Research on CO 2 Quality Pipeline Transportation Based on Yanchang Oilfield CCUS

. CO 2 capture,utilization and storage (CCUS) is now recognized as an important technology in the global scope of CO 2 emission reduction, pipeline transportation is the main center to connect the capture point and the use storage point, the first issue to CO 2 pipeline transportation is to solve CO 2 source quality research. Yanchang Oilfield has the advantages of CCUS, its coal chemical capture of CO 2 contains different impurities. In the CO 2 pipeline transportation, the impurity content in CO 2 is based on its end use and the actual situation of pipeline. The impurities will affect the efficiency of CO 2 -EOR, the choice of CO 2 state equation, the changes of CO 2 phase diagram and the capacity of pipeline transportation.

Yanchang oilfield has coal, gas and oil resources in the same area, the natural conditions lays the foundation for the comprehensive development of the CCUS work.Domestic modern energy and chemical base has been formed, especially Coal-to-Oil and Coal-to-Olefins as the representative capture large numbers of high concentrations CO2.
There are more than 9.5 million tons CO2 had emitted by coal chemical projects every year, of which about 60%, the concentration is above 85%.the use of CO2 for EOR to reduce CO2 emissions in Northern Shaanxi means that the coal utilization rate is higher than other areas and it's also the important module of coal chemical industry in Northern Shaanxi [4].In recent years, there are a total of about 10 million tons of high pure CO2 capacity in Northern Shaanxi, of which Yanchang oilfield has been formed a huge capture scale that is early 400000t CO2, mid 700000 CO2 and forward 4 million tons CO2, this huge CO2 capture is the prerequisite for the development of CO2-EOR.
Yanchang oilfield is extra-low permeability reservoir, which has low oil recovery, CO2-EOR can efficiently enhance oil recovery, this can ensure the long-term stable production of the oilfield; water resources in North Shaanxi is deficient, CO2-EOR saves a large amount of water;according to the abroad experience of CO2 pipeline and the characteristics of CO2, the CO2 pipelines are laid away from the crowd, sparsely populated of northern shaanxi for CO2 pipeline provides a good geographical conditions.
The nature conditions of Yanchang oilfield provided a successful foundation for CCUS: (1) It makes CO2 emission reduction successful and the benefits from enhanced oil and gas recovery can make up for the cost of CCUS.(2) It saves a lot of water resources, these advantages are the key reason that America and China choose Yanchang oilfield as demonstration base of CCUS [5].

Requirements of CO2-EOR for CO2 Quality
The requirements of pipeline transportation for CO2 quality are considered its final use: enhance oil recovery.For EOR applications, CO2 flow must has a high purity, which generally provides the minimum content of CO2 is 95% and the average content of CO2 is about 98.45%~98.72% in foreign countries, which is to ensure that CO2 and oil are mixed in the suitable geothermal basin.differentimpurities in CO2 flow has different effects on CO2-EOR.H2S can increase the solubility of CO2 in the oil and reduce the mixed phase pressure, increase the water solubility in CO2, so H2S is favorable for CO2-EOR.Therefore, the restriction on H2S is based on security perspective, the recommended of H2S concentration is below 0.02% [6], but in the actual operation, the concentration is determined according to the actual operation situation eventually;CH4 can affect the minimum miscible pressure , and it is an important factor for affecting the mixing capacity, the volume concentration of CH4 is 2% in EOR applications; while N2 and other non-condensable species have a negative effect on the miscible capacity, density and viscosity of CO2 flow [7,8].In conclusion, the impurities content in the CO2 flow is considered by having a minimal on EOR applications.

Requirements of Pipeline Transportation for CO2 Quality
The pipeline system is very sensitive to the impurities in CO2 flow, which will affect many parts of CO2 pipeline transportation.The impurities will affect the phase change of the whole CO2 flow, pipeline transport process, pipeline transport capacity, pipeline crack propagation and pipeline corrosion and protection, etc.

Requirements of pipeline transportation for water content
The impact of free water on the transport pipeline is mainly reflected in the following two aspects: (1) CO2 will lead to the electrochemical reaction between carbon steel and water, then make the pipeline and equipment corrosive and corrosion rate is high; (2) The free water will form hydrates to block the pipes and even damage equipment.So, before entering the pipeline systems, CO2 must be dehydrated.
Under super-critical conditions, the solubility of water in pure CO2 is 0.0026kg/m3 and it has been determined that the water content is less than 60% saturation (0.0015kg/m3), and carbon steel will not corrode.For super-critical CO2, DOI: 10.1051/ , 020 ( 2017) 710002004

2016
MATEC Web of Conferences 100 GCMM matecconf/201 04 along with the increase in pressure and temperature, the solubility of water in CO2 increased.In addition, the existence of free water also can cause hydrogen embrittlement or hydrogen brittleness, because the free water can cause the hydrogen atoms diffusing to the metal substrate to form hydrogen molecules, resulting in local internal pressure decreased the ductility and tensile strength of steel [9,10].
The formation of hydrate is not the main problem but it is a problem to be solved.The concerns about hydrates are mainly from the aspects of the flow protection, hydrate in the pipeline may cause the pipe blockage and flow content reduction and plug the pipeline system and accessories then causing accidents.In addition, if there is some water in CO2 flow, it is easy to generate hydrate in pipeline restart [11,12].In order to avoid the occurrence of corrosion, the water content in current CO2 pipeline is far less than 60%, and the operation of these pipelines are safe, so in our country designing CO2 pipeline is also to meet this requirement.If in transit appear free water, there is no definite response, the International Petroleum Exchange gives the suggestion is that using ethylene glycol or corrosion inhibitor [13,14].

Requirements of pipeline transportation for other impurities
The impurities affect the density, compressibility and viscosity of the whole CO2 flow.These thermodynamic parameters affect the physical properties of the fluid and the pipeline transport properties, they can determined by the equation of state.The most popular used to predict the CO2 phase diagram are: Peng-Robinson (PR) equation, Soave-Redlich-Kwong (SRK) equation, Benedict-Webb-Rubin-Starling (BWRS) equation and GERG-2008 equation.The differences of impurities content and kinds make the equation of state's more difficult, so the equation of state for CO2 pipeline design has not yet reached a consensus, currently used for simulation of the impurities in the CO2 state equation are based on the published experimental data available in the literature to evaluate, DNV RP J202 Design and Operation of CO2 Pipelines recommended PR equation is used for CO2 containing impurities.PR equation has higher accuracy, however, in the process of calculation, the equation of state needs use the experimental data to adjust and to assess the calculation of uncertainty.Impurities affect the phase behavior of the whole CO2 gas flow and then affect the process of pipeline transportation.

Figure 1
Figure 1 show the phase diagram of CO2, compared the different impurities on the critical point and phase curve of CO2.