Experimental investigations into stress sensitivity in three Chinese coal samples by NMR

ABSTRACT

The coal reservoirs have low mechanical strength and strong stress sensitivity. The study of stress sensitivity is of great significance to the efficient development of coalbed methane (CBM). In order to reveal the stress and strain characteristics of coal reservoirs, the low-field nuclear magnetic resonance (NMR) experiments with variable confining pressures were carried out on three coal samples from China. Based on the T₂ spectrum of coal samples under different confining pressures, the pore distribution, connectivity, and porosity variation rules of the coal samples were analyzed. Results show that the peaks of mesopore, macropore-fissure in the coal samples are well developed; however, the micropore peaks are poorly developed. There are differences in the stress deformation of pores and fractures in coals. With increasing confining pressures from 0 to 16 MPa, the peak of fractures decrease most obvious, and the peak of mesopore and macropore-fissure are relatively obvious, while the peak of the small pores is not significantly reduced. With the confining pressure increases, the total NMR porosity of coal decreases as the rule of negative exponential function, which is mainly controlled by changes of the porosity of seepage pore. The effect of adsorption pore on porosity changes is not obvious.

KEYWORDS:

• Coal
• nuclear magnetic resonance(NMR)
• stress sensitivity
• porosity
• adsorption pore
• seepage pore

Additional information

Funding

This work was supported by the the Major National Science and Technology Special Projects [2016ZX05065,2017ZX05064];the Natural Science Foundation of Shanxi Province, China [201701D221024,201701D221026];Shanxi Provincial Basic Research Program-Coal Bed Methane Joint Research Foundation [2015012011];the National Natural Science Foundation (NSFC) Project [41572144,41702175].

Notes on contributors

Qingzhong Zhu

Qingzhong Zhu, is a professor-level senior engineer of Huabei Oilfield Company, PetroChina. His research interests include exploration, development and management of coalbed methane.

Qianhua Xiao

Qianhua Xiao, is an associate professor of Chongqing University of Science & Technology. His research interests include unconventional oil & gas development and reservoir evaluation.

Yanjun Meng

Yanjun Meng, is a lecturer of Taiyuan University of Technology. His research interests are coalbed methane reservoir characterization and coalbed methane development geology.

Yinqing Zuo

Yinqing Zuo, is a senior engineer of Huabei Oilfield Company, PetroChina. Her research interest include coalbed methane development unit evaluation, coalbed methane well productivity analysis.

Xiangzhen Cheng

Xiangzhen Cheng, is an engineer of Huabei Oilfield Company, PetroChina. His research interest is coalbed methane development.

Yixuan Zhang

Yixuan Zhang, is currently a postgraduate student of Taiyuan University of Technology. Her research interest is coal and coalbed methane geology.