Study of reservoir rock and caprock integrity in geo-sequestration of carbon dioxide

Abstract

Carbon dioxide capture and storage in deep geological formations is proving to be a very promising method for sequestering anthropogenic CO2 emissions and isolating them from the earth’s atmosphere. The global CO2 storage capacity of saline aquifers is much greater than the other options and it is possible to find nearby major sources of CO2. The suitability of a potential geological storage site should be investigated carefully by taking into account a number of geochemical and geomechanical factors. Of these factors, it is important to analyse the effects of the induced changes in pressure and stresses caused by CO2 injection on the behaviour of the rock mass, especially the caprock and any major traversing low permeability geological structures. The geochemical immaturity of the reservoirs is also important. This study aims to develop a better understanding of aspects of the failure of caprock during and after the sequestration of carbon dioxide into geological formations deep below the surface. This project examines the various factors and processes affecting the geomechanics of rocks, since CO2 storage integrity may be seriously jeopardized by the fracturing or reopening of a pre-existing non-transmissive fault in the reservoir or caprock formation. In the present study, a comprehensive literature review of the geomechanical and geochemical processes involved in the carbon geo-sequestration was conducted which highlights relevant processes in conjunction with theories related to the mechanical and flow behaviour of formation rocks. The effects of pore fluid properties on the characteristics of reservoir rocks are also important when studying CO2 sequestration in saline formations and are investigated in this study using two types of locally-available sandstone specimens. The investigation also focuses on the experimental and numerical analysis of the flow and mechanical behaviour of the reservoirs and caprocks and their response to the injection of CO2. This is achieved by the investigation of the fluid flow-related properties of intact and fractured rock specimens obtained from the Otway Basin Pilot Project (OBPP) in Victoria, Australia. The main objectives of this study are as follows: • Development of a comprehensive literature review and a novel laboratory set-up for the testing of rock specimens close to realistic sequestration reservoir conditions. • A laboratory investigation and analysis of reservoir and caprock specimens to assess their hydromechanical characteristics. These tests are intended to be carried out at the pressure and temperature conditions which are expected to occur in the reservoir chosen for sequestration project. • Studying the effects fractures on the flow characteristics of rock specimens. • Development of exclusive models to understand, extrapolate and reproduce the intact and fracture flow behaviour of rocks as identified from the laboratory results. The development of a parametric reservoir-scale model to demonstrate the safety and efficiency of saline aquifers and support their suitability as promising CO2 storage reservoirs is also attempted. • Investigating the effects of geomechanical and geochemical interaction between rock minerals, brine and dissolved CO2 on the rock properties and the overall mechanical properties of reservoir rocks. • Analysing the deformation behaviour of reservoir and caprock specimens using the Acoustic Emission (AE) and visual surface strain measurement systems (ARAMIS). An elaborate high pressure and temperature-controlled laboratory apparatus was developed for the experimental investigation of rock specimens. This test set-up and the data acquisition system were developed in-house and were calibrated prior to the commencement of the test series. The set-up is presented and explained in detail in Chapter 3. The chapter also explains the methodology developed for the preparation, installation and testing of the rock specimens. Two locally-available types of sandstones (S and M-type) were used for the study of the effects of brine and CO2-rich brine saturation on the mechanical properties of reservoir rocks. Brine saturation at different values of NaCl concentration is found to cause a significant difference in the sandstone properties. Uniaxial compressive testing of the CO2-rich brine-saturated sandstone specimens revealed that sandstones cores with high clay content allow the dissolved CO2 and brine to chemically interact with the clay minerals and exhibit noticeable changes to their mechanical properties. These results will aid better understanding of the geomechanical response of saline formations with different types of reservoir rock materials and salinities to CO2 injection and dissolution. Visual and acoustic emission strain measurement systems were also used in addition to strain gauges; in order to carefully study the deformation-related engineering properties of the specimens under different brine saturations. Reservoir integrity is a critical factor for the long-term safe storage of CO2 in geological formations. Therefore, this study involved the hydromechanical testing of rock specimens from the OBPP reservoir and caprock layers. The triaxial and uniaxial testing set-ups and the acoustic emission measurement system explained in Chapter 3 were used for the extensive experimental study of the mechanical and flow-related properties of the rock specimens under brine and CO2 injection, and the results are reported in Chapter 5. The sandstone specimens from the reservoir region of the OBPP system possess very high porosity and permeability while they were also found to be physically and petrologically highly heterogeneous as thin layers of black coal and clay (mostly parallel to the stereographic alignment) were discovered in the core specimens. At the same time the specimens exhibit small weak matrix bands which can fail under axial compression resulting in the formation of compaction bands. Experimental results of triaxial testing of intact and fractured reservoir specimens reveal that these compaction bands can greatly affect the permeability of these sandstone specimens. On reservoir scale this may lead to compartmentalization of the storage reservoir, significantly hampering the overall storage capacity and integrity of the geological formation. Numerical models for different types of rock failure under triaxial stresses and their impact on the fluid flow properties of the rock specimens were developed using COMSOL Multiphysics (a FEM analytical software tool). These models also validate the triaxial experimental results by reproducing the observations of fluid flow similar to those recorded in the laboratory investigation. A reservoir-scale model was also developed to simulate the processes that take place in a saline aquifer CO2 storage system. The hypothetical reservoir model was investigated for a total period of 100 years (including 30 years of injection phase and 70 years of CO2 storage) in intact state. The laboratory scale models successfully determine the compaction-related hydromechanical characteristics of the reservoir rocks and the reservoir-scale model demonstrates long term safety of CO2 storage in an ideal, but hypothetical reservoir system.