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

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Gel Strength and Operational Impact

The gel strength is the shear stress of drilling mud that is measured at a low shear rate after the drilling mud has been static for a certain period of time. The gel strength is one of the most important drilling fluid properties because it demonstrates the ability of the drilling mud to suspend drill solid and weighting material when circulation is ceased.


How can gel strength is measured?

Gel strength measurement is made on viscometer using the 3-rpm reading, which will be recorded after stirring the drilling fluid at 600 rpm to break gel. The first reading is noted after the mud is in a static condition for 10 seconds. The second reading and the third reading will be 10 minutes and 30 minutes, respectively. Continue reading

Yield Point (YP) of Drilling Fluids

Yield Point (YP) is resistance of initial flow of fluid or the stress required in order to move the fluid. It can be simply stated that the Yield Point (YP) is the attractive force among colloidal particles in drilling fluid. As per Bingham plastic model, YP is the shear stress extrapolated to a shear rate of zero.

Figure 1 - YP is a shear stress at zero shear rate.

Figure 1 – YP is a shear stress at zero shear rate. Continue reading

Functions of Drilling Fluid

You may not know that drilling fluid or mud has several important functions helping us achieve goal to drill well. I would like to share about the functions of drilling fluid as follows;

1. Transport cutting and dispose to surface The drilling fluid brings the drilled material to the ground surface either by mud rheology and velocity.

2. Clean drill bitsAs drilling fluid exits the bit jets, fluid velocity removes cutting from the bit teeth and bit body. This prevents bit ball up situation.

3. Provide hydrostatic pressure to control well while drillingHydrostatic pressure provided from drilling fluid is the primary well control. Mud weight should be high enough to control formation pressure while drilling.

4. Prevent excessive mud loss While drilling, clay particle will form a thin layer over porous zones called “mud cake” or “filter cake”. Mud cake acts as barrier to prevent excessive drilling fluid loss into formation and provides wellbore stability.

5. Prevent formation damage by using reservoir drill-in fluidWhile drilling long reach zone in horizontal wells, the special drilling fluid will be utilized in order to prevent formation damage.

6. Provide hydraulic pressure to downhole assembly (BHA) as mud motor, measuring while drilling (MWD), logging while drilling (LWD), etcWithout enough hydraulic power, downhole tool will not be properly operated, hence, drilling fluid plays essential role to provide power to sophisticated downhole tool.

7. Facilitate downhole measurement as open hole logging, MWD, LWD, mud logging, etcMud will assist tool to measure everything downhole.

8. Lubricate drill string and BHA and cool the bit. The drill bit and BHA become hot due to friction during the drilling process. When the drilling fluid passes through the bit and exits the jets/nozzles, some extra heat is removed via mud.

Reference books: Drilling Fluid Books