Gas leakage through wellbore micro-channels is a major challenging problem for well cementing and integrity. One of the reason that leads to weaker cementing is the drilling fluid contamination within the cement plug, which provides weak parts for the flowing pathways of gas and fluids. The objective of this paper was to experimentally investigate the drilling fluids contamination in terms of liquid and dehydrated drilling fluid on wellbore cement in multiple scale to compare and quantify the impact of the drilling fluids on the cement plug. In the experiment, specific amount of drilling fluid was mixed with wellbore cement. Different level of contamination was introduced for each phase of the drilling fluid. Class H Portland cement slurries were then prepared using API-10 B standard for a density of 16.4 lb/gal (SG=1.97), and the control sample with 0% contamination is referred to as neat Class H cement slurry. The first part of the testing is to quantify the effect of the contamination on core scale. Samples were tested for petrophysical and mechanical properties in terms of porosity, permeability and unconfined compressive strength. Moreover, Microstructural properties and hydration process were evaluated using Optical Microscopy and Scanning Electron Microscope (SEM) on micrometer scale in order to investigate the fundamental mechanism of contamination on properties of cement matrix. The results showed that the presence of mud would highly harm the performance of the cement. Micro-fractures appeared at the interface of the cement/mud through the Optical Microscope. The porosity of the core increased from 30% porosity of the neat cement to approximately 40% porosity of contaminated samples at most. The compressive strength could drop to more than 50% of the original value. Micro-annular channel and irregularities in the structure can be observed which might explain the increased porosity and permeability. The results presented in this paper would lead to safe implementation of cementing, enable safe Oil and Gas exploration in deepwater without risks of environmental contamination, and contribute to improvement in overall wellbore construction and development of robust hydraulic barrier materials.

As we approach 2020, there are more well bores that require abandonment than well bores waiting to be drilled. The USA alone has ~1.7 Million well bores with an API number indicating that they are not permanently plugged, therefore will be in need of Plugging and Abandonment (P&A) at some point in the future. Some of these well bores are located in fragile ecosystems, such as the Gulf of Mexico, where it is estimated that 9,000 idle well bores are waiting permanent plugging and abandonment. The numbers of future P&As suggest requirement for an urgent improvement of current technology as many governments are preparing standards that require assurance for zero leakage over thousands of years. This paper/talk will present knowledge gaps we identified during literature review and the data obtained during first two terms of the ongoing project , under NASEM-GRP funding. Extensive literature review points to the following issues: 1) Portland-cement based materials, with their pH ~13, are geochemically incompatible with the reservoir conditions in the GoM. 2) Weak interfaces between and contamination with drilling fluids in well bore structures present high risk for Hydrocarbon leakage, which currently cannot be mitigated successfully. Placement and accessibility of well bore in offshore environment adds to technological complexity and increases the risk of leakage. 3) Monitoring and verification is currently not available for long-term assessment offshore. Some of our preliminary data suggests the following: 1) Plugging materials are impacted by contamination form drilling fluids and other well bore materials, such as fragment of dehydrated clays, oily rock fragments. 2) Casing corrosion might lead to deterioration of primary well bore cement, which can be difficult to locate and re mediate prior to plugging. 3) Geo-mimicry is a potential path forward in designing and developing barrier materials capable of serving over thousands of years.