A Field Applicable Laser-Based Apparatus for Mudcake Thickness Measurement
Authors: Amanullah Md. , Tan C.P. 
Published: December 2007

Abstract
A laser-based novel measuring apparatus has been developed to determine mudcake thickness with high precision and accuracy. The device is portable, easy to operate and suitable for quick measurement of mudcake thickness at the rig site. 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 apparatus needs no direct contact with the mudcake to measure its thickness and thus eliminates inaccuracies associated with the conventional direct contact methods of measurement. Cake thickness is determined by illuminating a 1 mm spot of the cake surface with a laser beam produced by an aluminium die-cast semi-conductor laser sensor. The peak quantity of the reflected light is detected by a light-receiving element of the laser head. The signal is then transferred to a signal conditioner to produce a current signal proportional to the distance travelled.
The newly developed apparatus is a precise and accurate tool for measurement of different mudcake thickness irrespective of the mechanical characteristics of the mudcakes. The proposed apparatus and the non-contact method of measurement will provide reliable inputs for cake thickness dependant parameters such as cake and formation permeabilities, numerical modelling of sticking forces, casing running loads, appropriate correction to log responses, design and formulation of muds with superior mudcake quality etc.
A correlation chart and equation having a coefficient of correlation greater than 0.999 were developed using a Standard Mitutoyo Gauge Block Set, Series-516. Statistical analyses of the measured thickness values of a solid disk showed a standard deviation of 0.025 mm and a coefficient of variation of less than 1%.
The performance of the newly developed laser apparatus was assessed by analysing the results of cake thickness measurements of a range of muds and compared with the thickness values obtained using a soft-touch digital dial gauge. The degree of accuracy and the precision of the measurement of the laser apparatus are independent of cake consistency i.e. mechanical characteristics of the mudcakes.

Introduction
A mudcake cake is formed on the wall of boreholes due to the deposition of mud particles and formation of a network of structures of colloidal and non-colloidal particles enclosing fluid-filled voids or inter-spaces of varying sizes during the period of circulation and non-circulation. The thickness of the mudcake and the quantity of fluid lost to the formation may exercise considerable influence on the drilling and production of a well and thus highlight their drilling and production engineering significance. The physical properties of mudcake such as porosity, permeability and thickness are some of the parameters used to assess the cake building properties of a mud system. For this reason, the routine measurement of mudcake thickness is an essential part of a mud-check for a qualitative assessment of a mud system.
The thickness of mudcake is also a critical factor in many drilling and reservoir engineering problems. Muds producing soft and thick cakes increase the potential of differential sticking and thus are not desirable for geological formations highly prone to differential sticking (Amanullah and Tan1). Mud related borehole problems such as tight hole, differential sticking, torque and drag usually have their root to the static phase of drilling and hence static mudcake thickness is a valuable piece of information in selecting a mud system. In addition, fluid loss and cake building properties of a mud system play an important role in making a logging programme successful. The decrease in effective hole size with increasing mudcake thickness makes the openhole logging more and more difficult. According to Dewan and Chenevert2, predictions of logging simulators have been hampered by a lack of knowledge of mud cake flow resistance, thickness, permeability and their variation with time.