An experimental platform for triaxial high-pressure/high-temperature testing of rocks using computed tomography [electronic resource]
Keywords: Energy Resources Engineering.

Over the last couple of decades CT based core analysis studies have become standard. Ideally, any core flood experiment should be carried out under reservoir conditions regarding pressure, temperature, and in-situ stresses to capture both the mechanical characteristics of the core sample and the flow mechanics of the process under investigation accurately. Higher temperatures and pressures are required during the investigation of enhanced oil recovery techniques based on gas injection or thermal injection. The study of reactive fluids in rocks as encountered during, for example, CO2 sequestration, acid gas injection, and geothermics are other examples. With the development of unconventional reservoirs for hydrocarbon recovery, capturing the influence of effective stress on fluid flow has become even more important. Compared to conventional reservoirs, hydrocarbons in unconventional reservoirs and self-sourced systems are located in the source rock or nearby layers. Consequentially, knowledge of generation history and storage capacity is essential in assessing retained oil and gas volumes. Even more, we need to understand the elastic behavior of these formations concerning maturity to gain insight into the amount and nature of hydrocarbons generated and to provide baselines for geomechanical attributes and electrical properties to elucidate seismic studies and well logs respectively. From an experimental point of view, the study of hydrocarbon formation and expulsion from kerogen, crucial for the understanding of unconventional reservoirs, on a lab scale requires all three components i.e. pressure, temperature, and stresses to be addressed simultaneously in an extreme setting. Temperatures greater than > 400 C are necessary to achieve hydrocarbon formation on a manageable timescale dramatically altering the rock matrix and thereby porosity and permeability. Previously described conventional high-pressure/high-temperature (HPHT) loading experiments are not X-ray compatible. Current X-ray transparent systems for operation in a medical CT scanner are limited to temperatures below 180 C. In this thesis, we developed an X-ray compatible, high-temperature (> 400 C), high-pressure (> 2000 psi/> 13.8 MPa confining, > 10000 psi/> 68.9 MPa vertical load) triaxial core holder for a medical CT scanner. Design requirements for HPHT and X-ray compatibility are inherently contradictive. From a pressure vessel perspective, HPHT requirements ask for thick pressure vessel walls. The thicker the walls, however, the smaller the X-ray transparency of any material. Casting a radiodensity and material failure model into an optimization problem allows determination of ideal dimensions of the core holder with respect to all principal stresses maximizing X-ray transparency. Rock samples have been subjected to uniaxial loads of up to 12450 psi (85.8 MPa) and confining pressures more than 2100 psi (14.5 MPa). To prove high-temperature capabilities, a shale sample was subjected to a constant vertical load of 1900 psi (13 MPa) in a 500 psi (3.45 MPa) nitrogen atmosphere and heated to 450 C at a heating rate of about 5.8 C/min. The temperature was maintained above 440 C for several hours allowing the sample to fail as a consequence of the thermal disintegration of the matrix. Besides, using krypton as a contrast agent, we elucidated the poroelastic behavior of a brittle Castlegate sandstone under varying hydrostatic and triaxial conditions. Furthermore, we summarize results of a successful laboratory investigation to develop a technique to visualize and quantify sub-core scale porosity evolution with time as a result of source rock pyrolysis, here oil shale containing type 1 kerogen, using X-ray computed tomography. For this particular well-laminated source rock, we noticed that porosity distributions for the immature samples show unimodal behavior. After maturation, the distributions shift to dramatically greater porosities and the distributions are multimodal. Within individual laminations, porosity remains normally distributed. The multimodality of matured samples reflects the varying kerogen content of the various laminations. Conventionally, the description of porosity of a core sample is limited by the number of bulk porosity measurements taken using, for example, helium porosimetry. This work shows that porosity evolution during pyrolysis (maturation) of a type I kerogen is not described adequately by bulk measurements. Combining CT imaging techniques with krypton as a pore contrast fluid reveals the in-situ porosity distribution of the immature and thermally matured shale sample on a volumetric scale of about 0.06 mm3. Porosity values rise dramatically due to the conversion of organic matter into hydrocarbons and thermal expansion of the shale matrix. In the sample studied, bulk porosity to krypton increased from about 9% to 25%.