Tuesday, 10 March 2020


Lost Circulation In Drilling

Lost Circulation Definition

Losses of whole mud to subsurface formations while any phase in drilling is called lost circulation or lost returns. Lost circulation has historically been one of the primary contributors to high mud costs. Other Hole Problems such as Wellbore instability, Stuck pipe and even Blowout have been the result of lost circulation. Besides the obvious benefits of maintaining circulation, preventing or curing mud losses is important to other drilling objectives such as obtaining good quality formation evaluation and achieving an effective primary cement bond on Casing.

Lost circulation occurs in one of two basic ways:
  1. Invasion or mud loss to formations that are cavernous, vugular, fractured or unconsolidated. 
  2. Fracturing which is mud loss due to hydraulic fracturing from excessive induced pressures.

Potential Types of Lost Circulation Zones

Lost circulation is classified into four basic types:
  1. Highly permeable formations
  2. Naturally fractured formations
  3. Cavernous formations
  4. Induced fractures due to a pressure imbalance
Lost circulations in drilling zones types

Permeable Zones (Pores/Matrixes)

Losses to drilled permeable zones may be anywhere up to 100% of the pumped volume. A permeable zone typically consists of coarse sands and/or gravel, particularly in surface holes. Shell beds, gravel beds, reef deposits and depleted reservoirs can also be classified as permeable formations. 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.

lost circulation in drilling Naturally Fractured Formations

Losses to naturally fractured formations can be 100% loss of returns with no preceding gradual losses. These losses may occur at overbalances as low as 50 psi. The hole normally will not stand full. Natural fractures can be encountered at any depth but most often occur at shallow to intermediate depths, particularly in tectonically stressed areas.

Cavernous/ Vugular Formations

Losses to drilled cavernous/vugular formations are normally the easiest type to recognize. Immediate 100% loss of returns takes place, accompanied by loss of weight on bit. Over geological periods of time, limestones, dolomites, and salts were dissolved by ground water, creating fractures and caverns which vary in size. The likelihood of success in regaining returns is limited. Some small zones may respond to diesel/oil/cement pills, however, several attempts are usually required. The most common solution is to drill blind for several feet below the zone, then to running casing.

Induced Fractures

lost circulation in drilling problem identication
Losses to induced fractures is the most common type of lost circulation. The losses can be slow, moderate or complete, at any depth. Induced fractures generally occur when ECD exceeds the fracture gradient causing the formation to break down. These losses can also occur during pressure surges, i.e. during connections or during trips. Induced fractures often occur during routine increases of mud weight or 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 being used in a particular area.

Classification of Circulation Losses

The correct treatment of lost circulation requires an appreciation of the loss rate and the type of loss zone. There are four primary loss types in drilling operations:

Lost Circulation severity classification

Seepage Losses

Seepage losses in drilling usually occur slowly and are sometimes very difficult to identify. There may be filtrate loss due to poor fluid loss control in some instances. Seepage losses can usually be controlled or prevented 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 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 be 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 Losses

Severe losses in drilling can be a serious problem. Large volumes of drilling mud may be lost in short periods of time. This can create a well control situation, as the fluid level falls in the annulus and hydrostatic pressure is reduced. Severe losses can also cause hole stability problems. It is important that the hole is filled with water or base oil and kept full to the equilibrium point. An accurate record of all volumes and pills pumped must be kept so that hydrostatic head can be calculated.

Complete Losses

Complete lost circulation is when there are no returns at all. The fluid level may drop out of sight. When a complete loss occurs the annulus should be refilled with monitored volumes of lighter mud and/or water or base oil. Determine the reduction in hydrostatic head and reduce the active system to this calculated equivalent mud weight. The hole must be monitored very closely for possible well control problems.

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 Assessing and Reporting Events

When reporting instances of lost circulation, the following is to be included:
  • Static Loss Rate bbl/hr. 
  • Dynamic Loss Rate bbl/hr @ gpm. 
  • Depth.
Note if the losses build to the loss rate gradually or suddenly. This is helpful for distinguishing losses through pores which require only fine LCM and losses into fractures which can require coarser grades.

Non Formation Circulation Losses

When lost circulation occurs the following procedure should be followed:
  • Establish loss rate.
  • Check all surface equipment to ensure no losses.
  • Check with mud logger/mud engineer that no mud has been dumped/transferred or no solids control equipment has been switched on.
Check that the riser slip joint packing is not leaking. Other non formation possibilities are:
  • Leaking casing. A packer should be run to determine the loss depth in this instance.
  • Riser leaks.

Formation Circulation Losses

If losses are experienced whilst drilling, it is likely that the losses are on bottom. 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 loss zone are:
  1. Temperature Survey. 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).
  2. Spinner Survey. Flow rate is indicated on film according to the speed of a varied rotor on the instrument.
  3. Tracer Survey. Using a gamma ray log and radioactive material.
    1. 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.
    2. Variation. 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.

Causes of lost of circulation

A) Invasion

In many cases, lost circulation cannot be prevented in drilled formations that are cavernous, vugular, fractured or unconsolidated. Depleted low-pressure formations (usually sands) are similar relative to lost-circulation potential. 

1 Coarse, unconsolidated formations
Coarse, unconsolidated formations can have sufficiently high permeability for whole mud to invade the formation matrix, resulting in lost circulation. This high permeability is often present in shallow sands and gravel beds. Formations that were once reefs and oyster beds also have similar tendencies. One important reason for preventing mud loss in shallow intervals is that it may cause these unconsolidated formations to wash out, forming a large cavity that is less stable which could cave in more easily from overburden and rig weight.

2 Depleted Formations
Another potential loss zone is in depleted formations (usually sands). Producing formations in the same field, or general vicinity, may cause subnormal (depleted) formation pressure due to the extraction of the formation fluids. In such a case, mud weights required to control other exposed formation pressures may be too high for the depleted formation, forcing mud to invade the lowpressure depleted formation. If this situation exists, plans should be formulated to prevent lost circulation or stuck pipe from occurring in the depleted zone. Special bridging agents and sealing materials should be used to form a good seal and filter cake on the depleted zone.

3 Cavernous or vugular zones
Cavernous or vugular zones are usually associated with low-pressure carbonate (limestone and dolomite) or volcanic formations. In limestone, vugs are created by the previous continuous flow of water that dissolved part of the rock matrix (leaching), creating a void space often later filled with oil. When these vugular formations are drilled, the Drill String may fall freely through the void zone and a rapid loss of mud is usually experienced. The volume and persistence of this kind of loss depends on the degree to which the vugs are interconnected. Similar vugs and caverns can develop during the cooling of volcanic magma or ash. Cavernous and vugular formations are often easily traceable from offset wells and predictable from mud logs and lithology.

4 Fissures Or Fractures
Mud loss also occurs to fissures or fractures in wells where no coarsely permeable or cavernous formations exist. These fissures or fractures may occur naturally, or may be initiated or extended by hydraulically imposed pressures. Natural fractures exist in many cases, which may be impermeable under balanced pressure conditions. Losses may also occur at unsealed fault boundaries.

B) Fracturing

5 Hydraulic fracturing is initiated and lost circulation occurs when some critical fracture pressure is reached or exceeded. Once a fracture is created or opened by an imposed pressure, it may be difficult to repair (heal) and it may never regain the original formation strength, as shown later in Figure 5. Lost circulation may persist even though the pressure is later reduced. This is one reason why it is better to pretreat for, and prevent, lost circulation than to permit it to occur. Lost circulation resulting from induced pressure is usually caused by one of two situations:

Setting intermediate casing in the wrong place. If casing is set above the transition zone crossing from normal to abnormal pressures, the pressures exerted by the heavier mud (required to balance the increasing pressures) will often induce fracturing at the weak casing seat. Losses due to fracturing are most commonly near the previous casing seat, not at bit depth, even if casing is properly set.

6 Excessive downhole pressures are the result of many conditions including: 

1) Mechanical forces.
  1. Improper hydraulics. Excessive pump rates and velocities causing high Equivalent Circulating Density (ECD) pressures.
  2. Increasing pump rates too rapidly after connections and trips. This can be extremely important when dealing with drilling oil-base fluids. Failure to bring the pumps up to speed slowly can put much higher circulating pressures on the formation due to the tendency of drilling oilbase muds to thin at higher temperatures generated while circulating and to thicken at lower temperatures during trips. It is common for circulating pressures to decrease 100+ psi as the mud heats to circulating temperature. 
  3. Raising or lowering the pipe too fast (surge/swab).
  4. Spudding bridges. 
  5. Excessive Penetration Rate (ROP) for a given flow rate will result in high cuttings concentration in the annular fluid causing a high ECD. 
  6. Pipe whipping.
2) Hole conditions.
  1. Sloughing shale or increased solids loading in the annulus and high equivalent circulating density.
  2. Accumulation of drilled cuttings in a washed-out portion of the hole or in the drilling mud. 
  3. Cuttings beds or Barite sag forming on the low side of a directional well, or possible slumping.
  4. Bridges. 
  5. Kick and well-control procedures.
3) Mud properties.
  1. Excessive viscosities and gel strengths.
  2. Buildup of drilled solids.
  3. Thick filter cakes that reduce the hydraulic diameter of the wellbore.
  4. Excessive mud density or increasing mud density too fast.
  5. Unbalanced drilling mud columns.
  6. Barite sag

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