Air drilling is a technique used in areas where formations are “dry”, i.e. there is no influx of water or liquid hydrocarbons. High volumes of low-pressure air is used in place of conventional drilling fluids to circulate the well bore clean of cuttings.
The amount of air required depends upon the hole size, presence of water and to a lesser degree the hole depth. This method is also called “Dusting” due to the finely ground rock cuttings that exit the return flow line, also called the “Blooie Line”.
The main advantages of Air Drilling are as follows:
- Increased penetration rates over other air assisted systems and drilling fluids, in many cases 2 – 5 times higher. This may improve even more by using air hammers.
- Elimination of mud losses into low-pressured formations.
- Continuous well test when drilling into gas zones; a real time indication of contact with the pay zone.
- Formation damage is minimized since to the lack of hydrostatic pressure prevents rock cuttings from being injected into the rock matrix.
- Extended bit life since the compressed air cools the bit and quickly transports the cuttings away from the drill bit’s cutting structure.
The disadvantages of air drilling can be its inability to handle formation fluids or to contain sloughing shales, thus reducing its application to consolidated “hard rock” country applications where there is little to no fluid influx. Also very large numbers of compressors would be required to drill larger hole sizes, which may be drilled more effectively using foam or aerated fluid.
Mist drilling is used when the rock formations begin to produce small amounts of water (10 to 100 bbls per hour) during air drilling operations. Air volumes are increased and a Mist Pump is used to inject small quantities of a solution of water and foaming agent. This solution entraps the water influx and enables the air phase to lift the cuttings and influx to surface. Without the addition of surfactant, wet cuttings may coalesce and form a “Mud Ring” in the annulus, usually just above the drill collars where velocities are reduced, resulting in lost circulation, stuck pipe, and/or downhole combustion.
The same advantages as for Air drilling also apply to Mist Drilling, though the increased air volume requirement for mist drilling will involve the 30-40% additional compression equipment.
Foam drilling is especially suitable for drilling large holes in formations that are prone to lost circulation. Foam is generated at the surface by mixing the air from the compressors with a foaming solution from the mist pump.
The consistency of this “foam fluid” is much like shaving cream with an interlocking bubble structure of encapsulated air providing a lifting capacity superior to that to any drilling fluid.
The adjustment of the Foam Quality (gas/fluid ratio) in conjunction with a back pressure valve allows the operator to produce a bottom hole pressure equivalent to a circulating fluid weight in the range 0.2 to 0.8 s.g.
AERATED FLUIDS DRILLING
Aerated fluids drilling is a technique whereby gas is injected into a column of unweighted drilling fluid (water-, oil-, or synthetic based) to reduce its specific gravity (s.g.), to as low as 0.6.
If there is a chance that hydrocarbons (oil, gas) may enter the wellbore then the gas used is usually membrane nitrogen or possibly natural gas from a nearby pipeline or producing wells. If no hydrocarbons are to be expected, such as on geothermal wells, then air can be used.
For aerated fluids drilling, the gas can be injected in a number of ways, namely:
UNDERBALANCED & MANAGED PRESSURE DRILLING
WHAT IS MPD & UBD?
In conventional drilling the hydrostatic pressure caused by the drilling fluid provides the primary well control mechanism. The imposed wellbore pressure is designed to fall between the pore and fracture pressures and arises from two different mechanisms:
- The hydrostatic (passive) pressure in the wellbore due to the density of the drilling fluid used, as well as the density contribution of any drilled cuttings.
- The dynamic pressure from fluid movement up the annulus due to the circulating friction of the fluid used and the relative fluid motion caused by the surge/swab actions of the drillpipe.
Sometimes it is impractical or even impossible to “fit” the mud weight pressure gradient between the pore and fracture gradients. This is especially the case in wells drilled in deep water, extended reach or drilled in HPHT or depleted fields.
BHP = SP + HH + AFP
BHP = Bottom Hole Pressure
SP = Surface Pressure
HH = Hyrdostatic Pressure, due to mud weight
AFP = Annular Friction Pressure, due to circulation
ECD = HH + AFP
STRAUME MODELING & SIMULATION TOOL FOR COMPLEX MULTIPHASE FLOW
The Straume® hydraulic simulator is a highly configurable high-end multiphase well simulator with full pressure and flow dynamics, rigorous fluid definitions – including multi-fluids, capable of simulating most MPD/UBD operations and contingency scenarios. The simulator is suitable for testing of an MPD control system and training of MPD/UBD personnel in a controlled environment.
MPD & UBD TRAINING
NEXT GENERATION AUTOMATED MPD/UBD TECHNOLOGY
The Leidar® system is the only genuinely automatic MPD control system in use today that can robustly control pressure in both single-phase and two-phase (gas-liquid) systems. It does this by using an adaptive model-based pressure control system that introduces unparalleled accuracy and level of control of the MPD chokes. The Leidar system is able to achieve:
- Uniform choke performance over the full operating range
- No overshoot in choke response
- No need for re-tuning of choke parameters
The significance of being finally able to robustly control the choke response in two-phase (gas-liquid) systems should not be underestimated. Conventional proportional-integral (PI) choke control systems are only “automatic” under very narrow operating conditions. In practice, such “automatic” PI-controlled systems require the operator to manually tune the PI variables each time an unexpectedly high gas/liquid flow and/or pressure comes across the choke. Only very few field operators are experienced enough not to overcompensate the choke response, as this could lead to an uncontrolled flow (not enough choke pressure) or a fractured formation (excessive choke pressure).
It is clear therefore that the genuinely automatic Leidar MPD control system provides tremendous scope for improvements in safety and cost reductions through automation and personnel reduction.
ADA extensively tested and verified the Leidar MPD control system over a period of six months prior to sending it out on projects for clients.
CONTINUOUS R&D FLOWLOOP FACILITY
THE LEIDAR MPD CONTROL SYSTEM CONSISTS OF THE FOLLOWING THREE MODULES
1. A high performance choke pressure control module that uses remotely operated electric actuators coupled with positive shut-off chokes that have a very high erosion tolerance.
2. The Straume real-time hydraulic model of the well that is automatically updated as drilling progresses.
3. An Influx-loss detection (ILD) module (or early kick detection, EKD) that can also be run independently on conventional (non-MPD) drilling jobs. The ILD system is a model-based algorithm that provides robust measurements of the dynamic mass balance of the closed-loop circulation system. This provides higher certainty of detection, as well as unique capabilities to detect during transients.
- Detection in flow transients, such as ramp-down in connections.
- Detection in case of surge & swab effects, i.e. enables detection of kicks while tripping out, and losses while tripping in.
- Detection during pressure set-point changes in MPD operations.
The system includes automatic calibration of important model parameters and sensors such as measurement offsets, fluid compressibility (bulk modulus) estimation, rig pump (efficiency) characterization and calibrating ultrasonic clamp-on meters. This removes the need for expert calibration of the system, making it easier to operate.
ADA can also design and install MPD control and/or early kick detection systems for use on existing and new-build drilling rigs. Please contact an ADA representative.
UNDERBALANCED DRILLING (UBD) EQUIPMENT
On an underbalanced drilling project the engineer is first faced with deciding what fluid system should be used “upstream” of the rotating control device (RCD). ADA can advise on the proper fluid composition as well as additives such as surfactants, corrosion inhibitors and so forth.
Invariably the drilling fluid may need to be lightened by injecting air, natural gas or on-site generated nitrogen. ADA has one of the largest and youngest fleets of air compression and membrane nitrogen separation equipment. We can deliver virtually any volume of high (95-99%) purity nitrogen at pressures up to 340 bar/5000 psi.
The “downstream” section of the UBD system deals with UBD “Drill Packs” that control and separate the return flow of gas, liquid hydrocarbons, drilling mud and cuttings. The oil and/or gas can be sent to an export pipeline or process facility, storage tanks, or a special burner. The drilling mud can be recirculated and drilled cuttings are sent to the shakers or confined storage.
The UBD choke manifolds can be upgraded with the Kelda Leidar choke control system to enable very precise bottomhole pressure control.
The four-phase UBD separators are designed to be capable of conducting long-term well testing operations.
Remote locations may require an oil burner designed for maximum clean burn capability with minimal fall-out by using compressed air as a propellant.
ADA has designed and built a large fleet of high pressure rotating control devices (RCD’s) for use on onshore as well as offshore jackup and floating rig applications. The RCD’s have been designed and rigorously tested to API specification 16-RCD. The RCD’s are available in sizes from 11“ to over 33“.
ADA designs and manufactures the Strata range of high pressure RCD’s, and also owns a wide range of Elite Drillmaster RCD’s. Some of these come equipped with a remote hydraulic clamp and a top flange for connecting to a riser spool on offshore rigs.
The Marine Diverter Series (MDS) is under development for subsea and below the tension ring applications on floating rigs.
The Switchfloat system comprises of flapper float valves that can be opened definitely by covering the opened flapper with a sliding sleeve. This way the Switchfloat flapper float valves eliminate any possibility of restricting the retrieval of wireline tools such as logging while tripping tools, freepoint tools and pipe severing tools.
The Strata push-pull machine (PPM) can control pipe movement into and out of the well in pipe-heavy situations such as rig floor snubbing in live well applications. It can be installed in most drilling and workover rigs and adapted to fit most rotary table bushing sizes or be supported from the BOP stack. On pipe-light situations the PPM can also be used to significantly improve ROP during the slide intervals of shallow TVD extended reach horizontal wells, increasing hole depth by as much as 1,000 feet over conventional methods.