Insight into the desorption behavior and mechanism of tight oil with in-situ low-temperature thermal

Abstract

A major challenge faced in using enhanced oil recovery techniques in tight sandstone reservoir is the unknown desorption mechanism of adsorbed oil in the reservoirs. In-situ low-temperature thermal desorption (LTTD) was used in this study to detect the desorption behavior and mechanism of tight oil found in tight sandstones. The study provides an insight into the micro-occurrence states, carbon number-based fractions, and functional group changes of adsorbed tight oil. The micro-occurrence states of adsorbed tight oil can be in the form of oil aggregate and oil film. The desorption capacity of tight oil present in tight sandstones at 120 °C, 160 °C, 200 °C, 240 °C and 280 °C were 0.028, 0.171, 0.886, 0.945 and 0.954 mg/g, respectively. The desorption behavior of tight oil reverses at approximately 200 °C. At temperatures below 200 °C, desorption of tight oil is dominated by the oil aggregate in the 20–200 nm reservoir space, while the desorption process itself is controlled by the original physical properties, skeleton minerals and original oil saturation level of the tight sandstone reservoir. However, at temperatures above 200 °C, clay minerals control the desorption of tight oil present in form of an oil film adsorbed on the 2–7 nm sized pores of the mineral surface. During the process of in-situ conversion of tight oil, the amount of aliphatic compounds in it decreases, the amount of aromatic compounds in it increases, and the branching chain degree increases. As the temperature increases, the oxygen-containing functional groups continue to increase in number.