中子井測應用於分析岩體裂隙及孔隙率之研究

Abstract

井測是將各種功能不同的探測儀器放入井孔內量測地層各項物理及化學性質。其中雙距中子探測儀（Dual-Spacing Neutron Probe）可以沿著鑽孔或井管連續偵測地層中的含氫量或含水量，進而得知地層的孔隙率指標，而地層中破碎帶通常具有相對較高之孔隙率，中子有可能偵測到地層的裂隙或破碎段位置，因此中子井測具有輔助調查地層之破碎帶及孔隙率分佈潛力。 本研究嘗試利用中央地調所於民國96年在樟湖及桶頭2處執行水文地質鑽探調查及孔內水文試驗分析研究計畫進行中子井測試驗。利用中子井測記錄來分析破碎帶或裂隙分佈，並以岩心記錄及孔內超音波攝影結果予以驗證。另回歸分析雙距中子偵測值導出之中子孔隙率指標與實驗室量測岩心樣本之孔隙率，以瞭解鑽孔內地層之連續孔隙率分佈。分析結果發現，樟湖井為桂竹林層關刀山砂岩段，主要為深灰色至灰色，極細粒至細粒，緻密塊狀混濁砂岩。判釋鑽孔內新鮮岩體裂隙或破碎帶分佈情形，利用短距及長距中子偵測值的方法，準確率均可達到75％，中子孔隙率指標的方法，準確率可以達到79％。估算樟湖井地層連續孔隙率成果，鑽孔中岩體最緻密，孔隙率最小值約為1％，地層中裂隙孔隙率最大值為14.4％。桶頭井為桂竹林層大窩砂岩段，主要以粉砂質泥岩至泥岩為主。由於鑽孔泥質含量頗高，加上多處破碎帶易坍孔，因此施以水泥灌漿保護孔壁，造成厚薄不均的水泥分佈於鑽孔中，嚴重影響中子偵測值而無法得到良好的結果。 本研究期望能逐步建立在台灣沈積岩環境，以中子井測試驗估算地層連續性的孔隙率，彌補實驗室量測岩心孔隙率面臨樣本有限及代表性不足的問題，進而估算裂隙位置孔隙率的大小。

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. This 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. After 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.