Underbalance drilling UBD is one of kind of drilling method have a principle is we cirulate mud while drilling, where our mud system pressure and the pumping pressure less than our formation pressure.
Hidrostatic Pressure + Pumping Pressure < Formation Pressure
Commonly Quastion ?
How can its will be happen without blowout happen in the well ….?
The answere is we using tools/equipment in our blow out preventif units the tools name is Rotating Blowout Preventive people always called by RBOP system.
Now will discuss about Underbalance drilling
The use of UBD method usually subnormal pressure on the area being able to minimize and avoid the problem of missing mud (loss circulation) and the occurrence of stuck pipe (pipe differential sticking). Also intended to avoid formation damage, and drilling can take place effectively and efficiently (increase rate of penetration , enhance formation evaluation results and the reduction of the use of mud fee.)
Identify the appropriate zone with the use of Underbalance drilling method
In general, a drilling operation has hope for can be done effectively and efficiently and gain the optimum results. In reality, not all drilling methods were suited to local conditions where the drilling is done. So the identification of the area drilling operations are to be considered in the implementation of drilling operations to obtain optimum results. Therefore, in the underbalanced drilling, the application of this method will potentially towards optimum results when performed in areas such as:
- Depleted Reservoir (subnormal pressure)
- Reservoir fracture
- Formation which consists of hard rock
- Formation with large permeability
- Formation damage
Depleted reservoir (area subnormal pressure), where the pressure gradient is lower than the hydrostatic pressure of drilling fluid column. If you use conventional methods, the area is potentially the occurrence of the loss circulation “phenomenon of mud into the reservoir (missing mud)” and stuck pipe. Missing this mud hole occurs if the pore size is larger than the particle size of the drilling mud. Pore ??size of the hole which resulted in lost circulation is in the range of 0.1 to 1.00 mm. It usually occurs in areas that have a layer with very large permeability, fracturing, such as sandstone and unconsolidated sand.
Reservoir with natural fractures usually show loss of drilling fluid is very large. Loss of this fluid will create problems such as well drilling control or provide the mechanical sticking, because the hydrostatic pressure fluid pemborannya greater than formation pressure. While in underbalanced drilling operations designed pressure is smaller than the formation pressure.
One of the factors that can increase the rate of penetration is the density chisels on rock drilling fluid. Laboratory and field studies to obtain the conclusion that the light density of the drilling fluid is used, the rate of penetration or chisel will be more rapid, because with the small differences in pressure or differential pressure, ie (ph-pf) will be smaller even in UBD, the pressure difference will be valuable negative. Penetration rate is also influenced by rock strength (compressive strength) are penetrated, by lowering the pressure difference is, the rock strength was going down and chisel drill can easily penetrate the layers of rock. Examples for this formation is solid Limestone (limestone) and the type of rock that big sementasinya factor (consolidated sand).
One of the causes of stuck pipe is the mud cake, which occurs if the difference (difference) between the hydrostatic pressure of drilling mud and pressure g
formation becomes very large at the time through a porous and permeable formations, such as sandstone (sandstone) and limestone (limestone).
Formation of potential damage (formation damage), if drilled overbalanced drilling method. One cause of formation damage (formation damage) is due to the use of mud that is too heavy so that the particles of solids sludge (innert solids) will enter into the productive formation. Solid particles and drilling mud filtrate into the formation will cause a few things, namely:
- Closes the pores of the productive formation
- Increasing the water content in oil-containing formation so that the oil saturation decreases and eventually occupied by water.
- Clay particles on the productive formation to expand and close the permeability of the formation.
- With the formation damage is of course will increase the cost of a well stimulation and result in disruption of productivity formation.
- All types of rock have a chance to be the place of the missing mud, but the formation of weak and caves is the most frequent. In soft formations such as sandstone, missing mud in principle caused by high permeability and the possibility of fracture. In hard rock, like limestone, dolomite and shale are hard, missing mud occurs as a result of the vugs, caverns, natural and induced fractures .
Wiil be present in next article
- All about realation underbalance drilling with our formation
- How To design Mud with underbalance drilling method