Thermal Cracking of Heavy-Oil/ Mineral Matrix Systems

Abstract

Summary. At temperatures and pressures encountered during thermal recovery, chemical reactions involving oil, possibly water. and mineral matrix may lead to significant changes in composition of the phases. This work focuses on thermal alteration of four crude oils with different geochemical compositions. The tests were performed in an autoclave at 350 deg. C [660 deg. F] for 200 hours. The oil was in the presence of water and a mineral matrix representative of reservoir rocks. The formation of significant amounts of insoluble organic material was observed in all tests. Gaseous hydrocarbons (mainly methane) formed by pyrolysis. CO2, evolution depended on oil and matrix compositions; moreover, water was found to be a reactant in the formation of oxygen-containing components.or crude oils investigated, the oil phase was significantly enriched in lighter hydrocarbons (mainly in saturates). However, aromatic content was less affected by the thermal treatment. On the other hand, asphaltenes and, to a lesser extent, resins were the main precursors of insoluble material. These results provide new insight on mechanisms of the cracking reactions involved in thermal recovery processes. Introduction During steam injection, it has been observed that chemical reactions can play an active role in the process and induce formation of gaseous components such as CO2 or H2S. These reactions, at high-temperature and high-pressure conditions, can affect both the matrix (carbonate transformations) and the oil (cracking of certain components). The results reported here are part of work related to the geochemical alteration of crude oils under conditions prevailing in thermal recovery processes. The objective of this work is to predict the effect of the crude oil's geochemical composition, the characteristics of the porous medium. and the pressure and temperature on the reactivity of crude oils. The tests reported in previous papers emphasized formation of an insoluble organic deposit, called pyrobitumen, during longterm exposure to steam of heavy crude oils and detailed the interpretation of the effect of the presence of minerals on oil alteration. The work in this paper deals with thermal alteration of four crude oils with different geochemical compositions in the presence of water and a mineral matrix representative of reservoir rocks. The four crude oils have densities ranging from 936 to 992 kg/m [19.7 to 11.1 deg. API] and viscosities ranging from 0.011 to 1000 Pa s [11 to 10 cp] at 40 deg. C [104 deg. F]. The influence of sedimentary rocks, such as carbonates and clays, will be evaluated. Fassihi et al. studied the thermal alteration of three different crude oils subjected to heating from 12 to 350 hours at 25 to 400 deg. C [77 to 752 deg. F]. They showed that the three oils studied, which were immature or biodegraded, underwent similar compositional changes-e.g., an increase of the C, to C fraction. Other authors concentrated their attention on mineral modifications, specifically permeability and porosity of the reservoir rocks. Recently, results of visbreaking reactions of heavy oils were used to simulate in-situ oil alteration; by taking advantage of these oil modifications, a better design of the steam process could be proposed. Consequently, it seemed important to study the effects of both oil composition and presence of a mineral matrix on oil alteration; moreover, these conditions are representative of those prevailing in reservoirs during thermal recovery. In addition, these experiments may give information on mechanisms of cracking reactions, which can significantly affect steam recovery processes. Experimental Apparatus. The reactor, with an internal volume of 300 cm, has been designed for a maximum operating temperature of 450 deg. C [842 deg. F] and a pressure of 50 MPa [7,250 psi] (Fig. 1). It is made of Hastelloy C 276 because of the good mechanical properties of this material and its high resistance to corrosion by H2S, H2, CO2, carboxylic acids, and chlorides in the presence of steam. The reactor is placed in an electrically heated furnace with electronic regulation and high heating power. permitting the vessel to be heated from ambient temperature to 400 deg. C [752 deg. F] within 1 hour. A ther-mowell placed in the axis of the vessel permits temperature to b followed during the test. The apparatus is equipped with manometers and pressure transducers. A system of valves permits placing the reactor under vacuum, sweeping the reactor with helium, and gas sampling. Different tests have been carried out with the reactor being agitated or not agitated. Because the results were quite similar. the following tests were performed with no agitation. Procedure. Four heavy crude oils were used for the experiments (Table 1). Their geochemical compositions are given in Fig. 2 in terms of saturates, aromatics, resins, and asphaltenes (SARA). Indian oil (Oil A) is naphthenic and contains practically no sulfur. Active infiltration of fresh water in the reservoir might have caused oxidation and degradation of crude oil to the present status. Bacterial degradation is also a possible cause. Nordhorn (Oil B) is an aromatic oil. Degradation by oxidation and bacteria is more pronounced for this oil than for Oil A. Boscan (Oil C), from carbonate source rocks, contains polar compounds consisting of very stable polycyclic aromatics. Athabasca (Oil D) contains aromatics that are less condensed and more reactive, and it is also degraded. The minerals have been selected to be representative of reservoir rocks, their characteristics are given in Table 2. The amount of oil placed in the reactor varied between 30 and 50 g. Generally, oil was present with an equal amount of previously deoxygenated water and a mass of powdered mineral equal to three times the oil mass (Table 3). Before the run, air in the reactor was evacuated and replaced by helium under 800 kPa [ 1 16 psi]. In this study, the tests were performed mainly at a temperature of 350 deg. C [662 deg. F] for 200 hours. For the tests performed in the presence of water at 350 deg. C [662 deg. F] both liquid water and vapor coexisted. According to the test conditions, pressure stabilized at 3 to 5 MPa [435 to 725 psi] or 18 to 20 MPa [2610 to 2900 psi] (Table 3). The gas and oil phases and the solids were analyzed after the test to study the modification of the crude oil. A gas sample was taken after the experiment when the reactor temperature had cooled to ambient. SPERE P. 1243^