Formation losses usually account for most of the additional costs when actual drilling fluid costs exceed estimated figures, which is why so much effort goes into preventing down-hole losses in the first place. If offset data is known, then wells can be designed to isolate lost-circulation zones behind casing before drilling deeper formations that require higher mud densities. However, this is not always possible and a range of Lost Circulation Material (LCM) treatments are available to try to prevent or cure formation losses in drilling oil & gas wells, which will be discussed later.
We recommend to visit the following links for complete understanding the lost of circulation problem:
- Causes Of Lost Circulation Problem
- How To Cure Lost circulation Problem,
- Precautions To be considered to avoid Mud loss problems
Main Types Of Lost Circulation Material
In General, These are the most common Lost Circulation Material which is used in drilling oil and gas wells:
- Cedar fiber (wood fiber).
- Sawdust (wood fiber).
- Drilling paper (paper).
- Magma Fiber (mineral fiber).
- Nut shells – (fine, medium & coarse)
- (coarse graphite).
- Calcium carbonate (fine, medium & coarse).
- Flake (cellophane).
- Mica (fine and coarse).
Fiber Lost Circulation Material
Fiber LCM type used in oil and gas wells is long, slender and flexible. It also comes in many sizes and lengths of fiber. Fibrous LCM can be added to the drilling fluid and pumped downhole to help in reducing losses & maintaining circulation if there are a lost circulation into fractures or highly permeable zones.
Granular Lost Circulation Material
This type of lost circulation material has a chunky shape and prepared in a range of particle sizes. Granular LCM is added to drilling fluid in oil & gas wells and pumped downhole to assist in delaying the loss of mud into fractures or highly permeable formations.
Actually, granular Lost Circulation Material should be insoluble and inert to any drilling mud system in which it is utilized.
- Ground and sized limestone or marble,
- Nut hulls
- Cotton hulls.
Flake type of lost-circulation material (LCM) is thin and flat in shape, with a large surface area. Flake LCM could be utilized in various sizes. It should be insoluble and inert to the mud system in which it is used. Its purpose is to shut off any mud loss zones in a well and help stop lost circulation.
A lot of laboratory studies were run to explain how LCM works when used in oil and gas wells, how to evaluate the performance, and how to improve the lost circulation material sealing ability in drilling operations (Scott and Lummus 1955; Abrams 1977; Nayberg 1986; Dick et al. 2000; Hettema et al. 2007; Kageson-Loe et al. 2009; Kefi et al. 2010; Clapper et al. 2011; Alsaba et al. 2014b, c, 2016).
Detecting Loss Zones For Better LCM Treatment
LCM treatments may be ineffective for the simple reason that they are not being spotted across the thief zone where formation losses are occurring. If this is the case, and if the loss rate is significant, then the following logging tools can be run to locate the thief zone:
Mud flowing into the formation will cool the area where losses are occurring and this can be detected by running a temperature log inside the drill pipe.
This is a propeller device that measures flow in the wellbore, enabling significant formation losses to be detected if the drill pipe is out of the hole.
This uses a very sensitive microphone (hydrophone) to detect noise created by turbulent flow in the region where fluid is passing into the formation.
Decision To Use Lost Circulation Materials (LCM) In Drilling Oil & Gas
Before taking the decision of using Lost circulation Materials (LCM) in drilling oil & gas wells, check the surface system to make sure that active fluid is not being lost at surface due to surface leaks or other factors (e.g. whole mud losses caused by blinded shaker screens or shale-shaker power failure).
Formation losses may not occur when the circulating rate is reduced or stopped, but the Equivalent Circulating Density (ECD) will increase while circulating or drilling, and this may result in losses. Circulation rates and ROPs should therefore be controlled when formation losses occur, in order to reduce the ECD and help to minimize losses.
Try Other Ways To Reduce formation losses
There are simple ways you could think of one of them before your decision of using lost circulation material as following:
- First, think about reducing the circulating rate and/or ROP, as this will reduce down-hole ECDs by reducing friction in the hole and reducing cuttings loading in the annulus.
- Second, try to reduce the drilling fluid rheology without affecting hole cleaning efficiency,
- Third, why not reducing the drilling fluid density without destabilizing the hole or inducing a kick.
- But, If formation losses are severe and cannot be controlled then it may be necessary to suspend drilling, or to drill “blind” while circulating with reduced or no returns, before setting an intermediary casing string to isolate the thief zone
Lost Circulation Decision Tree
A typical lost circulation decision tree appears overleaf. Decision trees are generally developed for each field and for each well interval, based on offset well data, so there is no guarantee that they will be effective if used in other fields. Below an example of a lost circulation decision tree for one of the drilling mud companies.
Selecting Lost Circulation Material Treatment
Several cheap Lost circulation Material are available for treating lost circulation, including Nut Plug, Mica, Sawdust, Wood Chips, Fibers, Calcium Carbonate and Marble Chips, along with more expensive specialist products.
The choice of treatments for lost circulation will depend on the the following factors:
- Formation being drilled
- Hole size
- Loss rate.
- Precious treatment results from offset wells
If you are designing mud program for an exploratory well, keep the density as low as possible to minimize losses.
Simple Tips While Lost Circulation Material Pill Preparation
LCM pills are usually prepared using mud from the active circulating system. However, this will produce a higher density pill once all the LCM products have been mixed and this might increase the hydrostatic pressure of the fluid and the loss rate to the formation when pumped down hole.
The density of the LCM pill should be checked before pumping down hole, to make sure it is the same as the active circulating mud density. LCM pills should not be lighter because this could destabilize the well bore or bring the oil or gas well under balance and create a well control situation.
The shale shakers should not be bypassed or fitted with coarser shaker screens when treating the circulating system with Lost Circulation Material because this will increase the drilled solids in the mud, which might result in an increase in density and rheology. If this happens then the circulating ECDs will increase and encourage more mud losses to the formation.
Limitation Of LCM While Drilling
- Take Care when using coarser grades of Lost Circulation Material while drilling because there are often limitations on LCM concentrations and particle sizes that can be pumped through the MWD and LWD tools.
- Remember that Coarse Drilling LCM may cause plugging the bit jets, especially if the LCM has been mixed quickly and is still lumpy as it is pumped down-hole.
- Don’t use Conventional LCM materials (e.g. Nut Plug, Mica, etc.) when trying to cure losses in the production zone because this might damage the formation.
Losses in the reservoir should be treated using Lost circulation material which is designed for use in production zones as sized Calcium Carbonate bridging material, which can be acidized.
Cure Different Types Of Circulation Lost With LCM
Cure Seepage Losses with Lost Circulation Material
Seepage losses (Read more about procedures to cure seepage losses) often continue as new formation is constantly being exposed while drilling, so it is usually more effective to treat the entire circulating system with Drilling Lost Circulation Material rather than prepare and pump LCM pills.
The LCM For Seepage Losses Recommendation:
- Treat the active circulating system with 5 ppb to 10 ppb fine grade lost circulation material, such as Nut Plug, Mica or Calcium Carbonate.
- Regular dosing with LCM at the mixing hopper to maintain concentration and compensate for Lost Circulation Material screened out at the drilling shale shakers.
- Don’t bypass shale shaker if LCM blinds its screens because drilled solids will then build up & will increase the drilling fluid density and rheology, with the potential of having more losses.
- Sweep the hole periodically with fine and medium grade Quick Seal and Mica, or fine and medium grade Calcium Carbonate bridging particles if in the production zone.
Cure Partial Losses with Lost Circulation Material While Drilling
Partial losses usually occur while drilling fractured formations. ( read more about procedures to cure partial losses)
The LCM For Partial Losses Recommendation:
- Spot a viscous pill comprising 30 ppb to 40 ppb fine and medium grade LCM (Nut Plug, Mica and K.Seal) across the suspected loss zone.
- Pull the drill string up into the casing shoe to minimize the potential for differential pipe sticking while leaving the LCM pill to soak in the loss zone for a few hours.
- Keep the annulus full with mud to maintain fluid hydrostatic throughout this period, before running back to bottom and gradually increasing the circulating rate while monitoring for losses.
- Repeat or adjust the LCM concentration if the first attempt is unsuccessful.
Cure Total Losses with Lost Circulation Material While Drilling
Complete losses usually occur while drilling large natural fractures. ( read more about procedures to cure Complete losses)
The LCM For Complete Losses Recommendation:
- Spot one or more pills comprising 100 ppb or more fine, medium and coarse grade LCM across the loss zone.
- Apply pressure to squeeze the LCM pill into the loss zone.
- Take care when using coarser grades of LCM to avoid any risk of plugging the down-hole tools.
- Remember LCM pills in the production zone should be formulated with sized Calcium Carbonate bridging particles in order to minimize formation damage.
Lost Circulation Material Cure Mechanisms
LCM Bridging Theory
When encountering losses, the priority is to determine the type of the losses which will help you choose the suitable size and material for bridging or plugging the loss zone. Below is how to differentiate between them:
- Losses into the formation structure are usually indicated by an increasing loss rate as drilling exposes more formation
- Losses into a fracture are usually sudden.
Formation structure Losses
If losses are into the formation structure then our task will be as following:
- First, plug the pore throats with larger-sized particles first (Coarse LCM),
- Second, plug the channels between the larger-sized particles with medium-sized particles (Medium LCM)
- Finally seal the channels between the medium-sized particles with a filter cake of small- sized particles (Fine LCM).
Barite and drilled solids (cuttings) also contribute to this bridging process.
How to effectively treat formation structures losses using Lost Circulation Material While Drilling
There is a correlation between pore size (in microns) and formation permeability (in milli-Darcies), as follows:
Pore Size = √ (Formation Permeability)
Research also confirms that particles with an average diameter one third the size of the pore throat diameter will form a bridge across the pore throat when subjected to differential pressure ( below diagram) which is provided by the over-balance between hydrostatic pressure and the pore pressure.
With this knowledge, effective LCM treatments can be formulated by measuring the permeability or pore throat size from core samples obtained from representative offset oil and gas wells.
Particle Size Distribution (PSD)
Bridging and LCM materials used in drilling oil & gas wells are generally categorized as following:
However, the particles in each category will range in size and this is called the Particle Size Distribution. Sophisticated electronic laboratory equipment is available for measuring the PSD of bridging materials in a fluid (e.g. laser diffraction technology), and the concentrations of these materials can then be adjusted to suit the formation being drilled in order to produce a low permeability filter cake.
Common particle size ranges for CARB Bridging Material grades are as follows:
- Fine: 5 to 400 microns
- Medium: 100 to 1000 microns
- Coarse: 60 to 2500 microns
- Ground Marble Grade E: 1 to 160 micron
Most Barite particles range from 2 to 44 microns and will therefore provide effective bridging for pore throat sizes ranging up to 132 microns, so pore sizes above this will require LCM bridging materials.
Based on the relationship between pore size and permeability, 2 micron Barite particles will theoretically bridge 6 micron pore throats, which equate to a 36 milli-Darcy permeability. Similarly, 44 micron Barite particles will theoretically bridge 132 micron pore throats, which equate to a 17,424 milli-Darcy (i.e. 17 Darcy) permeability.
The PSD for Barite includes particles that are larger than 44 microns, so Barite is therefore effective for bridging high permeability sands before whole mud starts to be lost to the formation. As a general guidline, whole mud losses do not occur in formations with permeabilities below 50 Darcies when Barite is used as a weighting agent.
Stress Cage & LCM Wellbore Strengthening Mechanism
What Is Wellbore Strengthen Mechanism For Lost Circulation Material in Drilling?
Wellbore strengthening is designed to strengthen the wellbore so that depleted reservoirs can be drilled with higher mud weights without inducing down hole losses due to formation breakdown (i.e. the fracture resistance of the formation is increased).
Application Of Wellbore Strengthening
- Drilling depleted reservoirs adjacent to formations with higher pore pressures.
- Deep water drilling with a narrow window between pore pressure and fracture gradient.
- Minimizing losses when running or cementing casing.
- Providing improved well control.
- Reducing the number of casing strings in a well.
For Example, in depleted reservoirs, the pore pressure gradually declines as the reserves decline, weakening hydrocarbon-bearing rocks. However, adjacent and inter-bedded low permeability shales may maintain their pore pressure, making drilling extremely difficult because the mud weight to support these shales exceeds the fracture resistance of the depleted sands and silts.
- How To Achieve Wellbore Strengthen By Stress Cage Theory ?
- Stress Cage Theory
- Permeable Formations
- Low Permeability Formations
- Operational Considerations For WellBore Strengthening By Stress Cage
Cross Linking Polymers
These pills are usually a blend of polymers and lost-circulation materials that crosslink with temperature and time to form a rubbery, spongy ductile consistency that effectively stops loss of fluid by sealing fractures and vugular formations. These pills may also be used to prevent water flow and consolidate loose gravels.
Depending on the manufacturer, the product may be a one-sack additive containing polymers and lost-circulation materials or be individually packaged. Most offer a retarder and accelerator for specific cases if needed. In most situations, the retarder will be required to delay premature crosslinking until the slurry can be placed opposite the loss zone.
In cross Linking Polymer Article you shall find more information about the following topics:
- How Cross Linking Polymer Affect Drilling Mud Properteies
- Cross-Linking Polymers for Severe Losses
- Application Of LCM Cross Linking Polymer In Drilling
- The Main Components Of Cross Linking Polymer System
- The Mixing & Pumping Procedures Of Cross Linking Polymer Pill
- Weighing Cross Linking Polymers Pill
- Pilot Testing