Effects of Thermal Shock on the Creation of Microfractures and Mechanical Properties of Various Volcanic Rock Types, including Samples from the Indonesian Geothermal Field (Tangkuban Perahu)

Abstract

This thesis aims at investigating the effect of a thermal shock of a couple of hundred degrees on the creation of the thermal microfractures and in what way the mechanical properties of volcanic rocks change. In the geothermal field, thermal microfractures can contribute to increase the permeability and produce more steam to the surface. We exposed basaltic andesite and andesite rocks from Tangkuban Perahu (Indonesia), a granite from Benin (West Africa), a basalt from the Eifel (Germany) and a basalt rock from Reynifsjara (Iceland) to temperatures of 200, 300, 400 and 500°C before cooling them rapidly by placing them in cold 20°C water. Initially, most of the sample rocks show less than 1% of porosity, excluding andesite Tangkuban Perahu (~6%) and basalt Reynisfjara (~14%), and permeability for all rocks is below detectable value (2.6 mD by Ruska gas permeameter for specified core geometry). In the geothermal field scenario, it indicates the pores were not connected and the steam may not easily flow through the rock. After the heating stage and thermal cooling, significantly increased values of porosity are observed in most of the rocks. However, only the two high porosity rocks gained permeability; the andesite Tangkuban Perahu (7-11 mD) and basalt Reynisfjara (4.45 mD). The changes in Young’s modulus, Poisson’s ratio and ultimate compressive strength of the various samples were also determined by using an unconfined Uniaxial Compressive Strength (UCS) apparatus in which both heat treated and non-heat treated samples were placed. The results show the rock strength decreases with increasing thermal shock. Similarly, we see a decrease in both Young’s modulus and Poisson’s ratio with increasing thermal shock. To recognize the thermal shock effect visually, the samples were imaged using a micro-CT scan before and after heating treatment. The resolution of the CT scanner was 30 μm at best when the whole sample is scanned. It is noticed that the porosity increases after the heating experiment. Additional mini-cores (~10 mm x 8 mm) from the whole core are also scanned at 14 μm resolution. The scans showed the porosity on the outside part of the cores is up to 10 times higher than the inner part. In Indonesia, the geothermal reservoir temperature varies from 200-300oC. By injecting cold water from the surface at a high rate, the water temperature remains low and can thus create the thermal shock that opens up existing fractures and forms new ones. In that way, it can increase fluid path ways around the well bore and along existing natural fractures.