Neutron Log Analysis of Rock Fractures and Formation Porosity
Date Issued
2008
Date
2008
Author(s)
Tseng, Hoteng
Abstract
The geophysical well loggings are useful and utility techniques for measuring the parameters of formation by operating different probes in the well or borehole. The Dual-Spacing Neutron well logging can continually detect the hydrogen or water content of formation along the borehole and then convert into the porosity index. Due to the fracture zones usually have relatively high value of porosity, the neutron well logging is an potential technique to investigate the distribution of fractures or fracture zones in the formation.his study was cooperated with the C.G.S. to complete the hydrogeological drilling investigation, hydraulic tests, and the neutron well logging at two test sites Changhu and Tonto in 2007. The neutron log data was used to analyze the distribution of fractures and fracture zones, and verify it with the core record and the televiewer logging results. In addition, we can obtain the continued formation porosity via regressing the neutron porosity index and the laboratory measured results of intact rock porosity. The rock type of Changhu site is Kuantaoshan sandstone of Kueichulin formation, which consists of dark gray to gray, very fine to fine massive muddy sandstone. According to the neutron log data analysis, the short distance neutron detect value method can reach 75% accuracy for distinguishing the locations of fractures and fracture zones in the fresh rock. But the neutron porosity index method can further improve the accuracy to 79%. The continuous formation porosity at Changhu site converted from neutron porosity index indicates the porosity distribution is ranging from about 1% to 14.4%. The rock type of Tonto site is Tawo sandstone of Kueichulin formation, which consists of silty mudstone to mudstone and has a higher argillaceous content. The test of this borehole was restricted by the well collapse and the cement grouting. The uneven thickness of cement influenced the results of neutron log quite seriously and was difficult to verify the fracture locations with the neutron log data. fter this verification of neutron logs at two test sites, it is expected to obtain the porosity of fractures progressively in sedimentary rock in Taiwan by using the neutron well logging to estimate the continuous formation porosity, and try to improve the problems of laboratory sample shortage and the insufficient information represented by intact rock.
Subjects
neutron log
fracture
porosity
neutron porosity index
Type
thesis
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