A Non-Destructive Method of Cake Thickness Measurement

Abstract

Abstract A novel non-destructive laboratory technique and apparatus have been developed to eliminate inaccuracies associated with the conventional direct contact methods of mudcake thickness measurement. The system consists of a laser head containing a transmitter and a position sensitive receiver, a signal conditioner, a digital read-out system and a DC power source. The position of the laser beam spot incident on the measuring surface is recorded by the receiver, transferred to the control box where it is amplified and then displayed as a standard current signal on a digital read-out system. As the method is non-destructive, the mechanical characteristic of the cake such as softness has no influence on the accuracy of the measurement. The conventional methods of cake thickness measurement provide an estimate only due to damage to the cake during measurement. The accuracy varies depending on the cake consistency. The variation is low for hard mudcakes but high for soft cakes. The newly developed apparatus and measurement technique eliminate the direct contact-related errors and thus can provide reliable input values for modelling mudcake thickness dependent parameters such as formation and cake permeabilities, sticking force, friction coefficient etc. Statistical analyses of the measured values show a standard deviation of less than 0.025 mm and a coefficient of variation of less than 1%. The apparatus is portable, easy to operate and suitable for quick measurement of mudcake thickness at the rig site. The performance of the system was reviewed by analysing the results of a range of muds and also by comparing with the measured values obtained using other methods. Introduction The thickness of deposited mudcakes varies considerably depending on the composition of the muds, depositional environments, nature of interactive forces prevailing during the period of deposition, etc. Hence, mudcake can provide valuable information about mud quality and can help take corrective measures in designing a mud. Formation of a thick mudcake during static filtration indicates the possibility of formation of a thick cake on the borehole wall during the period of non-circulation with a reduction in hole diameter. This will cause an increase in the angle of contact between the cake and drill collar in case of a pipe stuck problem. Flocculated muds usually form a thick mudcake with high a permeability compared to cake formed by a dispersed mud system. Hence, these two cake parameters together can provide an indication of the physical state of the mud solids of a mud system. This dictates the measurement of cake thickness as accurate as possible for proper assessment of different mud systems. When permeable formations are drilled with water-based muds, the infiltration of mud filtrate into the formation causes the deposition of a mudcake on the borehole wall. The deposited mudcake properties including its thickness are directly related to different drilling and production related borehole problems. Although cake thickness is associated with different borehole problems, little attention has been paid to its accurate measurement (Darley and Gray1). With increasing cake thickness, the pipe-cake contact area increases which in turn increases the magnitude of sticking force. The casing running load in the openhole section of directional and horizontal wells are significantly influenced by mud cake properties. Maidla2 showed how mud cake deposited on the wall of a borehole could affect casing running loads. The friction response of casing/drill strings motion depends on mud cake thickness and the applied pressure. Muds producing soft and thick cakes increase the potential of differential sticking and thus not desirable for geological formations highly prone to differential sticking. On the other hand, muds producing harder and thinner cakes reduce erosion of the particles from the cakes under the hydrodynamic action of the mud flow and pipe movement. Mechanically competent thin cakes also reduce the degree of drill string embedment, torque and drag. The above facts again highlight the importance of cake thickness and its accurate measurement.