Bullheading is a technique that may be used in certain circumstances during drilling operations to pump an influx back into the formation. In other words, it is a term to denote pumping into a closed-in well without returns. A major concern with this technique is that it may result in fracturing any exposed formation.
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Bullheading is, however, a relatively common method of killing a well during workover operations when there is adequate reservoir permeability. You might be interested in other killing methods as Wait & Weight Method and Driller’s method.
When to Think in Bullheading
During operations, bullheading may be considered in the following situations:
- When a very large well kick has been taken.
- When displacement of the influx by conventional methods may cause excessive surface pressures.
If well is flowing and is shut in and the gas is percolating with the bit a long way off bottom and tight hole conditions have been experienced, then consider bullheading.Abardeen Well Control Manual
Bullheading should be performed with an aim of not fracturing the formation. The surface squeeze pressure applied should not exceed the precalculated MAASP.Eni Well Control Manual
- When displacement of the influx by conventional methods would result in an excessive volume of gas kick at surface conditions.
- If the influx is suspected to contain an unacceptable level of H2S and can not be handled safely by driller, rig personnel and equipment.
If the influx is suspected of containing H2S, it may be acceptable to squeeze it away downhole provided that casing profile and nature of the formation overlaying the weakest strata, ensures proper isolation. In other circumstances, e.g. where surface permeable formations are exposed or only a surface casing is set not ensuring proper strata isolation, this practice is prohibited.Eni Well control Manual
All wireline or coiled tubing (CT) work in high H2S wells must not be carried out when the tubing is full of gas. The tubing must be bullheaded with inhibited fluid prior running wireline tools or CTAdco Manual
- When a kick is taken with the pipe off bottom and it is not considered feasible to perform stripping operations back to bottom.
- When an influx is taken with no pipe in the hole.
- When an influx is taken with no possibility of circulation.
- To reduce surface pressures prior to implementing further well control operations
- A combined kick and losses has arisen
- In Horizontal Well, If gas kick cannot be removed from inverted sections then consider bullheading gas into formation.
- The MAASP is likely to be exceeded by a large margin before the influx enters the shoe. This can be the case in high pressure high temperature wells where there is only a small margin between the drilling fluid gradient and the formation strength and where a high degree of expan sion (and thus high surface pressures) is required to bring a gas influx to surface.
- Bullheading still one of the options to control swabbed kick
- Concerning stripping operations, If surface pressures indicate that annular stripping is not possible, attempts should be made to reduce the pressures in order to enable annular stripping to be used. The most appropriate technique will depend on the position of the influx in the hole. The options are; to circulate out the influx, to lubricate the influx from the well or to perform bullheading
- A combined kick and loss situation is experienced (downhole annulus bullhead rates must exceed the gas migration rate to ensure the situation does not deteriorate further)
Prior To Bullheading
Consider using the volumetric method to eliminate the complication of migrating gas. If the gas can be largely removed this way, the bullheading operations are likely to be much easier and more effective in killing the well.
Pressure limitations of pumping equipment, wellhead equipment, and casing must be kept in mind throughout.
If a gas influx is suspected (shut-in pressures continue to rise indicating migrating gas), the pumping rate for bullheading must be fast enough to exceed the rate of gas migration. If pump pressures increase instead of decreasing, it is an indication that the pumping rate is too low to be successful. This can be a problem in large diameter holes. Note that increasing the viscosity of the kill mud mayor may not be Ilelpful in controlling this problem and could possibly even make it worse.
There is often a chance, particularly with relatively long open-hole sections beyond. the last casing setting depth, that bullheading could break down the formation at the shoe rather than at the producing formation. In this event, rather than killing the well, this procedure may aggravate the development of an underground blowout which could pose risks to nearby wells in communication with the formations involved. It could also increase risk of a blowout around casing in place with subsequent obvious risks. Thus, this method should be considered when these risks are considered the lesser of the potential evils.
A check valve is recommended between the pumping unit and the well to act as a failsafe valve in the event surface equipment should fail during the procedure. If possible, the cementing unit should be used for better control and adequate pressure rating.
Large mud volume and LC~v1 pills should be available In case major losses are experIenced during the operation.
Factors That Have an Effect on Bullheading
Bullheading during drilling operations will be implemented when standard well control techniques are considered inappropriate. During such situations, it is unlikely that accurate information is available regarding the feasibility of bullheading.
In most cases, therefore, the likelihood of successfully bullheading an influx will not be known until it is attempted. However, the major factors that will determine the feasibility of bullheading include the following:
- The characteristics of the open hole.
- The rated pressure of the well control equipment and the casing (making allowance for wear and deterioration).
- Rated pressure, reliability and output of available pumping equipment.
- The type of influx and the relative permeability of the formation.
- The quality of the filter cake at the permeable formation.
- The consequences of fracturing a section of the open hole.
- The position of the influx in the hole.
- Displacement rate required in larger hole sizes.
In general, bullheading procedures can only be drawn up bearing in mind the particular circumstances at the rig site. For example, there may be situations in which it is considered necessary to cause a fracture downhole to bullhead away an influx containing H2S. In another situation with a shallow casing set, it may be considered totally unacceptable to cause a fracture in the open hole.
Consideration should always be given to monitoring annulus pressure and applying backup pressure in the annulus to reduce differential pressures.
During a workover operation a procedure for bullheading will be drawn up along the following lines:
- Calculate surface pressures that will cause formation fracture during bullheading. Also calculate the tubing burst pressures as well as casing burst (to cover the possibility of tubing failure during operations
- Calculate static tubing head pressure during bullheading.
- Slowly pump kill fluid down the tubing. Monitor pump and casing pressure during the operation.
- Unfortunately, bullheading is not a routine method during drilling operations although it is often used when killing a well for workover using brine). A considerable amount of whole drilling fluid may have to be squeezed away in order to remove a migrating gas influx from the well. This might well result in considerable formation damage and a permanent loss situation, jeopardising the hole section and the objectives of the well.
- Bullheading can only be used if hole conditions permit and each case must be judged on its own merits. Whether bullheading is possible or not will depend on what additional pressure is required to inject the influx, especially if it is first necessary to squeeze drilling fluid into the recipient formation. Factors that will affect the success of the operation are formation permeability, the type of influx, contamination of the influx with drilling fluid and its position relative to the weakest formation exposed; the burst strength of the casing and the pressure rating of the BOP. Bullheading is, for these reasons, usually only considered in special circumstances.
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