Risky but rewarding approach calls for great attention to process and contingencies
- This schematic shows a temporary 9-5/8-in. casing string run to create a microannulus through which gas is injected into the return drilling fluid stream
- The surface setup for a parasite injection string is shown. The injection pipe is installed as coiled tubing and run separately or with the intermediate string.
- Graph A.
Drilling wells with a column of drilling fluid in the annulus that is lighter than surrounding formation pressures is undertaken offshore where downhole pressures are low, fields are mature, or the geology is well known.
Although not commonly practiced, drilling with a negative balance or under-balanced hydrostatic head of drilling fluid generally results in better well flow performance after completion. Low-pressure formations, already slow to release fluids and gases, are likely to be damaged by drilling fluids when borehole pressures exceed formation pressures.
Under-balanced drilling is related to gas or compressible fluid (mist and foam) drilling in the two make use of a similar hydrostatic lightening agent - a gas. Air, or preferably natural gas or nitrogen, can be used in place of a drilling fluid in dry well conditions. The gases are injected at adequate velocity to cool the bit and lift the cuttings to the surface, a process made difficult in horizontal wells. Generally, air or compressible fluid drilling is conducted in under-balance conditions to allow continuous sampling of formation fluids and gases, but the method fails when reservoir fluid influx creates heavy slugs in the air system or swelling formations pinch the drillstring.
When drilling fluid is used for well control, gases or air can be injected at the surface or downhole with special equipment. Downhole injection reduces the weight of the fluid column in the return annulus, but not in the drill pipe. Weight reduction in the drill pipe would create compressibility, which would not be desirable if well control is lost. The drilling fluid in the annulus is lightened to the point where it is below reservoir pressure. Contained fluids and gases will then flow into the wellbore.
The process is practiced offshore largely as a transient condition when there is a need to sample reservoir fluids, when fluid sensitive geological formations are encountered downhole, and while working over mature fields with low formation pressure. However, the process will get a lot more attention soon because of one factor alone - the large number of horizontal wells drilled.
In order to produce successfully, horizontal boreholes must expose a great deal of reservoir face. Severe damage in the reservoir can result from drilling fluids invasion, and the invaded fluids cannot be removed easily or inexpensively.
At the present time, most drillers try to achieve an exact drilling fluid balance when drilling into the reservoir horizontally in order to minimize damage. Balanced drilling already entails some risk in exploratory drilling and in some extended reach wells because little is known about pressure magnitudes or formation stability.
The classical drilling error in creating loss of control or blowout conditions downhole is drilling with insufficiently weighted drilling fluid. A sudden under-balance condition leads to a borehole invasion by reservoir fluids and gases. The drilling fluid, already under-weighted, is lightened further by the invading fluids and gases.
At the same time, allowing the system to become greatly over-balanced also creates control and cost problems. Porous or collapsing formations downhole caused by high pressures in the wellbore can drain volumes of drilling fluid from the annulus.
The ideal is a continually balanced mud system, with errors on the side of over-balance. Few drillers are willing to risk under-balance in the borehole fluids system, for reasons other than safety.
In the past, the drillers' major responsibility has been to turn over to the operator a reasonably straight borehole on time and under budget. The condition of the reservoir near the borehole was the completion or production engineer's problem.
Today, there is less distinction between drilling, completion, and production. The drilling engineer now has responsibility for the quality and condition of the producing formation surrounding the borehole. Operators often index the driller's pay and bonus to meeting prescribed borehole quality standards, the presence or absence of reservoir damage, or to achieving expected production volumes.
There is a second reality in fluids over-balance.
In the past, most operating companies were unaware of the extent of the damage wrought by drilling fluids invasion. Acidizing and fracturing were thought to remedy most problems with fluids invasion of the reservoir or under-performing wells.
It wasn't until the advent of horizontal drilling and focused research on the permanent impact of drilling fluids on formations that operators began to realize how severe a penalty they had been paying. Major reservoir exposure meant major reservoir damage.
Under-balance benefitsIn addition to minimizing formation damage by drilling fluids, there are other advantages to under-balanced drilling:
- Acidizing, fracturing: Since formation penetration is minimized, there is little need or benefit from acidizing and fracturing to open up the formation face.
- Bit penetration: At lower hydrostatic pressure, drilling fluid sweeps the drill bit face faster and bit cuttings are lifted away quicker, all resulting in faster rate of penetration. In turn, bit life is prolonged.
- Lost circulation: Since drilling fluids are drawn into the wellbore, porous formations and caverns no longer present a threat in terms of losing volumes of expensive drilling fluid.
- Formation swelling: With relatively little penetration by drilling fluids, formations swell very little, reducing the instances of doglegs, stuck pipe, and wellbore contortions.
- Stuck pipe: Over-balance in drilling fluids forces fluid into the formation, a process that pushes and sticks the drill pipe against or into the wellbore face. Under-balanced fluids minimize this situation.
DisadvantagesIn addition to the constant threat from loss of well control as a result of wellbore flow from the reservoir, there are other disadvantages with an under-balanced drilling fluids system:
- No mud pulsing: Because drilling fluids are more compressible with under-balanced systems and the hydrostatic head much lighter, mud pulsing in the wellbore in either direction functions poorly if at all. Conventional measurement-while-drilling, logging-while-drilling, and any other type of equipment that communicates by mud pulsing cannot be used.
This inability to communicate by pulsing is spurring on research on electromagnetic or radio systems. A type of electromagnetic systems now operates reasonably well in air or gas drilling, but the depth of transmission is limited to 6,000 ft. Tests have been conducted on one type of system that has successfully transmitted and received over 10,000 ft depths, and researchers believe they can extend that depth with signal repeaters installed in various points in the drill string.
- Need rotating BOP: There is a deficit in surface equipment for controlling annular fluid and gas surges that are common in under-balanced drilling. On wells with under-balanced fluid systems, a rotating blowout preventer or a fixed annular preventer is needed to strip pipe into or out of the well. Tripping, making or breaking of bottom hole assembly connections, and bit changes must all be done with the well shut in or flowing.
There are physical restraints on the size of a rotating or annular BOP preventer. At present, rotating equipment will restrain flows up to 2,000 psi, and developers will soon introduce a unit for 5,000 psi, but equipment with higher capabilities will be needed for deeper wells.
- Closed loop system: Most of the wells drilled with under-balanced fluid systems have been drilled onshore, where a substantial diverter system and a large mud pit are available to handle wellbore surges. Such a luxury is not available offshore.
A closed loop system with appropriate bleed valves and containment vessels is necessary to handle heavy annular flows and to separate gas, water, and cuttings quickly. With a rotating blowout preventer in place, annular pressures can be regulated and bled off when necessary using the diverter system. A closed loop system is also necessary in cases where hydrogen sulfide is encountered, and hydrogen sulfide can become a problem quickly with an under-balanced system.
- Formation washout: A balanced or over-balanced hydrostatic head also plays a role in maintaining formation integrity. During under-balanced conditions where the formation begins to flow, the sections nearest the borehole can break down and wash out. Such an event can leave large cavernous spaces that consume quantities of drilling fluid or cement and provide no lateral support for drill pipe or casing strings.
MethodsAn under-balance drilling fluid system can be created through a variety of methods: The addition of water or oil (dead crude) with drilling mud will reduce the hydrostatic head.
- Restrict materials that weight up the fluid or use lighter materials.
- Inject nitrogen (inert gas) or natural gas (no oxygen) to displace drilling fluid. Air can be used also, but in the presence of hydrocarbons, explosive conditions are created.
- Replace drilling fluid with mist or foam systems, which can be just as effective in lifting cuttings.
Mist systems can be used when water might be encountered downhole, and incorporated surfacants, corrosion inhibitors, and other additives take care of other problems. A high annular velocity is required to lift cuttings to the surface. Surfactants prevent drilling dust from sticking together in the presence of formation water. The same additives are used in foam systems, which are more effective than mist systems and do not require as high an annular flow velocity.
The use of water and crude oil as a drilling fluid or a supplement to other fluids provides mixed benefits. Water is inexpensive and can be easily handled, treated, and injected into the well, but it can damage a water sensitive formation downhole. Crude oil is ideal for water-sensitive formations, but it is readily soaked up by cuttings, which must be processed before disposal.
In situations where the addition of water or dead crude offers problems downhole and all heavy additives are removed from the drilling fluid, the next step to lighten the hydrostatic head is to inject a gas of some kind. The most common gas is nitrogen. The gas can be injected by three methods:
- Pump down: The drilling fluid and gas can be mixed at the surface and pumped down the drill string. However, unless a small amount of gas is involved, gas is rarely injected into the drilling fluid at the surface. It creates compressibility in the drill pipe flow and can produce control problems.
- Parasite string: Between the surface and intermediate casing, a small diameter pipe can be run on coiled tubing, providing a gas injection point at a selected point downhole. Typically, on high-angle, horizontal, or extended reach wells, the parasite string is run to the kickoff point where gas is injected into the return flow.
- Microannulus injection: A temporary inner casing string can be installed and a microannulus created to transport gas downhole. Typically, the inner string is run while the outer casing shoe is undrilled, even though it restricts the size of the drill bit. Centralizers stabilize the bottom of the temporary string.
While the last method is considered the most effective, it requires careful attention to the flow regime downhole. The bottom of the inner string can be packed off with drill cuttings if the annulus backflows (U-tube). This problem can be prevented with a parasite string, although this solution can produced problems in other areas.
The benefits of under-balanced drilling, particularly with horizontal or extended-reach wells, are so strong that operators are going to continue to develop methods and procedures to make the process less risky.
At the present, one of the greatest tools to reduce the well control risks associated with under-balanced drilling - real-time reservoir and drill bit monitoring - is not of much use because it involves mud pulse communications. The successful evolution of electro-magnetic or radio communications between the surface and bottom-hole assembly will provide the technology span needed to spur on the use of under-balanced drilling.
In addition to electromagnetic communications, more work is needed on closed-loop drilling fluid systems. The equipment to contain, divert, or handle a sudden influx of return fluids and gases is still not perfected. This is certainly the case for offshore platforms and mobile rigs, where systems tend to be minimized in order to meet weight and space requirements.
Almost certainly, drilling crews will require further training in under-balanced drilling methods and well control. Opinion is mixed as to whether the number of well control loss incidents would actually go up as under-balanced drilling becomes more popular. Better wellbore surge handling equipment, downhole communications, and better trained crews could actually lower the rate below the record low rates being experienced at the present.
Yee, S., Comeaux, B., Smith, R., "Recent Advances in Underbalanced Horizontal Drilling" (Sperry-Sun).
Shale, L. "Underbalanced drilling with air offers many pluses," Oil & Gas Journal, June 26, 1995.
Maurer Engineering, "Project to Develop and Evaluate Air/Mist/Foam and Underbalanced Drilling Technology," DEA-101, proposed for Drilling Engineering Association (US and Europe).
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