1:45 PM - 3:15 PM
[HCG21-P01] Verification study on borehole investigation technologies (2) Drilling and borehole investigations for swelling mudstones
Keywords:Radioactive waste, Geological disposal, Borehole investigation, Hayama Group, Swelling
1. Introduction
NUMO and CRIEPI have been conducting a verification study in the CRIEPI Yokosuka. The authors recognized the problems of core loss and borehole enlargement in the Hayama Group, which consists mainly of swelling mudstone. As a result of the examination on drilling technology using silicate mud in the borehole YDP-a, a prospect for overcoming these problems was obtained1). The authors also recognized the need to evaluate the effect of silicate mud on the geological environment data acquired in the borehole investigations. Therefore, in the new borehole investigation (YDP-4), the authors aimed to ensure both high core recovery rate and high borehole stability in order to comprehensively acquire data. The effect of silicate mud on the geological environment data was also examined.
2. Geological and mineralogical features of the Hayama Group
In the study area, the Hayama Group, presumed to be the Neogene accretionary complex composed mainly of mudstones, the Miura Group composed mainly of tuffaceous sandstones, and the Quaternary sediments are distributed in ascending order. The Hayama Group varies from hard facies to brittle and fractured ones that can be collapsed with fingers. The mineral analyses reviled that the mudstone consists of volcanic glass, clay minerals (illite, smectite, etc.), quartz, plagioclase, and opaque minerals. The smectite is Na-type and CEC value is 30meq/100g on average showing very high swelling feature.
3. Outline of borehole investigation
In drilling borehole YDP-4 (total depth 720m), 2 types of silicate mud were used in the coring section. 3 types of coring bit were also applied. Drilling parameters were constantly monitored. Using borehole and core samples, various investigations were conducted.
4. Results of borehole investigations
In the Hayama Group of YDP-4, the RQD was 75% against the 86% in the YDP-a. The core recovery rate was 90% compared to 99% for YDP-a. In the hard and sparsely fractured facies, it was as high as 98%. 76% in the fractured facies, that can be collapsed with fingers. The stability of the borehole wall was 106% compared to 103% in the borehole expansion rate of YDP-a. During the hydraulic packer tests, the mud cake protecting the borehole wall was removed, which caused stuck of the test equipment. The effect of changes in drilling parameters on borehole expansion rate could not be confirmed. Compared with the borehole expansion rate of YDP-2 and 3 which did not use silicate mud, the merit of using silicate mud for the stability of borehole walls was confirmed, because the expansion was restrained by about 10%. The influence of silicate mud on the data of geology, physical, mechanical and thermal properties of rock mass could not be confirmed. As for the hydraulic properties, hydraulic conductivity of less than the order of 10-9 m/s was obtained, and there was little difference between the tests in the freshwater and drilling mud environment. As for the groundwater chemistry, preparatory drainage was carried out to reduce the effect of drilling mud, and although the low K ion content was confirmed, it kept on decreasing slightly, suggesting the remaining effects of drilling mud. As for the microbial community, the damage on bacterial cells and a very high ratio of alkalophilic bacteria were confirmed.
5. Conclusion
Authors were able to accumulate know-how and experience on borehole investigation for swelling mudstone, including how to deal with the technical problems, which can be reflected in the future investigations. The effect of silicate mud on the geological environment data needs to be examined in more detail by comparing data from sections using different types of drilling mud, and by comparing pretreatment methods for laboratory analyses.
References
1)Nishio et al.,(2017):Borehole drilling using silicate mud for swelling mudstone,Proceeding of JSEG annual meeting, pp.73-74 (in Japanese).
NUMO and CRIEPI have been conducting a verification study in the CRIEPI Yokosuka. The authors recognized the problems of core loss and borehole enlargement in the Hayama Group, which consists mainly of swelling mudstone. As a result of the examination on drilling technology using silicate mud in the borehole YDP-a, a prospect for overcoming these problems was obtained1). The authors also recognized the need to evaluate the effect of silicate mud on the geological environment data acquired in the borehole investigations. Therefore, in the new borehole investigation (YDP-4), the authors aimed to ensure both high core recovery rate and high borehole stability in order to comprehensively acquire data. The effect of silicate mud on the geological environment data was also examined.
2. Geological and mineralogical features of the Hayama Group
In the study area, the Hayama Group, presumed to be the Neogene accretionary complex composed mainly of mudstones, the Miura Group composed mainly of tuffaceous sandstones, and the Quaternary sediments are distributed in ascending order. The Hayama Group varies from hard facies to brittle and fractured ones that can be collapsed with fingers. The mineral analyses reviled that the mudstone consists of volcanic glass, clay minerals (illite, smectite, etc.), quartz, plagioclase, and opaque minerals. The smectite is Na-type and CEC value is 30meq/100g on average showing very high swelling feature.
3. Outline of borehole investigation
In drilling borehole YDP-4 (total depth 720m), 2 types of silicate mud were used in the coring section. 3 types of coring bit were also applied. Drilling parameters were constantly monitored. Using borehole and core samples, various investigations were conducted.
4. Results of borehole investigations
In the Hayama Group of YDP-4, the RQD was 75% against the 86% in the YDP-a. The core recovery rate was 90% compared to 99% for YDP-a. In the hard and sparsely fractured facies, it was as high as 98%. 76% in the fractured facies, that can be collapsed with fingers. The stability of the borehole wall was 106% compared to 103% in the borehole expansion rate of YDP-a. During the hydraulic packer tests, the mud cake protecting the borehole wall was removed, which caused stuck of the test equipment. The effect of changes in drilling parameters on borehole expansion rate could not be confirmed. Compared with the borehole expansion rate of YDP-2 and 3 which did not use silicate mud, the merit of using silicate mud for the stability of borehole walls was confirmed, because the expansion was restrained by about 10%. The influence of silicate mud on the data of geology, physical, mechanical and thermal properties of rock mass could not be confirmed. As for the hydraulic properties, hydraulic conductivity of less than the order of 10-9 m/s was obtained, and there was little difference between the tests in the freshwater and drilling mud environment. As for the groundwater chemistry, preparatory drainage was carried out to reduce the effect of drilling mud, and although the low K ion content was confirmed, it kept on decreasing slightly, suggesting the remaining effects of drilling mud. As for the microbial community, the damage on bacterial cells and a very high ratio of alkalophilic bacteria were confirmed.
5. Conclusion
Authors were able to accumulate know-how and experience on borehole investigation for swelling mudstone, including how to deal with the technical problems, which can be reflected in the future investigations. The effect of silicate mud on the geological environment data needs to be examined in more detail by comparing data from sections using different types of drilling mud, and by comparing pretreatment methods for laboratory analyses.
References
1)Nishio et al.,(2017):Borehole drilling using silicate mud for swelling mudstone,Proceeding of JSEG annual meeting, pp.73-74 (in Japanese).