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.