What is Mud Loss | lost circulation drilling
Lost circulation In Drilling is any losses of whole mud to subsurface formations while any phase in drilling operations. Lost circulation is one of the primary contributors to high mud costs. In this article we will handle their effects, types, types of thief zones, first actions & methods for locating loss zones. Also we will cover the causes, remedial, LCM & preventive actions for lost circulation problems in other articles.
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For full understanding of Mud Loss or Lost Circulation Problem I recommend that you move to the following articles:
- Causes Of Lost Circulation Problem
- How To Cure Lost circulation Problem,
- Precautions to consider to avoid Mud loss problems
- Lost Of Circulation Material Used In drilling
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The reduction of flow in annulus above the loss can cause many other problems. Slow annular velocity lowers the carrying capacity of the mud. Cuttings may accumulate in low velocity region and fall back to the bottom when the pump stopped. This fall could cause pipe sticking.
The loss of mud into the formation may also lower the mud level in well bore which will lower the hydrostatic pressure in the hole. In shale sections, this induced lesser wall support may cause sloughing of shales which further may cause pipe sticking. I other words It can cause wellbore instability problems.
If hydrostatic pressure reduces against the permeable formations, the well may start kicking which is a very dangerous situation of lost circulation. If kick warning signs ignored & the kick fluid flow into the lost zone, this can definitely lead to an underground blowout, the worst situation for well control.
The benefits loss of maintaining circulation, such as obtaining good quality formation evaluation and achieving an effective primary cement bond on Casing.
Potential Types of Mud Lost Circulation Zones
We can classify Lost circulation while drilling operations into four basic types:
- Highly permeable formations
- Naturally fractured formations
- Cavernous formations
- Induced fractures due to a pressure imbalance
Permeable Zones (Pores/Matrixes)
Circulation mud Losses to drilled permeable zones may be anywhere up to 100% of the pumped volume as it consists of coarse sands and/or gravel, particularly in surface holes such as
- Shell beds
- gravel beds
- reef deposits
- depleted reservoirs
If some returns are evident at the flow line, the hole should stand full with the pumps off, however, the fluid level may start to fall slowly.
Mud Loss Caused By Naturally Fractured Formations
However, lost circulation while drilling by naturally fractured formations can be 100% loss of returns with no preceding gradual losses, it also may occur at overbalances as low as 50 psi.
- The hole normally will not stand full.
- Most often occur at shallow to intermediate depths.
- Particularly in tectonically stressed areas.
Mud Loss While Drilling Cavernous/ Vugular Formations
Over geological periods of time, limestones, dolomites, and salts dissolved by groundwater, creating fractures and caverns which vary in size.
Lost Circulation while drilling operations of cavernous/vugular formations are normally the easiest type to recognize as immediate 100% loss of returns takes place, accompanied by loss of weight on bit.
Curing Lost Of Circulation in Drilling:
The likelihood of success in regaining returns limited but usually you have two ways:
- Several attempts of diesel/oil/cement pills
- Drilling blind for several feet below the zone, then to running casing.
Mud Loss / Lost Circulation While Drilling Induced Fractures
Mud Losses to induced fractures is the most common type of lost circulation in drilling oilfield. The losses can be slow, moderate or complete, at any depth. Induced fractures generally occur when ECD in drilling exceeds the fracture gradient causing the formation to break down.
- Occurs during pressure surges, i.e. during connections or during trips.
- Often occur during routine increases of mud weight or
- Occurs during a Kick and kill operation.
- The hole may stand full or drop to an equilibrium point.
- In an induced case, it is possible that the hole will give mud volume back when the mud pumps are turned off.
It is very important to understand pore pressures, and fracture gradients versus equivalent circulating densities, and surge pressures with the mud properties used in a particular area.
Classification of Circulation Losses in Drilling
The correct treatment of lost circulation requires an appreciation of the loss rate and the type of loss zone. There are four primary lost circulation types in drilling operations:
Seepage Mud Losses
The Seepage Lost Circulation in drilling operations usually occur slowly and are sometimes very difficult to identify as there may be filtrate loss due to poor fluid loss control in some instances. Controlling and preventing seepage losses can be with the appropriate treatment. Seepage losses may be economically acceptable if there are high rig rates with a relatively low-cost drilling fluid. If pressure control is critical, the losses will have to be cured. It is always important to check all surface equipment and for any fluid transfers prior to diagnosing an apparent loss of returns.
Partial Mud Losses
Partial losses are more severe than seepage losses and usually require “lost-circulation material” additions to cure or slow down the losses. Normally drilling must stopped because the drilling fluid cannot perform its proper functions. The cost of mud and rig time lost becomes important. Logistics and mud building capabilities may be limited, so it may be necessary to take rig time to cure these losses.
Severe Mud Losses
Severe Lost circulation in drilling can be a serious problem. Large volumes of drilling mud may be lost in short periods of time.
Effects Of Severe Mud Losses
- This can create a well control situation, as the fluid level falls in the annulus and hydrostatic pressure reduced.
- Losses can also cause hole stability problems and to avoid that you have to keep the hole full as you can with water or base oil to the equilibrium point.
Note: An accurate record of all volumes and pills pumped must be kept so that hydrostatic head can be calculated.
Complete Mud Losses
Complete lost circulation in drilling is when there are no returns at all. The fluid level may drop out of sight. When a complete loss occurs refilling the annulus with monitored volumes of lighter mud and/or water or base oil is necessary. Determine the reduction in hydrostatic head and reduce the active system to this calculated equivalent mud weight. Monitoring the hole very closely for possible well control problems is a must.
Note: Prior to assuming that lost circulation to the formation has taken place, all surface equipment must be examined for leaks or breaks i.e. mud pits, solids control equipment, mud mixing system, riser slip joints, and/or incorrectly lined up pumps or circulating lines.
Lost Circulation In Drilling Assessing and Reporting Events
When reporting instances of lost circulation as this is helpful for distinguishing losses through pores which require only fine LCM and losses into fractures which can require coarser grades., consider the following:
- Firstly, Static Loss Rate Bbl/Hr.
- Secondly, Dynamic Loss Rate Bbl/Hr @ GPM.
- Thirdly, Depth.
- Fourthly, The loss rate behavior (gradually or suddenly)
First Action For Lost Circulation In Drilling Operations
When lost circulation occurs, we recommend following below procedure as it helps to make sure that the losses came from formation:
- Firstly, establish the loss rate.
- Secondly, check all surface equipment to ensure no losses.
- Thirdly, check with the mudlogger/mud engineer that there was no dumping or transferring for the mud and no switching on for solids control equipment.
- Fourthly, check that the riser slip joint packing is not leaking.
- Fifthly, you must consider these non-formation possibilities which are:
- Leaking casing. A packer should be run to determine the loss depth in this instance.
- Riser leaks.
Note : If losses are experienced whilst drilling, it is likely that the losses are on bottom and if losses are experienced whilst tripping or whilst increasing mud weight, it is likely that the loss zone is not on bottom.
Methods for establishing the Mud Circulation loss zones
Normally requires two runs. However, a variation of the technique can be used by stopping the sonde at a certain depth and pumping for a short period. This will give a temperature change. The instrument is lowered to successive levels in the hole until no temperature change is recorded (i.e. no flow to formation past this point).
Flow rate is indicated on film according to the speed of a varied rotor on the instrument.
Using a gamma ray log and radioactive material.
Run a base log through the Drill Pipe. Then pump a slug of mud with radioactive material down the drill pipe and repeat the log. Where the sonde encounters a high radioactivity it indicates the loss point.
If point of loss is suspected near last casing shoe, a small quantity of radioactive iodine can be pumped down the annulus. The sonde is run inside the drill pipe, following the tracer, while it is being pumped. The loss point has been reached when radioactive contact is lost.