Technological progress and prospects of reservoir stimulation

Technological progress and prospects of reservoir stimulation

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 46, Issue 3, June 2019 Online English edition of the Chinese language journal Cite this article as: PETRO...

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PETROLEUM EXPLORATION AND DEVELOPMENT Volume 46, Issue 3, June 2019 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2019, 46(3): 605–613.

RESEARCH PAPER

Technological progress and prospects of reservoir stimulation LEI Qun1, 2, GUAN Baoshan1, 2, CAI Bo1, 2, *, WANG Xin1, 2, XU Yun1, 2, TONG Zheng1, 2, WANG Haiyan1, 2, FU Haifeng1, 2, LIU Ze1, 2, WANG Zhen1, 2 1. CNPC Key Laboratory of Oil & Gas Reservoir Stimulation, Langfang 065007, China; 2. Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China

Abstract: Through reviewing the development history of reservoir hydraulic fracturing technology, this paper demonstrated the latest research progress at home and abroad and summarized six technical gaps between China and the world, that is fracture propagation mechanism, fracturing software development, fracturing vehicle equipment, downhole tools temperature and pressure resistance, proppant replacement and big data information database. The technical difficulties include lack of geological-engineering deep integration, unclear factors on horizontal well multi-fracture propagation, difficulty in reducing construction costs, environment protection pressure, lack of new experimental and field test equipment, immature techniques for fracturing fluid, and low efficacy of factorized fracturing equipment. We proposed six suggestions on China’s future reservoir hydraulic fracturing technology: (1) strengthen the mechanism study of unconventional reservoir hydraulic fracturing; (2) accelerate the development of geological-engineering integration software; (3) promote the upgrading of EOR fracturing techniques; (4) carry out low-cost multi-functional fracturing fluid formula experiment; (5) complete high-efficiency fracturing equipment; (6) build big database, informational and remote decision-making system of hydraulic fracturing. Keywords: unconventional reservoirs; hydraulic fracturing; hydraulic fracturing equipment; hydraulic fracturing materials; hydraulic fracturing design; remote decision-making; technological progress

Introduction Since hydraulic fracturing operation was successfully conducted for the first time in 1947, reservoir stimulation as a sustainable development science and technology has experienced more than 70 years of development[1–3]. The reservoir stimulation technology has achieved rapid development from basic theory and experimental research to equipment, tools, materials, software and field practice, and has been listed as one of the three key engineering technologies for exploration and development, along with drilling engineering and geophysical exploration[4]. Especially in recent years, under the background of the global exploration of unconventional oil and gas and undeveloped reservoirs, large-scale application of horizontal well multi-stage hydraulic fracturing technology in North America has led to the revolutionary breakthrough in shale gas and tight oil development. With the development of drilling and completion and reservoir stimulation technology, reservoirs with Nano-Darcy permeability can be effectively developed, making many traditional no-go exploration areas

realistic targets and changing the global energy setup[5–8]. The reservoir stimulation technology is currently the key technology for China Petroleum's upstream to lower cost, enhance well production and produce low-grade reserves. By reviewing the development history of reservoir stimulation technology, this paper summarizes the core elements, analyzes the technical characteristics at home and abroad, and systematically expounds reservoir stimulation technology, from simple to complex, from vertical well to horizontal well and from the traditional plug removal to efficient stimulation for low-permeability and unconventional reservoirs. At the same time, through comprehensive analysis and summary of the development status of reservoir stimulation technology at home and abroad, comparing and learning from North America, we can see the current situation and envision the overall development trend of China's reservoir stimulation technology. This study will provide a reference for reservoir stimulation in the development of reservoirs with low permeability, deep burial depth, offshore and unconventional resources.

Received date: 01 Aug. 2018; Revised date: 15 Nov. 2018. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2016ZX05023). https://doi.org/10.1016/S1876-3804(19)60040-6 Copyright © 2019, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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1. Development of reservoir stimulation technology 1.1.

Development at home and abroad

The reservoir stimulation technology development abroad has gone through four stages: (1) Vertical well conventional fracturing stage (before 1980), mainly based on single-layer moderate scale fracturing and near-well damage removal[9]; (2) Vertical well large scale fracturing stage, in order to meet the demand of tight gas fracturing in North America, in the 1980s, the Wattenberg gas field in the United States carried out large-scale fracturing technology research, with a sand adding volume of more than 100 m3, fracture length of 400–600 m, and stable production of (2.0–3.5)×104 m3/d[10]; (3) Vertical well multi-layer fracking, represented by the Jonah gas field in the Great Green River Basin in the United States, with 3–6 fracked layers or sections, longitudinal fracking degree 2 times of conventional multi-layer fracking, and output increment of more than 90% than conventional fracturing[11]; (4) Horizontal well multi-stage fracturing (since 2000), in order to meet the needs of tight gas-bearing reservoir stimulation, the horizontal well multi-stage fracturing technology has been developed, and through large-scale and batch production of wells, factory-like fracturing has been realized, with wells deployed from a wellpad rising from 8–16 in 2011 to 24–40 now, for example, Marcellus' Cogar platform has 40 wells, a super wellpad of Encana company in Permian Basin has 64 wells[12–14]. Horizontal well multi-stage fracturing technology has led to the second revolution in shale oil and gas in North America. By the end of 2017, the annual output of tight oil and shale gas in the United States were 2×108 t and 4 200×108 m3 respectively. In 1955, a well in the Yumen Oilfield was successfully fractured for the first time in China. After more than 60 years’ development and improvement, the reservoir stimulation technology has basically met the needs of the development of China's petroleum industry. The developing process can also be summarized into four stages: (1) Before the Eighth-Five Year Plan, mainly small-scale single well stimulation, Well N-1 in Laojunmiao block of Yumen Oilfield was fracked with 300 type cement truck, 30 m3 crude oil were used as fracturing fluid and 0.5 m3 quartz sand were pumped at the rate of 0.2 m3/min, and obtained an industrial oil flow of 15.4 t/d, opening a new era of China's reservoir stimulation. (2) During the period of the Eighth-Five Year Plan to the Tenth-Five Year Plan, in order to make the artificial fractures in low permeability reservoirs match with well patterns, the integral fracturing mode was put forward [15-16]. For example, the first integral fracturing practice was carried out in Shanshan low-permeability oilfield, which promoted the rapid construction of the 100×104 t productivity in Tuha Oilfield. ZJ60 in Changqing Jing’an Oilfield had 56 pilot fracturing wells, after hydraulic fracturing, their output has increased by 1.7 times and oil recovery increased by 7% averagely than wells in adjacent areas, apparently, hydraulic fracturing has become an

essential technology for the productivity construction of low-permeability reservoirs. (3) From 2000 to 2015, researchers have studied volume fracturing technology for low-grade reservoirs aiming to create complex fracturing networks[17–21]. Featuring long horizontal well sections (longer than 1 000 m), 1 000 m3 sand and 10 000 m3 fracturing liquid, this technology has propelled the commercial development of Fuling, Changning, Weiyuan and Zhaotong shale gas fields, making China the third country acquire the key technology for shale gas completion technology and realize commercial shale gas development after the United States and Canada. (4) From 2015 to present, a new technology named fracture-controlled reserves was put forward[22], which divides a well control target area into several units, each unit deploys one or more sets of interconnected complex fractures, from which the reserves in the unit would be recovered, greatly reducing the “blank oil and gas area” on the plane between wells or in the longitudinal range between layers. For example, since 2017, this technology has been used in Mahu glutenite reservoir of Xinjiang Oilfield, with the average daily oil production increased to 26 t per well, which has promoted the scale development of Mahu tight oil. 1.2.

Major progress in reservoir stimulation

In recent years, reservoir stimulation technology has made major progress in fracture propagation, full 3D software and fracture evaluation, high-power continuous operation equipment, multi-stage fracturing tools, multi-functional and lowcost materials, digitalization and remote decision-making. 1.2.1.

Fracture initiation model and propagation theory

The hydraulic fracturing theory model was put forward firstly in the 1950s, and the study of fracture propagation theory has gone through 2D models (PKN, KGD, radial models) to pseudo 3D and full 3D models[23]. Represented by the full 3D model developed by Yamamoto et al.[24] in 2004, the model established fracture tensile, shear failure and tensile-shear composite failure criteria. With this model, based on static and dynamic fracture mechanics theory, finite element, expansion finite element, discontinuous displacement and some other methods were used to simulate fracture non-equilibrium and network expansion mode, the contact angle between hydraulic fracture and different natural fracture, horizontal stress difference and the relationship between fractures under cementing, and the interaction between multiple non-planar fractures can be investigated. The complex network fracture propagation models such as discrete fracture network (DFN), orthogonal wire mesh (Wire-Mesh Model), unconventional fracture model (UFM) were also gradually improved, the full 3D model can effectively simulate the propagation of natural fracture in unconventional hydraulic fracturing, the propagation of fracture network in multi-stage and multi-cluster perforation, providing a new theory for the unconventional reservoir stimulation optimization design.

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In the physical experiment of fracture propagation, the fracture physical simulation method for large-scale rock is an emerging key technology that can directly show the dynamic characteristics of fracture propagation and fracture morphology. The early fracture propagation experiment device could be tested on small size rock of 30 mm×30 mm×30 mm, while this kind of method had low-pressure conditions and the simulation results were strongly affected by the boundary effect. In 2010, Terretak Company built a full 3D stress-loading hydraulic fracturing experimental equipment for a rock block of 1.0 m3 (Fig. 1). This set of physical modeling system has a maximum loading pressure of 69 MPa, a maximum stress difference of 14 MPa, tested rock sample size of 762.0 mm×762.0 mm×914.4 mm, the maximum center hole size of 125 mm, pore pressure of 20 MPa, and the maximum borehole flow of 12 L/min. Meanwhile, it has 24 sensors forreal-time monitoring the shape of the fractures in real-time by sound waves[25–26]. So far, large-scale physical experiments were conducted on more than 50 cores including sandstone, coal rock and shale via this equipment to find out the influence of shale brittleness, ground stress difference, weak surface (natural fractures, beddings) and net pressure in fracture (related to liquid viscosity and injection speed), and the influence of ground stress difference, induced stress, composite fracturing and temporary plugging diversion on the propagation of complex hydraulic fractures preliminarily. All these experiments results have strongly supported the optimal design of complex hydraulic fracturing, and also laid the foundation for the study of complex fracture propagation model. 1.2.2. Reservoir stimulation modeling software and fracture evaluation Basically, fracturing design software can be divided into two main categories: (1) Conventional fracturing software for optimal design, which can be used for 1D modeling, fracturing design, acidizing design, simple production simulation, mini-frac test analysis, real-time data analysis, and economical evaluation, etc. This kind of software is largely imported, such as FracPro, Goher, StimPlan, and Meyer, etc. (2) Geology-engineering integrated software, only by integrating geology and engineering can achieve good results for the

Fig. 1.

Large scale experimental apparatus.

unconventional reservoir complicated geological condition and non- planar fracture propagation. Hence, foreign companies outside China have developed geology-engineering integrated software based on conventional software. Geology-engineering integrated software can be used for 3D geological modeling, natural fracture modeling, in-situ stress modeling, pre-fracturing evaluation, fracturing design, microseismic interpretation, production simulation (reservoir numerical simulation) and economic evaluation, etc. The production simulation function of such kind of software is more advanced than conventional software. Software in this category commonly used includes Mangrove, JewelSuite, and Fracman, etc. Recently, many companies and institutes in China have also developed such kind of software, which are mainly used to model 3D symmetric bi-wing fracture propagation, single well production and economic evaluation. With the large-scale development of unconventional reservoirs, micro-seismic monitoring, micro-distortion inclinometer monitoring, distributed fiber optic temperature measurement, noise test and many other new fracture monitoring techniques have emerged. Micro-seismic monitoring is considered as one of the three most important techniques propelling shale gas revolution. Micro-distortion inclinometer monitoring technique used a micro-distortion inclinometer to acquire parameters of injected volume ratio, volume component differsence ratio and fracture complexity index, and effectively distinguish horizontal and vertical fractures in unconventional reservoir. Distributed fiber optic temperature measurement and noise detection techniques can be used to assist fluid flow monitoring and fluid distribution modeling[27]. These two techniques have been largely used in North America. However, this technique still lies in the research stage in China. Besides, tracer detection optimization technique is a good method to evaluate the amount of fluid produced in each stage, which can be quite helpful to analyze the opening degree of fracture and production effect after fracturing, and then evaluate the overall effect of horizontal well multi-stage hydraulic fracturing. In 2013, Ghanbari formulated the relationship between artificial fracture and matrix salinity and cumulative flowback quantity, and got the changing relationship between artificial fracture width and salinity. The fracture width mathematical model during flow-back was established based on characteristics of artificial fracture, fluid flow in matrix and salinity, which provides an effective tool for analyzing fracture complexity[28]. In 2018, the hydraulic fracture test on site (HFTS) was performed in the West Texas Permian (Midland) Basin[29]. This method involves drilling a high-angle core well near the fractured well at the test site (typically 60°, used to analyze the relationship between fracture length and well space) for continuous coring. Based upon observation of the acquired core, hydraulic fracture propagation and distribution of proppant can be found out, which can be used to evaluate the effectiveness of fracture and well interference and guide the optimization of well space and fracturing parameters.

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HFTS has been a hot topic and it has been gradually applied in Changqing, Jilin, Tuha and Xinjiang oil and gas fields. 1.2.3.

High power and continuously working equipment

In 2017, the total fracturing vehicle power in America was 1 641×107 W. Fracturing blender truck can be divided into two kinds: (1) One with output displacement of 16 m3/min and sand carrying capacity of 7.2 t/min; (2) The other with output displacement of 20 m3/min and sand carrying capacity of 9.5 t/min. Both are equipped with continuous mixing system and continuous water supply system. Model 2300 fracturing vehicle is most widely used in North America due to few high-pressure operations. In 2017, the total power of the fracturing vehicle in China was 239×107 W, which was only 14.5% of that in America. However, reservoir stimulation equipment has rapidly developed toward high power and automation. Currently, the widely applied Model 2500, 3000 fracturing pumping vehicles and matched blending truck, manifold truck, measuring truck (with the maximum pressure of 140 MPa and maximum output displacement of 16 m3/min) are able to automatically add water, powder and sand in proportion[30]. A type of 240-barrel closed blending truck with the maximum output displacement can pump sand at 38 m3/min. In 2015, China developed a turbine fracturing vehicle (Model 4500) with output power equal to the sum of Model 2500 and Model 2000. It is quite suitable for difficult and high-pressure fracturing of shale gas wells distributed in rugged areas as its turbine engine weighs only 0.7 t and its gross weight and chassis length are equal to that of Model 2000. Besides, in order to meet the demands of replacing oil by electricity, reducing noise, saving energy and protecting environment for large scale fracturing equipment, Model 6000 fracturing vehicle driven by electricity was successfully tested in a shale gas field in 2018, with noise-reducing from 115 dB to 95 dB and carbon emissions-reducing by 100 t/a, which foretells a broad application prospect. Piedmont reservoirs in Kuqa of Tarim Basin have the depth more than 7 000 m. A matching equipment of variable diameter coiled tubing for 7000 m deep formation was developed for these reservoirs. With lifting force of 680 kN, this tool has provided a guarantee for plugging removal in super-deep wells of Tarim Oilfield. Reservoirs in Ordos and Songliao Basins have low pressure and water sensitivity, so dry CO2 fracturing equipment has been developed[31]. With the maximum single layer sand dosage of 30 m3 and the highest sand ratio of 25%, this equipment can be used at the maximum well depth of 3 454 m and the maximum well temperature of 104 C.

cost and high efficiency. China, as an example, has developed several sets of fracturing tools after continuous research in the 11th and 13th five-year plan. Besides, the fracturing technique dominated by internal packer, sand jet fracturing and open-hole packer has gradually been replaced by fast drilling plug. With temperature resistance of 120 C, pressure difference resistance of 70 MPa and the highest operating pressure of 90 MPa, the fast drilling plug fracturing tool can satisfy the demand for massive reservoir stimulation. PetroChina has implemented horizontal well fracturing in 6 178 wells by the end of 2017 and wells treated by fasting drilling plug method account for 48.3% (Fig. 2). Besides, other advanced fracturing tools, such as soluble ball, controllable sliding sleeve, multi-sliding sleeve opened by a single ball, and electrically controlled sliding sleeve, mechanical coding sliding sleeve, balanced sliding sleeve and scalable prefabricated channel are also in research, which will definitely promote the development of multi-stage fracturing technique in China. 1.2.5.

Materials for reservoir stimulation

In order to adapt to the exploration and development for unconventional, ultra-deep and ultra-high temperature reservoirs, the fracturing fluid has been developed toward low damage, low cost, recyclable and high temperature resistant. And quartz sand has been gradually used as proppant to replace ceramic particles. (1) Low damage fracturing fluid. Guar-based polymers such as hydroxypropyl guar (HPG), carboxymethyl guar (CMG), and carboxymethyl hydroxypropyl guar (CMHPG) commonly used at present have insoluble substance of less than 2%. Moreover, significant progress has also been made in polymer-based clean fracturing fluid. Tight gas reservoirs in eastern Sulige are characterized by poor physical properties, poor connectivity, strong reservoir sensitivity and strong adsorption, etc. Study shows poor effect of reservoir stimulation in this area was mainly caused by clay swelling and residue of fracturing fluid. Duan et al.[32] developed new cellulose-based clean fracturing fluid. With breaking gel surface tension of

1.2.4. Effective tools for horizontal well multi-stage fracturing At present, horizontal well volume fracturing has been the major technique to develop unconventional reservoirs at home and abroad. Correspondingly, tools for horizontal wells staged fracturing are gradually developing toward multi-stage, low

Fig. 2. Proportion of stimulation methods for horizontal wells from 2011 to 2017.

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22.68 mN/m, viscosity reaching 97.5% of its maximum value in 2 min, almost no residue, and damage degree for core sample of only 13%, this new fracturing fluid can significantly decrease damage to reservoir and fracture conductivity. Field tests showed that average gas production per day after fracturing increased five times than adjacent wells. This fracturing fluid provides a new efficient fracturing fluid for the development of tight gas reservoirs in Sulige. (2) Low cost and recyclable slick water. New recyclable slick water system characterized by low viscosity, low friction and low damage has been developed to meet the demand of massive fracturing scale in tight gas and shale gas reservoirs since 2011. In Changqing Oilfield, slick water system of EM30 and EM50 with friction reducing rate of 70%–80% and concentration of 0.03%–0.08% were most widely used[33]. In addition, these kinds of slick water systems can also be used in reservoirs of Tarim Basin with ultra-deep depth, high temperature and high pressure. (3) Ultra-high temperature fracturing fluid. The maximum temperature in the fracturing operation is 210 C now. Yang et al.[34] and Wang et al.[35] developed a kind of polymer-based fracturing fluid that can work at 230 C though study on the temperature-resistant mechanism and rheological properties of polymer fracturing fluid. The polymer cross-linked gel with a concentration of 0.6% remained the viscosity of 170 mPas after sheared for 2 h at 230 °C and 170 s1 (Fig. 3). This fracturing fluid system was applied to the Niudong deep layer in the Bohai Bay Basin. After fracturing, the well had an oil production of 60 m3 per day and gas production of 10.3×104 m3, and economic cost reduction of 45.6%, which is a milestone for promoting deep exploration in the Bohai Bay Basin and China[36]. For 7 000 m ultra-deep, high-pressure and high-stress reservoirs in Tarim Basin, increasing the fracturing fluid density is an effective way to reduce the wellhead injection pressure. Cheng et al.[37] developed organic boron fracturing fluid weighing systems with KCl and NaNO3 respectively, with densities of 1.15 and 1.35 g/cm3. The weighing fracturing fluid system with NaNO3 can tolerate the temperature of 160 °C. The fracturing fluid weighing system has been

successfully applied on 26 wells in the Tarim Oilfield, with the maximum stimulation depth of 7 430 m (Well Keshen 13). Its cost for one well is 400×104 RMB lower than bromide weighing system. Recently, the research and application of proppant mainly focused on replacing ceramsite with quartz sand to greatly reduce the cost of fracturing materials. Since 2014, it has been a trend to increase the proportion of quartz sand in fracturing fluid in North America to reduce costs, which is of crucial for the development of shale oil and gas under low oil and gas prices[38]. According to statistics, the proportion of quartz sand in fracturing fluid in North America in 2018 has exceeded 90% (Table 1), and the sand demand for the year is up to 1.0×108 t. Among the 11 major conventional oil and gas basins in North America, the Permian Basin has the largest consumption of sand of over 360×104 t in a quarter, and the annual consumption is expected to exceed 1 500×104 t. Smallsize quartz sand has become the mainstream. The 0.150 mm (100 mesh) fracturing sand usage in the Permian Basin has dominated since 2015 and the proportion of its usage was more than 50% in 2016, while the usage of 0.425–0.850 mm (20–40 mesh) fracturing sand drastically reduced. Statistics on the cumulative production of shale gas wells in Bakken oilfield show that among the wells fractured with 100% quartz sand, 55% coated quartz sand+45% quartz sand and 55% ceramsite+45% quartz sand, the wells fractured with proppant of 55% coated quartz sand+45% quartz sand had the highest initial production. But after 1 year of production, the production of all the wells fractured with the three kinds of proppant tended to be stable and close. Apparently, the use of ceramsite proppant results in higher conductivity, but also higher cost. Although the use of quartz sand proppant results in lower conductivity, it is also lower in cost and can meet the long-term production needs of shale gas wells. The proppant currently used in China is still dominated by ceramsite, supplemented by quartz sand. Based on the successful experience of the United States, quartz sand is being evaluated and screened, meanwhile, the field tests are being performed in the shale gas area in the southwest China and the tight oil area in Xinjiang and Ordos. The application effect of typical wells in two shale gas wellpads in southwest China shows that there was no significant change in single-stage gas production with the proportion of quartz sand increasing from about 30% to 70%–80%[39]. Table 1.

Quartz Quartz sand dos- sand perage/104 t centage/% Eagle Ford 9.5 95 Appalachia 6.8 100 Permian 5.3 90 Bakken 2.2 92 Anadarko 2.1 91 Julesburg 1.3 98 Basins

Fig. 3. Rheological characteristic curve of 230 °C ultra-high temperature fracturing fluid system.

Quartz sand consumption in North America.

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Quartz Quartz sand dos- sand perage/104 t centage/% Haynesville 1.30 93 Barnett 0.90 99 Fayetteville 0.45 100 Unita 0.32 89 Piceance 0.26 96 Basins

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The quartz sand replacing the ceramsite tests in Block Fengnan 4 of Xinjiang Mahu Oilfield showed that in the cases of similar production systems, using quartz sand instead of ceramsite had basically the same daily oil production but a significant cost reduction of 20% to 30%. 1.2.6. Data informatization and remote decision in reservoir stimulation With the explosive increase of the number of stimulated wells at home and abroad, it is necessary to establish information platforms such as big data and cloud processing to get and analyze the fracturing dynamics and effects in time. In 2011, the United States Ground Water Protection Council (GWPC) created the FracFocus database[4041]. In the first batch, 17 companies uploaded fracturing data of 444 wells. The database has been developed to version 3.0 through gradual improvement. One of the databases, the chemical reagents and additives database (CAS), requires oil companies to upload chemical additives used in fracturing process in real time and release the data to the public through this database system (Fig. 4), which is convenient for users to call at any time. The system has collected fracturing data of 55 000 wells of more than 600 oil companies in the United States, realizing the sharing of big data. Some major international oil companies are developing remote control expert decision platform (RTOC) and have achieved initial results. For example, Shell has realized remote collaborative work in four locations, including Moscow and Salem in Russia, allowing experts to command fracturing operation remotely at any time[42]. China has also gradually carried out informatization construction work. CNPC has established a production-operation-management-decision support system for gas production and ground engineering, which are characterized by standard management, normalization, high efficiency, and safety. Some of the subordinate enterprises have begun the construction of remote control expert decision platform. 1.3.

Application effect of reservoir stimulations

In the past 10 years, CNPC has newly drilled about 1.6×104 wells per year. More than 70% of the wells need to be stimulated. Since 2000, over 22×104 wells have been stimulated,

Fig. 4.

Development history of the US FracFocus database.

that is an annual workload of 1.5×104 wells. The rapid development of reservoir stimulation technology has driven the exploration of oil and gas resources in China's low-permeability, shale gas, tight oil and deep reservoirs powerfully: (1) Effectively supported the construction of the large oil and gas field in Changqing with the production of 5 000×104 t, which now has the annual production capacity of 2 500×104 t crude oil and 350×108 m3 natural gas; (2) Supported the building of the first shale gas base in the Sichuan-Yunnan region, making China the third country have a complete set of engineering technology and realize commercial development of shale gas after the United States and Canada; (3) Fueled the exploration of the 10×108 t Mahu oilfield in Junggar Basin, which laid foundation for the oil production of 5 000×104 t in Xinjiang; (4) Ensured the continuous exploration and efficient development of two 300×108 m3 deep gas fields in Tarim and Sichuan Basin.

2. Development direction of reservoir stimulation technology 2.1.

Main technical difficulties in reservoir stimulation

The exploration and development of domestic and overseas oil and gas resources are gradually shifting to those in unconventional, high temperature and deep reservoirs, making reservoir stimulation crucial for the oil and gas development. In the new field, stimulation is facing more complicated challenges and difficulties: (1) Facing the complex geological conditions of unconventional reservoirs (tight oil, shale gas, shale oil), it is necessary to further improve the quality of reservoir stimulation operation and further promote the geological and engineering integration to enhance stimulation effect and realize economic development. (2) In horizontal well volume fracturing of unconventional reservoirs, the propagation and shape of multi-fractures and influencing factors are still unclear, especially the effects of weak surface of natural fractures, ground stress and horizontal two-direction stress difference, etc., so the expansion mechanism of fractures needs to be further studied, and the simulation method of fracture propagation needs to be improved. (3) Under the low-cost and environmental protection background, the cost reduction scope is getting smaller, the environmental protection pressure is getting bigger and bigger, and the commercial application of quartz sand to replace ceramsite proppant still needs theoretical research and a large number of field tests. (4) New technologies for maintaining stable oil production in the high water cut stage and in-situ support lack indoor experiment and field test equipment. (5) Fracturing fluid materials like guar gum and polymer still face technical problems to meet the requirements of environmental protection. Key technologies problems such as adsorption damage and control of slick water in shale[43], and ultra-deep ultra-high temperature (8 000 m, 200 °C) fracturing fluid system need to be tackled vigorously. (6) The current factory-like fracturing equipment has low efficiency and long operating cycle. For example, the efficiency of shale gas factory-like fracturing operation is 2 to

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3 sages every 12 h and the fracturing cycle is 30 d on average. The efficiency has been increased by more than 1 time, but the fracturing equipment is still with noise and low operating power. (7) The construction of big data and cloud processing information database for reservoir stimulation has just started, and there are problems such as difficult data collection and weak sharing foundation. Meanwhile, the real-time data transmission-reception, fracturing dynamic effect analysis and multi-node compatibility of the whole process remote decision-making system still have many problems to be solved. 2.2. Gap between the domestic and foreign reservoir stimulation Geological dessert evaluation, mechanical characteristics, mineral composition, and sensitivity, etc. of the unconventional reservoir are different from those of conventional reservoirs widely. There are certain gaps between China and foreign stimulation technologies in theoretical research, software development, fracturing equipment, and proppant selection, etc. (1) In terms of fracture propagation mechanism, foreign countries mainly consider multi-field coupling and multi-fracture expansion models, while China mainly focuses on simplified models, with multi-field coupling factor in secondary concern. (2) In China, the software is mainly imported, but China has independently developed software modules such as production prediction, proppant optimization, and multi-layer multi-segment fracturing optimization, which are poorer in sustainable development. Furthermore, the fracturing software isn’t closely combined with geological data and low in commercialization level, and weak in follow-up development and updating of the software. (3) Domestic fracturing trunk can basically meet the demand for production, but some core parts (engine, gearbox and chassis) are largely imported. Development of environmentally friendly and efficient fracturing equipment driven by dual fuels (or electrically driven), skid- mounted fracturing equipment and intelligent fracturing equipment are still in the exploratory stage. (4) There is an obvious gap between domestic tools and foreign tools in resistance to high temperature and high pressure, reuse and matching. (5) It has been confirmed that quartz sand can effectively replace ceramic particles to significantly reduce costs while meeting production requirement. Nevertheless, the fundamental study in this aspect has to be strengthened correspondingly. (6) Domestic big data of reservoir stimulation, the database of cloud processing information and remote decision-making system, etc. have a significant gap from those of North America. 2.3.

Development trend for main techniques

To achieve the goal of stable oil production (2×108 t) and rapid gas development, we think the following issues are the key ones in reservoir stimulation technology through reviewing target reservoirs of China’s oil and gas exploration and development combined with the demand, status, difficulties

and gaps of stimulation technology: stimulation mechanism of unconventional reservoirs, development of software integrating geology and engineering, upgrade of enhanced oil recovery technique, development of low cost and multi-functional fracturing fluid, development of efficient fracturing equipment and informatization construction, etc. (1) Fundamental theory and experimental study in reservoir stimulation, enrichment of unconventional reservoir fracturing theory. First, the mechanism of fracture initiation and extension under sophisticated geological and operation conditions should be further studied. Second, evaluation of geological recoverability and fracability of reservoirs should be strengthened. Third, physical modeling, numerical modeling, conditions for creating complex networks and corresponding controllable factors should be studied further. In foreign countries, slick water is widely used as fracturing fluid for reservoir stimulation. In North America, quartz sand with diameter of 0.075–0.150 mm (100–200 mesh) are commonly used as proppants in unconventional reservoir stimulation. Thus, it is of vital importance to study migration law of slick water that carries sand and sand-pack profile. Moreover, the relationship between forces exerted on proppants and conductivity needs to be figured out as well under efficient closure stress. (2) On the basis of the integration concept and technical innovations, we will develop domestic fracturing optimization software integrating geology and engineering. After that, fracturing software platform integrating geology, engineering and information will be constructed. (3) Further study on “fracture-controlled reserves” technology to enhance controlled area by fractures and oil recovery. With this technology, the effects of non-Darcy flow and start-up pressure on tight gas can be reduced, the contact area between fracture and matrix maximized, the distance and pressure difference from the matrix to fracture minimized, and drainage area controlled by fractures enhanced. We have come up with some new ways such as ultra-long horizontal section, tight cutting, multi-cluster perforation and shorten well spacing, etc. to expand well-controlled reserves. Finally, a new fracturing stimulation mode with multi-fracturing wells is established. (4) Development of low-cost and multi-functional fracturing fluid. While producing fracture network by large scale fracturing fluid injection, in low permeability and tight reservoirs with super-high capillary pressure, reducing or delaying flowback and extending imbibition time could enhance the imbibition of fracturing fluid and enhance recovery. Highdensity chemical fracturing fluid environmentally friendly and low in cost will be developed to apply in ultra-deep and ultra-high temperature reservoirs (8 000 m, 200 C). Meanwhile, according to reservoir characteristics and stress loading conditions in China, the field test of low-cost quartz sand must be strengthened so that this proppant can be widely used in the future. Furthermore, the proppant cost can be effectively controlled by speeding up domestic manufacture of quartz sand, economic evaluation and building sand manufacturing base.

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(5) Developing fracturing vehicles and operation tools to improve the ability of the equipment. According to defects of domestic fracturing vehicles, it is critical to developing three core parts (engine, gearbox and chassis) and five auxiliary systems (bifuel driving, electrically driving, skid-mounted, intelligent fracturing and deep coiled tubing management). It is particularly important to develop 5 220×107 W skidmounted fracturing equipment driven by electric power, which can be manufactured 100% domestically. Compared with other equipment, this new equipment can lower procurement cost by 20%–30%, reduce fuel cost and carbon emission by 25%–40% and 100 t/a respectively, dramatically increase work efficiency and satisfy environmental protection requirement. It has been tested in a shale gas field in southwest China. In terms of fracturing tools, five new, high efficient tools need to be developed, including soluble and high temperature resistant plug, tools for staged fracturing in deep wells, re-fracturing in old wells, slim hole fracturing, intellectual stimulation, etc. Soluble plug produced by degradable materials can eliminate formation pollution caused by milling, reduce risks of milling, realizing full clear borehole. These advantages will make a significant contribution to productivity, which will be the main direction of staged fracturing tools. (6) Complete construction of information system for reservoir stimulation. In an era of the rapid development of “Internet +”, it is inevitable to accelerate the development of information platform, remote control platform and expert decision-making platform based on the development of digital oilfield. Accordingly, the cluster effect of “Internet +” technique can make a great contribution to stimulation technique and practice by sharing information.

3.

Conclusions

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After years of development, China’s reservoir stimulation techniques can basically satisfy the demand for production in different historical periods, supporting productivity construction in several big oil and gas fields. Moreover, its role and status are continuously rising. As China’s oil and gas resources get poorer in quality and complex in targets, the four major areas of oil and gas exploration and development in the future, including low permeability, deep, marine and unconventional reservoirs, will have higher demand on reservoir stimulation and require highly advanced technology. We have good reasons to believe that the strategic objectives of stable oil production (2×108 t) and rapid gas development can be achieved with the following jobs done: study on stimulation mechanism of unconventional reservoir, development of geology-engineering integration software, upgrading oil recovery enhancement process, development of low cost and multi-functional fracturing fluid and efficient fracturing equipment, and information construction.

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