Abstract
Multi-gas assisted steam huff and puff process is a relatively new thermal recovery technology for offshore heavy oil reservoirs. Some blocks of Bohai oilfield have implemented multi-gas assisted steam huff and puff process. However, the development mechanism still requires further study. In this paper, high-temperature high-pressure (HTHP) PVT experiments and different huff and puff experiments of sand pack were carried out to reveal the enhanced production mechanism and evaluate the development effect of multi-gas assisted steam huff and puff process. The results indicated that viscosity reduction and thermal expansion still were the main development mechanism of multi-gas assisted steam huff and puff process. Specifically, CO2 easily dissolved in the heavy oil that made it mainly play the role of reducing oil viscosity, N2 was characteristics of small solubility and good expansibility, and it could improve formation pressure, increase steam sweep volume and even reduce the heat loss. Meanwhile, injecting multi-gas and steam could break the balance of heavy oil component that made the content of resin reduce and the content of saturates, aromatics and asphaltene increase so as to further reduce the viscosity of heavy oil. Compared with steam huff and puff process, multi-gas assisted steam huff and puff process increased the recovery by 2–5%. The optimal water–gas ratio and steam injection temperature were 4:6 and 300℃, respectively. The results suggested that multi-gas assisted steam huff and puff process would have wide application prospect for offshore heavy oil reservoirs.
Highlights
As one of main methods for producing heavy oil, steam huff and puff process is characterized of simple process and wide applicability for heavy oil reservoirs
Steam huff and puff process is classified into primary oil recovery mainly depending on natural energy (Liu 2013; Wang et al 2004; Sun et al 2017, 2018a, b, 2019; Yoelin 1970; Bao et al 2016)
CO2 is dissolved into crude oil, which increases the volume of crude oil by 10–30%, and the viscosity of crude oil will appear a certain degree decline which depends on pressure, temperature and the properties of crude oil
Summary
As one of main methods for producing heavy oil, steam huff and puff process is characterized of simple process and wide applicability for heavy oil reservoirs. For heavy oil with the viscosity of greater than 350 mPa·s, just limited reserves could be developed with the cold recovery technology At this time, steam huff and puff process will become an effective way of development. The reservoir type of Bohai oilfield is complex and diverse, viscosity of heavy oil in some blocks can exceed 10000 mPa·s, the development effect of simple steam huff and puff process is not ideal for this kind of heavy oil, a more reasonable development method is needed. Multi-gas assisted steam huff and puff process should improve the production effect of offshore heavy oil reservoirs like Bohai oilfield (Liu et al 2011; Gu et al 2007). The experimental apparatus mainly included sample mixer, intermediate container, flash separator, densitometer, rheometer and computer
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