North Buzachi field is structurally located in the western part of Ustyurt Basin on the edge of Caspian Sea in Kazakhstan. It's a heavy oil reservoir with viscosity nearly 400 mPa·s at 33°C. After nearly 20 years of cold-water flooding, the oil production has fallen sharply. Low sweep efficiency and high water cut come to be the main severe challenges at this stage. Ways to improve efficiency and productivity need to be tried and evaluated.

Water shut off tests, polymer flooding and adjusting perforations have been tried, but with little success. Thermal enhanced oil recovery (EOR) methods generally are effective options and cyclic steam stimulation (CSS) was considered. But the high-water saturation and water cut make CSS get rapid economic decline after 1 or 2 cycle. Therefore, the feasibility of Gas-Steam Composite Stimulation (GSCS) on this field is studied and evaluated. This paper shows the prospect of GSCS in this reservoir by studying flow mechanisms and numerical simulation analysis.

Gas-Steam Composite is a mixture of steam, carbon dioxide and nitrogen, which is produced based on a high-pressure jet combustion mechanism. The mixed components are simultaneously injected into the formation at the same pressure and temperature. GSCS has been tried in other similar oil fields (Mortuk oil field in Kazakhstan) before, and the actual production results are better than predicted. The application prediction of North Buzachi field was based on the previous theory and experience. During the GSCS progress, gas overlap occurs underground, resulting in secondary gas cap drive and higher reservoir pressure. Along with other synergistic effects, the oil rate can theoretically increase to 4-5 times of water flooding in North Buzachi field. By the results of numerical simulation, the upper residual oil was swept and the sweep efficiency increase from 14% to 40%. The daily oil production per well grows from the initial 3.5 t/d to 15 t/d. Compared with the final recovery of 12% by water flooding, the recovery may reach 33.4% after 7 cycles of GSCS. Meanwhile, the water cut declines from 97% to 50% and the average cyclic oil-steam ratio reaches 4.24.

Economical evaluation shows that cost per ton of oil can be reduced by 24%. With the remarkable increase in production, more economic benefits can be achieved with GSCS technology. This study not only lays the groundwork for future pilot of GSCS in this field, but also provides a significant reference for the development of similar heavy oil reservoirs.

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