DOB2C Procedures - DRAFT

DOB2C Procedures

1. Overview

DOB2C consists of 2 sacks of cement (188 lb) and 1 sack (100 lb) of bentonite mixed with 26.5 gallons of diesel. This mixture yields 38.2 gallons of slurry (0.91 bbl) at a density of 12.4 ppg.

DOB2C will be mixed in the cement batch mixer, 50-100 bbls as required, scaling up from the following recipe for 50 bbls DOB2C:

  • 34.7 bbls of diesel (check that there is no water in the diesel)
  • 110 sx cement (94 lbs/sx)
  • 110 sx of bentonite (50 lbs/sx)

2. Advance Pilot Testing of Mud:DOB2C Ratios

Pilot tests are necessary because bentonite and cement react differently in different muds. Testing must be done with the materials available on the rig before drilling out.The basic ratios of diesel, bentonite, and cement that go into DOB2C will always be the same. The ratio of water base mud (or seawater) that is added to the DOB2C will vary depending on the desired viscosity of the mixture, which is called M+DOB2C.

1.Take a sample of DOB2C from the batch mixer. Mix thoroughly to suspend the solids in the diesel, and check the density which should be ~12.4 ppg. Stir the mixture before each pilot test with the drilling mud.

2. Pilot test the following ratios of DOB2C to mud:

  • 1:3, 1:2, 1:1, 2:1, and 3:1
  • Stir vigorously, and use a glass container so that you can see the resultant viscosity changes as bentonite and cement take water from the mud. When this exchange has taken place, slowly pour the now clean diesel from the container, leaving the M+DOB2C mass in the jar.

3.Select the three ratios that best match the descriptions below. If the range of ratios recommended in step 2 does not produce the expected consistencies there is likely to be a product quality issue; check the bentonite quality, or water contamination of the diesel.

  • Soft Ratio: Readily loses shape when held in hand. Should be viscous, slowly pourable, with the approximate consistency of mayonnaise. This material is for filling the outer extremities of the vugs or fractures while providing a medium resistance to flow in the formation.
  • Medium Ratio: When squeezed, extrudes from the fist like toothpaste. The purpose of this mixture is to enter the fracture and provide a sufficient resistance to flow so that any fractures are widened considerably and the formation is stressed (compressed) around the borehole.
  • Hard Ratio: When squeezed, oil runs out and a solid mass is left in the palm with fingerimprints. A plug of 10 to 20 ft left inside the wellbore above the loss zone should allow circulation above it.

3. DOB2C Procedure

Ratios of M+DOB2C can be adjusted to suit operational objectives. A suggested initial sequence is 60 bbls soft, 20 bbls medium, and 20 bbls hard ratio. This will enable the fluid to penetrate the formation, and subsequently allow back pressure to be built.

As an example, to pump a one stage “hard” DOB2C the following generalized procedure should be followed, while continuing to fill up the annulus.

Ensure end of drill string is far enough above the suspected loss zone, preferably inside casing,to allow clearing the bit with the DOB2C before it reaches the suspected loss zone.

  1. Clean all mixing lines and tanks. Flush mixing lines and tanks with diesel.
  2. Pump 15 bbls diesel down drill string
  3. Mix desired amount DOB2C e.g. 100 bbls.
  4. Close annular or top ram. Reciprocate drill string slowly and continuously.
  5. Pump DOB2C plug down drill string at 3 bpm.
  6. Pump 7 bbls diesel behind DOB2C. Switch to mud (or seawater) and continue to displace at 3 bpm. Simultaneously start pumping mud down the annulus at 1 bpm.
  7. Continue to pump at this rate until all DOB2C and the diesel spacer behind has been displaced out of the drill string, and the lead DOB2C has started entering the loss zone.
  8. Once the lead DOB2C is at the loss zone, stop pumping, and use the hesitation squeeze technique. Pump in stages down drill pipe at 0.25 -0.50 bpm until the pressure reaches 200 psi over the pore pressure of the loss zone. Once pressure is attained, circulate bottoms up; or if no pressure is attained, leave approximately 100 feet of DOB2C in the borehole above the loss zone. Do not drill out until the cement or DOB2C attains a minimum compressive strength of 250 psi. If the procedure is not successful in stopping the loss, repeat with the same volumes as many times as required to regain circulation.

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