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Additives to drilling fluids and oil well cements play a significant role during wellbore drilling operations. These additives (such as carboxymethyl cellulose and starch) are often expensive and most times degrade at high temperature conditions. Although seashells (usually considered waste materials) have been utilized for various purposes in drilling muds as weighting agents and lost circulation materials, however, to the best of our knowledge, there is no research conducted yet to test their feasibility and performance as retarders or accelerators in oil well cements. In addition, no study has compared the filter loss capabilities of these seashells as drilling mud additives. Filling this gap is what makes this work novel and is the major contribution of this work to the extant literature on this subject. The specific objective of this work is to bring to light how useful seashells could be as a drilling mud and oil well cement additive from an experimental standpoint. To this end, the capability of the pulverized and calcined shells of egg, snail, oyster and periwinkle as mud filter loss control agents and as oil well cement additives were evaluated. Thirteen water based mud samples and one oil well cement sample were formulated with the shells as additives. The basic tests run on the formulated mud and cement samples were the filter loss test (using the American Petroleum Institute filter press) and cement setting time tests (using the Vicat’s apparatus) for the mud and cement respectively. The conventional filter loss control material - carboxymethyl cellulose (CMC) was used as the control. Results from the filtration tests showed that the filter loss capability of the shells decreased from egg shell to snail shell to periwinkle shell. With respect to cement, incorporation of calcined oyster and periwinkle seashells resulted in extended final setting times of the cement. In quantitative terms, it was observed that the final setting time of the cement was extended by an average of 14 and 28 ​min as the amount of oyster and periwinkle shell respectively introduced to the cement was increased from 5 to 15%. Conclusions drawn are that both the oyster shell powder and the periwinkle shell powder act as retarders. It is concluded that the filtration property of the seashells had a direct relationship with the amount of the seashell added. Economically, the pulverized seashells have comparable cost performance with the conventional filter loss and cement retarder additives. By utilizing these waste shells in this way, the overall well drilling cost would be reduced and the environment would be protected.