Experimental study on characteristics and mechanisms of matrix pressure transmission near the fracture surface during post-fracturing shut-in in tight oil reservoirs

Abstract

Tight oil reservoirs are usually exploited using the long horizontal well and multi-stage hydraulic fracturing technique. Meanwhile, post-fracturing shut-in has become a popular procedure in improving the performance of tight reservoir wells. However, field data indicate that there are significant differences in well production due to shut-in operation in various blocks. The matrix pressure transmission near the fracture surface plays an essential role. Up to now, the characteristics and mechanisms of pressure transmission are still unclear. In this study, we conducted the pressure transmission experiment and matrix water uptake testing. Besides, X-ray diffraction (XRD), capillary suction time (CST), cast thin section observing, and mercury intrusion testing were performed. A characterization method was established to analyze the characteristics and influential factors of pressure transmission in tight oil reservoirs. The mechanism of pressure transmission and the relationship between pressure balance time and injection depth were discussed. The results show that the pressure transmission efficiency (PTE) reduces spontaneously as the initial water saturation in pores decreases. The clay expands after absorbing water and then the particles are dispersed, which damages the pore structure and reduces the PTE. The PTE decreases with the high content of clay minerals, especially the great proportion of expansive clay minerals. The PTE increases with the growing of the permeability and the capillary radius in some situations. The junction of conglomerate complex and gravel easily break up to form micro-fractures, which improves the PTE significantly. The mechanisms of pressure transmission include injection pressure, capillary force, and chemical osmotic pressure. These mechanisms of pressure transmission will be different at each stage and various situation. The time of pressure transmission increases with the large injection depth. This study helps to reveal the characteristics and mechanisms of matrix pressure transmission near the fracture surface during the post-fracturing shut-in and provides guidance for the optimization of the shut-in schedule.