Optimum Timing for Miscible CO2-EOR after Waterflooding in a Tight Sandstone Formation

详细信息    查看全文

• 作者：Zeya Li ; Yongan Gu
• 刊名：Energy & Fuels
• 出版年：2014
• 出版时间：January 16, 2014
• 年：2014
• 卷：28
• 期：1
• 页码：488-499
• 全文大小：629K
• ISSN：1520-5029

文摘

In this paper, CO₂ dissolution into a light oil/reservoir brine and optimum timing for CO₂ enhanced oil recovery (CO₂-EOR) after waterflooding in a tight sandstone formation are experimentally studied. First, the gas鈥搊il ratio (GOR) of CO₂-saturated light oil and the gas鈥搘ater ratio (GWR) of CO₂-saturated reservoir brine are measured by using a PVT system. The measured GOR is found to be approximately six times of the measured GWR at the same pressure and temperature. Second, the equilibrium interfacial tensions (IFTs) between the light oil/reservoir brine and CO₂ are measured at different equilibrium pressures and the actual reservoir temperature by applying the axisymmetric drop shape analysis (ADSA) technique for the pendant drop case. The measured equilibrium IFT between the light oil and CO₂ is about one-third to one tenth of that between the reservoir brine and CO₂ under the same test conditions. Third, the minimum miscibility pressure (MMP) between the light oil and CO₂ is determined to be 11.0 MPa from the measured equilibrium IFTs by using the vanishing interfacial tension (VIT) technique. Lastly, a total of five coreflood tests are performed to determine the optimum timing for miscible CO₂-EOR after waterflooding. All five tests are conducted at the same production pressure of 12.0 MPa to ensure that CO₂ flooding is miscible. In comparison with CO₂ secondary flooding, CO₂ tertiary flooding has a much delayed CO₂ breakthrough (BT) because the initially injected water substantially reduces the mobility of the subsequently injected CO₂. Accordingly, the oil recovery factor (RF) of CO₂ flooding in terms of the residual-oil-in-place (ROIP) is much higher. By comparing the total oil RF of waterflooding and CO₂ flooding in terms of the original-oil-in-place (OOIP), the oil RF of CO₂ flooding in terms of the ROIP, and the pore volume (PV) of CO₂ BT from the beginning of CO₂ flooding, it is found that the optimum timing for starting miscible CO₂ tertiary flooding is when waterflooding reaches half of its maximum secondary oil RF.