A kick can be defined as a well control problem in which the pressure found within the drilled rock is greater than the mud hydrostatic pressure acting on the borehole . When this occurs, the greater formation pressure has a tendency to force formation fluids into the well bore. This fluid flow is called a kick. If the flow is successfully controlled, the kick has been killed. A blow out is the result of an uncontrolled kick.
The severity of kick depends upon permeability and porosity of the rock. A rock with high permeability and porosity has a greater potential for a severe kick than a rock with low permeability and porosity. Another controlling variable for kick severity is the amount of pressure differential involved. Pressure differential is the difference between the formation fluid pressure and the mud hydrostatic pressure. If the formation pressure is much greater than the hydrostatic pressure , a large negative differential pressure exists. If the negative differential pressure is coupled with high permeability and porosity in a rock, a severe kick can occur.
The pressure differential between the formation and well bore i.e the degree of underbalance, is proportional to the influx flow rate and kick volume for a given flow period. The situation can only deteriorate with time because less dense formation fluid volume further reduces well bore pressure and thereby serves to increase flow rate into the well. Permeability is another significant factor as are exposed thickness and fluid viscosity. We cannot control permeability or reservoir fluid properties, but the amount of exposed kick is governed by how long we continue to drill with kick entry.
CAUSES OF KICKS
Kicks occur as a result of formation pressure being greater than mud hydrostatic pressure which causes fluid to flow from the formation into the wellbore. The main factors which can lead to this condition can be classified as :
- a) Improper hole fill up on trips.
- b) Swabbing.
- c) Abnormal formation pressure.
- d) Insufficient mud density.
- e) Lost circulation.
- f) Gas cut mud
More than 50% of the kicks occur due to first two of the causes listed above.
- Improper hole fill up on trips
When the drill string is pulled out of the hole dry, the mud level decreases by a volume equivalent to the steel volume. If the hole does not take the calculated volume of mud, it is assumed a formation fluid has entered the wellbore. Even though gas or salt water entered the hole, the well may not flow until enough fluid has entered to reduce the hydrostatic pressure below the formation pressure.
Therefore, while pulling out the well should be filled continuously by using trip tank and differences of calculated and actual mud volume be recorded at regular interval. Similarly while running in drill string, trip tank should be used to monitor displacement volume correctly at regular intervals.
If the hole is not filled to replace the calculated volume, the fluid column in the wellbore shall go down and reduce the hydrostatic pressure. At the same time the pulling out of drill string causes a reduction in BHP due to swabbing effect. Therefore to avoid the possibility of any formation fluid entering the bore hole due to combination of above two factors the hole should be properly / regularly filled during tripping out. Displacement factors representing the pipe volume per unit length are used to determine the mud volume. Metal displacement of a pipe can be calculated using the following formula:
Metal displacement (bbl /ft) = (D2 -d2) /1029.4
Where “D” is the outside diameter and “d” is the inside diameter of the pipe in inches.
In the field normally the practice is to fill up the hole either on a regular fill up schedule or to fill up continuously with a re-circulating trip tank. Irrespective of the practice being used an accurate method of measuring the amount of fluid actually being taken by hole should be monitored and an accurate record of actual volume v/s theoretical volume should be kept. If at any stage during pulling-out it is observed that the actual filled in volume is significantly less than volume of steel that has been removed, it means that some formation fluids must have entered the well bore.
Prior to Tripping:
- Circulate the hole clean prior to the trip.
- Limit pipe speed to minimize surge/swab pressure
- Line up and use a trip tank
- Maintain hole fill /displacement data on trip sheets
Hole fill up = Displacement factor x Stand length x No of stands - Discuss with driller the purpose of a trip.
- Prepare the drill floor.
Tripping Procedure:- Flow check the well prior to tripping
- Pump a slug so that pipe can be pulled dry.
V slug = {MW (ppg) x planned length of dry pipe (ft) x Pipe capacity(bbl/ft) }/ (SlugWt(ppg) – MWt(ppg))
As a rule of thumb , the slug should be mixed to maintain a minimum of 2 stands of dry pipe. Accurate displacement is necessary so that the depth and height of the slug is known at all times during the trip.
- Ensure the level in the annulus is falling by monitoring the hole through the rotary for the first 5-10 stands pulled off from bottom. After confirming that there is no swabbing then install the pipe wiper.
- Circulate the hole across the trip tank and continue the trip out, check correct hole fill via trip tank and trip sheets.
- Conduct a flow check when BHA is first pulled into casing and prior to pulling the BHA into the BOP stack.
- Trip sheet: Trip sheet should be used to record hole fill up volumes for all trips. The trip sheet allows for comparison of actual vs calculated fluid volumes so that any discrepancies can be easily detected.
A trip tank should also be used during all trips to assist with accurate hole fill up requirement.
- Swabbing
SWAB pressures are created by pulling out the drill string from the borehole. It reduces the bottom hole pressure. If the reduced bottom hole pressure becomes less than the formation pressure, a potential kick may enter the wellbore. Various factors conducive to swab pressures are pipe pulling speed, mud properties, filtration cake, annular clearance, hole configuration and effect of balling up of BHA & bit.
Pulling speed is the only variable that can be controlled during the drilling process when a trip is made. In order to reduce the swab pressure , the pulling speed must be reduced.
It is important to remember that swab pressure aggravates the pressure reduction resulting from not keeping the hole full as pipe is pulled. Also the swab pressure is exerted at every point throughout the open hole below the bit, even though the drill string may be inside the casing string. - Abnormal pressure of reservior
In case of wild cat or exploratory drilling, most often the formation pressures are not known accurately. While drilling, sometimes the bit suddenly penetrates an abnormal pressure formation. As a result the mud hydrostatic pressure becomes less than the formation pressure and may cause a well kick. There are various geological reasons for abnormal pressures, some of these are discussed in 1.8 of this book. - Insufficient mud density
If a formation is drilled using a mud density that exerts less hydrostatic pressure than the pore pressure, the formation fluid may begin to flow into the well bore. Kicks caused by insufficient mud density seem to have the obvious solution of drilling with high mud density. The best solution is to maintain the mud density slightly greater than that required to balance the formation pressure in order to avoid mud loss.
- Lost Circulation
Lost circulation is another factor which reduces the hydrostatic pressure. When a kick occurs due to lost circulation, the problem may become more severe. A large volume of kick fluid may enter the hole before the mud level increase is observed at the surface. It is a recommended practice to keep the annulus always topped to avoid considerable reduction in BHP when lost circulation is encountered.
Severe lost circulation is one of the more difficult problems. Drilling into a depleted formation , pore pressure reversal or vugular rock can lead to a case where the bottoms falls out and a subsequent loss of hydrostatic head sufficient to permit entry from a shallower formation. Alternatively increasing the mud weight or annular friction may result in shallower losses and a kick from a deeper horizon. Serious well control problems resulting from lost circulation generally can be avoided by adequate planning and supervision. Failure to prevent such kicks often leads to an underground blow outs. - Gas cut mud
Gas contaminated mud will occasionally cause a kick. As the gas is circulated to the surface, it expands and reduces the hydrostatic pressure sufficient to allow a kick to enter. Although the mud density is reduced considerably at the surface, the hydrostatic pressure is not reduced significantly since the most gas expansion occurs near surface & not at the bottom.
KICK DETECTION
Detecting a kick early and limiting its volume by shutting in the well is critical to secondary control, and may make the difference between a manageable situation and one that leads to a loss of control. Thus it is important to have appropriate kick detection equipment in working order and to have crews alert to the warning signs and trained in the shut in procedure.
When a kick occurs, the surface pressure required to contain it will largely depend upon closing in quickly and retaining as much mud as possible in the well. The more mud unloaded , the higher the back pressure needed.
KICKS WHILE DRILLING
Following are the early warning signs & positive indications for kicks while drilling.
- Early warning signs
The detection of increasing formation or pore pressure is very essential in maintaining primary control of a well and preventing a kick. The early warning signs are indications of approaching higher formation pressure which means that the well may go under-balance if no appropriate action is taken. Correct action taken after review in consultation with Geologist and Mud person can help to avoid occurrence of kick while drilling further.
Early warning signs are listed below :
- Rate of Penetration Trends
When abnormal pressure formations are encountered, differential pressure & shale density are decreased causing a gradual increase in ROP. An increase in drilling rate can be masked by an increase in mud weight. Similarly bit weight changes can also mask the increase in drilling rate,but careful observation of drilling rate or some such related parameter as “d” exponent can provide a timely warning of increasing pressure.
- Drilling Break
The first indication of a possible well kick is a drilling break. For reservoir fluid to enter the well bore there must be a permeable section of reservoir rock. This will cause a change in drilling rate. In soft formation, a sand section usually causes a sudden increase in drilling rate. The increase in drilling rate varies. A 200% to 300% increase in drilling rate is not unusual. In hard formations a reverse drilling break to a slower drilling rate occurs in the reservoir like sandstone that are harder than the shale body.
The driller should be specially alert to signals when a positive drilling break is encountered . In fact company policy is to stop drilling and to conduct a flow check before proceeding vary far into any drilling break, especially when drilling in a known transition interval.
- Increase in Torque & Drag
As the difference between the mud hydrostatic pressure and formation pressure decreases (as a result of increasing formation pressure), the bit makes larger cuttings and the cuttings pile up around the collars and increase the rotary torque. Closing up of the hole may also increase torque. Increase in rotary torque is a good indication of increasing pressure and a potential well kick. Drag & fill up on connections and trips increase when high pressure formations are drilled.
- Decrease in Shale Density
Shale density usually increases with depth but decreases in abnormal pressure zones due to under compaction. The density of cuttings collected can be determined at surface and plotted against depth. A normal trend line is established and any deviation should theoretically indicate changes in pore pressure.
- Change in Cutting Size and Shapes
Cuttings from normal pressure shale are small in size with rounded edges and are generally flat. Cuttings drilled from abnormal pressured formation often become long and splintery with angular edges. As differential pressure is reduced due to increase in formation pressure, the cuttings have a tendency to explode off bottom. A change in cutting shape will be observed along with an increase in the amount of cuttings recovered at the surface and this could indicate that formation pressure in the well is increasing.
- Change in Mud Property
As the pressure in the formation increases faster than the pressure of the mud column, more cuttings & caving will dissolve into the mud and increase the viscosity of the mud.
- Increase in Chloride Content in Mud Filtrate
Drilling through high pressure formations having higher porosity results in contamination of drilling fluid with considerable volume of saline water from pores. This increases chloride content of the drilling fluid and its filtrate. A higher chloride trend can warn about increasing pore pressure.
- Increase in Flow Line Temperature
The temperature gradient in abnormal pressure formation is usually higher than normal pressure formation. The continuous measurement of the mud temperature at the flow line gives an indication of change in temperature gradient associated with abnormally pressured formation. The temperature may take a sharp increase (5-7oF/100 ft.) in transition zones.
- Increase in Trip, Connection and Back-ground Gas
An increase in trip and / or connection gas should be considered as an indication that pore pressure is increasing. Gas readings are arbitrary and are not proportional to actual gas concentration in the mud. These vary considerably from one mud logging unit to another. Therefore absolute values of gas readings do not have much significance in detecting abnormal pressures. Increase in back ground gas is not very reliable in detecting pore pressure increase. This is because gas concentrations can change drastically in the formation being drilled without any increase in pore pressure.
Gas analysers are used to establish trend line which is called background gas. A gas feed in from a permeable zone will change this trend line. The amount of feed in will determine the intensity of the trend change. Connection gas will normally occur on bottoms-up (calculated lag time) and if not re-circulated will not change the overall trend line except for short interval of time.
The most common error with gas cutting is the tendency to maintain the mud weight at its original value with addition of barite and without removing all the gas. Since moderate gas cutting contributes so little to bottom hole pressure reduction, additional barite may increase the mud weight enough to cause lost circulation.
- Change in ‘d’-exponent
Jordan and Shirley developed an equation for normalised penetration rate in which it was defined as a function of measured drilling rate, weight on bit, bit size and rotary speed in the equation as below :
Since the d-exponent tends to indicate the pressure differential between formation pressure and well bore pressure, mud weight will effect d -exponent. The original calculation should be corrected as follows :
dc values are plotted on a semi log graph paper at every 15 or 30 ft. interval depth to give normal trend line. Abnormal pressure transition zone top is detected at the depth where dc exponent values against shale tends to decrease in comparison to normal values.
- String weight change
With entry of lighter influx in annulus, there is slight change in buoyancy and hence minor string weight increase may be observed.
Positive Kick Sign
Positive kick indicators are different from kick warning signs. They indicate that the kick has already entered the well bore. Any of them indicate regular flow checks.
- Increase in return flow (Pumps On)
After the early warning signs the first positive kick sign is increase in flow rate at the flow line with pumps on. The entrance of any fluid into the well bore causes the flow rate to increase.
- Flow from well (Pumps Off)
Stopping the pump causes a reduction in bottom hole pressure equivalent to the annular pressure drop, so flow check is a reliable method of checking for a well kick. If the well does not flow when the pump is shut off and remains static for two or three minutes, then no well kick is entering.
- Pit Volume Increase
An increase in pit volume is obvious & positive indication of flow into the well bore & can be easily verified. If an increase in pit volume is seen, shut off the pump and make a flow check. If the well does not flow, no kick is entering.
- Decrease in Pump Pressure and Increase in Pump Stroke
In case of kick there is under balanced condition between the fluid in the drill pipe and the mixed column of mud and influx in the annulus. Therefore circulating pressure gradually decreases and unless the pump throttle is changed, pump speed slowly increases. This is not a confirmatory indicator as same indications may come due to pumping of light mud slug while circulation.
To determine kicks early while drilling , the driller should have instantaneous readouts or charts of average pit volume of the active pits, mud gained or lost and return flow rate. Preferably the pit volume and return flow rate will be recorded on the floor to establish trends. The driller should learn to monitor this outputs closely and insist that he be informed prior to any change in operating status of the mud equipment. As soon as an unexpected change in the monitors occurs, he should pick up and check for flow.
When the fast rise in pit volume /increase in flow rate is noted, pick up the string, stop the pumps, and check for flow. If the well is flowing, shut in or divert the well. Proceed to kill the well. When an unexpected , slow rise in pit volume occurs in areas or intervals not known to be tight formation , same action as stated above is taken.
KICKS WHILE TRIPPING
When the pump is switched off, a reduction in BHP equal to annular pressure losses occurs. Hence the beneficial aspects of the ECD will be lost during a trip and a flow check should be conducted on every well before starting pulling out. It is common in most areas to slug the drill pipe while preparing for a trip. Slugging refers to the procedure wherein a quantity of mud is densified relative to the mud in the hole and pumped down the drill pipe. The mud in the drill pipe fall to a stable level which allows the string to be pulled in dry condition.
Kick detection will be more definitive and any subsequent well control procedures will be less complicated if the pipe is slugged after rather than before the flow check. To prevent kick while tripping, basic requirement is that hole must be kept full of mud and the volume of mud required to fill the hole must be equal to the steel displacement of drill string pulled out.
The sequence of events to a kick while making a trip-out of hole is :
- Hole remains full or does not take proper amount of mud. Whenever such situation is
noticed the pipe should be run back to bottom and mud be circulated to clear the hole. - Flow from the flow line
- Increase in pit volume.
The sequence of events leading to a kick while tripping-in the hole is : - The hole does not stop flowing during making connection between the stands
- Increase in pit volume
In order to avoid well kicks while tripping, trip schedule must be made and trip tank must be used to monitor the hole fill up (in case of tripping-out) and mud displacement (in case of tripping-in).
A kick occurring while coming out of the hole will be indicated first by failure of the well to take a volume of mud equal to the pipe volume removed. When going in the hole, the pit level will increase more than the volume of pipe run. The Driller should know the volume of 10 stands of pipe and check the volume of pit gain at intervals when recorders are in use to make certain that the pits are not gaining too much mud. When a drill pipe in the hole and flow is noted during tripping, a safety valve is stabbed immediately and the well closed in observing pressure restriction.
Management of Non Shearable and Non Sealable tubulars: The drill string constitutes of Kelly, drill pipes and bottom hole assembly commonly known as BHA. The drill pipe section includes conventional drill pipe and heavy weight drill pipe. The BHA section includes drill collars, stabilizers, jars, reamers, directional drilling tools etc.
The heavy weight drill pipe and most of the BHA tools are thick walled tools. These tools and tubulars cannot be sheared in emergency with the pressure available from the BOP control unit and are categorised as non shearable tubulars.
The downhole tools like bit, stabilizers, casing scrappers, under reamers etc. are having irregular shapes and some have also got cutting structures on the outer body. In the event when these tools are positioned against the BOP and with some activity in the well it becomes to shut-in the well against these tools. The cutting structure will damage the rubber sealing elements of the preventers and will not seal properly on shut-in. These tools are categorized as non-sealable tools.
The following precautions must be taken while tripping out or tripping in of these non-sealable and non shearable tools and tubulars. - Proper flow check should be done prior to pulling out the last stand of drill pipe connected with the BHA.
- The safety valve (FOSV) should be made up with the proper size of the cross over as per the connection of the BHA prior to pulling out.
- Since RAM BOP will not close and seal on the BHA tubulars it becomes necessary to close the annular preventer for shut-in of the well.
- Attempts should be made to trip in one stand of conventional drill pipe stand so the well can be shut-in properly on the drill pipe.
- If situation does not permits to shut-in the well on the BHA tubulars or tools then arrangements should be made with all precautions to drop the string in the wellbore and shut-in the well on Shear/Blind ram.
If the flow is detected with the bit out of hole, blind rams should be closed immediately observing pressure restriction.
WHEN KICK OCCURS WHILE OUT OF THE HOLE
When crews are busy changing bits, breaking down core barrels etc. , they sometimes forget to observe the flow line and hole. Before sufficient mud flow shows on a pit level device, a small flow will begin at the flow line. If this noticed there will be time to take control measures before serious difficulties develop. If it is detected after the rate has built, control is more difficult. There is also a danger of the fluid level dropping when pipe is out of the hole, due to seepage or static losses. Therefore the well must be monitored while pipe is out of hole. - When one or more positive kick signs are observed, flow check is made. In case of self-flow well can be shut-in as per api RP 59.
Disclaimer: General reading purpose only.