Theory and Perspective on Stimulation of Unconventional Tight Oil 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 - 4923 - Vol. 21 [2016], Bund. 15 4924 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 source rock and gets close to tight sandstone 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 Jurassic 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 Vol. 21 [2016], Bund. 15 4925 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 unconventional oil 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 Silurian 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 limestone 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 sediment 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 shale 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 Vol. 21 [2016], Bund. 15 4926 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 Permian 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 calcareous 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 Vol.