When Will People Need OBM Instead of WBM? (And Vice Versa)

What is Drilling Mud?

Drilling Mud/ Drilling fluid is very important in drilling a well. If you’ve ever drilled a hole through a wood, you must have known that the wood chips should crash & fly outside the hole, or the bit will be buried with the wood chips (That is why people occasionally blow the hole to crash out the buried wood chip). How do people maintain those “chips” get out the well while drilling in the oil field, in the condition that the bottom hole might be kilometers away from the surface? Yes, this is the purpose of the drilling mud itself. We need to put the mud inside the hole, then circulating the mud while drilling to transport those “chips” (which is called ‘cuttings’ in petroleum engineering terminology) out from the hole. 


Simply we can imagine that a drill mud should be a fluid which able to capture those cuttings, and flow them to the surface and it should be easily remove the cutting in the surface with some treatments. The company will hire some experts to know what chemistry and compositions of the mud to make sure this mud do its function. But this function is not the only important thing to be considered in using the drilling mud. It simply explains that the cutting somehow should be brought out from the hole while drilling, just like other drilling activities in your garden, and drilling mud will do this job. 

The condition kilometers inside the earth is very complicated and really different than what we feel on land. Extreme conditions such as pressure, chemistry, and miscalculation of things happened below the land will have a huge impact, such as explosion and environmental damage. Drilling mud play a key role to keep every extreme condition stay in human’s control, and that leads to the question about the functions of drilling mud.

What are the functions of Drilling Mud?

Beside the basic function to transport the cutting to surface, the mud should also maintain the pressure from the depth (which is called “formation pressure”) stay in the depth. The system is similar with infuse in hospital. you need to keep the infuse bottle higher than your hand because the pressure from the infuse will withstand the pressure from your blood. this is called hydrostatic pressure. When the infuse is lower than your hand, then the blood have enough pressure to flow to your infuse. Now imagine that the blood is the pressured fluid from the soil (which is called formation fluid) and the infuse is filled with drilling mud.  When the hydrostatic pressure of the drilling mud is lower, then the blood/formation fluid could flow through the well. Unfortunately, unlike the blood pressure, the formation fluid pressure might kill people if it flows to the surface. That is why the drilling mud should be maintained in appropriate weight. Also, if the drilling mud is too heavy, it may break the formation, then flow into the formation, leaving enough place in the well for the formation fluid to flow in and dangerous to the people in the surface.

Drilling kilometers to the earth is not an easy job, and the drill bit should expose to frictions and high pressure. It will make the drill bit become extremely hot. It is very important to make sure the drill mud may lubricate and cool down the bit while circulating.

And the broadest function that leads to the topic of OBM and WBM is the capability of the drill fluid to minimize damage. There are many possibilities we can find when drilling a well. It may be exposed to saltwater, carbon dioxide, hydrogen sulfide, solids buildup, influxes, temperatures, shale, and a lot of other issue. The issues sometimes are very complicated, and there is no “one for all” solution. The mud should be designed, and then tested to the rock sample which is called core sample (it is obtained by using a special drill bit which will cut a sample smoothly and has a barrel to keep the sample until it is brought back to the surface) , and test the compatibility of the mud with the rock, before we use it in the well. There are two main type of drilling mud, Oil Base Mud and Water Base Mud.

What are the Differences of OBM and WBM?

Oil based mud contains diesel, mineral oil, or low-toxicity linear olefins and paraffins. Although it is called oil based mud, the mud itself will not only contain oil as the main contain, but also big portion of water (mostly, since the oil is expensive and has high viscosity, the emulsion with water will lower the price & viscosity). It is called OBM when the oil phase is the continuous phase with small amount of water as the non-continuous phase. OBM will have oil/water ratio in the range from 65/35 to 95/5, but the common value is about 70/30 to 90/10.

Water based mud is the most commonly used in the world. The main contain may be fresh water, seawater, brine, and saturated brine. The selection will depend on the environment which is anticipated. Also, some additive will be used to withstand different conditions from certain segment of the well. Thus, it is very important to keep attention to every change of the formation being penetrated.

When Do People Use WBM?

Water Based Mud is very popular since it is cheaper, less treatment and much easier to be found than the Oil Based Mud. It is very suitable for people who want fewer headaches of environmental concern, regulation, and disposal scheme. Unfortunately, water based mud is much related to the shale problem, which is called swelling. Clay will be instable when it react with the water, thus the clay will be swelling and making the hole become smaller. Our tools will be stuck inside and it will cost us a lot of money. Using the water based mud in the very reactive clay formation is not a wise option. We may move to the Oil Based Mud.

When Do People Use OBM?

The most obvious answer is if the clay is very reactive, then we use the Oil Based Mud. But selecting the Oil Based Mud is not an easy decision. The initial costs will be much higher than the WBM, thus it is a good only if we may reuse it in some wells. The SP log also won’t be able to be functioned in OBM, and the biggest consideration is the disposal scheme, as it is toxic and can’t directly dispose to land or sea. Another special case which is to be considered is in the high pressure and high temperature well. The OBM is very good to be used in this condition as in high temperature and pressure, the composition & quality of OBM will remain the same (unlike water which will evaporate in the high temperature).

And those are some considerations to choose between OBM and WBM. Sure, there are some efforts to reduce the disadvantages, such as additive for WBM for less swelling problem, or synthetic oil based mud to reduce the toxicity, but for sure, those considerations still remain the key between OBM and WBM.

Do you have additional opinion and information? Please kindly write in the comment section. Thank you very much!

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