Basic Drilling Formulas
Accumulator Capacity Calculation

Drilling Calculations -Accumulator Capacity
(Useable Volume per Bottle)

NOTE: The following will be used as guidelines:
Volume per bottle = l0 gal.
Pre-charge pressure = 1000 psi.
Minimum pressure remaining after activation = 1200 psi.
Pressure gradient of hydraulic fluid = 0.445 psi/ft
Maximum pressure = 3000 psi

Boyle’s Law for ideal gases will be adjusted and used as follows:
P1V1 = P2V2
Surface application

Step 1
Determie hydraulic fluid necessary to increase pressure from pre-charge to minimum:
P1V1 = P2V2
1000 psi x 10 gal = 1200 psi x V2
10000 / 1200 = V2
V2 = 8.33 The nitrogen has been compressed from 10.0gal to 8.33gal.
10.0 - 8.33 = 1.67 gal of hydraulic fluid per bottle.
NOTE: This is dead hydraulic fluid. The pressure must not drop below this minimum value.

Step 2
Determine hydraulic fluid necessary to increase pressure from pre-charge to maximum:
P1V1 = P2V2
1000 psi x 10 gals = 3000 psi x V2
10,000 / 3000 = V2
V2 = 3.33

The nitrogen has been compressed from 10 gal to 3.33 gal.
10.0 - 3.33 = 6.67 gal of hydraulic fluid per bottle. Determine useable volume per bottle:

Step 3
Determine useable volume per bottle:
Useable vol/bottle = Total hydraulic fluid/bottle - Dead hydraulic fluid/bottle.
Useable vol/bottle = 6.67 - 1.67
Useable vol/bottle = 5.0 gallons

English units
Volume delivered, gallons = bottle capacity, gals x (precharge, psi / final, ps) - (precharge, psi /system, psi)
Example: Determine the amount of usable hydraulic fluid delivered from a 20-gallon bottle:
Precharge pressure = 1000 psi.
System pressure = 3000 psi.
Final pressure = 1200 psi.

Volume delivered, gallons = 20 gallons x (1000 psi / 1200 psi) - (I000 psi / 3000 psi)
Volume delivered, gallons = 20gallons x (0.833 - 0.333)
Volume delivered, gallons = 20 gallons x 0.5
Volume delivered, gallons = 10gallons

Subsea applications
In subsea applications the hydrostatic pressure exerted by the hydraulic fluid must be compensated for in the calculations:
Example: Same guidelines as in surface applications:
Water depth = I000 ft
Hydrostatic pressure of hydraulic fluid = 445 psi

Step 1
Adjust all pressures for the hydrostatic pressure of the hydraulic fluid:
Pre-charge pressure = I000 psi + 445psi
Pre-charge pressure = 1445 psi.
Minimum pressure = 1200 psi + 445psi
Minimum pressure = 1645 psi.
Maximum pressure = 3000psi +445psi
Maximum pressure = 3445psi

Step 2
Determine hydraulic fluid necessary to increase pressure from pre-charge to minimum:
P1V,1 = P2V2
1445 psi x 10 = 1645 x V2
14,560 / 1645 = V2
V2 = 8.78 gal
10.0 - 8.78 = 1.22 gal of dead hydraulic fluid.

Step 3
Determine hydraulic fluid necessary to increase pressure from pre-charge to maximum:
1445 psi x 10 = 3445 psi x V2
14,450 / 3445 = V2
V2 = 4.19 gal
10.0 - 4.19 = 5.81 gal of hydraulic fluid per bottle.

Step 4
Determine useable fluid volume per bottle:
Useable vol/bottle = total hydraulic, fluid/bottle - dead hydraulic, fluid/bottle.
Useable vol/bottle = 5.81 - 1.22
Useable vol/bottle = 4.59 gallons

Accumulator pre-charge pressure
The following is a method of measuring the average accumulator pre-charge pressure by operating the unit with the charge pumps switched off:
P,psi = vol removed, bbl / totalacc.vol,bbl x ( Pf x Ps / Ps- Pf)
where P = average pre-charge pressure, psi.
Pf = final accumulator pressure, psi.
Ps = starting accumulator pressure, psi.

Example: Determine the average accumulator pre-charge pressure using the following data:

Starting accumulator pressure (Ps) = 3000 psi.
Final accumulator pressure = 2200 psi.
Volume of fluid removed = 20gal.
Total accumulator volume = 180gal.

P,psi = 20/180 x ( 2200 x 3000 / 3000 -2200)
P,psi = 0.1111x (6600000 / 800)
P,psi = 0.1111 x 8250
P, psi = 917psi

Find out the details of the preceding bit run. What factors improved/reduced drilling performance and can the lessons learnt be utilised in the planned run?

* Find out the condition of the preceding bit when it went in hole, ie: new bit, rerun, re-tipped, etc.

* Be on the rig floor to witness the preceding bit and BHA being pulled through the rotary table. This is the only way of ensuring maximum information is collected on the dull condition of the bit and the BHA, ie: sometimes bit/BHA balling is removed and not recorded.

* Collect the run details, dull grade the bit and take photos as outlined in the Dull Grading and Dull Bit Photos section. If a detailed run report is required this information may be critical.

* If it is planned to run a PDC bit and the preceding bit is pulled out of hole with severe damage; lost cutters or cones; or severely under gauge, the hole should be conditioned with a roller cone bit and a junk basket. (PDC bits are generally not recommended for long intervals of reaming or cleaning out junk).

These are final checks to ensure that the correct bit with the correct nozzle sizes is run hole and recorded accordingly.
- Record bit type, size and serial number.
- Ensure there is no debris inside the central feed bore and the individual feed bores that could potentially plug a nozzle.
- Ensure the bit is jetted with the correct size nozzles as indicated by the Drill Bits TFA calculation.
- If either damaged in transit, a rerun or a repaired bit.
- Record bit condition/dull grade
- Photograph the bit as outlined in the Dull Bit Photos section.Take extra shots of damaged/worn area as necessary.

- If a motor is to be tested in the casing recommend using a dull bit rather than the bit required to drill the section. This eliminates the risk of damaging the bit planned for the section in the casing.