3-reservoir Engineering For Completion

Basics of reservoir engineering for completion

SUMMARY

I.

What is a reservoir?

II.

Characterisation of reservoir rocks

III.

Fluid studies

IV. Reservoir knowledge V. Recovery mechanisms

2

I - What is a reservoir?

3

What is a reservoir? One or more RESERVOIR ROCKS: Porous to allow hydrocarbon storage Permeable to allow fluid flow

Containing HYDROCARBONS: Liquid or gaseous Water resources can be also targeted to be used for: − Water injection − CO2 underground storage

Which are TRAPPED: By a non-permeable barrier on top In an anticline structure, ...

A RESERVOIR: one or several pay zones 4

Conventional Representation of a reservoir Gas Oil Contact

Gas

Top

(impermeable layer)

Oil Water Oil Contact Bottom

Water

(impermeable layer)

Gas Oil Water

5

Reservoir Rocks Shaly SANDSTONES (80% of reservoirs) - Quartz and shale CARBONATED rocks - Calcites et Dolomites (40% of world production) Q

Shale

F Quartz and Feldspars with shale cement

Shaly cemented sandstone

Debris of various types (clasts) buried in a calcite cement

Skeletal limestone 6

Hydrocarbon Generation The type of hydrocarbon generated is strongly related to the conversion temperature of kerogen

7

Generation/Migration of Hydrocarbons Origin of Hydrocarbons Burial of source rock to temperature and pressure regime sufficient to convert organic matter into hydrocarbon Marine animal biomass : small shellfish (krill) and zooplankton Marine vegetal biomass : giant & microscopic algae's (phytoplankton)

Maturation from kerogen to hydrocarbon in the source rock Primary migration toward the reservoir, secondary migration inside the reservoir Closure Primary & secondary migration Secondary migration

Closure

Source rock Primary migration

8

Petroleum system processes Generation: Burial of source rock to temperature and pressure regime sufficient to convert organic matter into hydrocarbon

Migration: Movement of hydrocarbon out of the source rock toward and into a trap

Accumulation: A volume of hydrocarbon migrating into a trap faster than the trap leaks resulting in an accumulation

Preservation: Hydrocarbon remains in reservoir and is not altered by biodegradation or “water-washing”

Timing: Trap forms before and during hydrocarbon migrating 9

Geologic Time Scale

10

Petroleum System Events Chart

11

Classification of traps STRUCTURAL TRAPS: resulting from the deformation of rocks, simple anticlines or faults STRATIGRAPHIC TRAPS: due to facies variations, the rock becoming laterally impermeable. Examples are: sandstones lenses in shale/sandstone units, depositional or erosional pinch outs, and carbonate reefs COMBINED TRAPS: eroded anticlines, traps associated to salt domes

12

Different types of traps Anticline

Reef

Unconformity

Pinch out

Salt dome

Stratigraphic trap

13

II - Characterization of reservoir rocks

15

Characterization of Reservoir Rocks To be considered as a reservoir, a rock must have the following properties: • Must be a porous media able to store the hydrocarbons. This capability is called the rock POROSITY (noted Ø) • Allows the flow of hydrocarbon. This property is called the rock PERMEABILITY (noted k) • Contain enough hydrocarbons. This is called the hydrocarbon rock SATURATION (noted S)

There are several ways to determine these rock properties: • Analysis of cores samples taken during the drilling of the wells • Interpretation of well logs and well tests 16

The Porous Media

Porous media Residual porosity

Useful porosity

Cores

17

Porosity Definition : Ø = Volume of PORES / TOTAL Volume (current values between 0.01 and 0.35)

Cubic

(single size)

Ø # 0.476

Important Parameters:

porosity decrease when standard deviation increase

)

Different types of repartitions

– The grain shape and their organisation – The repartition of the grain sizes – Ø is not related to the grain size for a given assembly of same size spherical grains

Rhomboedric (single size)

Ø # 0.259

Cubic

(2 sizes)

0.35

0.50

1.00

2.00

< 0.259

18

Permeability Definition: The permeability k characterises the fluid flow trough a given porous media

Quantification – Darcy's law:

19

Saturation Definition: S = Relative amount of fluids inside the pores Sw = Water volume / Total pore volume = water saturation So = Oil volume / Total pore volume = oil saturation Sg = Gas volume / Total pore volume = gas saturation Sw+So+Sg = 1

Linked to the surface properties of the rock (wetability) Practical cases: Oil Water/Oil case - Water of often the wetting fluid Oil/Gas case - Oil is the wetting fluid Water/Gas case - Water is always the wetting fluid

Water

Rock

21

III - Fluid studies

28

Composition of hydrocarbons OIL = ɛ (C to C ) + C 1

4

+ 5

LIGHT oils (d<=0.86) MEDIUM oils (0,860,92)

GAS = C + C to C + C 1

2

DRY gas WET gas Gas CONDENSATE

4

5

Gas + Oil (surface conditions) Gas/Oil <<(surface conditions) ɛ Gas & Oil(surface conditions)

+

Gas (surface conditions) Gas & ε Condensate (surfaceconditions) Gas & Condensate (surface conditions)

Hydrocarbon components C1 methane C2 ethane C3 propane C4 butane C5 pentane C6 hexane C7 heptane

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Light and Heavy Oils Type of Oil Density(g/cm3) °API Volume Factor (volume reservoir/surface) Gas/Oil Ratio(m3gaz/m3oil) Viscosity (cP)

Light

Medium

Heavy

0.80 to 0.82

0.83 to 0.90

0.91 to 1

45

35

25 to 10

3 to 2

1.5

1.1 to 1

300 to 200

100

10 to 0

<1cP

Several cP

Up to1 Po

Viscosity of water at 15°and 1 atm. = 1cP Viscosity of gas 1/100 cP

⁰ API =

141,5 Sg

- 131,5

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Behaviour of a Pure Substance

Gas

Liquid Bubble point

Dew point

Vapor

Liquid and Vapor

33

Pressure – Volume diagram P TR

TC

TCC

(critical condensation temperature)

G Critical point

O Bubble point

• • O+G

• Dew point

V

Bubble point pressure: pressure at witch the first bubbles of gas evolves from the oil at a given temperature

35

Pressure – Temperature diagram P Critical Point TC TR

O

TCC (critical condensation temperature)

•M •

G

•A

Retrograde Condensation gas

• P R & TR

•R

Bubble curve

• P'R & TR • PS & TS • P'S & T'S

O+G

•B

Dew point curve Dry gas

Wet gas “Oil” reservoir

T

"Gas“ reservoir 36

PT diagram in function of the gas composition

37

Illustration of the PVT terms Rs, Bo & Bg

39

Definitions of the PVT terms Rs, Bo & Bg (1/2)

40

Definitions of the PVT terms Rs, Bo & Bg (2/2)

41

Example of calculation of PVT terms Bo & Rs

42

Some "Production operations" terminology

43

IV - Reservoir knowledge

45

Measurement of Rock Properties

Porosity Measurements on core plugs Well logs Interpretation

Permeability Measurements on core plugs Well tests Interpretation

Saturation Measurements on core plugs Well logs Interpretation

46

Open hole or Cased hole logs Well Logs are useful for: •

Recognition of reservoirs (lithology, porosity and saturation)

•

Knowledge

of

wells

characteristics (diameter, inclination,

cementing, formation-hole communication)

• Comparison between wells to identify well marker correlation

Different types of logs: •

Cable tension recorder

Electrical (PS, resistivity…)

•

Radioactivity (GR, Neutron, Density, TDT)

•

Sonic (Δt transit time)

•

Auxiliaries (Caliper, Deviation, Cementing…) Others (RFT, PLT…)

Recording system

Winch

Depth recorder

Cable Tools

47

Documents Schlumberger

Well logs and interpretation

48

Inter well correlations Well 3 Well 1

Well 2

Well 3

Well 1

Well 2

49

Well tests: basics gas sampling

Well surface rate

Separation gas

Psep-Tsep

Psto-Tsto

oil

sampling

Stock Tank Input

Pwf gauge Variation of Well rate

System

Well parameters + Reservoir properties

Output

Well pressure

Reservoir fluid Pres, Tres

The logical system The physical system 50

Well tests Goals: determination of: • • • • •

Well productivity index: PI Reservoir static pressure : BHP Well bore skin: Skin Drainage radius of the well during the test R Type and evolution of produced fluids

Well tests basics: To create a pressure perturbation around the well by producing the well at a given flow rate Utilisation of the basic fluid flow equations to relate the pressure transient measured in the hole to the characteristics of the well bore and the formation 51

Radial fluid flow around the well

rw

Pi Pwf

rw well radius R drainage radius h formation thickness

For a homogeneous infinite medium, constant thickness, constant flow rate: • The change of the pressure in the well with the time follow an integral exponential law • After a very short period of production time, the pressure drop P is proportional to the logarithm of the time log(t) 52

Typical well test layout

53

Schematic representation of a well test

Test period used for interpretation

55

Well test problems to be solved Skin:

Skin zone

The well bore is sometimes damaged by the drilling process (mud invasion,…). In some cases, the well bore properties can be enhanced by a mechanical fracturing as well as by formation acidification. The skin is: > 0 if flow restriction (well bore damaged,…)

rw

skin < 0

Pressure profile skin > 0

or < 0 if enhanced flow capacity

Well bore storage capacity: At early times of the test, part of the hydrocarbon comes from the well volume itself. During this period the sand face reservoir production rate is not constant.

Surface rate Formation rate

Down hole well rate: Since the well rate is never constant during the production period, build up period during which the flow rate is nil (excluding storage effect period) is used instead for well test interpretation

Well bore storage duration 56

Definitions Volumes of in place hydrocarbons: Oil and gas originally in place (OOIP, OGIP) Static evaluation

Reserves: Volume of hydrocarbon produced/to be produced Initial, remaining or ultimate reserves Dynamic evaluation requiring knowledge of the production profile

Recovery factor = Reserves/OOIP

57

V - Recovery Mechanisms

63

Recovery Mechanisms PRIMARY recovery: The reservoir energy is the only one used to produce hydrocarbons

SECONDARY Recovery: Energy used to produce the reservoir is external, such as water or gas injection

TERTIARY (Enhanced) Recovery: Complex methods such as miscible fluid injections, thermal methods, chemical methods …

64

Primary Recovery

65

Main processes of primary recovery Oil reservoir:

Monophasic Expansion:

− Production due to compressibility of the whole "oil + pore"

Dissolved gas expansion Aquifer action: − "Bottom coning" − "Edge coning"

Gaz cap expansion

+ Possible artificial lift process at the level of the well (pumping or gas lift)

Gas reservoir:

Gas expansion

66

Saturated or Under saturated oils

Infinite Acting Aquifer Ideal scheme

h'

OIL

Surface water

WATER

68

Evolution of the interfaces

Initial state Initial GAS-OIL Contact

Initial WATER-OIL Contact

State after oil production Initial GAS-OIL Contact

Gas-cap expansion Gas liberated by oil Aquifer expansion

Initial WATER-OIL Contact

Water encroachment

69

Primary recovery performances

Type of reservoir

Recovery

Single phase - OIL

P > Pb

< 10%

Two phase - OIL

P < Pb

5 to 25%

OIL with GAS CAP

10 to 40%

OIL with aquifer support

10 to 60%

GAS

60 to 95%

CONDENSATE

40 to 65% Average oil

25%

Average gas

75%

70

Secondary recovery Takes place when natural reservoir energy is too low to maintain primary recovery Requires external energy Principal methods: Water injection at the bottom of the oil zone or into the aquifer Gas injection at the top of the oil zone or into the gas cap Injection of gaseous hydrocarbons (dry gas injection into gas condensate reservoirs)

71

Water or gas Injection Production wells Water Injection wells

GAS INJECTION Gas Injection wells

Oil zone water

Production wells

WATER INJECTION

Oil zone

72

Exploitation scheme Low permeability area

North area

Organization of a production/injection scheme according to reservoir characteristics : Well spacing Location of water injector wells with respect to oil producers

High permeability area (20 time better than North area) South area

Oil producer Water injector

74

Tertiary recovery: costs

Gas injection Water injection

CO2 injection Miscible gas injection Polymers Micro-emulsion Steam, in situ combustion

0

5

10

15

20

25

30

35

40

45

$/bbl

75

Hydrocarbons recovery Conventional oil recovery

PRIMARY RECOVERY

ARTIFICIAL LIFT HORIZONTAL DRILLING

NATURAL FLOW

SECONDARY RECOVERY

WATERFLOOD GAS INJECTION

PRESSURE MAINTENANCE

GAS CYCLING

Enhanced oil recovery

TERTIARY RECOVERY

THERMAL

• Steam • In situ combustion

GAS

• Hydrocarbon miscible • CO2 • N2

CHEMICAL

MICROBIAL

•Polymer •Surfactant/polymer •Alkaline 76

Enhanced drainage schemes Horizontal wells:

Artificial lift

To enhanced drainage: two equivalent production systems

well

h

L

In case of some particular situation:

water

the horizontal well drains more faults

the horizontal well prevents water coning 77