Lost Circulation

Lost circulation is potentially one of the most expensive problems in drilling because of the large quantities of drilling fluid that can be lost before losses are cured or reduced to an acceptable level. Lost circulation does not necessarily imply total losses and includes partial or seepage losses.

2.1 Loss zones

The range of lost circulation problems extends from shallow unconsolidated sands, to well consolidated formations that can be fractured by the hydrostatic pressures imposed upon them by the drilling mud. Furthermore, associated activities such as fluid circulation and pipe movement can cause losses. Circulation is usually lost under one or more of the following conditions:

  1. Highly prorous and permeable formations such as gravel, cavernous or vugular limestones and faulted formations can allow mud particles to penetrate the formation. The degree of losses will depend on the size of the formation openings and the sealing properties of the mud in use.
  2. Weak formations can be broken down by the use of excessively high mud weights, high equivalent circulating densities, pressure surges caused by running in pipe too fast, packed off stabilisers and initial circulating pressure used to break circulation in thixotripic muds.
  3. Excessively high cement columns can break down weak formations.
  4. Lack of care while performing leak-off/limit tests at the casing seat can result in fracturing of the formation. When using very low fluid loss muds leak-off may occur very close to the fracture propagation pressure. The tests should always be stopped as soon as any leak-off indication has occurred. Not all formations exhibit healing properties.
  5. Excessive penetration rates while drilling can create a heavy mud weight in the annulus. This can cause the formation to fracture and result in losses.

If losses are experienced, they should first be quantified, analysed and then cured.

  1. 2 Quantification and analysis of tost circulation
  2. When losses are encountered, the first priority to establish is whether or not the well is kicking.
  3. Once losses are suspected, the surface equipment should be checked for leaks. On offshore rigs, check that there is not mud going into the water. On floaters run the TV camera down to visually check the riser and subsea BOP.
  4. It should be established whether losses are static or dynamic. Dynamic losses are only experienced during circulation, whilst static losses occur at all times.
  5. Losses occurring during drilling may be indicative of a formation change (check also the penetration rate and drilling rate), penetration of a fault, or a slight increase in mud weight and/or viscosity taking the equivalent circulating density over a critical point. In the latter case, a flow check with the pumps off will indicate whether a small reduction in mud weight and/or viscosity will suffice. Slight treatment with LCM may also be necessary.
  6. It there is any doubt as to where the losses are going, consideration could be given to running a casing packer to check for losses in the casing string by pressure testing. The ditch magnet should be checked for metal cuttings.
  7. An estimate can be made of the maximum weight the formation can withstand by filling up the hole with water. By measuring the volume of water taken to fill up the hole and converting this to a hydrostatic head, the formation strength can be estimated.
  8. If necessary, certain wireline logging tools can be used to detect the flow of mud into a lost circulation zone {e.g. temperature survey, spinner survey etc.).

Note: For proper analysis, it is important to have as much information available as possible from the time that losses were detected (e.g. ROP, Mud wts in and out, standpipe pressure, mud properties etc.)

  1. 3 Curing partial lost circulation
  2. As a first step, reduce the mud weight, provided it is safe to do so. Consider also reducing the ECD by reducing the PV and YP or lowering the circulation rate.
  3. Lost circulation normally occurs when the pores of the formation are larger than the bridging particles contained in the mud. Sealing materials (LCM) will be available on site and can be added to the mud to cure these problems.

When lost circulation is expected, use l6'32 inch or greater nozzles to allow LCM to be pumped. If smaller nozzles are used, employ a circulation sub. If it is considered necessary to pump LCM pills through the bit, the nozzles can be blown out using Schlumberger primer cord.

  1. A 25-50 bbls pill containing a mixture of lost circulation materials at a concentration of between 5 — 30 ppb, depending upon the degree of losses, can often cure lost circulation. The pill is preferably spotted where the losses occur and allowed to stand there for a short period. If the losses are not cured, a second pill can be used, or if there is a large section to be drilled, then lost circulation material can be added to the total mud system at about 2 ppb. Bypass the shakers when LCM plugs the screens.
  2. If losses are the result of a kick, then the kick should be controlled first by one of two means:
  • a) Displacing a heavier mud in the open hole below the lost circulation zone and decreasing the mud weight above it,
  • b) Setting a baryte plug (max. wt. 21 ppg) in open hole below the lost circulation zone.
  1. If a well kicks as a result of lost circulation then the lost circulation has to be cured first before the kicking formation can be controlled. This can be done by pumping LCM pills either down the annulus or DP.
  2. If none of the various LCM pills are successful, drilling without returns may be considered.
  3. After having experienced lost circulation problems, setting casing as soon as a non-permeable zone is encountered should be considered. If it is known that pore pressure gradients deeper down the hole do not increase, then it may not be necessary to set casing immediately. Never pull out when circulation is lost and the level cannot be seen until the situation has been fully accessed. The main danger in doing so is the possibility of swabbing the well in. With no level to be seen, it is impossible to detect swabbing, except by watching the weight indicator.
  4. 4 Lost circulation pills
  5. Slight losses can often be cured by:

Mica fine 3-5 ppb

2. Severe losses generally require a greater range of particle sizes and the following may be used; depending on severity:

Mica fine

3-10 ppb

Mica medium

3-10 ppb

Mica course

3-10 ppb

Cellophane

3-10 ppb

Nutplug

3-10 ppb

Mud fibre

3-10 ppb

The amount of LCM that can be added to a mud depends very much on the mud properties. High gels and YP will help keep the material in suspension. If too much material is added, then the excess will float on top. For this reason, add the material in stages, for example 5, 10, 15, 20, 25, 30, 35, 40 ppb. Stop after it is observed that the material cannot be held in suspension. If the LCM pill is left to soak for a period of time, you may find that more material can be added. It is useful to have a viscofier, such as CMC, available to thicken the mud before adding LCM material.

2.5 Gunk squeeze

If losses are particularly bad, a gunk squeeze can be tried. This method utilises the swelling property of bentonite in the presence of water and is done as follows:

  1. The cement pump and lines are thoroughly cleaned with diesel oil to remove all traces of water.
  2. A diesel spacer is then pumped down the drillstring with the bit or tail pipe just above the lost circulation zone.
  3. 300 ppb bentonite in diesel oil is pumped down in a 20 bbls pill.
  4. Follow this gunk mixture with diesel oil spacer.
  5. Displace gunk mixture to the bit with mud.
  6. Pump simultaneously down the drillpipe and the annulus and apply a squeeze pressure of 100-300 p.s.i.

While the bentonite is pumpable when mixed in diesel, as soon as it comes into contact with water it forms a highly viscous putty like material which can plug off the formation. This method is particularly effective if used in holes where considerable amounts of water have been lost downhole prior to attempting the gunk squeeze.

2.6 Use of cement plugs

When losses in excess of 100 bbl/hr are encountered, or when operational circumstances necessitates, cement plugs should be used to cure losses. Light weight cement should be used such as 1.67 SG 2 per cent bentonite slurry. The cement plugs should be placed over the loss zone and drilled our prior to full hardening to prevent the bit from wandering off them into the formation which is probably softer than the cement when it has fully hardened.

2.7 Recommended stocks of LCM

It is recommended to have the following minimum stocks of LCM at the rigsite at all times:

1 pallet mica (fine) 1 pallet mica (medium) 1 pallet mica (coarse)

1 pallet viscofier (to make the pill viscous before adding LCM) In areas of known losses, these stocks must be increased significantly.

2.8 Bit nozzle removal

Under certain conditions, it is desirable to remove the nozzles from a bit downhole to enable free passage of plugging/lost circulation materials. Nozzles, whether retained by circlips or nails, can be removed using explosives. In order to use sufficient primer cord the minimum ID, in the drillstring should be 21,ri6 inch. The charge is made up of a firing head of Yb inch mild steel bar approximately 7 ft long with a 1 inch OD, x2 inch long bull nose on the bottom (the basic string shot tool). One strand of primer cord is used for detonation with twelve lengths of 16 inch long primer cord laid along the bottom part of the bar. Wrap approximately 3 ft primer cord around the 12 lengths at approximately 9 inch from the bottom of the tool. Finally, wrap plastic tape over the cord to hold the charge in place. SO grain primer cordis ideal. The maximum OD of the charge should be 1SA inch. String connections adjacent to the bit can be slackened by this operation, therefore, care must be exercised when tripping out afterwards.

2.9 Total losses

Sometimes losses are not curable. If you drilled into a coal mine or subway, it would be approaching the impossible ever to fill the hole effectively. Some formations are analogous to this, it is just not feasible to cure the losses. This is not common in practice and tends to happen only in surface hole, fractured limestones and when drilling in caves and mine workings. At this time, it is a problem which must be tackled. The first question that we must ask ourselves after checking that the well is not flowing is, 'What is the hole going to do?' Having discovered the loss zone and lost the fluid head of mud which provided the primary well control in the borehole, any hydrocarbons would make their presence felt very quickly. At the same time unconsolidated formations higher up the borehole may fall in as the pressure differential across the borehole wall is increased with the loss of hydrostatic head of the mud. Consequently, our two main concerns can be addressed by the questions:

Will the well kick? Will the hole collapse?

As mentioned before, if the well is going to kick with no fluid in the hole, then we should know about it fairly quickly. A gas influx will almost certainly flow from the open hole and require closing in at the BOPs. Oil and water kicks could come to surface, but could in fact just produce into the loss zone. In this case, the well might be producing with little or no clear indication of this at surface. If this is the case, then we need to be aware of it to ensure that any future casing programme and cementations take this into consideration.

To check the likelihood of this eventuality, the geologist's records of the well to date should be checked, along with drilling data such as background gas, oil shows and salinity. If none of the formations drilled to date had gas, oil, or water shows and furthermore, if they were non-porous and impermeable, then there is little or no risk that any formation is producing. If there is a strong likelihood, however, that there is an exposed productive reservoir, then it must be assumed that this will produce under total loss hole conditions. Quite often with the drawdown created across the well bore interface during total loss downhole conditions, the productive formation will bridge itself off and seal off. This phenomenon, although forming the basis of a lot of wild well control planning, can never be relied on. If there is oil present in the well bore, it should be evident on the drillstring as and when it is tripped out of the hole, Whith total losses, we face the alternatives of curing losses or drilling ahead blind. Our initial attempt should be to cure losses using the methods mentioned earlier in this section and then, in the event of their failing, pouring in light weight or thixotropic cements to cure the losses. If formations have been fractured and unconsolidated since spudding the well or since the last casing was set, then it is worth considering drilling ahead. Oil and gas in a reservoir needs a 'cap rock' to retain it and prevent it from migrating to surface over the millions of years during which it was forming. If we have not drilled through any formation which could be considered to be a cap rock or a seal, then it is extremely unlikely that any hydrocarbons will be encountered until we do actually reach a formation which could be a cap rock. Casing should be set in the first firm formation reached.

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Responses

  • Lara Dickson
    How to estimate ECD with LCM in mud?
    9 years ago
  • irene
    How to do mud checks lost circulation material?
    9 years ago
  • katie-leigh jones
    Do you increase mud density for lost circulation?
    9 years ago
  • Genoveffa
    What is the meaning of ppb lcm pills?
    8 years ago
  • William
    How to calculate static and dynamic losses drilling?
    8 years ago
  • Charlie
    How to cure total losses circulation drilling?
    7 years ago
  • jack
    What is the permeability of lost circulation zones?
    7 years ago
  • roderic
    Can a viscofier increase the mud weight?
    7 years ago
  • savanna sackville
    How to test ppb lcm in drilling fluids?
    4 years ago
  • WAYNE
    How to measure high static mud loss in the well?
    2 years ago
  • Adaldrida
    What can i do when i lost rilling mud?
    2 years ago
  • thorsten
    What is circulation in oil and gas operation?
    4 months ago
  • Ryley
    What is lost of circulation during water borehole drilling?
    4 months ago
  • mhret
    What are losses encountered in circulation?
    4 months ago

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