Converting Development Wells to Observations Wells to Aid Reservoir and Overburden Monitoring for CO2 Sequestration in a Depleted Carbonate Field
Authors: Tiwari Pankaj K. , Das Debasis P. , Patil Parimal A. , Chidambaram Prasanna , Picha Mahesh S. , Abu Bakar Faiz A. , A Razak Zahin B 
Published: December 2021

Abstract
Measurement, Monitoring & Verification (MMV) is crucial to ascertain both containment and conformance in Carbon Capture & Storage (CCS) projects. The magnitude of parameters to be monitored along with the technologies to be adopted could be very cost intensive and impact overall project Net Present value (NPV). To rationalize the associated costs and maximize the value propositions of existing infrastructure, the development wells in depleted field provide the opportunity to reduce the MMV cost by converting them into observation wells. However, the wells are to be analyzed for their strategic location in the reservoir, fit for purpose plug & abandonment plan and the apt technologies that can be implemented for both reservoir & overburden monitoring.
Development wells in the identified depleted field are 30-40 years old and were not designed considering high CO2 concentration. In consequence, the possibility of well leakage due to accelerated corrosion channeling, cracks, along the wellbore cannot be ignored and requires careful evaluation. Rigorous process has been adopted in assessing the feasibility for converting existing producers into observation wells. Wells basis of designs disparity between the producer and the required observation well governs the selection for conversion to observation wells or plugging and abandonment. The reservoir simulation and coupled modelling predict that CO2 plume will reach all wells penetrating the storage reservoir during the initial injection phase. Out of 9 available producers, 2 strategically located wells have been evaluated for conversion based on end injection reservoir pressure of ∼3450psi. Quantitative CO2 leakage through the observation wells has been numerically computed based on all possible pathways for risk characterization. The permeable/perforated zones in these two wells are to be isolated along with the cap-rock restoration technique at deepest depth of ∼4000ft TVDSS. This will ensure the wells are safe & accessible for monitoring CO2 plume migration, CO2 leakage and well integrity by analyzing acquired DAS-VSP, DTS, DPS data and well logs.
This paper elaborates unique challenges associated with identifying strategic wells for conversion to observation wells. Minimum plug setting depths, ranging from 3720-3880ft TVDSS, for abandonment of 9 development wells are derived based on fracture gradient and maximum horizontal stress. 2 observation wells require deeper plug setting depth to make caprock accessible at ∼4000ft TVDSS to be restored by utilizing either perforate-wash-cement (PWC) or section milling. Based on the subsurface illumination modelling, deployment of fiber-optics sensors in observation wells promises a cost-effective solution for monitoring CO2 plume migration and leakage by acquiring 4D DAS-VSP survey. Conversion of producers to observation wells promises cost effective MMV application for CO2 plume migration and leakage monitoring along with periodic temperature, pressure, and CO2 concentration measurement in overburden.