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. Continue reading
Mud weight or mud density is a weight of mud per unit volume. It is one of the most important drilling fluid properties because it controls formation pressure and it also helps wellbore stability. Mud weight is measured and reported in pounds per gallon (PPG), pounds per cubic feet (lb/ft3), or grams per milliliter (b/ml).
Mud weight is normally measured by a conventional mud balance; however, if you have some air inside a fluid phase, reading from the conventional mud balance will give you an inaccurate number. Therefore, the most accurate method to measure the mud weight is with a pressurized mud balance.
Conventional Mud Balance
Pressurized Mud Balance
The pressurized mud balance looks like the convention one, but it has a pressurized sample cup. When you press a mud sample into the cup, any gas in a fluid phase is compressed to a very small volume so the mud weight measurement is more accurate. Continue reading
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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 bits – As 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 drilling – Hydrostatic 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 fluid – While 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), etc – Without 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, etc – Mud 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
