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Theory and Perspective on Stimulation of Unconventional Reservoir in China Zhifeng Luo Professor National key Laboratory of Oil and Gas Reservoir and Exploitation; College of Oil and Gas Engineering, Southwest Petroleum University, Chengdu 610500, China; e-mail: [email protected] Yujia Gao∗ Postgraduate College of Oil and Gas Engineering, Southwest Petroleum University,Chengdu 610500, China; e-mail:[email protected] Liqiang Zhao Professor National key Laboratory of Oil and Gas Reservoir and Exploitation; College of Oil and Gas Engineering, Southwest Petroleum University,Chengdu 610500, China; e-mail: [email protected] Pingli Liu Professor National key Laboratory of Oil and Gas Reservoir and Exploitation; College of Oil and Gas Engineering, Southwest Petroleum University,Chengdu 610500, China; e-mail: [email protected] Nianyin Li Professor National key Laboratory of Oil and Gas Reservoir and Exploitation; College of Oil and Gas Engineering, Southwest Petroleum University, Chengdu 610500, China; e-mail: [email protected]

ABSTRACT

China has tremendous unconventional tight oil resources which are widely distributed in lots of petroliferous basin. However, compared with North American, all the tight oil reservoirs discovered in China has its own characteristics including continental sedimentation, interbeded formation, worse reservoir physical properties and less oil reserves per unit volume, which make it much harder to stimulate. Based on thorough analysis of successful stimulation experiences in North American, taking the characteristic of China’s tight oil reservoir into consideration, this article emphasized that increasing stimulated volume must be paid attention to, what’s more, integrated fracturing technology must be adopted into tight oil reservoir stimulation to promote both production and recovery efficiency. Analysis results show that the widespread use of high-speed drilling technology and multi-interval integrate fracture technology for long horizontal interval are critical to successfully develop overseas unconventional tight oil reservoirs. Therefore, Researchers should 1)energetically study the physical properties and geomechanical condition of aim reservoir; 2)learn and digest

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horizontal well fracturing techniques to establish SRV stimulation technology suitable for the condition of China’s tight oil reservoirs; 3)actively explore the best stimulation pattern for different formations. We sincerely hope that through researches above, we can achieve large-scale exploitation of China’s tight oil reservoirs with the minimum cost and the maximum effect.

KEYWORDS: Unconventional Tight Oil Reservoir; Development Trend; Stimulation

Theory;

INTRODUCTION

Unconventional tight oil is the kind of oil which didn’t migrate in large-scale and long-distance way, adsorbed or separated in the reservoir; the overburden pressure permeability of which is less than 0.1mD (air permeability is less than 1mD). The reservoir containing unconventional tight oil interbeds with oil and gets close to tight and dense carbonate rock. Both the large quantity of adequate-maturity superior source bed and the large-scale tight reservoir intergrowth with the source bed are the critical conditions for the generation of unconventional tight oil reservoir. At the present time, several unconventional tight oil reservoirs have been founded in many hydrocarbon basins of China, such as Erdos, Sichuan and Songliao (Fig.1). There are three kinds of unconventional tight oil reservoirs: limnic carbonate rock, deep subaqueousdelta facies and gravity drainage sandstone; the total exploration area is more than 20×104km2 and the geological resource is about 106.7~111.5×108t1. Unconventional tight oil reservoir in Erdos is currently under developing; the unconventional tight oilin Three pond lake LuCaoGou group and The system of Sichuan basin is about to be developed; and except those, there are still large amount of potential tight oil resources. The unconventional tight oil reservoir developing technology is improved gradually and it’s necessary to develop the unconventional tight oil to ease the energy storage, change the energy structure and ensure the energy security of China.

Figure 1: The distribution of China’s unconventional tight oil reservoir

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CHARACTERISTICS AND CRITICAL STIMULATION BARRIER

While most of the overseas unconventional tight oil reservoirs are marine deposit, all the China’s unconventional tight oil reservoirs are continental sedimentation. The differences exist in sedimentation and tectonic environment make it more difficult to develop the unconventional tight oil reservoir in China.

Characteristics

The most extruded characteristic of unconventional tight oil reservoir is that the oil source rock also is the reservoir rock. Accumulation of the tight oil is controlled both by the source rock and the reservoir bed which makes significant differences between reservoir and traditional petroleum resource: 1) as the source bed or the tight rock could play the role of cap-rock, there is no obvious appearance about cap-rock and entrap; 2) with the tight oil distributing constantly, widely and gathering in the local fractured area, the reservoir boundary isn’t obvious; 3) the tight reservoir, of which the property isn’t good, leads to the pressure in the source rock rise to displace the oil, the non-floatage gather, bound water saturation turn up high, oil and water distribute complicated and oil aqueous interface doesn’t exist; 4) the source bed gets close to the reservoir rock and the short-distance migration is the major displacement; 5) the tight oil is mainly displaced by invading the hydrocarbon and in generally, the density of the tight oil is light and the pressure of the reservoir is always abnormal high. However, compared with the marine sedimentation in Bakken or Eagle ford, China’s tight oil reservoir which is continental sedimentation has its own special characteristics: (1) Multi-era sedimentation and multi interbeded layers The characteristics of continental sedimentation are the hydrodynamic conditions always change, the tight oil distributing area is limited and not stable in perpendicular and the reservoir has strong heterogeneity. Multi-era sedimentation and multi interbeded layers exist from system to Paleogene system. For the source rock interbeded with the tight reservoir, both the area and thickness are huge. The area of one single tight oil layer in Bakken, America is about 70000 km2. The tight oil reservoir of Sichuan which formed in Jurassic era re can be divided into 5 layers including shell and tight sandstone; while the single tight oil distribution area is inferior to 3km2, the total area can be 7~11km2. (2) Relative-poor reservoir property On one side, the and TOC in continental sedimentation are less than marine sedimentation; while the TOC in tight oil reservoir of Bakken is 10~14, it’s always less than 5 in China’s tight oil reservoir, most of which are between 0.8 to 3. However, compared to Bakken that the thickness of the layer is only about 10m, the accumulated thickness of tight oil reservoir in China is about dozen meters. On the other side, the tight oil reservoir

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property of China is pooer than Bakken and Eagle Ford: the oil is denser and more viscous, the porosity is always less than 10%, the permeability is much lower. The exploration of tight oil in China is more difficult and the complex tectonic movements in the late also has influence on the storage of tight oil. (3) Small scale of single tight oil area Although the total thickness and area of China’s tight oil reservoir is almost the same with marine sedimentation in North America, the total reservoir volume is smaller, the crude oil saturation is lower and the reserves of a single tight oil area is lesser for the lower TOC, poorer reservoir property, denser oil and more viscous oil. The distribution area and reverse of the tight oil reservoir in Yanchang Group of Erdos is about（ 8~10）×104km2and 35.5~40.6×108t respectively. The distribution area of Bakken is only about 7km2, but the tight oil reserves is 566×108t.

Table 1: Comparison between China’s tight oil reservoir and overseas typical area

Source rock Reservoir property Total Reverse Reservoir name Total area Porosity/ Permeabilit Total Pressure /108t TOC/% thicknes lithology /104km2 % y/mD thickness/m coefficient s/m Yanchang group of sequence in 2-10 20-110 Silt-fine stone 3-10 0.01-1.0 20-80 0.75-0.85 8-10 35.5-40.6 Erdos Permian sequence in siltstone 2-4 10-35 3-10 <1.0 80-200 - 6-8 15.0-20.0 Junggar Basin 、calcite dolomite

Jurassic system in Sichuan 100~15 Silt-fine stone 0.5-1.6 0.2-7.0 0.0001-2.1 10-60 1.23-1.72 7-11 15.2-18 Basin 0 Shell limestone Silt-fine stone、 ShahejieFomation in 1.5-3.5 100-200 calcareous sand stone 5-10 0.2-1.0 100-200 1.24-1.80 9-11 20.5-25.4 Bohai Gulf 、limestone Jurassic system in Tarim 0.28-16 85-380 Fine stone 4-7 0.01-1 200-300 1.73-1.78 - 10-15.9 Basin

Cretaceous system in 0.9-3.8 80-450 Silt-fine stone 2-15 0.6-1.0 5-30 1.2-1.3 8-9 19.0-21.3 Songliao Basin

Tertiary system in Qaidam 200-120 marlstone、algal 0.4-1.2 5-8 <0.1 100-150 1.3-1.4 2-3 3.6-4.4 Basin 0 limestone、siltstone

Cretaceous system in Jiuxi 1.0-2.5 400-500 carbonate、siltstone 5-10 <0.1 100-300 1.2-1.3 0.3-0.5 1.8-2.3 Basin marlstone、 Permian system in 260-120 1-6 calcareous sand stone 3-13 0.1-1.0 10-100 1.0-1.2 0.5-1 0.9-1.2 Santanghu Basin 0 、tuff、shale Jurassic system in Tuha 1-5 30-60 Silt-fine stone 4-10 <0.1 30-200 0.7-0.9 0.7-1 1.0-1.5 Basin greisen、 Bakken group in Williston 10-14 5-12 argillaceous siltstone 5-13 0.1-1.0 15-55 1.35-1.58 7 566 Basin of North America 、dolomite Eagle Ford group in 3-7 20-60 marlstone 2-12 <0.1 30-90 1.35-1.8 4 - American Gulf Basin

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Stimulation Difficulties

Characteristics, such as extremely lower permeability, tiny throat, heterogeneity caused by lacustrine deposit, interbeded layers and thin layers, etc., bring great challenges to the stimulation of China’s tight oil reservoir and the major difficulties are as follows: 1) As it is tight reservoir, high-displacement and large amount of liquid are needed in the stimulation process to form large-scale fractures with proper conductivity, which call for superior performance about fracture facilities, proppant( small volume, lower density and high strength) and frac-fluid (lower resistance, superior prop-carrying capacity, strong resisting shearing and low damage); 2) Micro-fractures are widespread in the reservoir and natural fractures are needed to be connected in the stimulation process to enlarge the fracturing scope. However, problems, such as huge filtration rate, high risk of screen out, strong heterogeneity, complex fracture extension pattern, hard to displace the sand and high risk, make it difficult to improve the sand volume and insure the fracture conductivity on the base safety; 3) It’s difficult to determine the location and development degree of the natural fracture; and also it’s hard to decide the fracture cracking and approaching pattern. All of those make it hard to optimize the stimulation parameters and the stimulation effect is uncertainty; 4) The stimulation mode is difficult to select for the multi-period sedimentation and complex bed series on vertical direction. There is no precedent to follow to develop the layers on the vertical; Besides, both the operation cost and the shortage of water limit the stimulation of China’s unconventional tight oil reservoir.

THE CRITICAL FACTORS OF OVERSEAS TIGHT OIL RESERVOIRS

In nowadays, North America, such as Canada and America, has developed the tight oil reservoir in large scale. The tight oil production of America and Canada are 100*104bbl and 20×104bbl respectively. By analyzing the successful field-experience, the key points to develop the tight oil reservoir successfully can be summarized as follows: (1) Fine reservoir conditions Although the reservoir is tight, it is easy to be drilled which conductive to long-interval horizontal well drilling; the high brittleness index and the amount of micro-fractures is conductive to the formation of fracture net; the heterogeneity of the thick single marine reservoir is not strong and the reservoir is easy to be stimulated. The middle of Bakken tight oil reservoir is complex carbonate shoal marine sedimentation buried 2000m beneath the earth, of which the major minerals are greisen sandstone and dolomite, the thickness is 15~55m, the brittleness index is high, the reservoir is duel media system, amounts

Vol. 21 [2016], Bund. 15 4928 of vertical fracture and high angle fracture are developed improving the effective permeability from 0.02~0.05mD to 0.6mD; The tight oil reservoir of Eagle Ford is Transgression Cretaceous system of which the buried depth is 1500~3500m, the thickness is 30~90m, the lithology is marlstone with percentage of the limestone and the I/S are 20~60% and 20~40% respectively and the other mineral is quartz; it is tighter than Bakken, but the brittleness is higher; (3) The long horizontal interval fast drilling technology The drilling time could be shortened by optimizing the wellbore configuration, increasing the casing depth and decreasing the drilling time; the contacting area between the well and reservoir might be increased by the use of large deflection curvature of which the angle is 8°~12°; drilling rig with good mobility could be used in the first drilling process to decrease the cost and improve the effectiveness; besides, PDC rig and hydraulic oscillator can also be used in the drilling process to promote the wellbore smooth, decrease friction and increase the length of horizontal interval on the base of delicate managed pressure drilling; drilling applied in horizontal interval ensures the wellbore extending in the reservoir and drilling rate be improved; compared that the water-based mud is used in the vertical well drilling, oil-based mud is used in the horizontal interval. All the technology mentioned improved the horizontal interval length from 800m to 1500m and the drilling time deceased from 60d to 20d which increased the developing effectiveness and deceased the cost enormously; (3) The applying of multi-integrate volumetric fracturing technology and the matching technology Integrate fracturing technology was used in the stimulation of long horizontal interval on the base of the application of the mature integrate fracturing tools, such as drilling bridge plug, open hole/casing packer+ sliding sleeve piecewise, fragment sand jetting by coiled tubing and so on; large sand volume mixed fracture with large displacement fluid volume and low sand ratio was used on the base of the using of high horsepower fracturing truck and high speed fracturing blender trailer; the mixed fracturing process is that forming the fracture net by the use of quick water first, supporting the secondary fracture by the use of 70/100 mesh sand carried by low crossing-link fracture fluid and supporting the major fracture by the use of 40~70 mesh sand carried by high crossing-link fracture fluid at last; the use of advanced volumetric fracturing technologies including multi-perforation for single interval, simultaneous fracturing and chain-cross fracturing for cluster well and branch well, proppant carried by fiber and HI-WAY proppant displacement technology make it possible to increase the fracture effective length and effective volume.

Figure 2: Wellbore structure and integrate fracturing design of the Eagle Ford

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Table 2: Fracturing parameters of the Eagle ford Parameters Min Max Avg TVD（m） 1725 3893 2498 Length of the horizontal interval - - 1774 Segmentation number 9 26 17 Perforation number per segmentation 3 10 6 Average fracturing pressure（MPa） 37.68 69.69 52.32 Average fracturing displacement（m3/min 6.75 15.85 13.00 ） Average fracturing fluid consumption for 6232.8 39336.6 16615.5 one well（m3） Average proppant consumption for one 848 3701 2225 well（t） Average fracturing fluid consumption per 403.86 3025.77 1001.7 interval（m3） Average proppant consumption per 70 215 132 interval（t）

STIMULATION THEORY

The pore-throat of dense oil reservoir is nanoscale and increasing the contact area between the wellbore and the reservoir is the only economic and effective way to develop the tight oil reservoir. In nowadays, almost all the stimulation of China’s dense oil reservoir draw on overseas dense oil reservoir stimulation experience that is long interval horizontal well + multi-integrate volumetric fracturing. The stimulation process is forming the major fractures by firstly, connecting the natural fracture with the reservoir and forming the secondary fractures by the use of multi-integrate perforation, high displacement rate, low viscosity fluid, diverting material and other matching technology. The complex fracture net help to maximize the contact area between fracture and matrix, minimize the distance of oil flowing to fracture, increasing the reservoir permeability greatly and realizing the 3-D stimulation of the tight oil reservoir. The technology can not only improve the single well production, but also reduce the limit of effective exploration and maximize the recoverable volume and recovery. With the development of dense oil exploration, the future direction of dense oil development is increasing the single well production as much as possible based on improving the traditional integrate fracturing which could maximize the contact area of wellbore with reservoir to obtain the maximum drain ability. Stress interference realized by multi-integrate perforation is one of the most important technology in volumetric fracturing.

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DEVELOPMENT TREND

It is more difficult to stimulate China’s tight oil reservoir than overseas. Although the dense oil has been developed at a certain degree, there are still so many problems, such as strong anisotropy and huge filtration which limits the improving of sand volume and fracture conductivity and pollutes the reservoir. Therefore, it’s still a long way to go to study China’ tight oil reservoir stimulating technology. Research about reservoir characteristics, fundamental study and experiments should be continued based on the advanced technology to develop the suitable stimulating technology for China. 1) The tight oil reservoir stimulation model should be optimized by in-depth geological mechanics and developing of stress system. The complex reservoir structure, natural fracture shape, interaction between artificial fracture and natural fracture in the process of fracturing and fracture net shape all are influenced by the distribution of tight oil reservoir stress and fracture and rock parameters. Therefore, it’s need to carry on multi-scale rock mechanics experiments, bring in the advanced formation rock mechanics and developed in-situ stress prediction technology to build the stress system to acquire the direction and the value of stress, horizontal stress difference, interlaminar stress and rock strength; Applying the geomechanics to the optimization of stimulation on the base of acquiring the varying characteristics of the stress in the same structure, analyzing the relationship between fracture and stress, building the fracture cracking and extending rules and putting forward the basis of segmentation. 2) To develop the suitable stimulation technology for China’s tight oil reservoir by bringing in advanced volumetric fracturing technology in the world and promoting the critical field experiment positively. On one side, it is need to develop a set of industrial technology and matching instrument system to lower the cost by learning the overseas advanced technology about horizontal well drilling and multi-integrate fracturing of cluster well and horizontal well based on the realistic characteristics of China’s tight oil reservoir; On the other side, integrated design method should be developed to optimize the pattern, scale and extending regulation of the fracture network and the distribution of multi-integrate fracture based on deepening the study of reservoir characteristics, considering the cost and stimulating effect and integrating geological model, geomechnanics model, rock crack mechanism, interaction of fractures, flowing model of the fracture networks and the diagnosis method of the hydraulic fracture. At same time, technologies, such as real-time monitoring and optimizing technology of staged horizontal well fracturing, fracturing fluid designing technology for horizontal well staged fracturing and fracturing optimizing method, should be developed to form a set of comprehensive optimization platform which could improve the fracturing effect greatly. Perfecting the dense oil stimulation technology and developing the suited stimulating technology for different tight oil reservoir. At the present times, the stimulation of dense oil reservoir always draws on shale-gas reservoir stimulating technology such as long horizontal well+ multi-staged volumetric

Vol. 21 [2016], Bund. 15 4931 sand-fracturing technology. But there are still many problems about the stimulating technology, for instance, the feasibility, operation risk, cost of the technology, etc. The lithology of China’s tight oil reservoir is comprehensive, major of which are silt-finestone, tuffaceous siltstone, marlstone, calcite dolomite and carbonate. The calcium levels in the dense oil reservoir is high(about 40%) except that the siltstone is the major component of the dense oil reservoir in Yanchang group of Erdos Basin and Cretaceous system of Songliao basin. Based on the stimulating experience of carbonate reservoir and comprehensive lithological reservoir, horizontal well+ multi-staged interval volumetric acid fracturing/hydraulic fracturing could be optimized to improve the effect of volumetric stimulation. The potential technologies are horizontal well+ multi-staged interval pad-acid volumetric sand fracturing, horizontal well + multi-staged interval volumetric closing acidizing and horizontal well + multi-staged interval(volumetric san fracturing + volumetric acid fracturing) combined stimulating technology. However, the (thin) multi-interbeded reservoir which couldn’t be well stimulated by interval horizontal well fracturing might be stimulated by the application of highly-deviated well, clustered well, radial well, segmented vertical well and the matching fracturing technologies. The superiority of the technologies above is improving the contact area between wellbore and reservoir to ensure the stimulating effect. 3) Fundamental research and more experiments should be carried on and the matching tools , equipment and materials with independent intellectual property for volumetric sand fracturing and composite volumetric stimulation technology should be developed to reduce the stimulating cost. As the segmentation of the horizontal well increases, core tools matching with segmented fracturing are developing to satisfying the requests of large scale segmented fracturing of which the horizontal well length is 1000~2000m and the number of segmentation is 10~20. Such as the FRAC-PointTM pack system and PSI system developed by Baker Hughes, the MSAF segmented acid fracturing equipment, liquid-packer sand fracturing and SSCD permanent sliding sleeve matching with coil tube made by Philips Petroleum Company, Stage FRACTM stimulating system developed by Packers Plus Energy Services and Baker Hughes and hydraulic jet developed by Halliburton Company. Some technologies, such as packer with double seal and single slips, packer+ slid sleeve and hydraulic jet, are developed at home and the advanced matching tools should be introduced or developed to satisfy the technological requirements of longer horizontal well with more segmentations and larger stimulating scale. Large amount of fracturing fluids are needed in the process of volumetric fracturing and the dispensing of the fracturing fluid is very complicated. Oxidation is the gel-breaking method of traditional guar gum which could short the chain length, remove the branch of the galactose, transform the major part into the spiral structure of cellulose, reduce the water solubility of the polymer and promote the formation of secondary deposition. The structure of guar gum changes too much to recovery that it can’t be recycled and the residual fluid is difficult to be disposed and might damage the environment. Therefore, further research about the guar gum and the disposing of the residual fluid should be deepen and the fracturing fluid system suiting for tight oil reservoir which owns the characteristics, such as reversible crosslinked, instant,

Vol. 21 [2016], Bund. 15 4932 stable rheological property, recycling and lower pollution, should be developed. The innovated fracturing fluid has many superiorities, such as lower cost, recycling, environmentally friendly and reducing the amount of storage device, all of those could promote the development of scale-fracturing technology. In the process of volumetric acid fracturing and composite volumetric fracturing, large acid volume, high displacement rate and long contacting time between acid and reservoir rock ask for superior properties of the acid, such as lower corrosion rate and reaction rate, lower damage and resistance and effective etching ability. Ensuring the development of matching equipment of scale-fracturing, such as continuous pumping system(injecting fracturing fluid and proppant into formation continuously), continuous sand-providing system(sending proppant into sand-blending machine), continuous water-supplying system(transporting the qualified water to the field), instant recycling fracturing fluid(swelling fast and satisfying the field requirements), continuous fluid-blending system(preparing the fracturing fluid in-situ) and the disposal system of the fracturing fluid(making the residual fluid satisfying with the discharge standard). All the system above should be integrated, standard, automated and benefit-maximized.

CONCLUSION

Because of the huge reserves and the gradually developing exploration technology, tight oil reservoir is a preferred candidate in unconventional oil reservoir development. As a vital technology in increasing individual-well production and achieving industrial exploration, volumetric fracturing technology is becoming more and more important. Simultaneously, it should be noticed that some problems still exist, such as the reserves is difficult to effectively develop, volumetric stimulating technology faces great challenges and the relative technologies and equipment are still far behind the overseas advanced technologies. Therefore, in order to develop a volumetric stimulating technology with proprietary intellectual property rights and realize the economical and effective exploration of the unconventional tight oil reservoir, we must face up to the reality, enhance exploring and developing strength, breakthrough the fixed form of thinking, insist on fundamental research , and strengthen field test.

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© 2016 ejge

Editor’s note. This paper may be referred to, in other articles, as: Zhifeng Luo, Yujia Gao, Liqiang Zhao, Pingli Liu, and Nianyin Li: “Theory and Perspective on Stimulation of Unconventional Tight Oil Reservoir in China” Electronic Journal of Geotechnical Engineering, 2016 (21.15), pp 4923-4936. Available at ejge.com.