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Loss of circulation is the uncontrolled flow of whole mud into a formation, sometimes referred to as a “thief zone.” This article discusses causes, prevention, and remedial measures for lost circulation.
Fig. 1 shows partial and total lost-circulation zones. In partial lost circulation, mud continues to flow to surface with some loss to the formation. Total lost circulation, however, occurs when all the mud flows into a formation with no return to surface. If drilling continues during total lost circulation, it is referred to as blind drilling. This is not a common practice in the field, unless all of the following criteria are met:
- The formation above the thief zone is mechanically stable.
- There is no production.
- The fluid is clear water.
- It is economically feasible and safe.
Causes of lost-circulation zones
There are several situations that can result in lost circulation:
- Formations that are inherently fractured, cavernous, or have high permeability
- Improper drilling conditions
- Induced fractures caused by excessive downhole pressures and setting intermediate casing too high
Induced or inherent fractures may be horizontal at shallow depth or vertical at depths greater than approximately 2,500 ft. Excessive wellbore pressures are caused by high flow rates (high annular-friction pressure loss) or tripping in too fast (high surge pressure), which can lead to mud equivalent circulating density (ECD). Induced fractures can also be caused by:
Eqs. 1 and 2 show the conditions that must be maintained to avoid fracturing the formation during drilling and tripping in, respectively.
where λmh = static mud weight, Δλaf = additional mud weight caused by friction pressure loss in annulus, Δλs = additional mud caused by surge pressure, λfrac = formation-pressure fracture gradient in equivalent mud weight, and λeq = equivalent circulating density of mud.
Cavernous formations are often limestones with large caverns. This type of lost circulation is quick, total, and the most difficult to seal. High-permeability formations that are potential lost-circulation zones are those of shallow sand with permeability in excess of 10 darcies. Generally, deep sand has low permeability and presents no loss-of-circulation problems. In noncavernous thief zones, mud level in mud tanks decreases gradually and, if drilling continues, total loss of circulation may occur.
Prevention of lost circulation
The complete prevention of lost circulation is impossible, because some formations, such as inherently fractured, cavernous, or high-permeability zones, are not avoidable if the target zone is to be reached. However, limiting circulation loss is possible if certain precautions are taken, especially those related to induced fractures. These precautions include:
- Maintaining proper mud weight
- Minimizing annular-friction pressure losses during drilling and tripping in
- Adequate hole cleaning
- Avoiding restrictions in the annular space
- Setting casing to protect upper weaker formations within a transition zone
- Updating formation pore pressure and fracture gradients for better accuracy with log and drilling data
If lost-circulation zones are anticipated, preventive measures should be taken by treating the mud with loss of circulation materials (LCMs) and preventive tests such as the leakoff test and formation integrity test should be performed to limit the possibility of loss of circulation.
Leakoff test (LOT)
Conducting an accurate leakoff test is fundamental to preventing lost circulation. The LOT is performed by closing in the well, and pressuring up in the open hole immediately below the last string of casing before drilling ahead in the next interval. On the basis of the point at which the pressure drops off, the test indicates the strength of the wellbore at the casing seat, typically considered one of the weakest points in any interval. However, extending an LOT to the fracture-extension stage can seriously lower the maximum mud weight that may be used to safely drill the interval without lost circulation. Consequently, stopping the test as early as possible after the pressure plot starts to break over is preferred.
Formation integrity test (FIT)
To avoid breaking down the formation, many operators perform an FIT at the casing seat to determine whether the wellbore will tolerate the maximum mud weight anticipated while drilling the interval. If the casing seat holds pressure that is equivalent to the prescribed mud density, the test is considered successful and drilling resumes.
When an operator chooses to perform an LOT or an FIT, if the test fails, some remediation effort—typically a cement squeeze—should be carried out before drilling resumes to ensure that the wellbore is competent.
A lost-circulation incident exacts a heavy cost that goes far beyond the price of products that are used to treat it. Lost circulation causes nonproductive time that includes the cost of rig time and all the services that support the drilling operation. Losing mud into the oil or gas reservoir can drastically reduce(or eliminate) the operator’s ability to produce the zone. Prevention is critical, but, because lost circulation is such a common occurrence, effective methods of remediation are also a high priority.
When lost circulation occurs, sealing the zone is necessary unless the geological conditions allow blind drilling, which is unlikely in most cases. The common LCMs that generally are mixed with the mud to seal loss zones may be grouped as:
- A combination of fibrous, flaked, and granular materials
These materials are available in course, medium, and fine grades for an attempt to seal low-to-moderate lost-circulation zones. In the case of severe lost circulations, the use of various plugs to seal the zone becomes mandatory. It is important to know the location of the lost-circulation zone before setting a plug. Various types of plugs used throughout the industry include:
- Bentonite/diesel-oil squeeze
- Cement/bentonite/diesel-oil squeeze
Squeeze refers to forcing fluid into the lost-circulation zone. See Remedial cementing
Use of loss of circulation materials
Rock mechanics and hydraulic-fracture theory indicate that it is easier to prevent fracture propagation than it is to plug the fracture later to prevent fluid from re-entering. Because of the high cost of most weighted, treated drilling-fluid systems, LCM routinely is carried in the active system on many operations in which probable lost-circulation zones exist, such as:
- “Rubble” zones beneath salt or in a known depleted zone
- Natural and induced fractures
- Formations with high permeability and/or high porosity
- Vugular formations (e.g., limestone and chalk)
Using an LCM that can be carried in the drilling fluid without significantly affecting its rheology or fluid-loss characteristics facilitates the preventive pretreatment. Pretreatment can mitigate wellbore breathing (ballooning), seepage losses, and/or potential lost circulation when drilling depleted zones.
When a loss zone is encountered, the top priority is keeping the hole full so the hydrostatic pressure does not fall below formation pressure and allow a kick to occur. The hydrostatic pressure may be purposely reduced to stop the loss, as long as sufficient density is maintained to prevent well-control problems. Loss zones also pose a high risk of differential sticking. Rotating and reciprocating the drillstring helps reduce this risk while an LCM treatment is prepared. If the location of the loss zone is known, it might be advisable to pull the drillstring to a location above the affected area.
A variety of LCM is available, and combining several types and particle sizes for treatment purposes is common practice. Conventional—and relatively inexpensive—materials include:
- Sized calcium carbonate
- Cottonseed hulls
Because lost circulation always has been one of the most costly issues facing the industry, a focus on healing the loss zone quickly and safely encouraged the development of proprietary materials that conform to the fracture to seal off pores, regardless of changes in annular pressure. In some cases, such deformable, expanding LCM is pumped ahead of cement jobs in which losses are expected. This type of material has a comparatively high success rate for the prevention and remediation of severe losses.
Severe lost-circulation problems that do not respond to conventional treatments might be curable by spotting a hydratable LCM pill, and holding it under gentle squeeze pressure for a predetermined period. At downhole temperatures, the LCM pill expands rapidly to fill and bridge fractures, allowing drilling and cementing operations to resume quickly, sometimes in 4 hours or less. Alternatively, rapid-set LCM products are available that react quickly with the drilling fluid after being spotted across the loss zone and form a dense, flexible plug that fills the fracture and adheres to the wellbore. In some cases, this type of plug has proved so effective that the natural fracture gradient of the formation actually increased, allowing the operator to resume drilling and increase the mud weight beyond constraints established before the treatment.
- Whitfill, D.L. and Hemphill, T. 2003. All Lost-Circulation Materials and Systems Are Not Created Equal. Presented at the SPE Annual Technical Conference and Exhibition, Denver, 5-8 October. SPE-84319-MS. http://dx.doi.org/10.2118/84319-MS.
- Sweatman, R., Wang, H., and Xenakis, H. 2004. Wellbore Stabilization Increases Fracture Gradients and Controls Losses/Flows During Drilling. Presented at the Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, 10–13 October. SPE-88701-MS. http://dx.doi.org/10.2118/88701-MS.
Noteworthy papers in OnePetro
W. Wayne Sanders, Roger N. Williamson, ConocoPhillips; Catalin D. Ivan, Donald Powell, M-I L.L.C.: Lost Circulation Assessment and Planning Program: Evolving Strategy to Control Severe Losses in Deepwater Projects, 79836-MS, http://dx.doi.org/10.2118/79836-MS.
James R. Bruton, Catalin D. Ivan, Thomas J. Heinz, M-I L.L.C.: Lost Circulation Control: Evolving Techniques and Strategies to Reduce Downhole Mud Losses, 67735-MS, ttp://dx.doi.org/10.2118/67735-MS.
Billingston, S.A. 1963. Practical Approach to Circulation Problems. Drilling Contractor.(July–August) 52.
Cagle W.S. and Mathews, H.D. 1977. An Improved Lost Circulation Slurry Squeeze. Petroleum Engineer (July): 26.
Clancy, L.W. and Boudreau, M. Jr. 1981. High-Water Loss High-Solids Slurry Stops Lost Circulation with Oil Mud. Oil & Gas J. (January): 99.
Goins, W.C. Jr. 1952. How to Combat Circulation Loss. Oil & Gas J (June): 71.
Howard, G.C. and Jr., P.P.S. 1951. An Analysis and the Control of Lost Circulation. J Pet Technol 3 (6): 171-182. http://dx.doi.org/10.2118/951171-G.