Abstract
Abstract CO2 injection is different than oil and gas injection, where CO2 phase changes is very likely to be occurred inside tubing at various operating conditions, especially in depleted gas reservoir during early stage where reservoir pressure is low, and CO2 can be changed from supercritical to gas phase at various depths inside tubing when flow approaches perforation intervals. This is a dynamic and transient behavior and modelling wellbore pressure and temperature during the life of the well (early, mid and late) is essential and can provide better understanding of the potential risks that are associated with CO2 injection and the appropriate actions and mitigation plans. Severe cooling due to phase changes and Joule Thomson effects inside tubing, as well as cycling of cooling and heating conditions, can negatively impact well and reservoir integrity including cap rock. This paper presents a case history of CCUS project in the Middle East where wellbore pressure and temperature modelling showed a significant impact on surface facility design, geomechanical model including cap rock integrity as well as the safe operating envelop for CO2 injection. The workflow included modelling of seven time-steps to represent CO2 injectivity performance and pressure/temperature profiles inside tubing from start to the end of the project. In addition, sensitivity analysis was considered for CO2 impurities at each time step. The results feed the geomechanical study to ensure reservoir and caprock integrity during injection as well as surface facility design for mitigation plan. The results showed that high purity CO2 can yield significant higher injection rate compared to low purity scenario. During early stage of injection, low bottomhole injection temperature was observed. Injection under low temperature may jeopardize reservoir and caprock integrity. Potential solutions to mitigate such issue is to consider heating CO2 at surface at early stage or to use smaller tubing size which may impact the injection capacity. Early stage during injection is the most critical period and may impact completion design and reservoir/cap-rock integrity. Hydrate risk assessment was conducted and showed that surface condition is close to the hydrate risk zone. Operation philosophy has been modified to consider a surface heating unit to be used during the early stage of injection until reservoir pressure builds up. This paper presents the importance of detailed wellbore pressure and temperature modelling during CO2 injection and how this can impact the integrity of the project as well as the operation philosophy. Workflow and risk assessment are also presented.
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