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 visiting the following links for a complete understanding of the loss 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 materials which are used in drilling oil and gas wells:
Fiber Lost Circulation Material
The 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 is a lost circulation into fractures or highly permeable zones.
- Cedar fiber (wood fiber).
- Sawdust (wood fiber).
- Drilling paper (paper).
- Magma Fiber (mineral fiber).
- shredded cane stalks
Granular Lost Circulation Material
This type of lost circulation material has a chunky shape and is 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.
- Nut shells – (fine, medium & coarse)
- cotton hulls
- (coarse graphite).
- Calcium carbonate (fine, medium & coarse).
The 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.
- Flake (cellophane).
- Pieces of plastic (cellophane)
- Mica (fine and coarse).
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 the 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 follows:
- Firstly, 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.
- Secondly, try to reduce the drilling fluid rheology without affecting hole cleaning efficiency,
- Thirdly, 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 is an example of a lost circulation decision tree for one of the drilling mud companies.
Lost Circulation Material Treatment Selection
In general, several cheap Lost circulation Materials 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 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 downhole.
The density of the LCM pill should be checked before pumping the downhole, to make sure it is the same as the active circulating mud density. LCM pills should not be lighter because this could destabilize the wellbore 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.
- 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.
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.
The LCM For Partial Losses Recommendation:
- Firstly, 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.
The LCM For Complete Losses Recommendation:
- Firstly, spot one or more pills comprising 100 ppb or more fine, medium and coarse grade LCM across the loss zone.
- Secondly, 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.
- At last, remember LCM pills in the production zone should be formulated with sized Calcium Carbonate bridging particles in order to minimize formation damage.