The dissolution characteristics of the Chang 8 tight reservoir and its quantitative influence on porosity in the Jiyuan area, Ordos Basin, China

Abstract

The Chang 8 tight reservoir in the area of Jiyuan holds characteristics such as high feldspar content and common feldspar dissolution phenomenon, which are vital to improve the physical properties of tight reservoirs. This paper simulates the interaction between the Chang 8 tight reservoir rock samples and organic acid, analyzes the types and characteristics of dissolution, explains the dissolution mechanism, and quantitatively calculates the influence of dissolution on the porosity of the reservoir on the basis of identifying the fundamental characteristics of the Chang 8 tight reservoir by utilizing the simulation experiment of fluid–rock interaction with high temperature and high pressure combined with polarized microscope and scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis techniques. With the purpose of making the simulation experiment closer to the actual geological conditions, 0.15 mol/L acetic acid solution with pH = 2.65 is used as the reaction fluid, wherein the simulated temperature range is set to 87–103 °C, and the simulated pressure range is set to 24.70–30.18 MPa. The result of the research shows that intergranular pores and feldspar dissolved pores are the primary pore types. The feldspar dissolved pores are more developed and accounts for 39% of the total porosity. The apparent solution ratio is 37.8%–50.0% with a moderate degree of dissolution. Additionally, carbonate minerals can also be dissolved in an acidic condition. The solution ratio of carbonate minerals generally exceeds that of feldspar. The dissolution degree is most robust at about 95 °C, and the burial depth is about 2370–2710 m by the geothermal gradient calculation, which should be the primary distribution area of the favorable tight reservoir of the Chang 8. Overall, dissolution is the main diagenesis type to improve the porosity that can result in the porosity increasing about 3.57%–3.69% in the tight reservoirs of Chang 8. Therefore, it is also the main controlling factor for the development of the "dessert" in the Chang 8 tight reservoir of western Jiyuan area.