K.H.A.A. Wolf

The influence of steam pressure on thermal spalling of sedimentary rock : Theory and experiments

Thermal spalling is the process by which surface material cracks and breaks off due to variation of temperature. The process is complex and the physical mechanisms that cause thermal spalling are not well understood. The theory and the results of experiments carried out to determine the effects of water and steam pressure on the phenomenon of thermal spalling are presented. It is shown that in theory the rise in pore pressure due to the presence of water as gaseous steam or as compressed liquid is insufficient to cause thermal spalling in sedimentary rock. A new criterion for spalling due to steam saturation pressure which is based on the concept of critical stress is presented. The results from uniform heating experiments and from linear heating rate experiments have led to the conclusion that explosive spalling is caused by saturated steam alone, at local pressures in access of the unconfined tensile strength. The results of thermal shock experiments are more difficult to interpret because of the high compressive stresses at the heated surface. Spalling due to thermal shock is probably caused by a combination of steam pressure and fracturing associated with high compressive thermal stress.

Experimental Study of Nonlinear Effects in Hydraulic Fracture Propagation(includes associated papers 29225 and 29687 )

The authors conducted fracture propagation experiments on blocks of cement paste, cement paste with sand, and a tight outcrop sandstone rock. A novel acoustic monitoring system was developed for measuring the fracture profile and radius during the tests. Results of laboratory tests on cement agreed with numerical simulations based on elastic rock deformation and linear elastic fracture mechanics. Tests at confining stresses of 20 and 23 MPa gave higher pressures than predicted.

A microstructural analysis of the compaction of claystone aggregates at high temperatures

Abstract The compaction behavior of rock aggregates at high temperatures and under intermediate stresses is investigated. A series of compaction experiments is performed in the laboratory on well-sorted claystone grain aggregates at temperatures up to 1000°C and under stresses up to 10 MPa. In order to investigate the mechanisms controlling the compaction process, we have developed a microstructural model based on the compaction of monosized spherical grains. In the model, compaction occurs through changes in the packing of the grains and by the indentation of grains. The grain contacts are assumed to be at yield stress, which is controlled by creep. We have analyzed the compaction-induced microstructural changes of the tested grain aggregates by using image analysis techniques. The spatial characteristics measured were used to make the model assumptions plausible. We found that the compaction model describes well the permanent volumetric strain in terms of microstructural parameters for relatively large strains. The model does not apply to the tested aggregates in the lower temperature range (550°C T T >650°C).

Thermo-mechanical properties of roof rock of coal for underground gasification

The mechanical stability of the cavity formed during underground gasification of coal is very important. Triaxial tests were performed at high temperature (up to 800 °C) and high confining stress (up to 15 MPa) to investigate the rock properties at in-situ conditions. The tested samples were taken from roof rock of a coal layer from a mine in Belgium. The thermal and mechanical properties of the tested rock types will be used for modelling. In this paper the possible consequences of these results for the stability and growth of underground cavities are discussed.

Effect of salinity and pressure on the rate of mass transfer in aquifer storage of CO2

The growing concern about global warming has increased interest in improving the technology for the geological storage of CO2 in aquifers. One important aspect for aquifer storage is the rate of transfer between the overlying gas layer and the aquifer below. It is generally accepted that density driven natural convection is an important mechanism that enhances the mass Transfer rate.There is a lack of experimental work that study the transfer rate into water saturated porous medium at in-situ conditions, i.e., above critical temperatures and at pressures above 60 bar. Representative natural convection experiments require relatively large volumes (e.g., a diameter 8.5 cm and a length of 23 cm). We studied the transfer rate experimentally for both fresh water and brine (2.5, 5 and 10 w/w %). The experiment uses a high pressure ISCO pump to keep the pressure constant. A log-log plot reveals that the mass transfer rate is proportional to t^0.8, and thus much faster than the predicted by Fick’s law. Moreover, the experiments show that natural convection currents are weakest in highly concentrated brine and strongest in pure water.

Effect of matrix wettability CO2 assisted gas-oil garvity drainage in naturally fractured reservoirs

The wettability behavior of the matrix block is one of the major factors controlling the effectiveness of the employed EOR methods in NFRs. Water injection in NFRs with mixed-wet or effectively oil-wet matrix blocks usually results in low oil recoveries. In this case, gas injection is considered to be an alternative process, where the process benefits from the gravity forces and the process is called gas-oil gravity drainage. In this study, the effect of matrix wettability on the efficiency of gravity drainage by CO2 injection is addressed. Laboratory experiments and numerical simulation were performed to analyze the process under different wettability conditions of the matrix. It is concluded that for a system with an effectively oil-wet matrix, water is the most non-wetting phase while CO2 is the intermediate-wetting phase. In the three phase setting, which includes carbon dioxide, this is considered favorable for oil production. However, with a strongly water-wet matrix, CO2 is always the least wetting phase. For this condition, it turns out that when water is displaced by the gravity drainage process part of the oil is also produced. It is observed that higher oil recoveries are achieved by CO2 injection in an oil-wet matrix block.

Investigation on Mechanical Behaviour of Coal and Overburden Rock for UCG

In recent years, the coupled UCG-CCS process has been considered as another potential CCS option, which can offer integrated energy recovery from coal and storage of CO2. However, existing potential problems may counteract its potential benefits. To develop a generic UCG-CCS site characterisation workflow, different aspects of this complex process, such as cavity progression and geomechanics, contamination of groundwater and subsidence impacts, need to be re-considered and understood. In this process, the thermo-mechanical behaviour of the roof rock and coal are the initial parameters to predict the stability and the development of the production cavity. These parameters affect heat conduction and the stability and caving of roof materials, especially under conditions of high stress and temperature. In this study, several experimental setups have been designed and built to study the thermo-mechanical properties of coal and overburden rock for UCG process. These experimental data can get an idea of elastic constants of rocks, the fracture growth mechanisms, the fracture orientations the maximum/yield stresses that the sample withstands, the conditions under which spalling occurs in overburden rock, as well as the rate which this take place. These results will be used as input for the modelling of the cavity growth of UCG.

Study of shale wettability for CO2 storage

For a water-saturated cap-rock, which consists of a low-permeability porous material, the wettability of the reservoir rock-connate water- CO2 system and the interfacial tension (IFT) between CO2 and connate water are the significant parameters for the evaluation of the capillary sealing. Also, the amount of capillary-trapped CO2 depends on the wettability of reservoir rocks. The wettability of the rock matrix has a strong effect on the distribution of phases within the pore space and thus on the entire displacement mechanism and storage capacity. In this work, the equilibrium contact angles of water/shale system were determined with CO2 for a wide range of pressures at a constant temperature of 318 K by using the dynamic captive bubble method. The results reveal that intermediate-wet conditions and hence possible leakage of CO2 must to be considered at relatively high pressures, however, the salt concentration of the water in the shales plays an important role too. The results show that this estimate is highly dependent on the pore structure, fluid composition and pressure/temperature conditions of the reservoirs. These properties need to be first evaluated before estimates for shale capillarity is used.

The effect of coal rank on the wettability behavior of wet coal system with injection of carbon dioxide and flue gas

The injection of carbon dioxide (CO2) or flue gas into coal layers enhances the coal bed methane production (ECBM) and is also an option for CO2-storage. The success of this combined process depends strongly on the wetting behavior of the coal, which is a function of coal rank, ash content, pressure, temperature and composition of the gas. Two coal samples have been used for this study representing different ranks: hvBb and semi-anthracite rank. The wettability behaviour of the wet coal samples upon injection of synthetic flue gas and pure CO2 was investigated in a modified pendant drop cell at a constant temperature of 318 K and pressures varying between 0.1- 16 MPa. For the hvBb sample, the wettability of the coal surface changed from intermediate-wet to CO2-wet at pressures above 8.5 MPa . When injecting synthetic flue gas, only a change from water-wet to intermediate-wet was observed. For the semi-anthracite rank Selar Cornish sample and CO2 injection this alteration was observed at about 5.3 MPa. Experimental results with synthetic flue gas revealed that the wettability of Selar Cornish coal is intermediate wet at all pressures and the contact angle only slightly increases with increasing pressure.