The alertness in determining Kick early warning signs in well control is of the uppermost importance to wellbore safety. Careful observance and positive reaction to these signs will keep the well under control and prevent the occurrence of a well flow situation. In this article we will discuss also the positive indicators for oil well kicks.
The major kick warning signs includes the following:
- Kick Early Warning Signs
- Drilling Break ( Increase in drilling penetration rate)
- Increase in drilling torque & drag
- Decrease in shale density
- Increase in cutting size and shape
- Mud properties changes
- Increase in trip, connection & background gases
- Change in temperature of the return drilling mud
- Decrease in d-exponent
- Kick Positive Indicators
- The gain in mud pit level
- Increase in the return mud flow rate
- Well is flowing while pumps are off.
- Improper hole fill-up.
The various signs that have been recorded as kick early warning indicators are not consistent in all situations. The signs however may have to be used collectively as one indicator may not accurately provide the warning of getting into an unbalanced situation. Even though the series of signs may change between wells, early warning indications can be found from the following list.
Kick Early Warning Signs
Why do We consider Drilling Break As A Kick Early Warning Signs?
When drilling ahead and using consistent drilling parameters, as the bit wears, a normal trend of decrease penetration rate (Check the factors that affect drilling penetration rate) should occur. If the differential pressure between the hydrostatic pressure of the drilling fluid and formation pore pressure decreases, an increase in the drilling rate occurs as the chip hold down effect is reduced.
A general and consistent increase in penetration rate is often a fairly good indicator that a transition zone may have been penetrated. A rapid increase in penetration rate may indicate that an abnormal pressure formation has entered and an under-balance situation has occurred.
Increased Torque & Drag
Increased drilling torque and drag are often noted when drilling into overpressured shale formations due to the inability of the underbalanced mud density to hold back physical encroachment of the formation into the wellbore. Drag and rotating torque are both indirect and qualitative indicators of overpressure. They are also indicators of borehole instability problems and other mechanical problems. Torque and drag trend increases often indicate to the driller that a transition zone drilled and it may consider as a kick early warning sign. Up drag and down drag as well as average torque figures should be recorded on each connection. These trends are valuable when comparing other trend changes.
How a Decrease In Shale Density Can Be One Of Kick Warning Signs?
The density of shale normally increases with depth, but decreases as abnormal pressure zones are drilled. The density of the cuttings can be determined at surface and plotted against depth. A normal trend line will be established and deviations can indicate changes in pore pressure.
Increase In Cutting Size & Shape
In transition zones or in abnormally pressured shales (sandy shales and bedding sand streaks) the shales break off and fall into hole because of underbalanced conditions (pore pressure greater than mud hydrostatic pressure). Water wetting may further aggravate this problem.
Changes in the Shape of Shale Cuttings can occur as an underbalanced situation is developing (check also underbalanced drilling). The particles are often larger and may be sharp and angular in the transition zone. Extra fill on the bottom may coincide with the trend change. Severe sloughing will often cause changes in pressure and stroke relationship.
Normally high-pressure shales produce small cuttings with rounded edges and are generally flat, while cuttings from an overpressured shale are often long and splintery with angular edges. As a reduction of the hydrostatic differential between the pore pressure and bottom hole pressure occurs, the hole cuttings will have a greater tendency to come off the bottom. This can also lead to shale expansion causing cracking, and sloughing of the shales into the wellbore. Changes in cuttings shape and cuttings load over the shale shakers needs monitoring at the surface as it is one of the major kick early warning signs.
Changes In Mud Properties
Water cut mud or a chloride (and sometimes calcium) increase that has been circulated from bottom always indicates that formation fluid has entered the wellbore. It could be created by swabbing or it could indicate a well flow is underway. A small increase of chlorides in drilling mud or calcium increases could be indicative of tight (non-permeable) zones that have high pressure.
In certain type muds, the viscosity will increase when saltwater enters the wellbore and mixed with the mud. This is called flocculation because the little molecules of mud solids, which are normally dispersed, form little “group” called flocs. These flocs cause viscosity and gel increases.
In other drilling fluids types, you might see a viscosity decrease caused by water cutting (weight decrease). This is true when operating with low pH salt-saturated water based drilling muds.
In oil based muds, any water contamination would act as a “solid” and cause viscosity increases. Gas cut mud would be fluffy and would have higher viscosities (and lower mud weight).
It Is Essential To Know That Trend Changes Are More Important Than Actual Value Of The Change As A Warning Sign For Oil Well Kicks.
Increase In Trip, connection & Background Gas
It is nevcessary to monitor return mud for contamination with formation fluids. Doing this will be by constantly recording the flowline mud density and accurately monitoring gas levels in the returned mud.
Gas cut mud does not in itself indicate that the well is flowing (gas may be entrained in the cuttings). However, we must treat it as an early warning sign of a possible kick. Therefore closely monitoring pit levels is a must if significant levels of gas are detected in the mud.
An essential part of interpreting the level of gas in the mud is the understanding of the conditions in which the gas entered the mud in the first place. In general, gas can enter the mud for one or more of the following reasons:
- Drilling a formation that contains gas even with a suitable overbalance.
- Temporary reduction in hydrostatic pressure result from swabbing as pipe moved in the hole.
- Pore pressure in a formation being greater than the hydrostatic pressure of the mud column.
Gas due to one or a combination of the above, can be classified as one of the following groups:
When porous formations containing gas are drilled, a certain quantity of the gas contained in the cuttings will enter the mud. Gas that enters the mud, unless in solution with oil base mud and kept at a pressure higher than its bubble point, will expand as it is circulated up the hole, causing gas cutting at the flowline. Gas cutting due to this mechanism will occur even if the formation is overbalanced. Raising the mud weight will not prevent it.
Note that drilled gas will only be evident during the time it takes to circulate out the cuttings from the porous formation.
Measuring Connection gases at the surface as a distinct increase above background gas as bottoms-up occurs after a connection. They are result from the temporary reduction in the effective total pressure of the mud column during a connection. This is due to pump shut down and the swabbing action of the drill pipe.
In all cases, connection gases indicate a condition of near balance. When an increase trend of connection gases is identified, consideration should be given to weighting up the mud before drilling, operations continue and particularly prior to any tripping operations.
Trip gas is any gas that enters the mud while tripping the pipe with the hole appearing static. Detecting trip gas will be when circulating bottoms up occurs after a round trip. If the static mud column is sufficient to balance the formation pressure, the trip gas will result from swabbing and gas diffusion.
Significant trip gas may indicate that a close to balance situation exists in the hole.
Gas Due to Inadequate Mud Density
The surface indication of an underbalanced formation depends on the degree of under balance, as well as the formation permeability. Drilling of a permeable formation that is significantly overbalanced will cause an immediate flow increase followed by a pit gain.
Change In The Temperature Of The Return Drilling Mud
The temperature will normally take a sharp increase in transition zones. The circulating rate, elapsed time since tripping, and mud volume will influence flowline temperature trends.
The temperature gradient in abnormally pressured formations is generally higher than normal. The temperature gradient decreases before penetrating the interface and, therefore marked differences can give an early indication of abnormal pressures. This is usually a surface measurement that has a tendency to be influenced by operating factors.
Decrease In D-Exponent
Plotting the D-Exponent will be by the well loggers and maintained current at all times. This value introduced in the mid-sixties to calculate a normalized penetration rate in relation to certain drilling parameters.
The D-exponent may need correction and normalization for mud weight changes and/ or ECD (equivalent circulating density) by the following:
A plot of Dc-Exponent versus depth in shale sections used with moderate success in predicting abnormal pressure. Trends of the Dc-exponent normally increase with depth, but in transition zones, its value decreases to lower than expected values.
Mud logging companies have further variations/models which try to normalize for other parameters (such as bit wear and rock strength) to varying degrees of success. An illustration of a Dc plot is as figure 3.
Positive Kick Indicators
A Kick in drilling occurs when the hydrostatic pressure of the mud column in the well is less than the formation pressure provided that the formation has the ability to produce. A Kick is a positive indicator that formation fluid is entering the wellbore and Secondary Well Control must be initiated.
Positive Warning Sign For Well Kicks While Drilling
- Flow into the wellbore causes two changes to occur in the mud circulating system:
- Increase of active mud system volume.” gain in pit level”:
The gain in the pit level detected by pit level measuring instruments with the recorder mounted at the driller’s console and supported by the mudlogger
Good communication between crew members is essential on the rig as it will detect any warning signs for a well kick. Drillers should make sure crew hands notify them if they do anything to change the level in the pits. If crew hands are adding volume to the pits, they should also notify the driller when they stop adding volume.
- The mud return flow rate exceeds the mud flow rate into the well. “flow monitoring system”
- When drilling a formation containing gas, a minor pit level rise will be noted because of the core volume of gas being drilled. However, this will occur only as the gas nears the surface, and is due to the drilled gas expanding and is not necessarily an indication that the well got under balance.
- The timing of the increase in pit volume is important in distinguishing between a true Kick and gas expansion only. The hole will also take the same volume of fluid that it gave up after the gas bubble has reached the surface. However, if there is any question as to the cause of the pit gain, stop the pump and check the well for flow.
- On trips, the drill crew should be able to recognize a 5-barrel Kick or less. During drilling, the crews are generally able to recognize a 10 barrel Kick or less.
5- The size or severity of a Kick depends on the volume of foreign fluid allowed to enter the wellbore, which depends on
- The degree of underbalance.
- The formation permeability.
- The length of time it takes the drilling crew to detect that the well is kicking.
Improper Hole Fill Up | Recognizing a Kick While Tripping
Flow into the wellbore will cause improper hole fill up, if this is seen a flow check should be performed.
- If the flow check is positive then the well should be shut-in.
- If the flow check is negative the drill string should be run back to the bottom to circulate bottoms up (stripping operation may have to be used here).
- Trip Tanks are the safest and most reliable method of monitoring mud volumes on trips. It is recommended that a continuous hole fill-up be used when tripping out of the hole. When tripping in the hole the, trip tank should be used to ensure the correct mud displacement is taking place.
- Rig movement with a floating drilling rig makes it more difficult to recognize Kick warning signs while drilling or tripping in with Bottom Hole Assembly BHA. For this reason, additional fluid volume detection equipment installed in the mud pits to compensation for rig motion. It is necessary for floating drilling units that performing flow checks on the trip tank with the hole fill pump circulating across the bell nipple to eliminate rig motion as much as possible.
Situations That Can Mask A Kick
- Mud weight adjustments while drilling.
- Mud transfers while drilling.
- Partially lost circulation.
- Solids control equipment and degassing mud.
- Spills and leaks in surface equipment.
- Drain back.
- Pump start-up and shut down.
Good Kick Warning Signs Resources:
Ref: Saudi Aramco Well Control Manual 2006 – (Saudi Arabia Oil History)