Abstract
Pore structure has been widely observed to affect 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 effects of the pore structure on the velocity are analyzed. The results show that the proposed image processing method is effective 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 different. 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.
Highlights
During the past 20 years, deep buried carbonate formations have become one of the major sources of natural gas resources in China
Unlike the carbonate formations discovered in the middle-east having a porosity of about 8–25% buried in depths of 2000–4500 m underground, lots of the carbonate gas formations discovered in China are buried over 5000 m in depth and these formations are usually ‘tight’ having a porosity of less than 6% [1]
The pore structure parameters were acquired basing on the characteristics of the constructed spaces of pores, vugs, and fractures
Summary
During the past 20 years, deep buried carbonate formations have become one of the major sources of natural gas resources in China. As well as the acoustic-logging evaluation, of the tight fractured-vuggy carbonate formations, a widely-aware challenge is that the seismic and acoustic properties of the carbonate formations depend, on the minerals and the porosity, and the pore structural characteristics, especially for the tight deeply buried carbonates [2,3,4,5,6,7] This issue becomes further complicated considering the fractures and the vugs developed in the carbonate rocks [8,9].
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