Civil Engineering ETDs

Publication Date

7-1-2016

Abstract

Fractures in wellbore cements can develop and provide pathways allowing fluids such as hydrocarbons or brines to communicate with freshwater aquifers and/or reach the surface. Once formed, these fractures can be altered by the stresses acting on them as well as reactions with the fluids moving through them. Previous studies, mostly with acidic brines flowing through a fracture, have shown dissolution/precipitation reactions cause permeability to decrease and mechanical properties to change as the acidic brine creates reaction fronts slowly propagate through the face of the fractures. This study investigated fractured cement reacted with de-ionized water and saline brine under varying stress conditions. These fluids also contact carbon steel that was used to represent interaction with a steel casing within a wellbore. Flow measurements were made on a fractured cement core using nitrogen gas, de-ionized water, and saline brine under confining stresses up to 16.5 MPa. Flow through the fractured cement samples are interpreted as effective hydraulic aperture. After the flow measurements, fracture surfaces were examined with X-ray diffraction analysis, scanning electron microscopy (SEM)/ energy dispersive X-ray spectroscopy (EDX), and inductively coupled plasma-optical emission spectroscopy (ICP-OES). Hardness values of reacted and unreacted cement fractures were measured. The hydraulic aperture measured with nitrogen flow decreased in response to the initial increase in stress applied to the fracture due to crushing of asperities. Subsequent stress changes while flowing nitrogen through the fracture produced recoverable changes in the hydraulic aperture. De-ionized water flow through the fracture irreversibly reduced the hydraulic aperture by about 20% compared to nitrogen, attributable to the dissolution of portlandite which reduces the stiffness of the cement adjacent to the fracture surface and allows for additional closure of the fracture. During flow of saline brine, the hydraulic aperture progressively decreased and eventually approached that expected for intact cement. This so-called "self-healing" is attributed to (1) decreased fracture aperture from dissolution of portlandite, and (2) precipitation of calcite along the flow path. Similar to many studies in this area of research, this study investigated the behavioral changes in fractured cement as it reacts with different fluids and is exposed to varying levels of confining stress. However, the physical and chemical impacts from brine that is not highly acidic are uniquely characterized to better understand what can be expected when fractured wellbores penetrate a saline aquifer.

Keywords

Saline Brine, Fracured Cement, Physical-Chemical Changes, Hydrofracturing, Wellbore, Wellbores, Cement, Oil, Gas, CO2, Sequestration

Sponsors

Funding for this research was provided by the UNM Center for Water and the Environment, an NSF funded Center for Research Excellence in Science and Technology (Crest), NSF Award #1345169

Document Type

Thesis

Language

English

Degree Name

Civil Engineering

Level of Degree

Masters

Department Name

Civil Engineering

First Committee Member (Chair)

Cerrato, Jose

Second Committee Member

Peterson, Eric

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