Coring

PDC core bit

Coring? Hmm,,,it’s kind of special ‘event’ you might find in drilling operation. Lucky me, I’ve met two times with it. So, I think it’s a good time to tell what happen on core drilling. And what should I do as a mud engineer to support the successful of coring itself. Well, first time I met with it in ZZZZ Field in Bekasi, West Java and the latest one is in Maleoraja Field in Central Sulawesi. Nothing special on mud preparation to support it but there’s some recommendation (if applicable). What I need to do is just following the mud program that have prepared before. Mud formulation should be discussed between the mud engineer, drilling engineer and Core Contractor specialist during the well planning stage, so that the mud can be optimised for all requirements whenever possible.

Firstly, I need to tell what coring is. Coring is special drilling that has a purpose to take rock sample in a certain reservoir. The rock is then tested in laboratory. It’ll give valuable information about the reservoir being drilled such as porosity, permeability, oil content, etc. Drilling core holes could be very expensive, depending on the depths of the pay zones. The company must used contractor services to conduct coring. There are 3 contractors worldwide that being main player in it: Baker Hughes INTEQ, Halliburton Security DBS and Corpro. But unfortunately, I was working with local companies for both. Hmm, actually just the same but I though working with an “experienced one” would have given me a different touch a little bit. Haha. Oke, Forget it! Oh please no offense. I’m just saying.

From two times working with core, that can be called “Unsuccessful”. I don’t know the problem was. The geologist just tell me that it could be come from unproper drilling practice (handling, tripping, etc), the parameter given while coring (the GPM, WOB, etc) and the formation. The first coring just get 37 cm from 2 core barrels used (recovery 2.1 %) and the second just get  60 cm ( around 6.3 % recovery). From both, I have seen the core sample which was taken from the mud logging unit, the first is the Marble formation (CMIIW) and the second is the limestone formation. That’s the original rock found the subsurface. It is interesting to such such thing 🙂

core sample

Coring used special tools consist of Core bit (core head), core barrel (a chamber to capture the sample rock), safety sub, and Core Catcher. For some companies there’re some additional tools such as anti Jamming tools, fluids collecting sub, flow diverter, etc.  The drilling practice and parameter take a really big deal in it. That’s why there’s a special engineer to control the coring process, Coring Engineer. He said, the important parameter is WOB (weight on bit) – Simply, it’s a force given to the bit. It’s measured in in Klbs. Mostly coring the WOB value must be kept between 1-5 kLbs, depend on the formation being drilled. FYI, in normal drilling the WOB is sometimes much higher than this can reached 15 kLbs or so. Hahaha. The Directional Drilling must be expert on it.

drilling practice and string handling play an important role for good core

Well, back to coring, another thing that needs to be concerned is Rig Pump. It’s totally must be in a good condition. No exception! So, before the coring is started, rig must ensure that the Rig pump is OK. The Coring engineer also told me for not to mix any polymer into mud as it possibly blinding the pump screen and stop the pump. Oke,,,fine! I won’t mix anything. What I need to do I just keep mud in a good shape and run all solid control to keep my mud away from high solid content. Coring is just like subjecting a woman. We must show our mercy as the the rock we are dealing with is a special rock which will give us tons of dollars. Remember that! haha

core barrel with the core, only 2.6 % recovery

normally, Mud should be conditioned pre-coring such that no modifications to the filtrate component are required during coring (so that mud properties can be estimated at any point during coring by interpolation between mud sample points). Here, another recommendation related to mud being used:

• Minimize differential pressure (overbalance) which will flush formation fluids from core

• Limit the water loss as low as possible to minimize core invasion by the mud filtrate.

• Minimize plugging of pore spaces by solids, especially insoluble or compressible solids.

• Use filtrate containing little/no surfactants and caustic soda that could affect wettability.

• Use solids that quickly bridge pore throats, forming a stable acid soluble filter cake. The particle sizing can be matched with the pore throat diameter, the rule of the thumb is the sizing of the particles should be one third of the pore throat diameter.

• Use acid soluble Polymers for viscosity and filtration control.

• Maximize Solid control Usange: Use shale shakers, desanders and desilters to limit the amount of solids and maintain the best mud characteristics all along the coring operation.

 Choosing which of these recommendations can be carried out and to what extent is a point that must be solved between drilling, subsurface and reservoir engineering. For economical reasons, in most cases the above mentioned requirements must be compatible with associated drilling operations and other acquisition tools (LWD, wireline logging). If possible do not use Barite or Bentonite (compressible solids). When residual saturation measurements are of crucial importance, the use of a specific coring mud may be considered. The multidisciplinary team should confront the evaluation advantages with the significant extra cost involved.

 Coring mud recommendations for unconsolidated formations

• Bridging agent to limit core invasion: to consolidate the core and minimize mud invasion the formation of a mud cake immediately after the core is cut is of primary importance. The addition of fine and medium grain size Calcium Carbonate is efficient. The quantity and grain size of CaCO3 is adjusted depending on expected pore size. A typical value is 50 lbs CaCO3/bbl, with 35 lbs medium size and 15 lbs fine size. Other bridging agents such as cellulose micro-fibers are proposed but they cannot be eliminated by acid. Particles approximately 1/3 of the maximum pore throat diameter should effectively bridge pore throats and help to build an effective filter cake, reducing spurt loss ahead of the core bit.

• Overbalance pressure: 200 to 300 psi of overbalance is a good value to create a protecting CaCO3 cake. Lower overbalance pressures present a high risk of core washing whereas too high pressures are penalizing in terms of mud invasion.

Recources: Coring guidelines from one of one of the Supermajor Oil Co.