1. Lecture 11 Oilfield Development
Fall 2012
Lecture 11 Oilfield Development

2. Main Points Reservoir production mechanisms (Primary recovery)
Introduction to waterflooding
(Secondary recovery)

3. What Is A Reservoir? A reservoir is……
formed of one (or more) subsurface rock formations containing liquid and/or gaseous hydrocarbons, of sedimentary origin with very few exceptions.
The reservoir rock is porous and permeable, and the structure is bounded by impermeable barriers which trap the hydrocarbons.

4. What Is Reservoir Engineering?
The goal of reservoir engineering, starting with the discovery of a productive reservoir, is to set up a development project that attempts to optimize the hydrocarbon recovery as part of an overall economic policy.
Reservoir specialists thus continue to study the reservoir throughout the life of the field to derive the information required for optimal production from the reservoir.
Volume of hydrocarbons (oil and/or gas) in place
Recoverable reserves
Well production potential (initial productivity, changes)

5. Reserves
The term “reserves” concerns the estimated recoverable volumes in place (to be produced).
The reserves obtained by primary recovery depend on the following:
Amount of oil and gas in place and their distribution.
Characteristics of the fluids and of the rock.
Existing drive mechanisms and production rate.
Economic factors.

6. Different “Reserves”...
Proven: Discovered reserves that can reasonably be expected to be produced in present economic and technical conditions.
Probable: Discovered reserves which have a reasonable probability of production with technology and profitability close to those that exist today.
Possible: Reserves not yet discovered, but whose existence is presumed with a reasonable degree of probability.
Ultimate: Proved + probable + possible

7. RESERVOIR PRODUCTION MECHANISM AND PRODUCTION CHARACTERISTICS1
RESERVOIR PRODUCTION MECHANISM AND PRODUCTION CHARACTERISTICS

8. Oil Reservoir Drive Mechanisms
Primary recovery-Definition
Hydrocarbon production resulting from natural reservoir energy.
Solution-gas drive
Gas-cap drive
Water drive
Combination drive
Gravity-drainage drive

9. Gas Reservoir Drive Mechanisms
Gas expansion drive
Water drive

10. Reservoir Energy Sources
Liberation, expansion of solution gas
Influx of aquifer water
Expansion of reservoir rock and compression of pore volume
Expansion of original reservoir fluids
free gas
interstitial water
oil
Gravitational forces

11. Compressibility Coefficients, Fluid Expansion
The orders of magnitude for oil, water and the porous medium are as follows:
For gas: P
Cg [ ] 10-6 psi-1

12. Solution-Gas Drive in Oil Reservoirs

13. 2. Solution Gas Drive Working condition Energy supply
Limited or closed reservoir without effective water drive.
The aquifer is of low porosity and permeability.
Energy supply
When the pressure in the vicinity of the wellbore drops below the bubble point pressure, gas will escape from the oil inside the reservoir and begin to expand.
The gas expansion will displace an increasing quantity of oil from the pore space in the rock.
When the pressure of the whole reservoir falls below saturation pressure, it is possible to form a “secondary gas cap” .

14. 2. Solution Gas Drive
Q: What will happen if there is a big drawdown in a solution gas drive reservoir?
The gas production rate will become excessive;
kro decreases sharply;
Lots of oil will be left in the reservoir.

15. Solution-Gas Drive in Oil Reservoirs
Typical production characteristics

16. Gas-Cap Drive in Oil Reservoirs

17. 3. Gas Cap Drive Working condition Energy supply
Limited or closed reservoir similar to solution gas drive case.
Energy supply
The gas cap expands to fill the pore space formerly occupied by the oil, and thus displaces oil downwards towards the producing well.

18. Gas-Cap Drive in Oil Reservoirs
Typical production characteristics

19. Water Drive in Oil Reservoirs
Edgewater drive
Bottomwater drive

20. Water Drive in Oil Reservoirs
Typical production characteristics

21. Water Drive in Oil Reservoirs
Working condition
The aquifer is as porous and permeable as the oil-bearing portion of the reservoir;
The aquifer is more or less in direct contact with oil.
Energy supply
The energy comes from the expansion of the aquifer water caused by the reduction in pressure resulting from the removal of oil from the reservoir.
Water compressibility: (~10-10 Pa-1)

22. Water Drive in Oil Reservoirs
Under this mechanism, the reservoir pressure will tend to be maintained to an extent depending on the size and permeability of the aquifer.
Production rate limit
If the production rate is proper, the aquifer water can enter the vacated section of the oil-bearing zones as fast as the oil is withdrawn.
If the oil production rate exceeds this limit there will be a decline in the reservoir pressure and a consequent reduction in the energy available to produce the oil.

23. Water Drive in Oil Reservoirs
Fingering (趾进）
Water (gas) coning (水锥进/气锥进)
Water cresting (水脊进）
Water breakthrough （水突破）
Watered out （水淹）
If a well is completed too near the WOC, it will tend to suck up water and its economic life may be short as it may soon become “watered out” (i.e. produce excessive quantities of water). This phenomenon is known as “water coning” and is induced by high rates of production.

24. Fingering (Fluid displacing a more viscous fluid)
Fingering: A condition whereby the interface of two fluids, such as oil and water, bypasses sections of reservoir as it moves along, creating an uneven, or fingered, profile. Fingering is a relatively common condition in reservoirs with water-injection wells. The result of fingering is an inefficient sweeping action that can bypass significant volumes of recoverable oil and, in severe cases, an early breakthrough of water into adjacent production wellbores

25. Water Coning
Coning.
The cone profile caused by preferential flow of different phase fluids surrounding the wellbore can be difficult to predict and control. Treatments against coning are typically conducted when the production or processing of the produced fluid requires that the production characteristics of the well be modified.

26. Water Cresting
Cresting. Similar to the coning profile encountered in vertical wellbores, cresting is the result of preferential fluid flow of different phase fluids near a highly deviated or horizontal wellbore. Due to the potentially extended contact area, cresting can be more difficult to control than coning.

27. Pressure and Gas/Oil Ratio Trends

28. Gravity Drainage in Oil Reservoirs
Working condition
Reservoir with a gas cap
Good permeability
Reservoir is steeply dipping
Energy supply
Some of the oil will drain downwards from the pores as a result of the difference in density between the gas and the oil.

29. Combination Drive in Oil Reservoirs

30. Recovery as A Function of the Type of Reservoir
Reservoir Type
Recovery
Remarks
One-phase oil
<10%
Pb<Pa
Oil with dissolved gas drive
5~25%
Pa<Pb
Oil with gas cap
10~40%
Oil with aquifer
10~60%
Aquifer active
Gas
60~95%
Average oil ≈ 30% and average gas ≈ 75%
(Recovery by natural drive mechanisms)

31. Introduction to WaterFlooding2
Introduction to WaterFlooding

32. Secondary Recovery and EOR
Secondary recovery = “Conventional” improved recovery
Water injection (water flood)
gas injection (gas drive)
Enhanced oil recovery = “Improved” or “tertiary” recovery
Steam, CO2, hydrocarbon gas injection
Chemical methods (polymers, microemulsions, etc.)
Thermal methods: heavy oils
(steam, in-situ combustion)

33. History of Waterflooding

34. Goal of Waterflooding
Increase the amount of oil recovered from the reservoir by
Maintaining reservoir pressure
Displacing (sweeping) oil with water

35. Peripheral Flooding

36. Line Drive Patterns

37. Different Well Patterns
5-spot pattern:
7-spot pattern:
4-spot pattern:
9-spot pattern:

38. Example of Well Sites on a Field
A large carbonate field in Abu Dhabi.
Wide variation in petrophysical properties from the south to the north of the structure.
South: h = 90m, k = 400mD -> peripheral flood
North: h = 30m, k = 50mD -> five-spot pattern
(Basics of Reservoir Engineering, R. Cosse)

39. Factors Affecting Selection of Waterflood Pattern
Provide desired oil production capacity.
Provide sufficient water injection rate to yield desired oil productivity.
Maximize oil recovery with a minimum of water production.
Take advantage of known reservoir nonuniformities - i.e., directional permeability, regional permeability differences, formation fractures, dip, etc.
Be compatible with the existing well pattern and require a minimum of new wells.
Be compatible with flooding operations of other operators on adjacent leases.

40. Pattern Orientation

41. Waterflood Performance Efficiency
Recovery efficiency
ER =
ER = EP EI ED
= EV ED
= EA EI ED
EP = Pattern sweep efficiency
EI = Invasion efficiency
EV = Volumetric efficiency
EA = Areal efficiency
ED = Displacement efficiency

42. Areal Sweep Efficiency [EA]
Fraction of the horizontal plane of the reservoir that is behind the flood front at a point in time.
Factors affecting EA:
Mobility ratio
Well spacing
Pattern geometry
Areal heterogeneity

43. Mobility Ratio
Mobility =
Mobility ratio:

44. Mobility Ratio Effects
Neutral
Water and oil move equally well
M < 1
Favorable
Oil will move easier than water
M > 1
Unfavorable
Water will move easier than oil

45. Areal Sweep Efficiency [EA]
(The Reservoir Engineering Aspect of Waterflooding, Forrest F. Craig)

46. Vertical Sweep Efficiency
Factors affecting EI:
Gravity
Barriers to vertical flow
Lateral pay discontinuities
Completion interval inconsistencies

47. Effects of Gravity

48. Barriers to Vertical Flow
Depositional
Shale streaks
Lithology changes
Evaporite streaks
Diagenesis
Cementation
Dolomitization

49. Lateral Pay Discontinuities

50. Completion Interval Inconsistencies

51. Injection Characteristics
Volume of injected fluid
Vinj = Vprod reservoir pressure is maintained
Vinj > Vprod recompression occurs
Vinj < Vprod reservoir pressure decline
Type of fluid
More viscous injected fluid is preferred since the mobility ratio M is lower.
(water, light oil, heavy oil)