Properties of drilling fluid you must know

Drilling fluids perform some very important functions during drilling. The efficiency of a drilling fluid to perform these functions depends upon its properties. The following are some important properties of drilling fluids that effect its performance.

Viscosity
It is the resistance to flow. Molasses has a higher viscosity than water. Viscosity is measured by the use of a Marsh funnel. The device measures the time required for a unit volume of fluid (one quart) to drain through the funnel. Fresh water at a temperature of 70° has a flow time of 26 seconds through the Marsh funnel.

Density
It is mass per unit volume. Drilling fluid densities are measured in pounds per gallon (ppg). The density of water is approximately 8.3 ppg.

Fluid Loss Control
Water loss and wall building (filter cake) tests are performed to API standards by measuring the amount of liquid forced from the mud, though a filter paper to a set pressure and time (normally 100 psi at 30 minutes). The filtrate or water passing through the filter paper and the thickness of the filter cake is measured. Please note that the filter cake does not structurally prevent the borehole wall from collapsing. The filter cake only minimizes the amount of drilling fluid that penetrates into the formation.

Sand Content
Sand content is measured as a percent of total fluid volume of particles retained on a 200-mesh sieve.

Gel Strength
It is a measure of a fluid’s ability to hold particles in suspension. Gel strength is measured on a concentric cylinder viscometer.

Field personnel on a periodic basis, normally measure density, sand content, and fluid loss, during drilling operations. The testing equipment is inexpensive and easy to use with minimal training. A typical range of fluid properties for drilling in unconsolidated formation are as shown below:
DENSITY Less than 9 pounds Per gallon (ppg)
FILTER CAKE Approximately 2/32”
SAND CONTENT Less than 1%
VISCOSITY 32–48 seconds
The above parameters should be modified on a site-specific basis. However, the sand content should remain below 1% in order to maintain the proper mud weight and viscosity.