Semester of Graduation

Summer 2021

Degree

Master of Science (MS)

Department

Craft and Hawkins Department of Petroleum Engineering

Document Type

Thesis

Abstract

The strength and stability of a well depends primarily on the seal that is created between a geologic formation and a wellbore cement. To protect and maintain this well integrity during production and after well abandonment, fixed seals are installed in the wellbore. For this purpose, Portland cement of class H and G are used throughout the world as a plugging and sealing material for offshore oil and gas wells. The cement-rock interface is an area of vulnerability with a pathway that provides possible routes for leakage out into the wellbore environment and in the proximity of the seals. Therefore, this plugging and abandonment process entails the need for understanding the bonding between subsurface rocks and cement.

The goal of this research work was to investigate and study the interface between high density class H cement and three different rock formations (sandstone, limestone, and shale) and the effect of mineralogical composition on the interfacial bonding. In order to analyze the influence of the different lithologies and formation rocks' microstructure on wellbore cement–rock bonding, semi-circular bend test (SCBT) experiments were performed, indentation tests at μm scale were conducted, and scanning electron microscopy (SEM) X-ray technique was used to map chemical and mineralogical compositions of the cement, rock and their interface. Microphotographs of the bi-material samples, prepared by curing cement to formation rock under high pressure and temperature, taken via backscattered electron and secondary electron modes helped in estimating the spatial chemical element distribution and microstructural properties of the samples.

Results from this study indicated 1) Cement-sandstone interface had the highest hardness values while cement-limestone interfacial zone showed ductile characteristics. The interface for both these bi-material samples were clearly visible under low resolution SEM images. 2) The cement-shale interface was brittle in nature and the boundary between the two composites was indistinct in low resolution SEM images. 3. Cement-sandstone bi-material had the highest fracture toughness. Significant contrast was observed in the fracture toughness of cement-shale samples while the cement-limestone heterogenous material had fracture toughness lower than cement- sandstone but within the wide range of cement-shale fracture toughness.

This research work intends to fill the current research gap in the field by identifying any similarities or differences amongst the bonded bi-material samples studied for improved understanding of rock-cement interfaces using a combination of SCBT, SEM, and indentation technique to monitor the interdependence of geochemical and geomechanical variations and microstructure characterization

Committee Chair

Gupta

DOI

10.31390/gradschool_theses.5346

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