10 UNDERBALANCED DRILLING
10.2 UNDERBALANCED DRILLING FLUIDS
The drilling fluids used in underbalanced drilling fall into the following classifications:
Gas (that is, air, natural gas, nitrogen, or other gases).
Gas with mist.
Foam with gas.
Aerated water, mud or oil, using one of the gases.
Oil, water, invert or direct emulsion muds (that is, conventional drilling fluids applied to provide hydrostatic pressure less than formation pressure).
10.2.1 Gas and Air Drilling
Since its inception, gas drilling is undertaken to increase the rate of penetration in hard rock formations.
With the introduction of the air hammer, it has become possible to drill a straight hole in hard rock, crooked-hole country using a simple pendulum assembly equipped with a hammer and slow rotation.
10.2.1.1 Advantages
Maximum ROP
Reduced cost to drill lost circulation zones
Reduced drilling fluid costs
Improved well performance
No corrosion (N2) 10.2.1.2 Disadvantages
Water wet formations
Cost. Nitrogen can be expensive if used, especially when drilling large diameter holes.
Possibility of downhole fires, when using air
No wellbore support
10.2.1.3 Equipment
The drillstring used for gas drilling is much the same as for mud drilling. However, the drillpipe must be strong enough to withstand the weight and shocks normally supported and absorbed by drilling mud.
Dried mud inside the drillpipe may loosen and plug an air bit or hammer, and the drillpipe may leak when returned to mud service.
Air drilling bits generally resemble mud drill bits, but have an open orifice to minimize pressure drop through the bit.
Sample catchers are used to gather cuttings samples to aid the mud loggers in evaluating the well.
Cuttings, however, are typically of poor quality because they are very fine (virtually powder) and do not always arrive at the surface evenly distributed.
10.2.1.4 Drilling Operations
Gas and air drilling operations typically fall into three general categories, depending upon the amount of moisture produced from the formations:
Dry gas (nitrogen or methane) is best used for weeping formations that dry very quickly and will not cause mud rings (Table 5).
Gas saturated with moisture from the mist pump will expand below the bit and carry formation moisture to the surface in droplets. This prevents the system from losing energy by absorbing formation water to its saturation point.
A light mist with a heavier than normal concentration of foam will not wet the side of the hole excessively, but will help dry dampness in the hole, thereby avoiding mud rings from forming.
10.2.1.5 Drilling problems
Table 5: Gas and Air Drilling Problems
Problem Description
Mud Rings When the formation is damp from water or oil, cuttings may form a mud that, due to poor hole cleaning, is deposited against the side of the hole. This tends to form rings of mud that can become larger and restrict the airflow, causing an increase in pressure and the possibility of downhole fires and stuck pipe.
Mud rings can be removed by adding detergents to the drilling fluid.
Floating Beds At high drilling rates, or with low gas volumes, cuttings are carried to the top of the collars where the annular area increases and the annular velocity drops to a point where it cannot lift the cuttings. This forms a floating cuttings bed that drops back to bottom as fill on connections when airflow is stopped. Floating beds may also occur opposite a washout where the annulus is larger.
Floating beds can be removed by increasing the flow rate briefly, before making a connection, in order to lift the cuttings. Thus, in terms of cuttings, the mud logger sees nothing while the joint is being drilled, but receives all the cuttings at once when the flow rate is increased, making analysis very difficult.
Problem Description
Fires Drilling into gas or oil bearing zones using air, leads to the possibility of fires, either downhole or at the surface.
The use of nitrogen or methane eliminates this, because there is no oxygen for combustion.
Tight Hole Tight hole problems commonly extend from mud-rings and floating beds. It is important to keep the gas circulating and continue working the drillpipe to minimize such build-ups.
Weeping
Formations Low permeability formations will weep fluid, which in turn leads to bit balling and / or the forming of mud rings. Weeping may stop when adjacent fluids are depleted.
Nitrogen or methane, since they are so dry, are particularly effective at drying a damp or weeping formation.
Key Seats Since gas drilling is typically done in hard-rock dipping formations, key seating, while not common, does occur.
10.2.2 Mist Drilling
A mist is formed by suspending fluid droplets within an air, or gas, stream.
The particular fluid, whether water, mud, or even oil, depends on local lithologies and conditions.
For example, water droplets may lead to reaction, swelling and destabilizing of shale, but the use of mud or water with polymers may prevent this. However, misting slows the penetration rate and requires more air volume and, sometimes, more injection pressure. The use of injection pumps and misting agents add to the cost of mist drilling.
10.2.2.1 Advantages
Can drill in wet formations
Represses downhole fires 10.2.2.2 Disadvantages
Lower drilling rate than air / gas
Requires more air volume and injection pressure
Dampness may allow corrosion
No wellbore support
10.2.3 Foam Drilling
Whereas mist has liquid droplets suspended in a continuous gas phase, foam is a two-phase fluid with gas bubbles suspended in a liquid phase.
Foam is typically used because it is not affected by formation fluid influxes to the extent of air or gas, and because it has extremely efficient cuttings-lift and hole-cleaning characteristics.
Foam quality is a term used to describe the proportion of gas to liquid. For example, a foam quality of 0.80 would contain 80% gas. (Above 0.97, that is, 97% gas, the fluid would be termed a mist.)
The liquid phase of a foam-liquid contains a surfactant, or soap foaming agent, that helps to bind the fluid together and prevent the gas phase from separating from the fluid system.
10.2.3.1 Advantages
Fluid lifting capacity; it is able to lift and remove large influxes of formation liquids (for example, water).
Excellent cuttings sample / lift / removal (compared to air / mist) due to its viscosity, requiring lower velocity.
Needs less gas than air / gas / mist.
10.2.3.2 Disadvantages
Wets the formation, although minimized with additives.
Corrosion if mixing with air.
Disposal concerns (requires more surface equipment).
High cost because the foam is not reusable and must be constantly generated.
10.2.4 Aerated Mud Drilling
The term aerated fluid is given to a two-phase fluid with a foam quality of less than 0.55 (that is, 55% gas).
Aerated mud was developed to reduce lost circulation when using conventional muds, by reducing hydrostatic pressure. The single most critical problem with aerated mud however, is pressure surges.
Aerated mud is therefore best suited for drilling hard rock formations that will not immediately cave in reaction to pressure and velocity changes.
Any conventional drilling fluid, whether water, brine, oil or mud, can be aerated with gas, be it air, nitrogen or methane. An aerated fluid maintains the benefits of the original fluid, such as viscosity, hole cleaning, filter cake, inhibition, and so on, while reducing the potential for lost circulation.
10.2.4.1 Advantages
Mud Properties (for example, density, filter cake, inhibited muds)
Pressure control
Reduced risk of lost circulation 10.2.4.2 Disadvantages
Pressure surges
Corrosion (with certain drilling fluids)
Additional cost of equipment and gas generation
10.2.5 Mud Drilling
Any conventional drilling fluid can be used in underbalanced drilling, provided it is capable of handling the formation fluids without destroying its own properties or creating uncontrollable situations on the surface or other unacceptable contamination.
Conventional drilling fluids, when the hydrostatic pressure is less than the formation pressure, will result in underbalanced drilling with the principal benefits of improved penetration rates, reduced formation damage (since mud will not invade the formation) and minimal risk of lost circulation.
The term flow drilling is used when underbalanced drilling, and formation properties such as permeability, lead to continual influxes (that is, the well is flowing as it is being drilled).
Advantages and disadvantages really depend upon the type of drilling fluid being used, but generally:
10.2.5.1 Advantages
Increased rate of penetration
Less formation damage
Better productivity
Reduced lost circulation
Real-time testing of zone productivity 10.2.5.2 Disadvantages
Surface handling.
Salt systems are extremely corrosive, but can be used in higher pressures than fresh water.
Water-base mud systems are subject to solids control and separation of oil, but inexpensive and easily modified.