ABSTRACT:

Hydraulic fracturing plays an important role in developing tight sand gas. Large amount of fracturing liquid was injected into the formation to produce fracture. The flowback rate usually is very low. The retention liquid can cause serious aqueous phase trapping (APT). Some tight reservoir can auto-remove its APT after well shut-in for a period of time. The nuclear magnetic resonance (NMR) associated with imbibition was carried out to research the mechanism of APT auto-removal in tight reservoir. From Taiyuan Formation NMR experiment, liquid preferentially filled into small pores, and then gradually filled into the medium pores. In H8 Formation, liquid fills into both small pores and medium pores uniformly. No preferential displacement appears. After flow-back, liquid is prone to move from medium pore to small pores in Taiyuan Formation because small pores have strong imbibition force; liquid has less capacity to move from medium pores to small pores in H8 Formation because all pores have equal imbibition force. Therefore, the aqueous phase trapping (APT) is less serious in Taiyuan Formation than H8 Formation. Our research is conducive to learning the mechanism of APT auto-removal in tight gas reservoir.

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