Lost circulation In Drilling is any loss of whole mud to subsurface formations during 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.
For a full understanding of Mud Loss or Lost Circulation Problems, 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
What Is The Effects Of Lost Circulation Drilling Problem?
Reducing flow in the 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 regions and fall back to the bottom when the pump stops. This fall could cause pipe sticking.
The loss of mud into the formation may also lower the mud level in the wellbore, 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. In other words, It can cause wellbore instability problems.
If hydrostatic pressure reduces against the permeable formations, the well may kick, which is a hazardous situation of lost circulation. If kick warning signs are ignored & the kick fluid flows into the lost zone, this can definitely lead to an underground blowout, the worst situation for well control.
The benefits of maintaining circulation include 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
Signs:
If some returns are evident at the flow line, the hole should stand full with the pumps off; however, the fluid level may fall slowly.
Mud Loss Caused By Naturally Fractured Formations
However, lost circulation while drilling by naturally fractured formations can be a 100% loss of returns with no preceding gradual losses; it also may occur at overbalances as low as 50 psi.
Signs:
- 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, limestones, dolomites, and salts dissolved by groundwater create fractures and caverns that vary in size.
Signs
Lost Circulation while drilling operations of cavernous/vugular formations are usually the most accessible type to recognize as immediate 100% loss of returns occurs, accompanied by loss of weight on bit.
Curing loss of Circulation in Drilling:
The likelihood of success in regaining returns is 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 Loss to induced fractures is the most common type of lost circulation in drilling oilfields. 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.
Signs:
- Occurs during pressure surges, i.e., during connections or trips (check also: tripping pipe procedures).
- Often occur during routine increases of mud weight or
- It 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.
Understanding pore pressures, fracture gradients versus equivalent circulating densities, and surge pressures with the mud properties used in a particular area is essential.
Classification of Circulation Losses in Drilling
Correctly treating 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 loss circulation in drilling operations usually occurs slowly and is 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 done with the appropriate treatment. Seepage losses may be economically acceptable with high rig rates with 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 before 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. Usually, drilling must be stopped because the drilling fluid cannot perform properly. The cost of mud and rig time lost becomes essential. Logistics and mud-building capabilities may be limited, so it may be necessary to take 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.
Effects Of Severe Mud Losses
- This can create a sound control situation, as the fluid level falls in the annulus and hydrostatic pressure is 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. Refilling the annulus with monitored volumes of lighter mud and/or water or base oil is necessary when a complete loss occurs. 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 helps distinguish 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 the 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 of the mud and no switching on for solids control equipment.
- Fourthly, check that the drilling 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
Temperature Survey.
It usually 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).
Spinner Survey.
Flow rate is indicated on film according to the speed of a varied rotor on the instrument.
Tracer Survey.
Using a gamma ray log and radioactive material.
Conventional.
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.
Variation.
If the loss point is suspected near the last casing setting depth, 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.
