Drilling of oil well is a capital intensive project; hence, the importance of selection of proper mud system that is free from contamination cannot be overemphasized. A poor design and contaminated drilling fluid can be very costly in the life of any well. Therefore, during drilling operations care must be taken to avoid contamination of mud with cement among others. This project details experimental work and findings associated with mud contamination with cement. Also, solutions were proffered on how to manage contaminated mud systems with cement during drilling operations. The result of this work will in no small way help not only the operators but Engineers to manage this concern with dispatch when it arises, thereby minimizing down time and associated costs which affect drilling operations negatively.

Drilling fluid is an essential element to the drilling process as most drilling challenges can be traced to the condition of the drilling mud used. Density and rheology are among the basic properties of drilling fluid usually defined by a well program and closely monitored during drilling operation. The sustainability of the density of drilling fluid is a key factor that needs deliberate monitoring to ensure that its density is higher than the formation density, so as to avert unexpected complications. Hence this research was aimed at investigating the effect of barite concentration on oil base drilling fluid density and rheology. Formulations and testing procedures were conducted in accordance with American Petroleum Institute (API) specifications. Experimental results showed that the increase of drilling mud is a direct function of increase in concentration of barite. It was also observed that when the barite concentration was 0%, the plastic viscosity decreased as the temperature increased meaning that the drilling mud sagged whereas an increase in barite concentration to 10% revealed an opposite relationship, the plastic viscosity increased as the temperature increased. The yield point at 0% barite concentration gave the least yield point; this signifies that the barite concentration aids in the carrying capacity of the drilling mud. It was also observed that the barite concentration can be used to make the fluid more pseudo-plastic in nature. The consistency index (K) increases as the barite concentration is increased, which indicated that the fluid becomes more viscous when barite concentration is increased. Therefore, results have showed that concentration of barite affect the density of drilling mud as well as rheological properties.

Shales are considered to be the most challenging formation to drill. Over 90% of the formations drilled worldwide are classified as shale formations and about 75% of drilling operation problems are related to shales. There are many factors affecting shale stability, which can be classified into three. The first factors are in relation to geological mechanics, the second type is the chemical factors of the mud which is the main focus of the work and the third type is the engineering factors, which includes the time when the wellbore is opened, the parameters of the depth of the well, inclination and azimuth angles, pressure excitation and drawing. All these factors and parameters governing the shale stability react upon and affect each other, making the problem of wellbore instability extremely complicated. The osmotic effect which is the principal driving mechanism causes water to enter either in the shale or drilling fluid depending on the strength each other. The shale acts like a membrane, but a very poor membrane, so water with improper salt concentration could increase the rate of imbibitions of water into the shale or the reverse may be the case. Therefore, there is need to strike a balance in the design of water activities of the mud and shale in order to achieve wellbore stability. The focus of this research therefore is to design balanced activity drilling mud with the shales from Niger Delta that will prevent wellbore instability. Experimental results for four shale samples collected from the Niger Delta was able to establish the balance activities points for; shale A is 0.86, Shale B is 0.73, shale C is 0.72and shale D is 0.79 respectively. It is recommended that the activities of the shale and oil based mud must strike a balance in order to achieve wellbore stability chemically.