Drilling and Production Technology

1st Semester In-Class Test

Question (1): Describe the components of drill string and the bottom-hole assembly and their

functions.

Solution (1): The term drill string is used to describe the tubular and accessories on, which the drill bit is run to the

bottom of the borehole. The drill string consists of drill pipe, drill collars, the Kelly and various

other pieces of equipment such as stabilisers and reamers, which are included in the drill string just

above the drill bit (see figure below). All of these components will be described in detail below. The

drill collars and the other equipment which is made up just above the bit are collectively called the

Bottom-Hole Assembly (BHA).

The functions of the drill string are:

• To suspend the bit

• To transmit rotary torque from the Kelly to the bit

• To provide a conduit for circulating drilling fluid to the bit.

Question (2): Describe the difference between over-balance pressure and under-balance

pressure.

Solution (2): The differential between the mud pressure and the pore pressure at any given depth is known as the

Over-balance pressure at that depth. If the mud pressure is less than the pore pressure then the

differential is known as the under-balance pressure. It will be seen below that the fracture pressure

gradient of the formations is also expressed in units of psi/ft.

Note: It is highly recommended to include graphs in your solution as shown below (Most

students lost marks last year because of this).

Question (3): (A bit tricky):

I remember we were given values for pore pressure and fracture pressure in psi at a certain

depth. Following these values, we were also given three values (or options) for mud weight

in ppg:

a) State which mud is NOT safe to be used for drilling at the given depth?

Solution (a): Firstly, you need to convert the values for pore and fracture pressures from

(Psi) to (ppg), then the mud weight which exceeds the pore pressure and fracture pressure

should be eliminated and it must not be used for drilling.

Note: To make the answer for this question easier, you need only to plot a graph showing

pore and fracture pressure trends versus depth (See Graph below)

b) By using the SAFE mud weight (value above) at a certain depth (15000 ft TVD for

example), Determine the leak-off pressure at the surface?

Solution (b): USE the following equation (ALWAYS REMEMBER IT):

1- Max. Bottom hole pressure during leak-off = Pressure when the formation already

fractured (fracture pressure value given above in Psi) / TVD (15000 ft).

2- Hydrostatic pressure of mud colom = (0.052 * TVD (15000 ft) * SAFE mud weight).

3- From equation above, Leak-off pressure at surface = (1) – (2).

Question (4): (Drill pipe, Drill Collars, Collapse Loading and Total Tension Load):

NOTE: FOCUS on Tutorial (1), Problems (1) and (2):

a) Calculate the number of 8 1/4” x 2 13/16” (83.9 Ib/ft) drill-collars (25ft/joint), drill

pipes (19.5 Ib/ ft) and Total Length of Drill string that would be required to provide

20,000lbs WOB and keep the drill pipe in tension in 12 ppg mud.

Solution (a):

1- Length of Drill Collars = WOB / (Ib/ft * fraction (if it is NOT given, always assume

it 1))

= 20,000 / (83.9 * 1) = 238.4 ft

2- Number drill collars= 238.4/25 = 10.

3- Length of Drill Pipe = [WOB (20,000)/ (19.5 Ib/ft)] – Length of Drill Collars (NOT

SURE)

4- Number of Drill pipes = Length of Drill pipe / 25 ft/joint

5- Total Length of Drill string = Length of (Drill Pipe + Drill Collars)

b) From Data above, calculate the collapse loading and total weight of drill string at a

TVD of 12,000 ft (This part is easier than the one in Tutorial 1, because the area was

given)?

Solution (b):

Step (1): Collapse loading at 12,000 ft = 0.052 * MW * TVD.

Step (2): Follow STEP 2 in TUTORIAL 1.

Step (3): Total Weight of Drill String (EASY) = Wc + Wp

(NO NEED TO CALCULATE THE WEIGHT FOR EACH SECTION, AT THE TOP AND

BOTTOM OF DRILL COLLAR AND DRILL PIPE).