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Li, Yuwei (Northeast Petroleum University) | Peng, Genbo (Northeast Petroleum University) | Jia, Dan (Northeast Petroleum University) | Zhang, Jun (Northeast Petroleum University) | Cong, Ziyuan (Northeast Petroleum University) | Fu, Xiaofei (Northeast Petroleum University) | Tian, Fuchun (Engineering and Technology Research Institute, PetroChina Dagang Oilfield Company) | Tang, Jizhou (School of Engineering and Applied Sciences, Harvard University)
ABSTRACT Pulse fracturing is a new technology for producing the coalbed methane (CBM). Pulse fracturing can increase the stimulated reservoir volume (SRV) and enhance the productivity of CBM wells. However, the research on mechanism of fracture initiation and propagation in pulse fracturing in a coal seam is still not popular. On the basis of the elastic mechanics theory, the initial geostress field is superimposed with the pulse stress field, and a perfectly matched layer (PML) is introduced in this study. The numerical model of stress field in pulse fracturing in a coal seam is established. The effect of pulse frequency and amplitude, horizontal principal stress difference and coal seam elastic modulus and Poisson's ratio on fracture initiation is analyzed by solving the numerical model, and the effect of pulse frequency and amplitude and the angle between the fracture and the horizontal maximum principal stress on fracture propagation is studied. The results show that the optimal pulse frequency for fracture initiation and propagation in pulse fracturing in a coal seam is 50 Hz in this study. The elastic modulus and Poisson's ratio in a coal seam have significant effect on fracture initiation. The amplitude and frequency of pulse fracturing should be optimized according to the coal seam elastic modulus and Poisson's ratio. Under a certain pulse frequency, a reasonable increment of the pulse amplitude is beneficial to fracture initiation and propagation. The results in this study fill the gap in numerical simulation of fracture initiation and propagation mechanism in pulse fracturing in a coal seam and provide theoretical basis to optimize operation parameters of pulse fracturing for CBM wells. 1. INTRODUCTION Pulse fracturing, a new technology for producing CBM, expands the stress disturbance zone, increases the SRV, and enhances the productivity of CBM wells (Ghamgosar et al., 2017; Vogler et al., 2016; Munoz et al., 2017; Li et al., 2018e) compared with conventional hydraulic fracturing (Li et al., 2018a, 2018b, 2018c, 2018d, 2019, 2020; Lecampion et al., 2018; Katsuki et al., 2019; Barton et al., 2017; Sheng et al., 2018, 2019; Zhang et al., 2017). In pulse fracturing in a coal seam, the stress wave is acted on the coal bed through the medium like the fracturing fluid. The failure of the coal bed and the size of stress disturbance zone are affected by the stress wave area (Zhai et al., 2011; Gao et al., 2018). The pulse load acted on the borehole wall and the fracture walls is resulted from the periodical rate from the pulse pump on the ground. The flow is output from the pulse pump in the form of the pulse wave (Qi, 2001; Zhai et al., 2015). Four types of pressure pulse waves are detected by the pressure sensor, as shown in Table 1. Some research results on pulse fracturing have been published. Sakhaee et al. (2018) theoretically and experimentally studied fracture propagation under pulse stress and proved that repeated variations of some physical variables in pulse fracturing enhances fatigue failure and permeability of the coal. Xie et al. (2015) designed a high pressure pulsed water injection system and carried out a simulation experiment on pulse fracturing in a coal seam, confirming that pulse fracturing results in better stimulation effect than conventional hydraulic fracturing does. Lv et al. (2018) and Sher et al. (2002) found that a single pulse fracturing can lead to several small fractures and that repeated pulse fracturing can make the fracture fully propagate, forming a complex fracture network. Li et al. (2013) studied the mechanism of pulse wave generation and propagation and coal failure and found that reflection and superimposition of pulse waves make the stress in some areas increase, which improves the stimulation. Li et al. (2014) studied the effect of the pulse frequency on fracture propagation, discussed the relationship between the pulse frequency and pressure amplitude and the fracture initiation pressure, which provides the basis for improving the technology of pulse fracturing in a coal seam, and proposed the “double pulse-double pressure” pulse fracturing technology by carrying out experiments in simulated rock samples with different fracturing modes, different pulse frequencies and different parameters of the pulse hydraulic fracturing (PHF) system. Mao et al. (2019, 2020) and Zhao. (2008) found in experiments that the variable frequency pulse water injection technology can generate high pressure pulsed water and result in a fracture network in a coal seam. The fractures are more likely to be initiated and crossed under high pressure water. Zhai et al. (2011) studied the failure mechanism of original fractures under strong pulsed hydraulic pressure in a coal seam and proved that pulse fracturing produces alternating stress at the fracture tip and generates repeated "compression-expansion-compression" in the fracture pores, causing propagation of weak surfaces of fractures and forming the interpenetrating fracture network. Xie. (2014) and Hou et al. (2016) verified in experiments that pulse fracturing produces fatigue failure, clears pores and increases permeability. Olsen et al. (2007) and Dehkhoda et al. (2013) carried out the field pilot of pulse fracturing, which quantitatively evaluates the effect of pulse fracturing on enhancing the CBM recovery, and proposed the guidance on pulse fracturing design, which helps pulse fracturing operation. Zhu et al. (2013) microscopically studied the fracture initiation and propagation mechanism under pulse fracturing and proposed the critical pressure computation model of fracture initiation in a coal seam on the basis of sliding friction force and effective stress. Li et al. (2017), Tang et al. (2018, 2019) and Zhu et al. (2014) analyzed the coal failure and deformation caused by pulse water injection pressure on the basis of energy theory. The research shows that the higher water injection pressure causes the higher the dissipative energy and more serious plastic failure and deformation. Erarslan et al. (2012a, 2012b) established a numerical model of coal seam pulse fracturing and simulated the effect of pulse amplitude and pulse frequency on the fracturing effect and found that pulse fracturing can reduce the fracturing operation pressure. The research conducted by Wang et al. (2015) and Xu et al. (2017) also shows that pulse fracturing requires lower fracture initiation pressure than conventional fracturing does. Li et al. (2013) and Lu et al. (2014) used the method of cross grid finite element analysis to establish a numerical model of PHF stress disturbance, and the model computation shows that pulse fracturing generates larger stress disturbance area than conventional static fracturing does and is beneficial to fracture propagation.
A rapid in situ stress measurement technique was developed for exposed underground surfaces. The method applies radial stress in small diameter (38mm) boreholes, initiating fracture propagation which is monitored by measuring ultrasonic pulse velocity. Theoretical relations for stress concentrations near boreholes provide independent expressions for unknown principal field stresses at incipient fracture; fracture orientation defines principal stress orientation. Accuracy and precision were evaluated using blocks of brittle material subjected to known stresses. Results were comparable to those obtained by other in situ stress measurement techniques.
The U.S. National Committee for Rock Mechanics reported a need to make existing test methods more cost effective. In situ stress data is rarely used because of high cost and questionable accuracy. Some in situ stress measurement methods require costly non-recoverable components; others demand tedious slot cutting or overcoring. In some materials, usable results come from only 20% of the tests performed (Van Heerdon and Grant, 1967). This paper presents preliminary work toward a rapid, accurate technique for in situ stress evaluation at a free surface. The method may also serve in deep hole applications.
In situ stress by induced fracture involves assessment of pressures required for incipient crack formation on the periphery of a borehole. Crack initiation during borehole pressurization is controlled by the material's tensile strength and in situ field stresses at the borehole boundary. However, cracks reopened upon repressurization are no longer dependent on tensile strength, so stress concentration theory taken with known borehole pressures as cracks are caused to reopen provides information about in situ field stress. A two stage process providing information sufficient to calculate both principal stresses in a plane perpendicular to a borehole axis is proposed. Cracks are first induced from a single borehole, where theory predicts fracture should propagate in a direction normal to the minor principal stress (Hubbert and Willis, 1957). Next, simultaneous pressurization of a pair of bore- holes placed close to each other and aligned parallel to the minor principal stress is performed. This should result in a fracture normal to the major principal stress. Theories for stress concentration near single and double holes provide two relations in terms of the two unknown principal stresses.
Jeffrey (Timoshenko and Goodier, 1967) showed that the tangential component of a uniform radial pressure, Pi, acting on the inside boundary of a cylindrical hole is: (mathematical equation) (available in full paper)
The negative sense of Eq. 1 indicates tension. The tangential stress component as provided by Kirsh (Timoshenko and Goodier, 1967) for field stress concentrations at the wall of a single hole is: (mathematical equation) (available in full paper)
where: s1 = major principal stress, s2 = minor principal stress and ¿ = angle measured from the direction of major principal stress. For unequal compressive principal stresses, Eq. 2 shows minimum compressive stress occurring at ¿ = 0 and p as: (mathematical equation) (available in full paper)
When considering the behavior of rocks under dynamic loading, there are three zones of interest: (1) the source zone; (2) the transition zone; and (3) the seismic zone. The discussion at the buzz session involved all three zones.
The source zone is concerned with the origin of the dynamic load, which may be high explosive, nuclear explosive, high-velocity mechanical impact, or the sudden release of stored elastic strain energy. If the source is either a high-explosive charge or a nuclear device placed in a cavity in the rock, hydrodynamic theory can be employed to describe the phenomena that takes place in the cavity.
In general, the energy released per unit volume is so large that the resulting pressures far exceed the elastic limit of rock. Consequently, shock waves can be generated in the rock at the cavity boundary. Therefore, equations of state for rocks beyond the elastic limit are needed to develop satisfactory theories to describe the resulting effects in the rock.
The transition zone is that zone of rock immediately surrounding the source zone in which some type of failure results. Depending upon the intensity of the source, the rock surrounding the source may be vaporized, melted, crushed, caused to flow plastically, or fractured. To date there is no satisfactory theory to account for the various phenomena that occur in this region--or to predict the size of this region from a knowledge of the rock properties and source properties. In fact, only a minimum of experimental data are available. In traversing the transition zone the shock wave which was generated at the source zone boundary decays to an elastic wave.Normally a strong compressive stress pulse is radiated outward from the transition zone. Experimental data are available which indicate that the size and shape of this compressive stress pulse is a function of the intensity of source and the properties of the rock. In general, the shape of the pulse radiating out of the transition zone is controlled more by the rock properties than the source properties.
Lv, Yanjun (China University of Petroleum-Beijing) | Zhang, Guangqing (China University of Petroleum-Beijing) | Zhao, Zhenfeng (Oil & Gas Technology Research Institute) | Wang, Yue (State Key Laboratory of Petroleum Resources and Engineering)
ABSTRACT: Hydraulic pulse fracturing technology, which is applied to the reservoir rocks by instantaneous pulse pressure, can form fractures not perpendicular to the direction of minimum stress in the tight rocks. It is of great significance for the production development of low permeability reservoir and repeat treatment of old oilfield. In this paper, the influence of hydraulic pulse on hydraulic fracture is studied by a large scale of true triaxial physical simulation test. Test conditions of four experiments focus on the influence of peak pressure and pulse times on hydraulic fractures, respectively. The experiment is divided into two stages, conventional hydraulic fracturing and hydraulic pulse fracturing. Experimental results show: Hydraulic pulse fracturing can form fractures in different directions, especially not perpendicular to the direction of minimum stress. A single hydraulic pulse can form many small cracks, and repeated pulses can make cracks extend fully and eventually develop into a network of complex fractures. In hydraulic pulse fracturing with a higher peak pressure, there are fewer hydraulic fractures being formed, but the size of them are larger than fractures of conventional hydraulic fracturing.
In the 1850s, people began to study how to create artificial fractures in the reservoir to improve the efficiency of oil and gas production. The methods of forming artificial fractures can be classified into three types: hydraulic fracturing, gas fracturing and explosion fracturing (Ma and Zhang, 2002, King, 2010, Gandossi, 2013, Zhou 2017).
Gas fracturing and explosion fracturing can form a network of complex fractures. Their influence reach only a small area because of limited energy. And they may cause formation damage, casing failure, and unpredictable fracturing effect (Liu and Ma, 1991, Han and Yang, 1992, Pu, 2015). Hydraulic fracturing has predictable results and causes less damage to the formation and casing. However, hydraulic fracturing often only forms a single fracture, the direction of which is controlled by ground stress. Sometimes, the stimulation of hydraulic fracturing may be difficult to reach the target expected and with a huge cost (Wang, 1987, Zhang, 2013, Fan, 2014).
ABSTRACT: This study presents the feasibility of performing permeability measurement by interpretation of consolidation-induced time-dependent deformation of sample when an instant pore pressure pulse is applied. For the interpretation of measured transient behavior, a procedure to back-calculate permeability from measured consolidation strain is developed. The estimation shows promising results that gave similar order of magnitude of permeability compared to that measured from steady state method. However, this study shows that creep behavior affects the interpretation of permeability significantly as the effective stress increases. This study indicates that proper calibration of creep effect is important to get reliable values. This study also present some recommendations to optimize the results.
Permeability measurement using the steady-state method requires much time for low permeable lithologies (e.g. shale or mud stone), leading to very long experimental time if the aim is several permeability measurements per test. For example, it may take more than one week for one permeability measurement for low permeable rock. However, if an instant pressure pulse loading can capture the transient/consolidation behavior within some hours and the measurement can be converted to the permeability, it can provide a cost and time effective alternative test method.
Change in pore pressure of porous material change the stress conditions acting on its grain structure, which is known as the effective stress. Consequently, the pore volume changes due to expulsion/influx of the pore fluid that occupies the void spaces, a process known as consolidation (or primary consolidation) (Biot, 1941; Terzhaghi, 1943). Thus, hydraulic parameters, especially the permeability, can be deduced from the consolidation-induced deformation, which is strongly coupled with hydraulic behaviour.
As schematically illustrated in Figure 1a, we can assume two-way 1-D consolidation condition when the pore pressure at the top and bottom boundaries change rapidly with a step function of ΔΡ, which is labeled ‘pulse-loading’ hereafter. After the pulse loading, the pore pressure will be gradually dissipated from the top and bottom to the middle of the specimen over time (i.e. red curves in left Figure 1a). Consequently, as shown in Figure 1b, associated volume changes may continue until the pore pressure becomes fully dissipated or reaches steady-state condition.