CO2 Assisted Steam Flooding Technology after Steam Flooding - A Case Study in Block J6 of Xinjiang Oilfield

Abstract

Currently Block J6 is in the later stage of steam flooding after 27 years’ steam injection, its recovery factor is about 50%, and the water cut is more than 95%. Particularly, the present steam oil ratio is about 12 m3(CWE)/t which has reached the economic limit and is in ineffective development. Cores from four post steam flooding drilling wells show that only top 2-3m of the total 25-30m pay zone has a steam chamber which is the main steam channel and its residual oil saturation is about 20%, the other 22-27m pay zone is displaced by hot water and its oil saturation is 40-55%. A 3D physical simulation show the conventional steam flooding with full interval perforation quickly broke through from the top of reservoir, and the steam oil ratio rose rapidly from 5 m3(CWE)/t to 10 m3(CWE)/t. The recovery factor was only 20.1% at the time of steam breakthrough, and then it was in the phase of high steam oil ratio for a long time. During CO2 assisted steam flooding the whole perforated producer is switched into a low half perforated well, and the recovery factor increases from 20.1% to 81.1%, the steam oil ratio is 3.3m3(CWE)/t. There are three characteristics in CO2 assisted steam flooding stage, firstly there is a steam and CO2 assisted gravity drainage mode, steam chamber expands from the top 2-3cm to the total 20cm oil layer. Secondly, there is a stable emulsion foam oil, its water cut is 60-70%, CO2 liquid ratio is about 5:1 Sm3/t, CO2 is a kind of dispersed bubble so it is much more than the dissolved CO2 liquid ratio 2:1 Sm3/t. Thirdly, CO2 lows the heatloss to overburden and keeps the formation pressure. The calculation shows that the heat loss can be reduced by more than 10% in the top layer. A pilot test including 9 well patterns(49 wells) has been established, and its recovery factor will be up to 75%, and the steam oil ratio will up to 2 m3(CWE)/t, a good production performance is predicted optimistically.