Experimental study on expansion characteristics of core-shell and polymeric microspheres

Traditional polymeric microsphere has several technical advantages in enhancing oil recovery. Nevertheless, its performance in some field application is unsatisfactory due to limited blockage strength. Since the last decade, novel core-shell microsphere has been developed as the next-generation profile control agent. To understand the expansion characteristic differences between these two types of microspheres, we conduct size measurement experiments on the polymeric and core-shell microspheres, respectively. The experimental results show two main differences between them. First, the core-shell microsphere exhibits a unimodal distribution, compared to multimodal distribution of the polymeric microsphere. Second, the average diameter of the core-shell microsphere increases faster than that of the polymeric microsphere in the early stage of swelling, that is, 0-3 days. These two main differences both result from the electrostatic attraction between core-shell microspheres with different hydration degrees. Based on the experimental results, the core-shell microsphere is suitable for injection in the early stage to block the near-wellbore zone, and the polymeric microsphere is suitable for subsequent injection to block the formation away from the well. A simple mathematical model is proposed for size evolution of the polymeric and core-shell microspheres.Pengxiang Diwu, Baoyi Jiang, Jirui Hou, Zhenjiang You, Jia Wang, Liangliang Sun, Ye Ju, Yunbao Zhang and Tongjing Li

Effect of low velocity non-Darcy flow on pressure response in shale and tight oil reservoirs

Low velocity non-Darcy flow in shale and tight oil reservoirs is described by nonlinear or nonhomogeneous models. These models, especially for well shut-in period, are usually solved by numerical method, since the traditional pressure superposition principle is no longer applicable. The current paper presents a modified pressure superposition principle, accounting for the pseudo Threshold Pressure Gradient (TPG), and its mathematical proof. The proposed principle indicates that the total change of bottom hole pressure (BHP) in shut-in period is equal to the superposition of BHP change in a real well with pseudo TPG and that in a virtual well without pseudo TPG. The new principle is applied to the derivation of an analytical solution to the nonhomogeneous problem during the well shut-in period. Type curves calculated from the analytical solution show that the pseudo TPG leads to curve up-warping in switch-on period but down-warping in shut-in period, which agree with previous numerical results, and can be explained by the moving-boundary theory. Throughout the switch-on period, a closed moving-boundary is generated when the pressure gradient is less than the pseudo TPG. The boundary is closer to the well with higher pseudo TPG. However, during the shut-in period, a supply moving-boundary, which was generated during previous production or injection period, is earlier to be reached for virtual well with higher pseudo TPG. The flow is steady state afterwards. Matching of field data by the analytical solution results in the pseudo TPG in the investigation zone. The interpretation of the field case shows that pseudo TPG equals 0.104 MPa/m, generating a pressure drop as high as 6.35 MPa across the investigation zone during the well testing period.Pengxiang Diwu, Tongjing Liu, Zhenjiang You, Baoyi Jiang, Jian Zho

New evidence for ~4.45Ga terrestrial crust from zircon xenocrysts in Ordovician ignimbrite in the North Qinling Orogenic Belt, China

Evidence for the earliest known terrestrial crust comes predominantly from Jack Hills in Western Australia, where hafnium isotopic results from >3.8. Ga detrital zircons indicate crustal precursors as old as ~4.4-4.5. Ga. We present evidence from magmatic cores in >3.9. Ga xenocrystic zircons from a felsic volcanic rock in the North Qinling Orogenic Belt, China, of similar Hf crustal model ages up to 4.45 Ga. These lie on the same Lu/Hf trajectory as the least disturbed Jack Hills and Apollo 14 zircons, therefore providing only the second example of the earliest known generation of continental crust on Earth. In addition, the rims of two zircon grains record later growth at 3.7. Ga and, when combined with the fact that the grains are incorporated in Paleozoic volcanic rocks, imply long-lived crustal residence within the basement of the North China Craton. These results therefore establish the wider distribution and survival of the most ancient crustal material on the Earth and highlight the possibility for the further discovery of ancient crustal remnants