A theoretical discussion and case study on the oil-charging throat threshold for tight reservoirs

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 41, Issue 3, June 2014 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2014, 41(3): 408–416.

RESEARCH PAPER

A theoretical discussion and case study on the oil-charging throat threshold for tight reservoirs ZHANG Hong1,2,3,*, ZHANG Shuichang2,3, LIU Shaobo2,3, HAO Jiaqing2,3, ZHAO Mengjun2,3, TIAN Hua2,3, JIANG Lin2,3 1. School of Earth and Space Sciences, Peking University, Bejing 100871, China; 2. Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China; 3. State Key Laboratory of Enhanced oil Recovery, Beijing 100083, China

Abstract: By analyzing the relationship between throat threshold and fluid forces of oil charge in tight reservoirs and according to the oil-charging mechanical conditions, the lower limits of throat at the interface between source and reservoir rocks and in the middle of reservoirs were determined theoretically. On the basis of Young-Laplace formula and the equilibrium between driving forces and capillary resistance, the threshold models were set up by using the maximum driving forces near the source-and-reservoir interface and inside reservoirs respectively. They were applied to the Yanchang Formation in the Ordos Basin, the middle-lower Jurassic in the Sichuan Basin and the Bakken Formation in the Williston Basin in America. The corresponding results near the interface are 15.74 nm, 29.06 nm, and 14.22 nm, and the ones in the middle of reservoirs are 39.45 nm, 37.20 nm, and 52.32 nm respectively. Accordingly, the threshold per− − − meabilities of the three typical tight oil reservoirs calculated are 0.002 1×10 3 μm2, 0.006 1×10 3 μm2, 0.001 8×10 3 μm2 near the inter−3 −3 −3 2 2 2 face and 0.010 0×10 μm , 0.009 4×10 μm , 0.016 9×10 μm at the inner reservoirs. The rocks near the interface are complex, so there is a poor correlation between porosity and permeability, while inside reservoirs, homogeneous lithology results in good correlation between porosity and permeability. The porosity thresholds were determined as 2.16%, 2.00% and 3.50% respectively. Key words: tight oil; oil-charging throat threshold; fluid forces; driving forces; theoretical discussion; case study

Introduction The oil-charging throat threshold for tight reservoirs refers to the minimum diameter of throats through which oil can charge from the source rocks into reservoirs under geological conditions [1−2]. Studies on the threshold of throat in conventional reservoirs at home and abroad have mostly been based on statistical analysis of reservoir physical property data. Although this method is helpful in figuring out the throat threshold of oil-charging to a certain extent [2−5], it is hard to determine if it fully reflects the reservoir conditions or not, due to a lack of sufficient theoretical evidences. At present, for unconventional tight oil reservoirs, the threshold is generally worked out by summing up the thickness of bound water film and oil molecule diameter [1,6−8]. Although easy and simple, the method is limited in static conditions, and thus does not agree with the real geological conditions which are dynamic. The throat threshold of oil-charging is affected jointly by reservoir intrinsic characteristics, source rock characteristics, oil properties, and burial depth and history of reservoirs [9]. In this article, the process of oil charging into tight reservoirs is

studied from the perspective of fluid forces; then relationship between the throat threshold of oil charging and mechanical action of fluid is analyzed; furthermore, the theoretical models of throat threshold of oil charging for tight reservoirs were established, according to the mechanical conditions for the charging process at the sourcerock-reservoir interface and inside the reservoir, as well as the Young-Laplace Equation [6,10] . Finally the throat thresholds of oil charging of Yanchang Formation in the Ordos Basin, Middle-Lower Jurassic Series of the Sichuan Basin and Bakken Formation of the Williston Basin in the US were worked out to provide references for reservoir evaluation.

1 Fluid mechanics factors affecting throat threshold of oil charging Oil charging is mainly controlled by fluid mechanics. The driving force for oil charging near the interface between source rock and tight reservoir mainly comes from the pressure caused by hydrocarbon generation; the force inside the reservoir is the state pressure of tight oil under the corresponding conditions. Resistance preventing tight oil from

Received date: 06 Jun. 2013; Revised date: 20 Feb. 2014. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Science and Technology Major Project (2011ZX05003-001,2011B0403); the PetroChina Science and Technology Research Project (2011A-0203, 2012E2601-01); the RIPED Science and Technology Research Project（2012Y-062, 2011Y-004） Copyright © 2014, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

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getting into the reservoir is capillary pressure, and it increases with interfacial tension [10−11]. Formation-brokendown pressure affects the charging process by restricting the charging force, and in turn the throat threshold of oil charging [12]. Therefore, driving force, capillary resistance and formation-brokendown pressure are the main factors influencing the fluid mechanics of oil-charging in tight reservoirs, which will affect the throat threshold of oil charging directly [6−7]. 1.1

Driving force of oil charging

1.1.1 Overpressure caused by oil generation near the interface between source rock and reservoir Thin but tight, the interface between the source rock and the reservoir is the key part of oil charging. The characteristics of tight oil accumulation show that the process of tight oil expelling from source rocks and filling into the reservoir is driven by the overpressure caused by oil generation [13]. The higher the pressure increases caused by hydrocarbon generation, the more likely the tight oil fills the small throats near the interface where the source rock and the reservoir meet, and the lower the throat threshold of oil charging; and vice versa. Based on the mechanism of hydrocarbon generation from kerogen and formation compaction, the theoretical calculation model of overpressure caused by oil generation is as follows [13]: FYM k [ aD (1 − ph Co ) − 1] pg = (1) CwVw1ρ k + (1 − YF )Ck M k + aYFM k DCo

ps = ph + pc =

RTVm α − Vm 2 − AVm + B Vm β

Statistics show that the global tight oil is mainly light oil, with samiliar physical and chemical characteristics [6−7]. In order to determine the four parameters in the state equation (2), namely, A, B, α and β, tight oil samples were selected from Well Gong-26 in the Gongshanmiao Oilfield in central Sichuan, and their chemical components were analyzed using liquid chromatography, and results show that the main components C13, C14 and C15, are 4.13%, 4.03% and 3.89% in relative content respectively. Taking their relative contents as weight coefficients and according to the Van der Waals volume of C13, C14 and C15 (139.87 mL/mol, 150.10 mL/mol and 160.33 mL/mol respectively) [15] and their acentric factors (0.623, 0.679 and 0.770 respectively) [16], the average Van der Waals volume of tight oil is worked out as 150.303 8 mL/mol and the average acentric factor as 0.692 1. According to the relationship between the coefficient A, B, α and β, and Van der Waals volume of tight oil and average acentric factors, the equations describing the charging state of tight oil are established with the determined coefficients: (3) A = 3.034 6Vw − 5.43

B = 0.197 5 A2.067 9

(4)

α = 3.172 5 A β = 1.539ω + 2.071 8.314TVm ps = 2 − Vm − 450.68Vm + 60 744.24

(5) (6)

3.045 6

(Y in the equation above is a parameter relating to HI, Y = HI /1 000 )

383 732 419.10 Vm 3.14

1.1.2 State pressure of tight oil inside the reservoir

When tight oil overcomes the resistance at the source -reservoir interface and enters the reservoir, the throat space for migration becomes larger and as a result, the oil-charging force drops suddenly, and thus the force of driving oil into small throats decreases accordingly. Inside the reservoir, the state pressure driving tight oil fights against pore resistance. When the charging is balanced, the higher the state pressure of fluid, the lower the throat threshold allowing oil into the tight reservoir will be; and the lower the fluid state pressure, the higher the throat threshold will be. Therefore, the state pressure of tight oil inside the reservoir and the throat threshold of oil charging are negatively correlated. Based on the van der Waals model of compressed fluid [14−15], the theoretical state equation for balanced tight-oil charging is established according to tight-oil components and the balancing pressure and temperature for charging. When tight oil charges from source rock to the nearby reservoir, the state pressure of fluid is balanced against the sum of capillary resistance and static water pressure. At a certain temperature, the state pressure and the volume of tight oil charging into the reservoir satisfy the modified van der Waals model [14]. Therefore, the state pressure of tight oil inside the reservoir is expressed as follows:

(2)

1.2

(7)

Resistance of oil charging

The charging of tight oil is hindered by capillary pressure, viscous force and inertial force [6, 11]. Among them, viscous force and inertial force can be neglected due to low viscosity and low expulsion velocity of tight oil [17]. Therefore, only the capillary pressure needs to be considered as the resistance for oil charging at the source-reservoir interface and inside the reservoir. The higher the capillary pressure, the stronger the resistance and the higher the throat threshold of oil charging are. According to the Young-Laplace equation, the formula to calculate the capillary pressure is [10]: 2σ cosθ (8) pc = r 1.3 Formation-brokendown pessure restricting the maximum driving force of oil charging

If the overpressure caused by hydrocarbon generation at the source-reservoir interface and the tight oil state pressure inside the reservoir are both higher than formation-brokendown pressure, micro-fracturing will happen in the source rock and reservoir. As a result, the driving force will decrease near the formation-brokendown pressure [18]. With the generation and expulsion of tight oil, overpressure caused by oil generation

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will no longer be influenced by the rate of oil conversion, rather than it will change with the formation-brokendown pressure, thus influencing the throat threshold of oil charging at the source-reservoir interface and inside the reservoir. Based on the effective stress of fluid and the formation tension mechanism, the model for calculating formation-brokendown pressure is expressed as the following [12, 18−19] : pf = hg ρ /(m − 1) + Tt (9)

2 Determination of the throat threshold of oil charging for tight reservoirs The process of tight oil charging is jointly influenced by driving force, capillary resistance and formation-brokendown pressure. Charging occurs when the driving force of tight oil (pd) is greater than capillary resistance (pc) [6, 11]. Therefore, the condition of tight oil charging can be expressed as: pd≥pc Combining the Young-Laplace equation, the condition can be converted into: r≥2σcosθ /pd Therefore, the minimum throat diameter the tight oil can get into is: d min,i = 2rmin,i = 4σ cosθ / pd,i (10) Generally, for a certain oil group in a block, the fluid property is relatively stable and the oil-water interfacial tension is close to constant [20]. Therefore, the larger the driving force is, the smaller the minimum throat (dmin,i) the tight oil can get in will be. In real geological history, oil charging might have happened many times, and the throat threshold of oil charging near the source-reservoir interface is determined by the maximum pressure caused by oil generation of source rocks during the entire history. For the tight oil accumulations, the largest overpressure at the interface corresponds to the largest state pressure inside the reservoir, and the throat threshold of oil charging inside the reservoir is determined by the maximum state pressure in the charging history. Therefore, the throat threshold can be expressed as: d min = 2rmin = 4σ cosθ / pd max (11) The maximum overpressure formed by hydrocarbon generation from kerogen changes with different oil-expulsion modes in charging history. The relationship between the overpressure and the rate of kerogen oil conversion shows that the maximum overpressure is determined by this rate of the deepest source rocks during continuous oil expulsion process [13] . While the relationship between the overpressure of the source rock and the formation-brokendown pressure shows that the maximum overpressure is determined by the formation-brokendown pressure of the deepest source rocks during oil expulsion process, because the formation-brokendown pressure increases with the increase of burial depth [12]. Similarly, the state pressure of tight oil is at its highest in the maximum burial depth. That’s because the same changing tendency exists between the tight oil state pressure in the res-

ervoir and the overpressure of source rock. Therefore, the throat threshold of oil charging near the source-reservoir interface and that inside the reservoir can be divided into the cases under fractured formation and non-fractured formation, which can be expressed respectively as follows: (1) When fluid pressure in pores at the deepest burial is less than formation-brokendown pressure, i.e. ppmax pfmax, tight oil is filled into the reservoir from source rock episodically. The maximum formation-brokendown pressure can be found at the deepest of burial source rock and reservoir. The maximum oil-driving force occurring at the maximum burial depth can be taken as the difference between formation-brokendown pressure and hydrostatic pressure at this depth. Based on Equation (11), the model for throat threshold of oil charging in tight reservoirs is: 4σ cosθ 4σ (m − 1)cosθ d min = = (13) pfmax − phmax hmax ρ g + (Tt − ρ w ghmax )(m − 1)

3 Case study The Yanchang Formation of the Ordos Basin, the Middle-lower Jurassic Series of the Sichuan Basin and the Bakken Formation of the Williston Basin are typical tight oil reservoirs [6, 11]. The above models for throat threshold of oil charging are adopted to work out their throat threshold values respectively. 3.1 Geological settings of the three typical tight reservoirs

Buried at the maximum depth of 3000m, with mainly type II kerogen, Chang-6 Member in the Yanchang Formation of the Ordos Basin and the underlying Chang-7 Member source rock constitute a reservoir-above-source combination [6,11,21]. While, the tight sandstone of the first member of the Shaximiao Formation in the middle-lower Jurassic of the Sichuan Basin and the underlying black mud shale of the upper Lianggaoshan Formation form a reservoir-above-source combination; the black shale in the upper Member of the Lianggaoshan Formation and the thin siltstone interbed in it form a source-reservoir in one combination, with the maximum burial depth of 3 100 m and mainly type II kerogen [6, 11]. Whereas the middle Bakken Formation in Williston Basin of America,

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Fig. 1 Source-reservior combination of tight oil accumulations in the three typical regions

held in between the upper and lower source rocks of the same Formation, forms a ‘sandwich’ reservoir-source combination with a maximum burial depth of 2 900 m and mainly type I kerogen [6,11] (Fig. 1). The tight sandstone reservoirs directly contacts with the source rocks in all these 3 regions. 3.2 The fluid mechanics at the maximum burial depth of typical tight reservoirs

The oil-water interfacial tension should be worked out firstly. Under ambient temperature and pressure, the standard physical parameters of tight oil of Well Gong-26 in the Gongshanmiao Oilfield were measured, and the relationship between oil-water interfacial tension and oil density was fitted, with a multiple correlation coefficient of 0.9117 (see Fig. 2).Based on the oil density of the Yanchang Formation of the Ordos Basin, the middle-lower Jurassic of the Sichuan Basin and the Bakken Formation of the Williston Basin [22−23] (see

Table 1), the oil-water interfacial tension of them is 32.74 mN/m, 36.18 mN/m and 29.30 mN/m respectively. Based on physical parameters of kerogen and the properties of crude oil[18−21] (see Table 1), the four pressure values, namely the hydrostatic pressure, overpressure caused by hydrocarbon generation, fluid pressure in pores and formation-brokendown pressure in the deepest burial of source rocks, all can be worked out by the models of classical mechanics presented above (see Fig. 3). These are 31.80 MPa, 9.75 MPa, 41.55 MPa and 40.12 MPa respectively for the Yanchang Formation of the Ordos Basin, 32.86 MPa, 4.98 MPa, 37.84 MPa and 41.26 MPa for the middle-lower Jurassic of the Sichuan Basin and 30.74 MPa, 20.41 MPa, 51.15 MPa and 38.98 MPa for the Bakken Formation of the Williston Basin. The maximum driving force inside the reservoirs can be calculated by the equation describing the corresponding state pressure of tight oil. Tight oil is usually buried shallow, and relatively stable in property. Also factor analysis tells that, comparing with the chemical composition of the oil, the influence of temperature on the oil state pressure can be neglected. Under the ambient temperature of 293.15 K the pressure in the state equation is approximated related to the molar volume of tight oil only (Vm), which is the key to determining the state pressure value. According to definition, molar volume equals the value of molar mass divided by density of oil. Thus molar mass and oil density should be determined firstly. (1) Molar mass is 198.28 g/mol by weighting the relative contents of C13, C14 and C15 of the typical tight oil in Well Gong-26. (2) Density of tight oil inside the reservoir under equilibrium state equals the mass of the expelled oil divided by the oil volume in the pores of the reservoir. Assuming that the volume of the source rock is “1” and the maximum oil-expulsion coefficient is 72% [11, 21], the mass of the oil charged into the nearby tight reservoir can be obtained according to the law of conservation of mass; and the volume of oil, under equilibrium state after charging in the reservoir, equals the total porosity multiplied by oil saturation. The oil saturation of the Yanchang Formation in the Ordos Basin, the middle-lower Jurassic in the Sichuan Basin and the Bakken Formation in the Williston Basin is 70%, 85% and 70% respectively [6−7, 11, 21]. Thus the molar volumes of tight oil inside the reservoir can be determined and their state pressures at the maximum burial depth can be obtained, which are 41.18 MPa, 45.74 MPa and 44.90 MPa respectively; and the formation-brokendown pressures of the reservoirs are 35.12 MPa, 36.75 MPa and 32.98 MPa respectively. 3.3 Throat threshold of oil charging for the typical tight oil reservoirs and discussions 3.3.1

Fig. 2 Relationship between oil-water interfacial tension and density of tight oil in Well Gong-26 (tested under ambient temperature pressure)

Theoretical throat threshold of oil charging

The throat threshold of oil charging near the source-reservoir interface was determined by overpressure caused by oil generation. In the Yanchang Formation of the Ordos Basin,

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Table 1 Physical properties of source rock, reservoir and the fluid in the three regions [6-7,11,21]

Strata

TOC/ HI/ % (mg·g−1)

Yanchang Formation 6.0 of the Ordos Basin Middle-lower Jurassic of the Sichuan 1.7 Basin Bakken Formation in 11.0 America

Hydrocarbon Maximum Ro/ generation burial % conversion depth/m rate/%

Ratio of resCompressibilDensity Oil-water in- Porosity of Tensile ervoir thickity coefficient of oil/ terfacial ten- tight sand- strength/ −3 ness to source of oil/(10 (kg·m−3) MPa sion/(mN·m−1) stone/% −1 rock thickness MPa )

600

3 000

1.2

80

1.0

850

2.4

32.74

7.0

1.0

600

3 100

1.4

85

0.5

870

2.3

36.18

3.5

1.5

900

2 900

1.0

80

1.5

830

2.5

29.30

9.0

0.7

Note: oil-water wetting angle is 180°; residual coefficient is 80%; density of kerogen is 1 200 kg/m3; compressibility coefficient of kerogen is 1.4×10−3 MPa−1; porosity of kerogen is 1%; density of formation water is 1 060 kg/m3; compressibility coefficient of formation water is 0.44×10−3 MPa−1; oil productivity index of source rock is 400 kg/t; density of source rocks is 2 450 kg/m3; density of the overlying formation is 2 650 kg/m3; Poisson ratio is 0.3; tensile strength of source rocks is 6 MPa.

kendown pressure, so the oil expulsion is also episodic. Placing the formation-brokendown pressure and hydrostatic pressure at the maximum depth of 2 900 m, into the model (Formula (13)), the throat threshold was estimated as 14.22 nm (see Fig. 4). Similarly, the throat threshold of oil charging inside the reservoirs can be worked out by the state pressure of fluid. The maximum state pressure of tight oil in the three basins is all higher than their reservoir breakdown pressure correspondingly, so their oil-charging processes are episodic. The throat threshold of oil charging for the three basins can be obtained by the model (Equation (13)), which is 39.45 nm, 37.20 nm, and 52.32 nm respectively (see Fig. 4). 3.3.2 Discussion on theoretical throat threshold of oil charging

Fig. 3 Comparison of overpressure caused by oil generation, static pressure and formation-brokendown pressure of the three typical tight oil reservoirs at the maximum burial depth

the fluid pressure in the pores of source rocks is higher than its formation-brokendown pressure, so the oil expulsion here is episodic. The throat threshold of oil charging was estimated as 15.74 nm (see Fig. 4) by placing the formation-brokendown pressure and hydrostatic pressure at the maximum depth (3 000 m) of the source-reservoir interface into the theoretical model of oil-charging threshold (Equation (13)). The fluid pressure in the middle-lower Jurassic of the Sichuan Basin is lower than its formation-brokendown pressure, so the oil expulsion from source rocks is continuous. Putting the overpressure value into the model (Equation (12)) at the maximum depth of 3 100 m, the throat threshold calculated was 29.06 nm (see Fig. 4). In the Bakken Formation of the Williston Basin, the fluid pressure is higher than the formation-bro-

The common diameter of throats in the tight sand reservoirs is between 50−900 nm, while the maximum bitumen molecule-diameter is 4 nm [6, 11, 24−27]. Calculated by the model of classical mechanics and the actual parameters of tight oil, the throat threshold of oil charging in this study ranges between the throat diameter of the reservoirs and the bitumen-molecule diameter (see Fig. 4), which reflects the geological conditions of oil charging to a certain extent. The throat threshold of oil charging calculated previously, by adding the thickness of bound water film and the maximum bitumen diameter together was 54 nm, higher than the result of this study, which may be explained from the following four aspects: (1) The thickness of bound water film in the reservoir rocks for static calculation was based on the equation of bound water film thickness in soil, possibly causing overestimation of the thickness of bound water film in rocks; (2) Tight oil, relatively light, mainly is composed of saturated hydrocarbons and aromatic hydrocarbons [6,11,24], so the throat threshold calculated by the maximum size of bitumen molecule might not reflect the real characteristics of tight oil; (3) The fluid mechanics were not considered in the static calculation, driven by overpressure from source rocks under geological condition,

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Fig. 4

Comparison of throat threshold of oil charging and the diameters of throats in tight reservoirs

the charging ability of tight oil could be strengthened considerably; (4) Mainly considering rock samples from the reservoir, the static calculation doesn’t take comprehensive consideration of the source rock, reservoir and fluid; in comparison, the calculation in this study was mainly based on the real parameters of tight oil reservoir and took into consideration the fluid forces during charging. Therefore the results of this study can reflect the minimum throat in the oil charging history and are the thresholds of throats for oil charging in the true sense. Moreover, the results obtained in this article are close to the threshold calculated previously by using the chart of throat size and oil saturation [28], but is lower than the results obtained from statistical analysis of oil test data. Compared with the threshold parameters, used in the calculation of reserves in industry (throat diameter: 100 nm, permeability: 0.1×10−3 μm2 and porosity: 7%) [29], the results of this study are also smaller. The main reason lies in the fact that the driving force of fluid during oil accumulation should not be neglected, because the mechanical mechanism of the original accumulation of tight oil is a primary factor influencing the throat threshold; but the statistical analysis only reflects the present conditions of reservoirs rather than the charging and accumulating process. In this study, the estimation of throat threshold from the perspective of the mechanical mechanism of accumulation reconstructed the process of oil charging to a certain degree. In addition, this study also tried to convert the throat

threshold of oil-charging into physical properties threshold of reservoirs. The permeability threshold at the source-reservoir interface and inside the reservoirs (see Table 2) were calculated according to the regressive relationship between the diameter of median throat and the permeability of tight sandstone [29]. Since the correlation between porosity and permeability in tight reservoirs is not clear, due to the complex lithology near the interface between source rock and reservoi [6, 11, 21] , only the porosity threshold inside the reservoir was discussed [30] (see Table 2). The middle-lower Jurassic of the Sichuan Basin has the highest permeability threshold near the source-reservoir interface, at around 0.0061×10−3 μm2; while the Bakken Formation of the Williston Basin has the lowest permeability of about 0.0018×10−3 μm2; and the Yanchang Formation of the Ordos Basin is in the middle, with the permeability of 0.0021×10−3 μm2. Inside the reservoirs, the middle-lower Jurassic of the Sichuan Basin has the lowest physical threshold, with the permeability of about 0.0094×10−3 μm2 and the porosity of 2.00%; the Bakken Formation of the Williston Basin sees the highest threshold, with the permeability of 0.0169×10−3 μm2 and the porosity 3.50%; the Yanchang Formation of the Ordos Basin is in the middle, with the permeability of 0.0100×10−3 μm2 and the porosity 2.16% (see Table 2). The results show that the charging and accumulation process would come to the strongest resistance near the source-reservoir interface in the middle-lower Jurassic of the Sichuan Basin, and the potential for accumulating here

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Table 2 Threshold of physical properties of different tight sandstones corresponding to throat threshold [28−32] Range Low-porosity and low-permeability sandstone Deep clasolite Chang-6: ‘continuous’ tight oil

Diameter threshold of throats/nm

Method Statistical method Oil test Method Calculation by water film

54.00

Tight sandstone oil Environment scanning and test Unconventional tight sandstone Chart method Yanchang Formation of the Ordos Basin Interface between source rock and Middle-lower Jurassic of tight sandstone the Sichuan Basin reservoir Bakken Formation of the Williston Basin Yanchang Formation of the Ordos Basin Inside the tight Middle-lower Jurassic of sandstone reservoir the Sichuan Basin Bakken Formation of the Williston Basin

is the smallest. However, if the oil breaks through the interface and charges into the reservoir, the resistance would decrease. In fact it has the smallest throat threshold inside the reservoir among the three regions, showing the largest accumulation potential in the middle of reservoir of the Sichuan Basin.So it can be speculated that tight oil tends to accumulate inside the reservoir in the Sichuan Basin, near the source-reservoir interface in the Bakken Formation of the Williston Basin and both in the reservoir and near the interface in the Yangchang Formation of the Ordos Basin.

4 Conclusions The throat threshold of oil charging in tight reservoirs at the source-reservoir interface and that inside the reservoir are influenced by the charging mechanics: the larger the overpressure produced by hydrocarbon generation near the interface is, the higher the state pressure of tight oil inside the reservoir will be; and the smaller the capillary resistance is, the smaller throat threshold at the source-reservoir interface and inside the reservoir will be. The driving force for continuous oil expulsion at the interface between source rock and reservoir is determined by the hydrocarbon conversion rate of source rocks, while that inside the reservoir is affected by the state pressure of fluid. Moreover, the driving force of episodic oil expulsion is controlled by formation-brokendown pressure when micro-fractures occur. Based on the equilibrium of oil-charging mechanics, the theoretical models of throat threshold of oil charging for continuous oil expulsion and episodic oil expulsion at the source-reservoir interface and in the reservoir were established. Based on the parameters of source rocks, reservoirs and fluid properties of the Yanchang Formation of the Ordos Basin, the middle-lower Jurassic of the Sichuan Basin and the Bakken Formation of the Williston Basin, the throat threshold near the source-reservoir interface

Permeability/ 10−3 μm2 1.000 0 0.800 0

Porosity/%

Source

6.00 9.00

[31] [32]

6.69

[29]

2.00

[11] [28]

0.059 0

44.00−58.00 30.00 15.74

0.002 1

29.06

0.006 1

14.22

0.001 8

39.45

0.010 0

2.16

37.20

0.009 4

2.00

52.32

0.016 9

3.50

This article

was worked out by the models, which is 15.74 nm, 29.06 nm and 14.22 nm respectively; while throat threshold inside the reservoirs is 39.45 nm, 37.20 nm and 52.32 nm respectively. From the diameter-permeability relationship and the porosity-permeability correlation of tight sandstone reservoirs, the permeability threshold corresponding to the throat threshold at the source-reservoir interface is 0.0021×10−3 μm2, 0.0061× 10−3 μm2 and 0.0018×10−3 μm2 respectively in the three regions; and the permeability threshold inside the reservoir is 0.0100×10−3 μm2, 0.0094×10−3 μm2 and 0.0169×10−3 μm2 respectively, and the porosity threshold inside the reservoir is 2.16%, 2.00% and 3.50%. The middle-lower Jurassic of the Sichuan Basin has the highest throat threshold of oil charging at the source-reservoir interface and the lowest throat threshold inside the reservoir, revealing that the massive tight oil in the Sichuan Basin tends to gather inside the reservoirs after the oil breaks through the interface. The Bakken Formation of the Williston Basin has the lowest throat threshold of oil charging at the source -reservoir interface and the highest throat threshold inside the reservoir, which suggests a higher accumulation potential near the interface. The Yanchang Formation of the Ordos Basin has a throat threshold between those of the above two regions, revealing the similar accumulation potentials both at the interface and in the reservoir. Overall, the study of the throat threshold of oil charging in this article based on the actual parameters in tight oil accumulations provides a new reference for the evaluation of tight oil accumulations and tight reservoirs.

Nomenclature

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pg—overpressure caused by oil generation, Pa; F—oil conversion rate of kerogen, %;

ZHANG Hong et al. / Petroleum Exploration and Development, 2014, 41(3): 408–416

Mk—mass of kerogen, kg;

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a—residual coefficient of oil, dimensionless;

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D—density ratio of kerogen to oil, dimensionless;

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Lu Xuesong, Liu Keyu, Zhuo Qingong, et al. Palaeo-fluid ev-

ph—hydrostatic pressure, Pa;

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Vw1—volume of water in pores, m3;

[3]

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ρk—density of kerogen, kg/m3;

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ps—state pressure of tight oil inside the reservoir, Pa;

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pc—capillary resistance, Pa; [4]

R—molar constant of gas, 8.314 5 J/(mol·K);

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T—temperature, K;

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Vm—molar constant of fluid, mL/mol;

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A, B, α, β—characteristic constants related to fluid properties but [5]

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Vw—Van der Waals volume, mL/mol;

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ω—acentric factor, dimensionless; [6]

σ—interfacial tension, N/m;

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ppmax—pore fluid pressure at the maximum burial depth, Pa; pfmax—formation-brokendown pressure at the maximum burial

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3

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