Well control for slim hole wells can be split into two areas for both continuous coring and downsized conventional drilling: Kick detection/ shut-in procedures and Well killing.
1 Kick detection and shut-in procedures for slim hole wells
1.1 Need for early kick detection system
Early kick detection enables the volume of influx to be minimised, thus minimising the shut-in pressures at the casing shoe and on surface. As the diameter of a well decreases, the height of a given volume of influx increases; and with it the reduction in hydrostatic pressure on the formation. During the drilling/ killing process the dynamic circulating pressure drops in the annulus are greater.
Traditionally kick detection while drilling has been by active mud system pit level measurements. While tripping a smaller trip tank is used. In these systems the sensors are simple and the information processing is limited to summing up the outputs and plotting against time. A manually set high/low alarm is usually used. This results in a simple, cheap system easily maintained in the field.
Such systems are adequate for conventional hole sizes, but are insufficient for many slimmer holes.
1.2 Kick detection while drilling by pit level increase
During a kick, the formation fluid starts to mingle with the mud flow up the annulus. The flow velocity and hence pressure drop up the annulus is increased. This increase in annular velocity travels up the annulus as a pulse or wave. When the wave of increased velocity reaches the bell nipple the mud level rises very slightly. The level in the flow line then rises slightly, thus absorbing some of the influx volume. The level in the sand trap then rise. Only then does the level in the active pits start to rise.
Once the pit level increase has been alarmed, and the driller has often mentally checked that the level increase is not due to transferring of mud, he then traditionally performs a flow check. If the well is observed to be flowing he shuts in the well.
Because of this process, for a non-heaving rig the volume of kick actually in the well at the end of the shut-in procedure is about three times the volume detected by mud pit level increase. Thus, if a pit level system is set to alarm at 10 barrels then, with a competent alert crew, the actual influx volume after the well has been shut-in and stabilised cannot be guaranteed to be less than 30 barrels.
The reasons for this additional volume of influx are summarised below:
·The mud level rise in the bell nipple, flow line, shaker pan, and sand trap absorb an additional volume of mud before the pit level sensors respond. This delays detection, thus allowing more formation fluid to enter the well bore.
·This additional influx is usually of lower density than the mud in the well and thus the pressure on the formation decreases, therefore the flow rate of formation fluid into the well bore increases.
·When drillstring rotation is stopped much of the dynamic pressure drop up the annulus is removed, the pressure on the formation is reduced and the flow rate of formation fluid into the well increases. For slim wells this loss of ECD can be significant.
·When the pumps are switched off the remainder of the dynamic pressure drop up the annulus is removed and the flow rate of formation fluid into the well increases. For slim wells this can be a significant effect.
·The delay while closing the BOP and then shutting the choke also increases the volume of the influx.
1.3 Kicks at connections and during flow checks
The above analysis can be extended to connections. During normal drilling the flow out of the well decays gradually after the pumps are shut down. This is principally due to:
. the expansion of fluid in the drillpipe, and to a lesser extent in the annulus, as the stand pipe pressure is reduced.
·The contraction of the wellbore as the dynamic component of the annulus pressure is removed.
In the event that the well is being drilled close to balance the most likely time for a kick is during this decay period. Therefore sensitive delta flow kick detection systems require software to be able to rapidly and reliably distinguish the decay when there is no influx from the decay when there is a kick. When the pumps are shut off prior to making a connection or making a flow check, the flow out decays to zero. If a kick is taken the flow first decreases and then starts to increase rapidly.
1.4 Kicks during tripping
Many kicks occur when tripping out of hole, and the tight clearances used in slim wells means that sensitive kick detection instrumentation should be used during this phase of the operations.
1.5 Kicks during wireline logging
The clearances between logging tools and the borehole are typically reduced in slim wells. Long logging sondes can form excellent "swab tools". In addition, thick mud and external filter cakes can improve the effectiveness of these "swab tools" as well as making running into hole difficult. Therefore kicks while logging in slim wells are more likely than in conventional wells. Modern kick detection systems can record the change in mud volume due to the wireline running in and out of the well.
Indications of swabbing the well in can come from:
·surface cable tension,
·downhole cable tension,
·kick detection system on the trip tank,
·visual observation of the bell nipple.
1.6 Kicks on floating rigs
For a floating rig which may be heaving, pitching and rolling the situation is much worse, because:
·the pit level sensors are less sensitive due to mud movement in the tanks,
·the flow line is usually longer than on a land rig,
·the changing volume of the slip joint at the top of the marine riser,
·the longer time to shut-in and hang off.
2 Shut-in procedures for slim hole wells
The establishment and monitoring of safe shut-in procedures are a critical part of the planning and drilling of slim wells. Precise procedures have to take into account individual rig and surface geometry, the geometry and conditions of the well, the sensitivity of the kick detection system, casing and formation strengths, mud density, etc. as well as safety of personal.
The shut-in procedures should be integrated with the detection equipment and procedures. These include delta flow kick detection system, pit levels, visual observation, drilling breaks, trends extrapolated from Logging while drilling and mud logging, etc. Automatic alarms should be set at a level commensurate with minimum false alarms and lowest influx volume.
A hard shut-in procedure should be used for the following reasons:
·The influx volume and height is smaller than when a soft shut in (i.e. closing in the well on the choke after the BOP is closed) is used.
·The pressure at the casing shoe due to the water hammer effect during a hard shut-in is less than that due to circulating out the larger kick which would be taken were a soft shut in to be used.
·A flow check with the BOPs open allows additional formation fluid to enter the well bore. Therefore a pressure check performed after a preventer has been closed is substituted (dependent on having sensitive gauges).
·Keep the drillstring rotating and the mud pumps on until the drillstring is in the correct position for shutting the annular preventer. This maintains the dynamic component of the annulus pressure as long as possible, thus minimising the volume of the influx.
·If the kick detection system is giving a large number of false alarms the traditional "open BOPs" flow check may be done to minimise wear on the BOPs.
·Care should be taken not to shut in before the pumps have fully stopped (more likely with a hard shut in) which can lead to accidentally fracturing the well. This can permanently weaken the formation, and the later return flow of mud into the well can be mistaken for a fresh kick.
3 Improved kick detection systems for slim hole wells
The shortcomings of simple monitoring of the pit levels to detect kicks and losses has long been recognised. Numerous improved systems have been proposed. The more important are:
·paddle flow meters in the flow line,
·delta flow meters,
·trip tanks,
·acoustic influx detection systems.
4 Paddle flow meters
Paddle flow meters (Flo-Shos) in the flow line are deflected by the mud, and in use, plane or bounce on top of the moving mud stream. They have the advantages of simplicity, cheapness and reliability. They can be quickly fitted to any geometry of flow line, including an open flow ditch which in many areas, especially those prone to clay balls, is a common and practical arrangement.
Due to the bouncing effect, they have a highly non-linear response and have an accuracy of 10%. The readings are also influenced by mud viscosity. This leads to a tendency to set the alarm points fairly wide to avoid false alarms. There is no compensation for the dynamics of starting or stopping circulation, or for vessel heave on floating rigs. The alarms are usually set once circulation has been established, and the device should be considered as an indicator of flow change. Kick detection by paddle meter should be considered for Slim hole Drilling (SHD) only when the chances of a kick are small and the consequences of late detection of a kick are not serious.
Many attempts have been made to improve the sensitivity of these devices, of which three will be briefly discussed.
1.Computer processing of data. The principle is to apply data processing to the output of this sensor to overcome its inherent deficiencies.
2.Use an acoustic level detector on the flow line. This measures the level of mud in the open flow ditch using an acoustic transmitter/receiver system. The velocity of sound in air changes with temperature and gas concentration, so alternate pulses are deflected to a fixed target at a constant known distance from the transmitter/receiver unit. This system does not measure the return flow rate directly but only the height of mud in the flow ditch. As with the paddle it should only be considered as a change of flow indicator.
3.Rolling paddle. The principle is that a small rubber-tyred and buoyant wheel is substituted for the paddle. This is allowed to free wheel in the mud stream. The concept is that the rotation of the wheel reduces the size of the "bow wave". The device is then floating rather than planning/bouncing in the mud stream. Initial tests showed that the signal variation or noise was smaller than with a paddle, and that the response is more linearly related to flow. Tthe device was tested on a number of wells, but found to give no significant practical advantage over the paddle.
5 Delta flow kick detection systems
Several delta flow kick detection systems have been developed.
5.1 Sedco Forex MDS system
One approach is to take conventional sensors and to apply sophisticated computer modelling to their outputs. This has been done by Sedco Forex as part of their MDS system. This should be taken as representing the best which can be achieved with conventional instrumentation. The methodology is to use a statistical trend analysis to derive a delta flow for drilling using inputs from pit level, paddle flow and pump stroke data. Mud pit level is modelled for response at connections. There is a trip tank algorithm. The system is part of the MDS comprehensive drilling parameter instrumentation.
The following influx detection levels were established which were in line with published experience.
·Drilling:
1.pit level increase 8 bbls
2.Delta flow 4 bbls at alarm (Integrated over time)
·Connections:
Pit level increase 9 bbls
·Tripping:
Trip tank 2 bbls
The system still lacks the sensitivity required for all but the most benign slim hole drilling applications, and the addition of magnetic flow meters would seem to be a cost effective upgrade. Unless a MDS system is already installed on the rig it is possible that other solutions would be cheaper.
5.2 Other Delta flow systems
More sensitive systems have been developed specifically for slim wells by various operators and services companies.
This guide will concentrate on the Kick Detection System (KDS) from BHI:
·it is believed to be the most sensitive,
·it is believed to be the most robust operationally,
·it can detect kicks during most rig operations,
·it can and has worked from floating rigs.
The system was originally specified to be able to alarm on an influx of 50 metres of annular influx in 8 1/2" hole which is equivalent to:
Influx equivalent to 50 m metres of annular volume
Hole BHA Influx
8 ½ 6 ¼ 5.3 bbl
5 7/8 4 ¾ 1.9 bbl
4 1/8 3 ¾ 0.5 bbl
2 5/8 2 3/8 0.2 bbl
Field experience in hole sizes from 4 1/8" up to 8 1/2" suggests that the system can be relied upon to detect an influx of about one barrel (159 litres), at a rate of about 25 litres/min. (Larger hole sizes can be monitored with a larger diameter flow out meter).
6 The Baker Hughes INTEQ Kick Detection System (KDS)
The system was designed for downsized conventional drilling. Drilling with mining or continuous coring geometry is described in section 15.
Normally, three flow meters are installed to accurately measure the delta flow; one on the flow line, one on the stand-pipe, and one on the flow line to the trip tank. For water-based mud operations, electromagnetic flow meters have been successfully field proven to be reliable and accurate to within ±1% of the total flow rate. These can only function effectively over a limited fluid velocity range. In practical terms this means that they have to have approximately the same diameter as the well being drilled. Usually this means that the flow line flow meter is only used for the final two or three hole sizes. It has to be operated fully flooded at all times and this can give some practical problems with clayballs.
Magnetic flow meters can only measure flow of conducting fluids, i.e. water and brine-based muds. Various "Coriolis effect" mass flow meters offer the desired accuracy for Kick Detection System (KDS) application and can be substituted for the electromagnetic flow meters when drilling with non-conducting fluid (OBM). They can also be used for drilling underbalanced with nitrified fluid returns. However, when the Krohne meter was tested on an underbalanced well, accuracy deteriorated rapidly when there was more than 1% gas or nitrogen by volume in the returns. The meters also have a higher pressure drop and on many rigs a froth pump needs to be installed after the meter to lift the returns back up to shale shaker level.
The flow rate into the well can also be obtained by using pump stroke counters. The Kick Detection System (KDS) also includes a hook load transducer, a hook position sensor and a stand-pipe pressure transducer to determine the operating mode of the rig.
The signals from the sensors are relayed via an analog/digital converter to a computer where they undergo real-time processing by means of dedicated software, which acts to approximately model the dynamics of the mud in the hole. The software requires various parameters to be calibrated on-site in order to successfully model and predict the flow out anomalies created by pump speed changes and pipe movement for a given particular rig/mud/hole configuration. A flow out, compensated for such pump and pipe movement effects, is thereby calculated and compared to the actual measured value. The difference between calculated and measured flow out, termed the "kickflow", will remain close to zero except for when formation influx or loss occurs. If this difference exceeds a certain practically determined threshold (ca. 25 litre/min), set to minimise false alarms, and then if the time integrated difference becomes larger than a pre-set volume limit (i.e. 1 barrel), a kick alarm is given. Losses are similarly detected.
Various onshore field trials on slim hole/high pressure wells, for hole sizes ranging from 8.5" to 4.125", have shown that the Kick Detection System (KDS) can detect influxes of less than one barrel (159 litres), if the compensated difference between flow-in and flow-out exceeds ca. 25 litre/min. This sensitivity constitutes a major improvement over conventional kick detection methods.
7 "Stand-alone" Kick Detection System (KDS) provides cost reductions
A "stand-alone" Kick Detection System (KDS) can be used to reduce costs. The system include a driller's control/alarm console, so that the Kick Detection System (KDS) and driller can always directly communicate without the requirement of the operator. Also, the Kick Detection System (KDS) software contains various different automatic features which, for instance, allows the system to make minor temporary adaptations to its kick detection sensitivity depending upon the complexity of rig operation, thus permitting a reduction in false alarms. A statistical analysis of the flow data is also in place, allowing the driller to be warned of delta flow offsets below 25 litres/min., which may correspond either to a flow meter calibration drift or a very slow kick.
8 Kick Detection System (KDS) for floating rigs
Direct application of the Kick Detection System (KDS) on floating rigs is complicated by the heave motion of the vessel. The heave induces flow-out variations which are sufficiently large to reduce kick detection sensitivities and give erroneous alarms. Thule Rigtech have developed a flow compensation system (FCS), which is available commercially.
The mud flow is taken from just below the riser slip joint, via a riser pup joint through a second riser, to an outlet manifold tank. The mud subsequently discharges, via a flexible hose, into a buffer tank, from where it is continuously pumped back into the original flow line. Heave compensation is accomplished by attaching the second riser flow line manifold to a counter weight, in reality a 2 ton water bag, via a sheave system, thus enabling the counter weight to travel up and down with the movement of the rig, whilst the outlet manifold remains stationary. Both the flexible hose and the counterweight sheaves system represent couplings which allow for the relative movement between the floating rig and the stationary riser. The flow-out meter is placed on the stationary second riser just below the outlet manifold, instead of on the normal flow-out line. The only (negligible) variations therefore associated with heave in the return flow measurement are caused by the steel displacement of the moving inner barrel.
Alternative software-based solutions are also available. These employ real-time sensor measurements of either the fluid level in the riser, or the movement of the riser slip joint, so that the Kick Detection System (KDS) computer model can compensate the flow out data for rig heave motion effects. Kick detection accuracy depends on the heave. The complete drilling and kick detection systems can be mobilised and installed within one week without adapting the riser, a major operational advantage over the Thule system.
9 Trip tanks
Trip tanks for slim wells are similar to those on conventionally sized wells. Enhanced sensitivity is required to detect small influxes. The BHI Kick Detection System (KDS) has a mode for kick detection when the main mud pumps are not in use. A key part of this system is that the mud flow through the tank is monitored, thus enabling the system to determine rig status.
10 Acoustic influx detection systems
These systems rely on the difference in acoustic impedance between mud and gas. Normally an acoustic sensor is positioned in the bell nipple. This listens to the pump strokes. When gas enters the well bore the signal undergoes a phase shift. When the phase shift exceeds a threshold value an alarm sounds. They are not a substitute for a delta flow kick detection system and are unlikely to be cost effective in many routine slim well operations.
They have the following features:
·They detect only gas influxes.
·The influx has to be in its gas phase when it enters the borehole. This means that the system does not work when the bottomhole pressure exceeds 13,000 psi for water-based mud and not at all for oil-based mud.
·When the gas from one influx is in the annulus the system cannot detect a second influx.
·The system does not work when there is no circulation.
·The system has potential as a connection gas detector in deep wells as part of an integrated mud logging system.
11 Killing of for slim hole conventional wells
11.1 Moderately pressured wells
Killing procedures for downsized conventional wells are in principle the same as for conventional wells. The only modification to conventional practices is to take the dynamic component of the annular pressure drop into account during the killing process.
11.2 High pressure/high temperature wells
Usually in the deep small sections of HP/HT wells there is a small drilling margin between pore pressure and formation strength. This means that the drilling system has to be optimised to give maximum penetration rates while maintaining adequate flexibility to cope with kicks and losses or a combination of the two. The high circulating and static borehole temperatures, heavy (and often thick) muds and the fact that often operations are conducted from a heaving rig, add to the operational problems.
The fastest way to drill small, deep sections of HP/HT wells is often to use a high power Moineau HT motor, thruster, and MWD while rotating the drillstring very slowly. Often the optimum combination is an impregnated diamond bit and high speed motor.
The rationale for this optimum combination is that the overriding constraint when designing the drilling system is to contain the dynamic pressure drop up the annulus within the gap between formation strength and pore pressure.
In many cases there may be reluctance to run such a BHA in view of potential problems of pumping LCM and cement through the BHA. In many cases this is based on experience with turbines which are more susceptible to plugging than Moineau motors. To provide flexibility in overcoming some of these potential problems, a 3 3/4" drilling safety joint compatible with the HP/HT downsized conventional approach to slim wells has been designed, although not yet built.
12 Kick detection and killing of slim wells drilled with continuous coring geometry
The geometry of continuous coring means that about 90% of the circulating pressure drop takes place in a very small annulus, resulting in high ECDs. If a (gas) influx is taken, a very small influx volume can dramatically reduce the hydrostatic pressure in the annulus, and this could in extreme cases, fracture the casing shoe.
12.1 Dynamic killing of slim wells with continuous coring geometry
Link to alternative methods - continuous coring
12.2 Modified "shut-in-and-think" well kill for wells with continuous coring geometry
BP and Statoil have used continuous coring on four wells in the Congo using a recognised oilfield drilling contractor, Parker, and rather larger rig and a "retrofit" package of downhole equipment and top drive. They have published extensively, and these papers form a useful starting point if it were decided to proceed further with this technique. From the well safety perspective they increased the annular clearance between the pipe and hole, developed a flow in/flow out Early Kick Detection (EKD) system and modified the standard "shut-in-and-think"-based well killing procedure.
During the final stages of circulating out a gas kick surface pressures will be higher than with downsized conventional geometry wells.
Provision should be made for the use of a lubricator when recovering the core barrel insert with wireline.
12.4 Kick detection for slim wells drilled with continuous coring geometry
Kick detection for slim wells drilled with continuous coring geometry also employs a delta flow kick detection system. There are however several critical differences between the two applications which the potential user should be aware of. These are:
·The drillpipe/hole annulus is very much smaller compared with the downsized conventional system where the narrow annulus is only over the BHA.
·Typically drillpipe rotational speeds are higher.
·Influxes are longer.
·ECDs are higher.
·Dynamic effects during the decay of flow out of the well are different from those in the conventional geometry as they are much more influenced by the DP rotation.
·When tripping out of hole with a full faced bit or core barrel with the insert in place, sensitive detection is required to detect swabbing in of the well.
·Kick detection has to be provided when recovering the core barrel insert on wireline.
From the perspective of a kick detection system this means that:
·The system has to be more sensitive than for a downsized conventional well.
·A computer model of the decay at connections and flow checks which has been developed for downsized conventional geometry should be revalidated for the precise geometry in use, and will require DP rpm as an input.