Hydrocarbon Generation from Oil Shale Using Thermochemical Fluids: Integration of Geochemical, Petrophysical, and Core Flooding Methods

Abstract The oil and gas industry has shifted its focus towards maximizing hydrocarbon production from unconventional reservoirs. Oil shale is an important unconventional resource that contains significant amounts of organic materials known as kerogen. This study aims to examine the effect of using thermochemical stimulation on fracture propagation and hydrocarbon generation from oil shale and characterize the rock sample using integrated geochemical and petrophysical methods before and after the stimulation process. An integrated workflow to characterize the petrophysical and geochemical properties of oil shale was used before and after thermochemical stimulation. Laboratory measurements such as X-ray diffraction (XRD), X-ray fluorescence (XRF), Rock-Eval pyrolysis, and micro-CT were carried out to characterize the oil shale. The thermochemical stimulation was done using two chemical fluids, which are sodium nitrite (NaNO2) and ammonium chloride (NH4Cl). The coreflooding system was utilized to mimic reservoir conditions, where the fluids were injected separately and allowed to interact only upon reaching the core inlet. The sample was ultimately cleaned with methanol to remove the precipitated salts in order to be able to capture induced fractures effectively. The stimulation of organic matter can lead to several alterations such as the enlargement of pore space and pre-existing fractures. The proposed workflow was successfully applied to the hydrocarbon generation from the oil shale sample. The TOC value declined after the stimulation due to the conversion of organic matter into hydrocarbons resulting from the thermochemical fluid reactions. Furthermore, the micro-CT images show that the pre-existing fractures were enlarged and formed a bath for new fracture networks. Along with this, the porosity of the sample increased due to the generation of new fractures. The more fluid cycles there are, the more efficient it is to convert organic matter into hydrocarbons. Also, the characteristics of the sample itself can affect the results of the reaction; thus, the higher the silica content, the more challenging the reaction. The XRD results after stimulation show that the calcite percentage decreased from (37.6%) to (19.2%) and quartz from (60.5%) to (48.1%), suggesting dissolution driven by the acidic thermochemical fluids. Overall, this study introduces a new technique that can be utilized to stimulate and generate hydrocarbons in situ from oil shale source rocks. Oil shale is not exploited properly due to the complex nature of organic matter. So, the suggested workflow can pave the way to introducing a downhole technique to stimulate and produce these resources more effectively.