EFFECT OF POROSITY

Thus, 10 bbl of formation solids arrive at the surface. However, consider drilling 10 bbl of hole. Ground rock occupies the same volume as the rock before it is drilled. If a drilled formation has a 10% pore volume, or porosity, the rock added is 9 bbl and the volume of fluid in the pore space is 1 bbl. If the fluid in the pore space is liquid, no volume change will occur when the formation is ground into small pieces and enters the drilling fluid. The pit levels remain constant except for the volume of the drill string added to drill the hole. If the fluid space is filled with gas the pit levels decrease as the gas is liberated at the surface. The pit levels decrease only by the volume removed from the system by the solids control equipment.

The 9 bbl of rock remaining in the system increase the drilled-solids concentration. Most drilling-fluid systems require maintaining drilledsolids concentration at some predetermined value.

How much fluid would be required to dilute these 9 bbl of rock to a concentration of 4%vol? Answer:

. The requirement will be that the 9 bbl of rock will be 4% of the volume of new drilling fluid built, or 9 bbl=0.04 (new drilling fluid built).
. The volume of new drilling fluid built would be 225 bbl.
. The 225 bbl of new drilling fluid would contain 9 bbl of drilled solids and some volume of clean drilling fluid, that is, clean drilling fluidþ 9 bbl¼new drilling fluid built=225 bbl.
. So, the clean drilling fluid volume would be 216 bbl. The volume of liquid added would depend on the other ingredients in the drilling fluid, such as barite, deflocculants, filtration control additives, other chemicals, and low-shear-rate viscosifiers.

Solids-control processes are designed to remove drilled solids so that such large quantities of additional fluid will not be required to keep drilled solids at their prescribed values with dilution only. If the 216 bbl of clean drilling fluid cost $50 / bbl, the drilling fluid would cost $10,800 plus eventual disposal costs. For comparison, 100 feet of 97 8-inch hole would be about 10 bbl, and few drilling budgets can afford to tolerate over $10,000 for drilling fluid for 100 feet of hole.

decanting-centrifuge
Picture 1. Centrifuge cutaway.

The clean drilling fluid added would also increase the system volume by 216 bbl. This volume would need to be sent to a reserve pit or discarded.Normally, drilling fluid is processed through equipment to remove drilled solids when the drilling fluid reaches the surface.

solids control equipment

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