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Tariq, Zeeshan (King Fahd University of Petroleum and Minerals) | Mahmoud, Mohamed (King Fahd University of Petroleum and Minerals) | Abdulraheem, Abdulazeez (King Fahd University of Petroleum and Minerals) | Al-Nakhli, Ayman (Saudi Aramco) | Bataweel, Mohammed (Saudi Aramco)
Abstract The enormous resources of hydrocarbons hold by unconventional reservoirs across the world along with the growing oil demand make their contributions to be most imperative to the world economy. However, one of the major challenges faced by oil companies to produce from the unconventional reservoirs is to ensure economical production of oil. Unconventional reservoirs need extensive fracturing treatments to produce commercially viable hydrocarbons. One way to produce from these reservoirs is by drilling horizontal well and conduct multistage fracturing to increase stimulated reservoir volume (SRV), but this method of increasing SRV is involved with higher equipment, material, and operating costs. To overcome operational and technical challenges involved in horizontal wells multistage fracturing, the alternative way to increase SRV is by creating multiple radial fractures by performing pulse fracturing. Pulse fracturing is a relatively new technique, can serve as an alternative to conventional hydraulic fracturing in many cases such as to stimulate naturally fractured reservoirs to connect with pre-existing fractures, to stimulate heavy oil with cold heavy oil production technique, to remove condensate banking nearby wellbore region, and when to avoid formation damage near the vicinity of the wellbore originated due to perforation. Pulse fracturing is not involved with injecting pressurized fluids into the reservoir, so it is also a relatively cheaper technique. The purpose of this paper is to present a general overview of the pulse fracturing treatment. This paper will give general idea of the different techniques and mechanisms involved in the application of pulse fracturing technique. The focus of this review will be on the comparison of different fracturing techniques implemented normally in the industry. This study also covers the models developed and applied to the simulation of complex fractures originated due to pulse fracturing.
ABSTRACT: Hydraulic fracturing is one of the most effective conventional reservoir-stimulation techniques currently utilized in the petroleum industry. Hydraulic fracture (HF) – Natural fracture (NF) interactions during the hydrofracturing process are complex in space and time. Although in the past five decades, several numerical, analytical and experimental studies have explored and characterized the interactions between natural fractures and hydraulic fractures, there remains differing opinions on the fundamental behaviors of hydraulic fractures when and where they interact with natural fractures. Here, we present a comprehensive review of the propositions and limitations of these studies in order to emphasize the need for further studies on NF-HF interactions and highlight the important fundamental questions that are yet to be answered. Our review focuses on previous experimental, analytical and numerical studies that investigated the impact of geomechanical and geometrical properties, and operational parameters on the result of NF-HF intersections during fracture initiation and propagation. We envisage that advancement in our understanding of NF-HF interactions will help in optimizing the fracture-stimulation processes during hydrofracturing operations.
In the oil and gas industry, hydraulic fracturing is one of the most effective conventional reservoir-stimulation techniques that has been utilized over the past seven decades. Hydraulic fracturing operations have been extensively investigated and widely published (Clark, 1949; Soliman et al., 2014; Feng and Gray, 2018; Elwegaa and Emadi, 2018; Susi et al., 2018a; Susi et al., 2018b; Wigwe et al., 2018; Ramezanian and Emadi, 2019; Elwegaa et al., 2019; Ramezanian et al., 2019). The United States has been named as the largest global crude oil producer according to U.S Department of Energy report (EIA, 2018), the tight shale gas and tight oil formations in the Permian and Bakken regions are two (2) among the top three (3) formations that has contributed to this feat. Hydraulic fracture (HF) interactions with pre-existing natural fractures (NF) in unconventional reservoir, geothermal systems, and block cave mining, is a complex process which has not been fully understood. In unconventional reservoirs, geothermal wells and block cave mining, hydrofracturing process has proved to be efficient (Gil et al., 2011) in: improving the stimulated reservoir volume (SRV), providing faster and high-conductivity pathway into the reservoir, and overcoming near-wellbore damage and sand production in to the wellbore (Kolawole et al., 2018a) during drilling and completions operations. The underground complex-fracture induced system of NF-HF behavior is highly important to be meticulously and efficiently characterized during hydro-fracturing process (Xiao et al., 2017). The complex behavior created by HF-NF interaction was by virtue of merging and/or branching of the HFs upon intersecting with NFs in unconventional reservoirs.
Hou, Yanan (China University of Petroleum (Beijing)) | Peng, Yan (China University of Petroleum (Beijing) (Corresponding author) | Chen, Zhangxin (email: email@example.com)) | Liu, Yishan (University of Calgary and China University of Petroleum (Beijing) (Corresponding author) | Zhang, Guangqing (email: firstname.lastname@example.org)) | Ma, Zhixiao (China University of Petroleum (Beijing)) | Tian, Weibing (China University of Petroleum (Beijing))
Summary Pulsating hydraulic fracturing (PHF) is a promising fracturing technology for unconventional reservoirs because it could improve the hydraulic fracturing efficiency through inducing the fatigue failure of reservoir rocks. Understanding of the pressure wave propagation behavior in wellbores and fractures plays an important role in PHF optimization. In this paper, a transient flow model (TFM) was used to describe the physical process of pressure wave propagation induced by PHF, and this model was solved by the method of characteristics (MOC). Combination of the TFM and MOC was validated with experimental data. The impacts of controlling factors on the pressure wave propagation behavior were fully discussed, and these factors include the frequency of input loading, an injection mode, an injection position, and friction. More than 10,000 sets of pressure wave propagation behaviors in different scenarios were simulated, and their differences were illustrated. In addition, the generation mechanisms of different pressure wave propagation behaviors were explained by the Fourier transform theory and the vibration theory. The important finding is that there is resonance phenomenon in the propagation of the pressure wave, and the resonance frequencies are almost equal to the natural frequencies of a fluid column. As a consequence of resonance phenomenon, the amplitudes of bottomhole pressure (BHP) and fracture tip pressure will increase sharply when the input loading frequency is close to the resonance frequency and less than 5 Hz; otherwise, the resonance phenomenon will disappear. Furthermore, an injection mode can alter the resonance frequency and the amplitude and frequency of the induced pressure wave. In addition, a friction effect can significantly decrease both the resonance frequency and the resonance amplitude. These findings indicate that the optimized input loading frequency should be close to the natural frequency of a fracturing fluid in a wellbore to enhance its BHP.
Li, Minghui (China University of Petroleum (Beijing)) | Hu, Xiaodong (China University of Petroleum (Beijing)) | Zhou, Fujian (China University of Petroleum (Beijing)) | Wang, Bo (China University of Petroleum at Karamary) | Han, Shaobo (China University of Petroleum (Beijing)) | Huang, Guopeng (China University of Petroleum (Beijing))
Hydraulic fracturing is the most extensive stimulation method for unconventional reservoirs (such as shale and tight formations). Existing diagnosis indicates pre-existing natural fractures in shale provide opportunities for complexity to arise, which have a significant influence on hydrocarbon production. Therefore, it is particularly necessary to study the interaction between hydraulic fracture (HF) and natural fracture (NF). However, most of the current researches are limited to two-dimensional models, which assumes that NFs are vertical fractures. Such assumptions are not consistent in real formations. The objective of this paper is to explore the interaction between HF and NF under different in-situ stress in three-dimensional space. The factors explored include NF geometry (dip angle, approach angle) and physical properties (natural fracture cement strength).
In this paper, a three-dimensional model based on the cohesive zone method coupling stress-seepage-damage filed is developed to simulate the interaction between HF and NF without introducing the crossing criterion. Firstly, the simulation results of HF&NF at dip angle of 90° are compared with published experimental results to verify this model. Secondly, a series of three-dimensional models of HF&NF at the different vertical differential stresses and dip angles will be simulated to explore the HF propagation pattern. Subsequently, three-dimensional simulations of different NF cement strength will also be performed. Eventually, these results will be presented in several charts to determine the boundaries of interaction behavior.
Firstly, this newly proposed three-dimensional model can be effectively used to simulate the interaction behavior of HF&NF in three-dimensional space and the simulation results are in good agreement with experimental results. Secondly, when the dip angle is 90 °, the interaction behaviors of HF&NF are consistent with the two-dimensional simulation results, which means the three-dimensional model can be effectively represented by the two-dimensional model and greatly improve the calculation speed. Thirdly, the geometry of NF in three-dimensional space has a great influence on HF propagation. Specifically, HF will grow along NF at low approach angles and high dip angles but cross through NF in the opposite condition. Besides, HFs are more likely to propagate along weakly cemented natural fractures.
Sufficient numerical simulations elucidate the role of NFs in hydraulic fracturing in three-dimensional space. Many important factors of NFs and HFs (such as natural fracture geometry and properties) are investigated and used to forecast the development of HF when NF exsit. This simulation method is very helpful for engineers to design the directions and scales of HF in industry application. In this way, it can make more hydrocarbon production and less stimulation cost for each well.
Yang, Ruiyue (China University of Petroleum) | Hong, Chunyang (China University of Petroleum) | Huang, Zhongwei (China University of Petroleum) | Wen, Haitao (China University of Petroleum) | Li, Xiaojiang (Sinopec Research Institute of Petroleum Engineering) | Huang, Pengpeng (China University of Petroleum) | Liu, Wei (China University of Petroleum) | Chen, Jianxiang (China University of Petroleum)
Summary Multistage hydraulic fracturing is widely used in developing tight reservoirs. However, the economic and environmental burden of freshwater souring, transportation, treatment, and disposal in hydraulic fracturing operations has been a topic of great importance to the energy industry and public alike. Waterless fracturing is one possible method of solving these water‐related issues. Liquid nitrogen (LN2) is considered a promising alternate fracturing fluid that can create fractures by coupled hydraulic/thermal loadings and, more importantly, pose no threats to the environment. However, there are few laboratory experiments that use LN2 directly as a fracturing fluid. In this work, we examine the performance of LN2 fracturing based on a newly developed cryogenic‐fracturing system under true‐triaxial loadings. The breakdown pressure and fracture morphologies are compared with water fracturing. Moreover, fracture‐initiation behavior under cryogenic in‐situ conditions revealed by cryo‐scanning electron microscopy (cryo‐SEM) is presented, and the role of thermal stress is quantified by a coupled thermoporoelastic‐damage numerical simulation. Finally, the potential application considerations of LN2 fracturing in the field site are discussed. The results demonstrate that LN2 fracturing can lower fracture initiation and propagation pressure and generate higher conductive fractures with numerous thermally induced cracks in the vicinity of the wellbore. Thermal gradient could generate enormously high‐tensile hoop stress and bring about extensive rock damage. Fracture‐propagation direction is inclined to be influenced by the thermal stress. Furthermore, phase transition during the fracturing process and low fluid viscosity of LN2 can also facilitate the fracture propagation and network generation. The key findings obtained in this work are expected to provide a viable alternative for the sustainable development of tight‐reservoir resources in an efficient and environmentally acceptable way.