Experimental Study and Pilot Tests of Autothermic Pyrolysis in situ Conversion Process for Oil Shale Recovery

Abstract

Abstract Oil shale has not been applied in large-scale industrial processes owing to its poor efficiency and high energy extraction costs. The autothermic pyrolysis in situ conversion process (ATS) is an oil shale high-efficiency heating extraction method that uses the residual potential heat after kerogen pyrolysis. Herein, a large-size oil shale (Φ: 100 mm, L: 500 mm) in situ pyrolysis experimental apparatus, which considers the overburden pressure, was designed to study the feasibility, characteristics, and energy efficiency of ATS. Two ATS pilot tests were conducted at different depths (approximately 70 and 480 m) in the Songliao Basin, China. The pilot tests were performed for nine years, from 2013 to the present. The results demonstrate that the ATS of oil shale is successfully triggered at 300 °C. The autothermic pyrolysis reaction is steadily advancing, proving the feasibility of the method in laboratory-scale experiments. According to their optical characteristics and chemical composition, ATS reaction zones can be divided into residue, autothermic, pyrolysis, preheating, and virgin zones. Compared with the high-temperature nitrogen in situ conversion process, pyrolysis oil obtained from ATS contains more light components. When the oil recovery via ATS reaches 69.1%, the energy efficiency reaches 3.46, which is much higher than 0.51 by nitrogen. The cumulative oil and gas production of the shallow-ground pilot test was 1.6 ton and 1500 m3, respectively. In addition, the cumulative oil and gas production of the deep-ground pilot test was 3.0 ton and 18,000 m3, respectively. During the two pilot tests, long-life downhole heating technology and equipment were developed and applied. The compound reservoir reconstruction technology, which includes shock wave fracturing, hydraulic fracturing, and acid fracturing, was studied. To improve the pyrolysis efficiency, oil recovery, and quality to the oil shale was studied. This study shows the advantages and feasibility of ATS using experiments and pilot tests, which can be used for the large-scale commercial development of oil shale.