API and HTHP Fluid Loss

Fluid loss is one of important drilling fluid properties and two type of fluid loss are API and HTHP fluid loss. This article will explain about both of them and effect on the drilling operation.

API Fluid Loss Test

API Fluid Loss Test (low-pressure, low-temperature filtration test) is a test used to measure a filtration of mud with ambient temperature and 100 psi differential pressure. The API fluid loss testing equipment is shown below (Figure 1).

Figure 1 - API Fluid Loss Test Kit

Figure 1 – API Fluid Loss Test Kit

How will the drilling mud be tested?

  • Place a filter
  • Add the sample into the testing chamber
  • Place the chamber in the testing kit
  • Apply 100 psi pressure
  • Record volume for 30 minutes; at the end of the test the volume of filtrate will be recorded.
  • Record thickness of filter cake

If drilling fluid has good fluid loss property, it will show a thin and impermeable mud cake. Please keep in mind that this test is based on the surface condition, and it may be in error because it does not simulate downhole conditions. The API fluid loss test can lead you to the wrong conclusion, because at the surface condition the test demonstrates very good fluid loss and a very thin filter cake.When the drilling mud is in a downhole condition, wellbore temperature and pressure can dramatically change drilling fluid properties.The best way to test the fluid loss is to simulate wellbore condition at high pressure high temperature in order to see what the fluid loss property will be. The procedure is called” HTHP Fluid Loss.”

 HTHP Fluid Loss Test

The HTHP fluid loss test is similar to the API test because it indicates information on drilling mud filtration into the formation under a static condition over a certain period of time.

Figure 2 - HTHP fluid loss test kit

Figure 2 – HTHP fluid loss test kit

For the HTHP fluid test, both temperature and pressure can be varied to represent an expected downhole condition. The HTHP testing equipment has a heating jacket so you can heat up the drilling fluid sample to the expected wellbore temperature. Typically, the recommended temperature in the heating jacket should be above the estimated temperate of about 25 F to 50 F. The test pressure is normally at 500 psi differential pressure.  Normal test conditions are 150 F and 500 psi differential pressure and the maximum allowable test temperature is 300 F with the standard equipment. Mud filter cake thickness must be maintained below 2 mm. The HTHP test is performed for 30 minutes, just like the API fluid lost test.

The video below shows how to do a HTHP fluid loss test. Even though it does not describe anything, it still gives you some ideas on how to do it.

Drilling Operational Impacts of Fluid Loss

 The impacts of fluid loss on a drilling operation are listed below;

Formation damage

If the drilling mud does not have good fluid loss property, fluid with small particles in the drilling mud can be invaded into formations and it will cause formation damage. This will directly affect the production rate of the well once it starts producing.

Differential sticking

The drilling fluid that has bad fluid loss will form a very soft and thick mud cake across permeable formations. This can lead to differential sticking because of an increase in the contact area between formation and drill string.

Torque and drag

A thick mud cake across porous zones can be easily formed because the drilling mud has high fluid loss values. The thicker the  mud cake is, the more torque and drag are experienced while performing the drilling and tripping operations.


Andy Philips, 2012. So You Want to be a Mud Engineer: An Introduction to Drilling Fluids Technology. Edition. CreateSpace Independent Publishing Platform.

Ryen Caenn, 2011. Composition and Properties of Drilling and Completion Fluids, Sixth Edition. 6 Edition. Gulf Professional Publishing.

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