Investigation of Imbibition in Shale by Nuclear Magnetic Resonance

Abstract

Horizontal wells combined with multistage hydraulic fracturing plays an important role in developing shale gas. During hydraulic fracturing, large amount of fracturing fluids usually retains in formations. Flow-back rate can be as low as 5%. Thus, aqueous phase trapping (APT) occurs. However, after well shut-in for a period of time, The APT can be auto-removed. Nuclear magnetic resonance (NMR) associated with spontaneous imbibition was carried out to investigate the development and removal of APT. Shale samples were obtained from Lower Silurian Long-ma-xi formation in Chongqing, China. Tight sands from upper Palaeozoic H8 formation of Ordos basin in China was tested for comparison. Distilled water was used as imbibition liquid. During imbibition, liquid filled into small pores preferentially in shale samples. Gas was displaced from small pores to large pores. It improves the efficiency of displacing gas, because large pores are easier for gas flowing. After well flow-back, liquid can move from large pores to small pores, due to the strong imbibition force in small pores. Thus, APT can be auto-removal. However, in tight sand, liquid enter into different pores uniformly. It shows that different pores have similar imbibition force. Therefore, the liquid displacement efficiency is less than the shale samples. After well flow-back, liquid difficultly move between different pores, due to similar imbibition force. Thus, the APT couldn’t be auto-removed. Comparing these two types of formation, shale has strong capacity of liquid displacement and APT auto-removal. Our research is conducive to analyzing APT auto-removal of some shale reservoirs during well shut-in for a period of time.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018