There are four principal causes of loss of primary control: swabbing, insuficient fluid density, losses, failure to fill the hole correctly.
1 Swabbing
Swabbing can also be caused by pulling full gauge tools which restrict the passage of fluid in the annulus. This creates a piston effect and the volume below the tool would be filled by the fluid inside the drillstring, causing a large drop of the fluid level in the drillstring and consequent dangerous reduction in hydrostatic head. "High Volume" swabbing is especially dangerous in large diameter holes.
Swabbing is one of the greatest pressure control hazards of drilling operations. The danger is not necessarily related to the volume of fluid swabbed. A small volume of gas for example, may migrate and expand, ultimately displacing (sometimes violently) a significant volume of mud from the well. This may occur when there is little, if any, pipe left in the hole. The possibility of losing control of the well when the drillstring is run back into swabbed gas must also be recognised. Safety requires continuous monitoring of filled up and return volumes throughout the roundtrip. This should be done with a trip tank and trip tank chart recorder or trip sheet.
A short roundtrip followed by circulating bottoms up before pulling out of the hole completely will provide the most reliable information on the swabbing characteristics of the system. This might indicate whether pulling speeds and/or mud properties should be adjusted.
When partial losses are experienced and a roundtrip is being made, the degree of losses should be accounted for in the trip tank measurements during the roundtrip, otherwise swabbing may not be detected.
The three principal factors which contribute to a swabbing tendency are annular clearance, pulling speed and mud properties.
1.1 Annular clearance
Annular clearance is determined by the drillstring and assembly configuration for a given hole size. However, hole size can be reduced by excessive mud cake, swelling formations, or an accumulation of cuttings (i.e. on low side in deviated holes). With a large annular clearance, the tendency for swabbing to occur is reduced.
1.2 Pulling speed
Low pipe speeds reduce the possibility of swabbing.
1.3 Mud properties
Overbalance is the main factor in reducing the possibility of swabbing. Controlling the yield point of the mud is also very important, since the swabbing effect is almost proportional to it. Minimising the solids content will help maintain a low yield point.
A low water loss assists in obtaining a thin filter cake on the borehole wall, and in minimising the swelling of shales. This will reduce the balling-up of bit and stabilisers and thus reduce swabbing tendencies.
When swabbing is anticipated it is recommended not to pump a heavy pill prior to commencing a roundtrip. This permits a more accurate check for swabbing. The heavy pill can be pumped when it is certain that swabbing will not occur. If low pipe pulling speeds and good mud properties are not sufficient to allow a roundtrip to be made safely, consideration should be given to circulating whilst the string is pulled, or to increasing the overbalance factor. It is important to pump at a rate to prevent any loss of positive bottom hole pressure.
2 Insufficient fluid density
An underbalanced situation may result from the dilution of the drilling fluid (e.g. by rainwater, or errors in opening/closing valves). Any influx of formation fluids or gas can also result in a reduction of the hydrostatic head in the hole.
Fast drilling rates may increase the mud weight considerably. Drilling with heavier mud returns could obscure indication of drilling through higher pressured formations. Primary control may be lost during circulating the hole clean.
Annular pressure drop due to friction whilst drilling/circulating will disguise an underbalanced situation and primary control may be lost as soon as circulation is stopped.
During drilling, an underbalanced condition will occur if a permeable formation is penetrated which has a pore pressure higher than the hydrostatic pressure of the column of mud in use, i.e. the density of the mud is insufficient. Under these conditions primary control can no longer be maintained and secondary control measures are normally implemented.
Sometimes mud having insufficient fluid density can be circulated safely and operations can proceed temporarily without increasing the mud gradient and without the need for secondary control measures. This occurs when drilling into abnormally pressured zones with low permeability and hence, low productivity. In such cases the well will not flow measurably, but hydrocarbons or salt water will show up in the returns from bottom after a trip. Also traces will always be present in the drilling mud. (Under the conditions of insufficient fluid density, crews must be alert to detect the inflow which will occur when a more permeable formation is encountered).
3 Losses
Losses may occur as a result of naturally fractured or pressure-depleted formations, excessive pipe running speeds, balling up of bottom hole assemblies, excessive high annular friction losses, high mud weights, or excessive pressures caused by breaking circulation.
If loss of circulation occurs in drilling or well servicing operations, the mud level may drop. Depending on the severity of the losses, the hydrostatic overbalance in the well will then be reduced or eliminated.
In areas where loss of circulation is regularly experienced, the following preventive measures should be considered prior to and whilst drilling into the potential lost circulation zones:
·drill with the lowest mud density that can be safely used;
·use mud with a low yield point and low plastic viscosity;
·lower the circulation rate to reduce annular friction losses;
·adjust the drillstring configuration (DC size and length, pipe weight, stabilisers, etc.) to minimise pressure surges and annular friction losses. This may also reduce the risk of getting stuck;
·remove the jets from the bit or run regular bits with enlarged water courses to allow pumping of LCM pills;
·limit the running-in speed of the pipe to minimise pressure surges;
·use proper techniques for breaking circulation during and after the trip;
·include a circulating sub in the assembly to allow pumping of LCM pills.
In order to restore circulation, the severity of the losses must first be evaluated.
3.1 Evaluation of losses
Losses are usually classified as follows:
- ·static losses (downhole losses which occur under static hole condition, e.g. no circulation or string movement);
- ·dynamic losses (downhole losses which occur during circulation. The difference between mud volume in/out will indicate the magnitude of the losses);
- ·total losses or loss of circulation (no returns will be observed).
3.2 Restoring full circulation: slight losses
In the case of slight losses, both of the following techniques should be applied:
- ·drill ahead carefully whilst adding fine lost circulation materials to the mud (this may seal the loss zone);
- ·reduce the mud yield point and circulation rate (this will reduce the pressure on the loss zone).
3.3 Restoring full circulation - severe losses
In the case of severe losses, the fluid gradient which can be supported by the loss zone must be determined. To do this, light mud or water is pumped into the annulus and the volume required to fill the hole noted (be aware of possible hole collapse if water comes in contact with water sensitive formations).
The equivalent fluid gradient can then be calculated by working out the heights and hence, the hydrostatic heads of both the light mud column and the original mud column.
3.4 Sealing the loss zone
If circulation cannot be restored by the use of LCM, attempts should be made to seal off the loss zone by the use of special lost circulation "soft plugs" such as salt gel, diesel/bentonite, etc. Alternatively the use of a cement plug may be considered.
Plugged off lost circulation zones might hold some borehole pressure depending on the type of formation and the lost circulation material used.
However, after sealing a loss zone, casing should normally be set as soon as a non-permeable zone is penetrated. An exception to this rule may be made when it is known that pore pressure gradients do not increase with increasing depth.
3.5 Drilling without returns
If the loss zone cannot be sealed and the formation will not support a column of light mud (or water), drilling without mud returns may be considered.
An NRV or Drop In Sub (DIS) should be installed in the string when drilling with total losses or with a floating mud cap. The dart of the DIS should be dropped and pumped down/latched prior to each trip.
3.6 Loss of circulation in offshore operations
In offshore drilling operations primary control is more complicated than in onshore operations. This is due to the difference between the pressure gradient of the mud column from the flowline to well depth and the greatly varying overburden and formation strength gradients over the same interval.
The overburden and formation strength gradients are nil from the flowline to sea level; from sea level to sea bed an 'overburden' gradient of sea water is present; below the sea bed the formation strength gradient gradually increases. The strength of the high porosity sediments within 600 m (2000 ft) of the sea bed is usually very low.
Particularly when operating in deep waters (say beyond 600 m), great care must be exercised to avoid fracturing the formation. Therefore, the lowest practicable overbalance should be employed to prevent mud losses. Accurate information regarding the strength of exposed formations, based on carefully conducted tests, is particularly valuable.
It should be kept in mind that when mud losses are gradually reducing while drilling/circulating, the well may have actually started to flow. In well servicing operations, loss of fluid to the productive zone can occur during initial well control or remedial operations. In multiple zone completion wells with considerable differences in formation pressures between separate producing zones, lost circulation can only be remedied by mechanically isolating the loss zone (with a packer or cement) or by temporarily bridging the loss zone.
4 Failure to fill the hole properly