Experimental Study on Displacement Characteristics and Water/Air Ratio Limit for Wet In-Situ Combustion in Ultraheavy Oil Reservoirs

Abstract

Summary An ultraheavy oil block is undergoing a dry in-situ combustion field test in the Xinjiang oil fields, China. However, this method faces the problems of high air/oil ratio and low oil production rate. Researchers are exploring the feasibility of using wet in-situ combustion to improve economic efficiency and oil production rate. For this purpose, it is necessary to study the displacement characteristics and the water/air ratio (WAR) limit of wet in-situ combustion in ultraheavy oil reservoirs. In this study, we investigated the differences in displacement characteristics between wet and dry in-situ combustion by conducting 1D combustion model tests. The results show that wet in-situ combustion can transfer heat from the burned zone to the vicinity of the combustion front, forming a high-temperature area upstream and a steam zone downstream. The length of the high-temperature area reflects the stability of the wet in-situ combustion, and the length of the steam zone reflects the amount of heat that contributes to the oil displacement. We also designed a wet in-situ combustion experiment with a variable WAR to study the WAR limit. We found that wet in-situ combustion has three critical WARs in ultraheavy oil reservoirs—optimal, maximum, and minimum. If the WAR is too small, the effect of wet in-situ combustion will not be obvious enough. If the WAR is too large, it may affect the stability of the combustion. The length of the high-temperature area and the steam zone can be used to determine these three WAR limits. For the experimental crude oil, the optimal WAR was 8.182×10-3 m3/m3(st) at a ventilation intensity of 20 m3/(m2·h), which resulted in a high-temperature area of 20.8 cm and a steam zone of 41.6 cm in length. The maximum and minimum WARs were 13.636×10-3 m3/m3(st) and 2.727×10-3 m3/m3(st), respectively.