Lecture 11 Oilfield Development Fall 2012 Lecture 11 Oilfield Development

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

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.

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)

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.

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

RESERVOIR PRODUCTION MECHANISM AND PRODUCTION CHARACTERISTICS 1 RESERVOIR PRODUCTION MECHANISM AND PRODUCTION CHARACTERISTICS

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

Gas Reservoir Drive Mechanisms Gas expansion drive Water drive

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

Compressibility Coefficients, Fluid Expansion The orders of magnitude for oil, water and the porous medium are as follows: For gas: P 1000 3000 5000 7000 Cg [1000 333 200 143] 10-6 psi-1

Solution-Gas Drive in Oil Reservoirs

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” .

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.

Solution-Gas Drive in Oil Reservoirs Typical production characteristics

Gas-Cap Drive in Oil Reservoirs

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.

Gas-Cap Drive in Oil Reservoirs Typical production characteristics

Water Drive in Oil Reservoirs Edgewater drive Bottomwater drive

Water Drive in Oil Reservoirs Typical production characteristics

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)

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.

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.

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

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.

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.

Pressure and Gas/Oil Ratio Trends

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.

Combination Drive in Oil Reservoirs

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)

Introduction to WaterFlooding 2 Introduction to WaterFlooding

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)

History of Waterflooding

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

Peripheral Flooding

Line Drive Patterns

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

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)

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.

Pattern Orientation

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

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

Mobility Ratio Mobility = Mobility ratio:

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

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

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

Effects of Gravity

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

Lateral Pay Discontinuities

Completion Interval Inconsistencies

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)