Properties of mud
Plastic Viscosity (PV)
•Control the magnitude of shear stress develops as one layer of fluid slides over another.
•Measure of friction between layers.
• Provides a scale of the fluid thickness.
• Decreases with increasing temperature; with liquids; the reverse with gasses .
•Effective viscosity depends on fluid velocity flow pattern, difficult to measure, but can be calculated.
• PV is the difference between readings at 600 rpm and that at 300 rpm
•A measure of the attractive forces between particles due to positive and negative charges
• Measure the forces causes mud to gel in case of motionless
•Shows a minimum level of stress must be provided before mud flows
•Expressed in lb/lOOft2
•A parameter of the Bingham plastic model. YP is the yield stress extrapolated to a shear rate of zero
•It can be increased by Bentonite .
Gel strength
•Ability of mud to develop gel structure .
•Defines the ability of mud to held solids and measures thixotropy
• Is Determined using viscometer .
• The sample stirred at high speed and the allowed to rest for 10 sec or 10 m.
•The torque readings at 300 rpm is taken as gel strength at specified time.
• Expressed in lb/100 ft2 .
•Converted to metric by multiplying by 0.478 .
Filtration and filter cake
• Filtrate is the liquid that passes through the porous and permeable rock medium, leaving the cake on the medium .
• A layer of solids deposited on the rock is described as filter cake
• Loss occur when mud pressure is higher than formation pressure
•Quantity of mud loss depends on volume of filtrate and thickness and strength of filter cake, and differential pressure .
•Ideal mud gives small filter loss and thin and tough mud cake
•Filtrate is a disadvantage due to formation damage & plugging pores by shale swelling .
•Mud cake is an advantage due to stabilizing the well bore but when increasing it decreases the hole diameter
•Polymers are used in mud cake .
•Drilling muds are tested to determine filtration rate and filter-cake properties.
• Cake properties such as cake thickness, toughness, slickness and permeability are important because the cake that forms on permeable zones in the wellbore can cause stuck pipe and other drilling problems.
•Reduced oil and gas production can result from reservoir
damage when a poor filter cake allows deep filtrate invasion.
• Can be determined by filter press, called API filter press
pH of mud
•Describe the acidity or alkalinity of mud
•Defined as the negative logarithm of the hydrogen ion (H)
•Measured by pH meter or strips or dyes
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