Potential Leakage Paths along Cement-formation Interfaces in Wellbores; Implications for CO2 Storage

Abstract

The concept of geological CO2 storage relies on the capability of the storage site to contain the gas within its intended location. Leakage on a large scale will render the capture, transport and injection processes futile. Flow paths enabling stored CO2 to re-enter the atmosphere can either be caused by geological conditions (e.g. failed caprock integrity) or it can be related to failed integrity of wells that penetrate the reservoir. Leakage along wells, which can occur through the cement porosity, through cracks in cement or along the casing-cement or cement- formation interfaces, has been appointed the “weak link” of safe and efficient geological CO2 storage.In the present work, the cement-formation interface has been studied in detail for various well situations in order to enable estimation of flow through this potential leakage pathway. Two rock formation types, Castlegate sandstone and Mancos shale, were cemented without the use of drilling fluids, with water based mud (WBM) at the interface and with oil based mud (OBM) at the interface. The motivation was to study the degree of adherence, or inversely the degree of debonding, of the well cement as a function of varying surface properties of the rock.The cement-formation interfaces were characterized by X-ray micro computed tomography (μ-CT), which enabled three-dimensional (3D) interface visualization and quantification. It was found that the interface porosity, defined as the volume of interface pores divided by the total sample volume, was strongly dependent on the surface properties of the rock. The cement-sandstone interface porosity was 0.06% without mud, 1.11% with WBM and 0.35% with OBM. The cement-shale interface porosity was 0.69% without mud, 0.76% with WBM and 0.13% with OBM. This underlines that fluid choices made already during drilling affect the long-term sealing ability of a well – and thereby the success of a CO2 storage project.