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Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A large amount of pulverized coal is easy to be produced in the process of hydraulic fracturing of coalbed methane reservoir. The accumulation and settlement of pulverized coal will cause serious blockage to the fractures formed by fracturing, and reduce the permeability and conductivity of fractures and affect the effect of fracturing stimulation of coalbed methane wells. Therefore, a new type of coal powder dispersant MFFS-2 was developed through a large number of laboratory experiments and combined with the reservoir characteristics of the target CBM block, and its comprehensive performance was evaluated. The experimental results show that when the mass concentration of MFFS-2 reaches 0.3%, the surface tension of water solution can be reduced to less than 25 mN/m, and the contact angle of coal surface can be reduced from 115.3° to less than 60° with good surface activity and wettability; when the suspension of coal powder is standing for 10 hours, the suspension rate of coal powder can reach more than 30% with the addition of 0.3% MFFS-2. When 0.3% MFFS-2 solution is used to displace for 2 hours, the yield of pulverized coal can reach more than 60%, which indicates that MFFS-2 has good suspension dispersion and carrying effect on pulverized coal; in addition, the damage rate of MFFS-2 to the matrix permeability of block coal core in the target area is less than 10%, and the damage degree of fracturing fluid system to the permeability of coal core is reduced after adding MFFS-2 into the active hydraulic fracturing fluid. The field fracturing operation results of coalbed methane well show that the fracturing operation of S-1 well is smooth after adding new coal powder dispersant MFFS-2, a large amount of coal powder is carried out in the backflow fluid, and the gas production is high, which achieves the expected fracturing effect. Moreover, the construction effect of the new active hydraulic fracturing fluid in the same block is significantly better than that of the conventional active hydraulic fracturing fluid.

Supercritical CO 2 fracturing can form a more complex fracture network in rocks than hydraulic fracturing and avoid aqueous phase trapping damage in reservoirs. Thus, it is a promising alternative to hydraulic fracturing for enhancing the production of low-permeability hydrocarbon reservoirs. In this study, a new numerical model for predicting the wellbore temperature and pressure during supercritical CO 2 fracturing was established based on thermodynamics, heat transfer, fluid mechanics, and a numerical solution method. In the new model, the physical properties of CO 2 are calculated with the Span–Wagner and Vesovic models, and the heat generated by fluid friction losses is absorbed by the tubing and CO 2 according to the contact coefficient. The model was used to examine the influences of the injection rate and temperature on the wellbore pressure and temperature. The results indicated that both the heat transfer and pressure in the wellbore are transient processes in the initial stage of injection; as the injection time increases, the heat transfer and pressure in the wellbore can be considered steady processes. The CO 2 temperature in the wellbore is considerably affected by both the injection temperature and rate, whereas the wellbore pressure is greatly affected by the injection rate but weakly affected by the injection temperature. The CO 2 pressure in the wellbore decreases rapidly as the well depth increases because of high fluid frictional resistance, so a drag reducer suitable for liquid CO 2 needs to be developed. [ABSTRACT FROM AUTHOR]

Coal rock is a type of fractured rock masses in which lots of cleats, fractures and weak planes develop, and these structures make the coal rock mechanics properties largely different from those of conventional sandstone rock. Previously established calculation models for the fracture initiation pressure of fracturing of conventional reservoirs cause large deviations when used to calculate the fracture initiation pressure of coalbed methane fracturing. In order to accurately calculate the fracture initiation pressure of coalbed methane fracturing, a new calculation model needs to be established while taking coal rock mechanical properties into consideration. In this paper, first, we carry out the macroscopic measurements and scanning electron microscopy (SEM) observation experiments on coal rock samples to describe the cleats distribution characteristics of coal rock samples. Second, we have carried out tests on rock mechanical parameters of different coring directions to obtain the anisotropy of coal rock mechanical parameters. Third, a calculating model has been established to determine the elastic modulus and Poisson’s ratio of coal rock based on the equivalent continuum mechanical methods, and then the model gets validated using the results of mechanical parameters tests. Finally, we have established the calculation models for borehole wall stress and fracture initiation pressure of hydraulic fracturing, which is based on the calculation model for equivalent coal rock mechanical parameters, to analyze the influence of mechanics properties of coal cleats on initiation pressure. The research result provides a new kind of research idea and approach for the calculation of fracture initiation pressure of coal hydraulic fracturing. [ABSTRACT FROM AUTHOR]

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)