5-Prod Mechanisms - Lecture notes 6 PDF

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

PRODUCTION MECHANISM OF A FRACTURED RESERVOIR Introduction Reservoir description during depletion Reservoir zoning Zoning vs. pressure distribution Reservoir zoning vs. production mechanism Production mechanism in the gas-invaded zone Production mechanism in the gasing zone Production mechanism in the undersaturated zone Convection process in gasing and undersaturated zones Production mechanism in the water-invaded zone Specific aspects of a fractured reservoir Material balance relationship Migration of hydrocarbons in a fractured reservoir

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

INTRODUCTION The behavior of a conventional reservoir where fluid and rock characteristics are in the same range of magnitude as a fractured reservoir is different:  The reservoir gas-oil ratio, GOR vs. recovery is substantially lower in a fractured reservoir

 The rate of pressure decline per unit of oil produced is normally low in fractured reservoirs

 The absence of transition zones in a fractured reservoir  Pressure drop around a producing well in a fractured reservoir is very low  Free-water oil production in a fractured reservoir is essentially a function of production rate,  Constant PVT properties with depth usually occurs in a fractured reservoir 2

IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

RESERVOIR DESCRIPTION DURING DEPLETION Fracture network is divided into a number of zones, each of them practically saturated with only one phase, while inside each zone the matrix block may be saturated with one, two or even three phases. A given reservoir zonation will already exist before reservoir production begins (under static equilibrium) and another zonation will result from reservoir production conditions during field exploitation (dynamic state). Reservoir Zoning Division of a reservoir into zones depends essentially on the fractured network saturation. Sub-zones may also develop during the production of the reservoir, as a result of fluid equilibrium inside the matrix block as well as fluid exchange between matrix and fracture. The extension of zones and subzones is continuously changing during reservoir production 1. Reservoir zoning under static conditions

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

2. Reservoir zones under dynamic conditions a. Main zones. - water-invaded zone between WOLO and WOL - gas-invaded zone between GOLO and GOL - oil zone between WOL and GOL b. Sub-zones As a result of pressure variation with depth the oil zone may be divided into two additional zones: - the gassing zone, between GOL and Ps= Bpp - the undersaturated zone, between Ps = Bpp and WOL

c. Saturation distribution - In the gas invaded zone: Sg=1-Swi-Sorg - In oil gassing zone: So >>Sorg - In undrsaturated oil zone: Interstitial water and initial oil - In water invaded zone: So=Sorw 4

IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

Zoning vs pressure distribution 1. The role of fracture pressure vs. depth Reservoir zoning may be continuously evaluated by pressure recording vs. depth in an open hole observation well. The four zones may be delimited if the pressure vs. depth variation is associated with the bubble point pressure Bpp.

If the single blocks are separated in the invaded zones:

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

2. Zoning at a late stage of reservoir depletion Superposition of zones becomes possible especially if the reservoir height is not very significant. A stage may be reached when reservoir pressure is below Bpp in the water-invaded zone. In the upper part of the water-invaded zone (between WOLO and WOL) there will be a sub-zone where, in the matrix beside the water displacing oil, a saturation in gas will result from gas liberated from solution. The residual oil saturation in this case will have to decrease.

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

RESERVOIR ZONING VS. PRODUCTION MECHANISM In the four zones which are developed during the depletion of the reservoir different production mechanisms is due mainly to the difference in fluid saturation of matrix and fractures.

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

a. Gas-invaded zone: Gravitational drainage displacement b. Gassing zone: Liberated gas expansion + buoyancy + imbibition + convection mechanisms Two sub-zones may be developed as a result of critical gas saturation vs. effective gas saturation in the matrix. The circulation of liberated gas in fractures saturated with oil, as well as the contact between the heavier oil of fractures with the lighter oil remaining in the matrix, develops more complex transfer processes. In the case of non-uniform distribution of matrix pores and low decline rate of reservoir pressure, supersaturation pressure phenomena can occur. c. Undersaturated zone: Simple expansion drive mechanism The expansion drive mechanism will be bigger if the compressibility and pressure decline rate are higher while production rate increases if block dimensions are smaller. d. Water-invaded zone: Gravitational + capillary imbibition Oil recovered from the matrix pores as a result of progressive exposure of the matrix pores to a water environment in the fractures, is rate sensitive to rate of water table advancement. The situation may be simplified in the case that only part of this production is present in the reservoir.

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

- Strong water drive may maintain a reservoir pressure. Recovery is then the result of imbibition displacement only, through capillary and gravity forces. This corresponds to the behaviour of the Gela (Italy), Amposta (Spain) and Nido (Philippines) reservoirs. - If the aquifer is limited or non-existent, the oil will be produced as a result of the expansion of gas liberated from solution in the oil zone, and of gravity drainage in the gasinvaded zone. 10

IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

- The oil produced from the blocks during drainage displacement, if moving downward in the fracture network, may be re-imbibe the lower located matrix blocks with oil, if these are desaturated in oil, or may move towards the wells if the pressure gradients in the fractured network are big enough to overcome the segregation effects. - Since gas-oil interfacial tension increases with reservoir depletion, the hold-up height (in gas invaded zone) may increase in the absence of a pressure maintenance by gas injection. Production Mechanism in Gas Invaded Zone The interaction effect of the blocks in gas invaded zone May be called block-block interaction, as a result of oil entering (supplied) into the upper face of a tilted block, or infiltrating from the surrounding fractures after having left an upper located block. Between the two adjacent blocks 1 and 2 a wet region may create an oil bridge and thus a continuous oil phase among the blocks, due to irregularities in narrow fractures.

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

Drainage rate of single block is expressed:

Maximum drainage rate is when Pc=0:  =  = 

 ∆ 

- the supply rate from block i-1 towards block i along the wet region is Qi-1,sup,i which may be higher or lower than the drainage maximum rate Qmax of block i.

if Qi-1, sup, i < Qmax all the oil is suck in if Qi-1, sup, i > Qmax excess oil pass through fracture network i 12

IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

the supply rate is expressed: Qi-1, sup, i = QDR, i + Qpass, i the reinfiltration rate is then equal to the rate of supply under the condition that the supply rate is lower than the maximum drainage rate: Qreinf, i = Qi-1, sup, i ; if Qi-1, sup, i < Qmax the passing rate is: Qpass,i= Qi-1, sup, i - QDR, i ; if Qi-1, sup, i > Qmax

Degree of block interaction An interaction parameter α is introduced: - Full interaction: α=1

→

Qi-1, sup, i < Qmax

- Partial interaction: 0< α Qmax

- No interaction: α=0

→

Single block problem

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

Experimental Observations - the oil from block 1 moved through block 2 instead of travelling along its surface - even though the wet region of supply was small, infiltration rates were often high - reduction of infiltration rates was observed in flat blocks where the wet region was located at one side of the blocks’ surface - the infiltration rate depended on block shape and size, as well as on supply point location, matrix isotropy and presence of impermeable layers. Saturation behavior Cons. entering rate Qsupp + cons. produced drainage rate (gravitational QMAX) → a steady-state internal saturation. The ratio of the two rates  = QSUPP/QMAX will determine a capillary hold-up height and an oil desaturation distribution.

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

For Kro=(So*)7 and RF=31.5%

At the top of the block, So* is large even for reduced wet zones and Ko is high enough to assure the entrance of oil into the matrix. If RF = 100%, corresponding to Pc = 0, the curve is similar to the Kro curve which the variations of Q behave as a Kr curve. For lower final recoveries, the shape of the curves remains the same, which means that the curve Ko may be shifted downward according to the product Ko (1-RF).

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

Gravity drainage from stacks of equal blocks Oil will move through all blocks: tstack=tsingle ×number of blocks Due to this long period of time, the rate in a stack will evidently be very slow (semi-steady state) compared with the drainage rate of a single block. The semi-steady-state stack production rate will be determined by the pseudo relative permeability curve for the oil phase in relation to the recovery RF , at an average saturation in oil So *. The function F(S), which is similar to a pseudo-relative permeability curve, takes into consideration the capillary holdup height, the process is similar to a gravity drainage process, but without the capillary effects. Case of 50 equal blocks: The characteristics of the single block are: height 3 meters; (1- Swi) = 0.083; Kro = (So*)7; Ko (Swi) = 1.26md; Δρ = 0.6 gr/cm3 and µo = 1.c.P.

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

Gravity drainage from stacked heterogeneous blocks The variations in dimensions and physical rock properties influence the supply and drainage rates. From one block to another the degree of interaction α will be different; for each block in a stack, the relationship between the drainage rate and its average oil saturation will be given by a pseudo-relative permeability F. the ratio of the drainage-gravity ratio vs. time

Saturation vs time is:

Example: A stack of 100 matrix blocks having an average height of 3 meters, but with different physical properties. The viscosity of oil used was µo = l0 c.P., Δρ = 0.6 g/cm3 and Kro = (So *)7

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

- The drainage rate was initially controlled by high K blocks. - When the drainage rate of the stack decreased, the lower K blocks are also involved in the drainage process. - The parameter α greatly influences the recovery-time relationship, and thus, RF is proportional to α. - When (α = 1) the ultimate RF did not show any sensitivity, but it seemed to be very sensitive through the relationship recovery vs. time for a moderate (α = 0.25) or no interaction (α = 0). - A sudden increase in recovery from 30 % to 50% after 25 years of production showed that in the case of α = 1 (total interaction), negligible additional oil could be produced; but in the case of α = 0 (no interaction), significant improvements in recovery became possible in the same period of time.

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

b. a comparative calculation for various driving conditions The constant yearly rate was 1 % of original oil in place.

In the case A, recovery was stopped in each block when oil saturation corresponded to the Pc hold-up. The difference in recovery between α= 0.75 and α = 0 is not too sensitive. But repressuring or pressure maintenance at an early stage showed significant increases in recovery. If repressuring or pressure maintenance began at a later stage of depletion. The improvement in recovery was less significant.

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

c. Comments on calculation example: Since any additional recovery obtainable from gas injection seems very favourable, the understanding and evaluation of the block-block interaction process with all implications is very important for the design of reservoir development. The evaluation of interaction through matching the past history of reservoir performance of the reservoir with calculated results is the general procedure. In the case of depletion independent of block-block interaction, matching may be obtained by adjusting block height which influences the recovery RF, and permeability, and consequently, adjusts the time scale of the gravity drainage process.

Production mechanism in the gasing zone During the reservoir depletion, different fluid exchanges between matrix and fractures take place. The liberated gas from matrix blocks percolates upward in fractures while the oil from matrix moving in a downward direction may re-imbibe to lower blocks. Presence of fractures changes the production mechanism from a conventional solution gas-drive to a more complex production mechanism and flowing process.

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

1. Oil circulation and implications on PVT properties

P increase with depth

→

Oil compression

T increase with depth

→

Oil expansion

Above a critical temp. grad., and excellent intercommunication between fractures, circulation may occur causing constant PVT properties with depth. a. Segregation vs. convection of oil in a fractured reservoir Segregation means that the heavy and light components in the reservoir are distributed under gravity forces. b. Condition of convection and segregation For a given oil composition and a given geothermic gradient: (δp – δT) > 0 → Convection (δp – δT) < 0 → Segregation 22

IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

2. A different concept of solution gas-drive as a result of diffusion (non-dispersion) Applying the conventional methods for simple expansion and solution gas-drive of a fractured reservoir → high OOIP or an extremely efficient water-drive for history match. Very low gas saturation in the gassing zone, which implies a higher gas displacement efficiency in the gas-invaded zone → reasonable match. At low pressure decline rate (dp/dt < 10-6 atm/sec), a supersaturation phenomena would be developed in the pores. - The largest pore having a lower Pc would be the first pore in which a gas bubble would develop. - Because of supersaturation, a gas concentration gradient exists between the bubble of gas in the large pore and its surrounding. If the reservoir pressure drops very slowly there will be enough time for the diffusion of gas through liquid in the surrounding area to feed the bubbles of gas. In this manner the bubbles of gas grow larger and larger until the gas bubble channel reaches the upper boundary of a block, where it delivers gas to fractures. - At any time a new gas nucleus developed into the largest pore undergoes a similar process, since the pore pressure is below the bubble pressure, but greater than a critical supersaturation pressure.

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

- For a pressure decline rate (10-6 atm/sec) and a nonhomogeneous reservoir rock, a gas saturation of about 1% would be developed in the gassing zone. - If diffusion takes place in the undersaturated part of the reservoir, where the bubble point pressure drops considerably below the original value, the free-gas saturation is low when this part of the reservoir becomes a gassing zone. If, on the contrary, the pressure decline rate is high, supersaturation oil capacity is low and if in addition the reservoir rock is homogeneous, a tendency of gas dispersion phenomena occurs and a classic solution gas-drive calculation must be applied even in the fractured reservoir. a. Experiments and interpretation of results

A: coarse model, Dispersion

;

B: fine model, Non-dispersion

- In a non-homogeneous grain pack the non-dispersed single channel follows a more tortuous pattern than in a more uniform grain pack. 24

IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

- The transition of upward gas flowing from dispersion to nondispersion conditions is restricted to a particular permeability interval or a particular permeability -porosity ratio. - The upward gas flow at low rates is governed mainly by gravity and capillary forces alone, which helps a non-dispersion state of flow. b. Qualitative explanation of dispersion and non-dispersion Gas can leave the pore only if pore pressure P is higher than the capillary pressure in the two restrictions.

Leverett’s expression, the limit of the states of dispersion and non-dispersion: 25

IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course



σ ≅ 2 × 10 / c. Large scale experiments The two essential parameters resulting from supersaturation flowing conditions are the mean average saturation g and the mean supersaturation pressure  ss, expressed in relation to the mean oil pore pressure  , which is associated to reservoir depletion. Relationship of pressure vs. gas liberation is different on the surface of oil-gas contact, at the boundary of gas channels and inside the oil in the interchannel pores. The oil at the contact surface of the gas channels will liberate the gas according to the relationship Rs vs. P obtained in the laboratory PVT analysis, while outside the channels the oil will not liberate the gas as result of oil supersaturation, but diffuse to gas channels. If the rate of pore pressure decline is high, a non-equilibrium will be created between pore pressure and channel pressure. This overly high supersaturation pressure will reach a new equilibrium only by developing new gas channels. It becomes evident that the number of gas channels, the spacing between channels, and the mean gas saturation are all associated to the rate of pore pressure decline.

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IPE-UT ; Fundamentals of Fractured Reservoir Engineering ; Graduate Course

1. Role of permeability Whenever capillary pressure increases, the saturation Sg is small G...