A Brief History
The first wells drilled in the 1800s were drilled underbalance with insufficient pressure in the annulus. This resulted in uncontrolled flow.
UBD advanced through the 1900s as engineers began to understand the use of mist and multiphase fluids. In 1930 multiphase fluids (gas or air with water or oil) became popular in the US. Foam became popular in the 1960s because of its hole cleaning ability compaired to air or mist although by 1970 environmental concerns arose because of the large amount of waste generated in single circulation systems. Although most wells UB drilled before 1985 were to increase ROP, new technology led to a renewed interest in UBD with better rotating heads, recycling foam systems and Electromagnetic MWD tools. Improved technology and the introduction of horizontal drilling boosted UBD in particular as it limited the damage typically found in horizontal wells.
UBD is no longer a niche technology and is being successfully used throughout the world both on and offshore.
Reduced Formation Damage
Reducing the formation damage caused by conventional drilling can dramatically increase productivity from a reservoir ias has been noted in several large projects. Most notable was the Exxon/Mobil ARUN project where production increased between two-to-six times compared to conventionally drilled wells.
Wells damaged by overbalanced drilling often require expensive stimulation programs to fix the associated problems although as Exxon/Mobil have found out, in many cases the original formation productivity is never regained.
Minimized Loss Circulation
Loss circulation can be defined as the loss of mud in to the formation. Losses occur when the hydrostatic pressure of the mud exceeds the fracture gradient of the formation. Due to the nature of conventional drilling fluids, loss circulation is always a risk. However if an underbalanced state is maintained, lost circulation cannot occur. In severely depleted reservoirs with high permeability the ability to remove drilled solids from the wellbore is lost. If pore spaces are not large enough to take the drilled solids, solid buildup and mechanical sticking of the drill string results. For this reason, severely depleted fields or under-pressured reservoirs cannot be drilled with conventional drilling fluids. ......read more
|Gas Drilling||Drilling process using only gas as the drilling medium; no intentional fluid added.|
|Mist Drilling||Drilling with liquid entrained in a continuous gaseous phase; typical mist systems have <2.5% liquid content.|
|Foam Drilling||Drilling with a two-phase fluid and a continuous liquid phase generated from the addition of liquid, surfactant, and gas; typical foams range from 55% to 97.5% gas.|
|Gasified Liquid Drilling||Drilling with a gas entrained in a liquid phase.|
|Liquid Drilling||Drilling with a single liquid phase.|
|Low-head or near-balanced drilling (MPD)||Condition where the hydrostatic head of the wellbore fluid column is reduced to be either in balance or slightly greater than the formation pressure, thus not planning to induce hydrocarbons or formation fluids into the wellbore.|
|Underbalanced Drilling (UBD)||Planned condition where the bottom-hole pressure exerted by the hydrostatic head of the fluid column is less than the formation pressure being drilled.|
Better Formation Identification/Evaluation
Another significant advantage of UBD is that it allows continuous reservoir evaluation and characterization. Not only can fluid types, flow rates, and pressures be identified but also reservoir parameters such as static pressures can also be estimated while drilling underbalanced. Also natural fractures and the resulting flow/pressures may be identified during UBD. Most importantly underbalanced drilling allows formation pore flow into the wellbore and therefore allows detection at the surface that would not be seen with conventional overbalanced drilling.
No Differential Sticking
In conventional drilling operations, all the necessary ingredients are always present, and differential sticking is always a concern. With UBD, there is no hydrostatic pressure differential to the formation and no filter cake. It is impossible to get differentially stuck while drilling underbalanced.
It is well known that heavy muds combined with drilled solids impact penetration rate. Conventional drilling is a process of grinding where re-circulated drill solids that are not removed by the shakers are re-introduced into the wellbore and subjected to re-grinding.
In UBD, there is no pressure on the rock to hold the solids in place and create a filter cake. And because the UBD fluid is free of solids, they cannot be re-introduced into the circulation system for re-grinding. Furthermore, since the formation pressure is greater than the wellbore pressure, less energy is expended in breaking the rock, which can mean extraordinarily high rates of penetration (ncreases of penetration rates by a factor of ten are not uncommon) .
The graph belows shows the ROP correlation developed by Bourgoyne and Young. This correlation shows the relationship of ROP and mud weight as compared to the pore pressure. As shown, the greatest ROP % increase occurs with harder formations.
In general, with the current state of the technology, the key drivers for the selection of UBD have been:
Generally the presence of one or more of the above is sufficient to consider UBD drilling.
Underbalanced drilling is usually more expensive than conventional drilling and has safety issues of its own.
Technically the well is always in a blowout condition unless a heavier fluid is displaced into the well.
Air drilling requires a faster up hole volume as the cuttings will fall faster down the annulus when the compressors are taken off the hole compared to having a higher viscosity fluid in the hole.
Because air is compressible mud pulse telemetry measurement while drilling (MWD) tools which require an incompressible fluid can not work. Common technologies used to eliminate this problem are either electromagnetic MWD tools or wireline MWD tools.
Downhole mechanics are usually more violent also because the volume of fluid going through a downhole motor or downhole hammer is greater than an equivalent fluid when drilling balanced or over balanced because of the need of higher up hole velocities.
Corrosion can also be a problem, but can be largely avoided using a coating oil or rust inhibitors.
Notwithstanding these potential disadvantages, UBD may serve as an additional tool for an operating company to drill and produce from those reservoirs that cannot be exploited by conventional drilling methods. UBD has proven to be not only safe, but also cost effective and can result in an overall more efficient drilling program.
The IADC Underbalanced Operations Committee has adopted the following standard classification system for UBO and a set of standard nomenclature:-
Performance enhancement only; no hydrocarbon containing zones
Well incapable of natural flow to surface, inherently stable, and a low-level risk from a well control point of view.
Well capable of natural flow to surface but enabling conventional well control methods and has limited consequences in the case of catastrophic equipment failure.
Geothermal and non-hydrocarbon production. Maximum shut-in pressures are less than UBD equipment operating pressure rating. Catastrophic failure has immediate serious consequences.
Hydrocarbon production. Maximum shut-in pressures are less than UBD equipment operating pressure rating. Catastrophic failure has immediate serious consequences.
Maximum projected surface pressures exceed UBO operating pressure rating but are below BOP stack rating. Catastrophic failure has immediate serious consequences.
Increased Bit Life
The considerable heat generated by friction at the bit, between the drill string and wellbore is removed by the circulating drilling fluid. Additional frictional heat is also generated by the inert solid content of the mud. Heat removal from the bit is more efficient in underbalanced operations. Since there is no additional force holding the formation in place (less frictional force), the bit does less work to cut the formation. By using UBD, the fraction of retained solids is maintained at a minimum.
As UBD also requires less weight on the bit to obtain optimum ROP, the reduced load on the cutters and bearings increases bit life. During the Exxon/Mobil PASE Project, a single bit drilled the entire (production) hole section of 754 feet. In contrast, the average bit life for this field was 150 feet per bit for conventionally drilled wells.
Reduction/Elimination of Expensive Drilling Fluids
Conventional drilling fluids consist of a mixture of chemicals that are added to control fluid properties such as viscosity and fluid loss. In the case of loss return zones, additional chemicals and sized particles are added to control losses. These systems can be very costly. Because simple fluids (such as, KCl water or produced oil) are typically used in UBD, costly drilling fluid programs can be eliminated for the hole section drilled in an underbalanced mode. Additionally, significant cost savings may also be realized by not losing expensive drilling fluids to the formation.
Improved Safety and Reduced Environmental Impact
Conventional drilling is designed such that reservoir fluids do not enter the wellbore. But if they do, the system relies on personnel to recognize the inflow and control the well pressures correctly. Most blowouts occur, not because of poor engineering or planning, but due failure of personnel to correctly recognize an inflow and properly handle it.
A properly designed UBD system is less reliant on personnel recognizing an unplanned event. The system is designed to safely handle a continuous inflow from the formation and give a continuous positive BHP reading throughout the drilling operation.
Conventional drilling fluids are heterogeneous mixtures of organic, inorganic, and inert constituents. Regardless of the efficiency of the solids removal equipment, continued dilution is necessary to maintain a usable drilling fluid. Cosequently on a well it is not uncommon to generate thousands of barrels of waste material ranging in various levels of toxicity. On the other hand, UBD requires naturally occurring constituents, such as air and water plus fairly harmless foamers plus perhaps some corrosion inhibitor. The concentration of foaming additives are very low while the majority of surfactants used for foaming agents are biodegradable. As air/gas is the largest component, very little waste is generated as compared to conventional drilling fluids. Therefore disposal problems are a fraction of that for conventional drilling.
Variables that influence the final configuration of a UBD system.
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