Experimental Study of the Flow Mechanism of Deep Lacustrine Carbonate Reservoirs in the Yingxi Area of the Qaidam Basin

Abstract

Many of the exploration and development theories derived for conventional clastic rock reservoirs are not applicable to pore-fracture lacustrine carbonate reservoirs. The fluid flow mechanisms under reservoir conditions are still unclear. Therefore, in this study, the rock samples were characterized using X-ray diffraction (XRD), porosity-permeability analysis, scanning electron microscopy (SEM), plain thin sections, and casting thin sections. The core samples were classified into two types (fractures and matrix pores) based on their reservoir spaces. The core flow experiments were performed under reservoir conditions using reservoir core plugs. The experimental results demonstrate that the cores, especially the fractured cores, have a strong stress sensitivity. The oil phase flow in the core has the characteristics of non-Darcy flow, and the threshold pressure gradient is 0.01–0.003 MPa/m. Additionally, for the oil-water two-phase flow in the fractured core, the water phase relative permeability of the residual oil is high. In contrast, the water phase relative permeability of the matrix core is less than 0.2. The nuclear magnetic resonance (NMR) transverse relaxation time ( T 2 ) spectra were used to analyze the differences between the water flooding characteristics of the two pore structures. The experimental results show that the peaks of the T 2 spectra after water flooding are lower than those before water flooding, and the matrix cores have a better oil displacement effect. The relaxation time of 0.1–10 ms makes the greatest contribution to the water flooding efficiency. The micropores smaller than 10 μm in diameter play an important role in the water flooding of the matrix core. These results will provide theoretical basis for solving the difficult problems of developing deep lacustrine carbonate reservoirs.

Pore structure and fluid mobility of tight carbonate reservoirs in the Western Qaidam Basin, China

“…For MF‐ and M‐type lithologies, the first factor played a dominant role, whereas for MFD‐type lithologies, the second factor was dominant. Water displacement efficiency is also related to the pore structure 10,32 . We found that the water displacement efficiency of the MFD‐type with developed microfractures was significantly lower than that of the MF‐type with poor microfracture development.…”

“…The boundary layer theory can be used to explain low‐velocity non‐Darcy flows, which have been studied in low‐permeability sandstone or shale reservoirs 16,43–46 . Non‐Darcy flows have also been observed in the carbonate reservoirs in the Yingxi region 32 . Under a low‐pressure gradient, the fluid boundary layer in the micropore channel was extremely thick, and marginal amount of free internal fluid participated in the flow.…”

“…The design of conventional core flow equipment is shown in Figure 2. Details of the equipment composition can be found in a previous study 32 . According to calculations based on the test data, the overburden pressure reached 30 MPa.…”

“…16,[43][44][45][46] Non-Darcy flows have also been observed in the carbonate reservoirs in the Yingxi region. 32 Under a low-pressure gradient, the fluid boundary layer in the micropore channel was extremely thick, and marginal amount of free internal fluid participated in the flow. The thickness of this layer decreased with an increasing pressure gradient, and more free fluid in the micropore channel participated in the flow.…”

“…Details of the equipment composition can be found in a previous study. 32 According to calculations based on the test data, the overburden pressure reached 30 MPa. Thus, the maximum confining pressure in the experiment was set to 30 MPa to simulate actual reservoir conditions.…”