Influence of steam on the coking characteristics of heavy oil during in situ combustion

When in situ combustion is applied to reservoirs that have been exploited by steam recovery, the influence of high water saturation on the thermal conversion of heavy oil into coke needs to be evaluated. A Xinjiang heavy oil was used as the sample. A thermogravimetric analyzer and a pressurized reactor were used to study the influence of steam on the pyrolysis and oxidation characteristics of heavy oil under atmospheric pressure (0.1 MPa) and reservoir pressure (4 MPa), respectively. Under atmospheric pressure, the presence of steam showed little effect on the pyrolysis and low temperature oxidation characteristics of heavy oil but accelerated the rate of coke oxidation. Under reservoir pressure in an inert atmosphere, steam showed little influence on the coke yield but increased the proportion of alkanes and hydrogen in gas products due to the hydrogen donation of steam. Additionally, the oxidation activity of coke produced with steam decreased. The main reason was a decrease in the number of active sites of coke oxidation according to kinetic analysis. Under reservoir pressure in an oxidizing atmosphere, the coke yield decreased significantly because of the existence of steam, but the coke oxidation activity remained unchanged. The main reason for this coke yield decrease was that steam accelerated coke oxidation and increased the oxidation consumption of the produced coke in the coking temperature range of 250-400 degrees C. In addition, steam promoted the cleavage of oxygen-containing functional groups in heavy oil conversion intermediates, thus inhibiting the formation of coke precursors.