Experimental study on development of CO2 driven low permeability reservoirs based on harsh climate background

GONG Yufei

Abstrast：CO2 geological sequestration and Enhanced oil recovery（EOR）application is one of the possible way to manage the risk of global climate change.Since 2005，PetroChina has conducted pilot and extended test of CO2 flooding and storage in Songliao basin，from which significant progresses have been achieved in CO2，EOR technologies for low permeability reservoirs.Main progresses that we achieved include oil-CO2 phase behavior studying and minimum miscible pressure（MMP）reduction technology，water alternating CO2 EOR method and recovery optimization stratagems.We also formed two types of separated layer CO2 injection technologies，bottom hole real time corrosion monitoring system，effective lift technologies for three different types of reservoirs based on gas-oil ratio classification，long distance pipeline transportation and supercritical CO2 injection technology，produced fluid treatment with CO2 EOR application and cycling gas injection technology，CO2 flooding reservoir monitoring，performance analysis and evaluation method，potential evaluation and strategic planning for CO2 EOR and storage in PetroChina.

Key words：CO2 EOR Addressing Global Climate Change Green Operations Low Permeability Reservoirs Stratified Gas Injection

1 The miscible phase behaviour of oil-CO2 system

We have studied the formation crude composition profile feature for 22 low permeability blocks in 8 oil-field and also observed the miscible process between CO2 and non -marine crude.We observed firstly a low Interfacial Tension（IFT）rich hydrocarbon transition phase formed by mass transfer between oil and gas component，and then this low IFT transition phase is further miscible with heavy hydrocarbon components.The cognitions we obtained from the phase behaviour in oil/CO2 systems could be summarized as follows：（1）com-pared to marine crude，the C2-C6 components in continental crude are much lower while C11+ and gum asphaltene components are relative higher；（2）besides C2-C6，C7-C15 also has strong ability of mass transfer and is useful to miscible.Hydrocarbon surface active agent CAE that has low molecular weight was synthesized.The slime tube test show that a front-facing CAE slug（about 0.2PV）could reduce the minimal miscible pressure（MMP）from 27.3MPa to 21.2MPa in block Huang48，and thus effectively improve the miscibility in crude oil/CO，system.The relationship between in crude oil/CO2 system.The relationship between components classification and MMP was determined from analyzing formation crude experimental data for 30 well in 25 blocks，and then we defined a new hydrocarbon component factor Xf =（C2-C15）/（C1+N2+C16+）.The newly proposed hydrocarbon component factor Xf characterizes the relationship between component and MMP much better compared to the old one（see Figure1）if the effects of temperature and gum asphaltene content are considered.Finally，a new two stage MMP prediction correlation was proposed.

2 Separated layer CO2 flooding，anti-corrosion and high efficiency lifttechnology

PetroChina has conducted research on concentric double-barreled andsingle-barrelled separated layer gas injection technologies，and transformed from commingled gas injection to separated layer gas flooding strate-gy.Separated layer gas injection pilot test in Daqing oil field were implemented，which effectively reduce gas cross flow between layers that caused by commingled gas injection.We also developed the first set real-time anti-corrosion monitoring system by independent intellectual property rights.Compare with conventional coupon monitoring technology the new real-time system has plenty of advantages，such as long-term monitoring，online monitoring，short time（single point ≤30s）.In addition ，we have developed core lift equipments，such as sleeve valve，hollow gas proof pump，effective gas proof equipment，and 3 kinds of effective lift technology，which apparently improves the technology，which apparently improves the efficiency of the pump.PetroChina finished efficient lifting test of 17 wells in Jilin and Daqing，which can effectively solve problem of pump efficiency drop under the condition of high gas/liquid ratio.

3 Produced fluid treatment for CO，flooding and cycling gas injection technology We have formed several technologies and methods like circularity

We have formed several technologies and methods like circularity water，gas/liquid mixture transportation，centralized separation and measurement，and thus allowed us to meet the industrialized application in term of airtight transportation workflow.The demulsifier KD-1 for CO2 flooding produced liquid by using orthogonal experiment was selected，and experimental data shown that the dehydration rate is larger than 95% after 2bours’ test.We also compound flocculants PAFC，experiment shows that an optimized quality of re-injected water could be obtained if composite 95mg/L inorganic flocculants PAFC and 10mg/L nonionic PAM.Studies on separation and purification technologies，such as amine，membrane separation，pressure swing adsorption，rectification and low temperature stripping were also performed.From these method，pressure swing adsorption one shows the largest wide range of adaption capability，from 3% to 90% and whish also has high CO2 recovery rate（up to 99，4%）and low energy consumption，Three types of CO2 flooding produced gas re-injection

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