquisition

Marine data acquisition

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