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.

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