Barite plug use is usually limited to extreme or emergency conditions where it is imperative that some measures be taken to seal off the bottom section of the wellbore. This type of plug is applicable in several situations, including:
- Simultaneous kicking and lost circulation.
- Well Abandonment procedure allows the safe withdrawal of drill pipe to allow setting of cement plug.
- Withdrawal of drill pipe to either set casing or repair existing casing strings.
- Plugging drill pipes in emergencies.
- High-pressure salt water flows, where required, kill mud weight approaches or exceeds the formation breakdown equivalent at some point in the open hole, usually the last casing shoe.
Objectives of Setting Barite Plugs
A Barite plug is a barite slurry pumped down the drill pipe and placed at the bottom of the wellbore. A successful Barite plug should accomplish two things:
- Initially, the weight of the barite slurry should kill the well.
- After some time, the settled barite plug should mechanically block any flow up the wellbore.
The well should be killed before a mechanical blockage is established in the wellbore.
Barite Plug Design
Designing a barite plug for killing a well or curing circulation loss is straightforward. The barite slurry pumped into the well must be heavy enough and fill enough of the wellbore to increase the bottom hole pressure to a level exceeding the formation pressure. Problems arise when formation pressure is unknown or when the weight or volume of the required barite slurry becomes excessive.
Designing a barite plug to physically block the wellbore is somewhat more complicated. The generally accepted method is to mix a slurry so that the barite settles out from the slurry into a hard plug, which will block the wellbore. The rate at which barite will settle into a hard plug is usually slow and predictable. Fairly accurate field predictions may be made by observing a small container’s initial barite settling rate. The initial rate is constant and independent of the height of the slurry.
The initial settling rate lasts for a short period, after which the settling rate decreases as fewer barite particles remain in suspension. The initial settling rate applies in a container one foot high for approximately five minutes. The initial rate may apply for a day or longer in a field situation with 500 or more feet (150 or more meters) of barite slurry. The amount of barite settling in a shorter period can be computed as the product of the initial rate times the waiting time.
Field experience has shown that slurries of up to 20 lbs/gal (2.40 SG) are relatively easily prepared using only Base Oil, EZ MUL (Oil Wetting Agent), DRILTREAT, and Barite for oil mud. Water, SAPP, caustic soda, and barite are used for Water Based Mud.
Barite Plug Preparation
From a practical point of view, the following points should be considered: Using a cement unit is preferable. This requires that either bulk barite be fed directly to the cement unit surge tank or sufficient stocks of sacked barite be available at the rig site.
Standard plugs can be mixed to the desired density with no problems of massive settling before displacement.
Oil Based Mud
Oil Mud Application
Barite plug settling rates in oil muds usually depend on the slurry’s density and the type and concentration of oil-wetting agents. Laboratory studies have shown that oil-based plugs tend to settle, on average, more slowly than water-based slurries.
At too low a concentration of EZ MUL and DRILTREAT, the barite is insufficiently oil-wet and is not self-suspending. The barite becomes extremely well suspended at too high a concentration, and the settling rate is reduced. It is, therefore, essential to carefully select the optimum concentration of EZ MUL for the plug density required.
If a cement unit cannot mix barite, use a slug pit or the reserve mud pits, depending on the total volume of slurry required. The length of the plug is a wellsite determination and should be based on the severity of the situation. A plug in the range of 250 – 500 ft (75-150 m) is usually sufficient.
Oil-Based Mud Procedure
Oil-based mud slurries can be mixed as follows:
- Transfer sufficient oil-based mud to the slug pit to maintain circulation through the mixing pump.
- Fill the pit to half its capacity with a base oil and add approximately 4 lbs/bbl (11.4 kg/m3) EZ MUL and 4 lbs/bbl (11.4 kg/m3) DRILTREAT.
- Weight up with barite to the required density; the pit should be nearly full.
- If the total capacity of the slug pit is insufficient for the required volume of the plug, transfer the slurry already mixed to a reserve pit, ensuring that agitators are used constantly and another mixing pump is put on to circulate that pit.
The Mud Engineer on site should ensure that the following measures are also adhered to:
- Excessive additions of base oil are not made at any stage to avoid the chance of initiating rapid settling.
- Small additions of up to 1.5 lbs/gal (4.3 kg/m3) EZ MUL may be made to control viscosity increases noted during barite additions.
- The barite addition rate is controlled to avoid excessive increases in viscosity or possibly initiating settling.
Formulations for oil-base muds:
Water Based Mud
Water-Based Mud Application
The slurry comprises barite, fresh water, sodium acid pyrophosphate (SAPP), and caustic soda. SAPP, a thinner, increases the barite settling rate by lowering the yield point (check also Yield Point In Drilling Mud Formula) and gel strength of the slurry, and the caustic soda is added to provide an alkaline environment (pH = 10).
The formulation for one barrel of a 20 lbs/gal barite slurry is:
OR
Displacement
Displacement techniques are the same as in cementing; i.e., the slurry should be under-displaced so that the height of the slurry in the drill pipe is 2 bbls greater than in the annulus. This allows the drill pipe to be withdrawn with a natural slugging action. It will also minimize the slurry movement in the hole, reducing contamination.
Because of the high density of these slurries, high differential pressures can be created by under or over-displacement. Care must be taken when calculating volumes. After the plug is spotted in place, tripping out of the hole should be done as quickly as possible, allowing the plug to settle for several hours. The well should be observed to ensure there is no flow. When tripping the pipe back into the hole, the “feeling” for the plug should begin near the theoretical top of the plug.
Operations can then be started to set a cement plug above the barite, and the well can be safely secured.