*Article* **Pore-Structural Characteristics of Tight Fractured-Vuggy Carbonates and Its E** ff**ects on the P- and S-Wave Velocity: A Micro-CT Study on Full-Diameter Cores**

#### **Wei Li \*, Xiangjun Liu, Lixi Liang, Yinan Zhang, Xiansheng Li and Jian Xiong**

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China; liuxj@swpu.edu.cn (X.L.); lianglixi@swpu.edu.cn (L.L.); 201721000587@stu.swpu.edu.cn(Y.Z.);201821000182@stu.swpu.edu.cn(X.L.);201599010137@swpu.edu.cn(J.X.)

 **\*** Correspondence: liwei2014@cugb.edu.cn

Received: 8 October 2020; Accepted: 17 November 2020; Published: 23 November 2020

**Abstract:** Pore structure has been widely observed to a ffect the seismic wave velocity of rocks. Although taking lab measurements on 1.0-inch core plugs is popular, it is not representative of the fractured-vuggy carbonates because many fractures and vugs are on a scale up to several hundred microns (and greater) and are spatially heterogeneous. To overcome this shortage, we carried out the lab measurements on full-diameter cores (about 6.5–7.5 cm in diameter). The micro-CT (micro computed tomography) scanning technique is used to characterize the pore space of the carbonates and image processing methods are applied to filter the noise and enhance the responses of the fractures so that the constructed pore spaces are reliable. The wave velocities of P- and S-waves are determined then and the e ffects of the pore structure on the velocity are analyzed. The results show that the proposed image processing method is e ffective in constructing and quantitatively characterizing the pore space of the full-diameter fractured-vuggy carbonates. The porosity of all the collected tight carbonate samples is less than 4%. Fractures and vugs are well-developed and the spatial distributions of them are heterogeneous causing, even the samples having similar porosity, the pore structure characteristics of the samples being significantly di fferent. The pores and vugs mainly contribute to the porosity of the samples and the fractures contribute to the change in the wave velocities more than pores and vugs.

**Keywords:** fracture; vug; micro CT; carbonate; pore structure; wave velocity
