هدف از سيمانكاري چاه نفت

Zonal Isolation: Isolate hydrocarbon pays, water pays, shale, etc.

Seal off lost circulation zones

Hydraulic Seal: Prevent pressure from the undesirable migration of fluid coming up to surface via annulus between casing and hole.

Protect casing: Cement is a protective sheath around casing.

Hold casing and completion string: Cement supports weight of whole string of casing or completion.

The importance to accomplish the primary oil well cementing is  as follows:

Good reservoir management: Good cement will let engineer design to produce hydrocarbon from reservoirs easier because there is no concern related to cement.

Maximize Reserves: Good cement sheath allows good pay sands to produce as maximum as possible because good sands will stop producing earlier due to water load-up or permeability damage.

Save time and cost: A lot of time and money must be spent to perform remedial cement operation which is cement squeeze job by coiled tubing or Hydraulic Work Over. If primary cement can be accomplished, no more time and money is spent out to do remedial work.

The key factors necessary to ensure a successful primary oil well cement job are as follows:

1. Good information: Related information such as calculated hole diameter, hole volume, depth, etc from related personnel is required in order to design cementing programs.

2. Good cementing design: There is a number of cement purposes such as primary cement, cement squeeze, cement plug. To meet the purpose of each job, cementers must design a cementing program in correct and good way.

3. Good procedures: Clear and concise procedure will lead to good job because cementer and assistants can perform the cementing job correctly, fast, safely.

4. Good equipment and experienced personnel: Experienced staff with good equipment can perform great jobs, no lost time, safe and fast.

5. Good centralizer placement: Centralizers help casing in the center of hole. Casing with good centralizer placement, cement can form properly cement sheath around casing.

محاسبه Lag Time در عمليات حفاري چاه نفت

Lag time is traveling time interval required for pumping cuttings from each particular depth to surface. It can be expressed in terms of time (minutes) and pump strokes.

The lag time always changes when a well becomes deeper and/or pumping speed change. Two factors, affecting lag time calculation, are annulus volume of drilling fluid in and drilling mud flow rate.

With certain annular volume, the lag time, normally expressed in minutes, can be determined by dividing the annular volume (bbls) by the flow rate (bbl/min).

If there are changes in mud flow rate, the lag time figure will be changed as well. In order compensate for any changes, the lag time is transformed into pump strokes too; therefore, a change in speed of pump will not affect the lag time.

How to Calculate Theoretical Lag Time

There are 3 steps to do in order to calculate lag time as listed below;

1. Calculate pump output [1]
2. Calculate annular volume at certain depth of hole [2]
3. Calculate the theoretical lag time

Example – Determine lag time from bottom to surface with the following information;

Bit depth = 9500’ MD
Pump rate = 300 GPM
Annular volume at 9500’ MD = 250 bbl
Pump details: Triplex pump, 97% efficiency, liner size 6” and stroke length 12”

Solution;

Triplex Pump Output Formula is listed below;

Triplex Pump Output in bbl/stk = efficiency x 0.000243 x (liner diameter in inch) ² X (stroke length in inch)

Triplex Pump Output in bbl/stk = 0.97x 0.000243 x (6) ² X (12)
Triplex pump output = 0.102 bbl/stroke

Pump rate = 300 GPM ÷ 42 = 7.14 bbl / minute

Lag time in minutes = 250 bbl ÷ 7.14 bbl / minute = 35 minutes
Lag time in strokes = 250 bbl ÷ 0.102 bbl/stroke = 2451 strokes

[1] Rig pump out put, normally in bbl per stroke, of mud pumps on the rig is important figures that we really need to know because we will use pump out put figures to calculates many things such as bottom up strokes,  wash out depth, tracking drilling fluid, etc. In this post, you will learn how to calculate pump out put for triplex pump and duplex pump.

Triplex Pump Output Formula

Triplex Pump Output in bbl/stk = 0.000243 x (liner diameter in inch) ² x (stroke length in inch)

Example: Determine the pump output in bbl/stk at 100% and 97% efficiency
Linner size = 6 inch
Stroke length = 12 inch
Triplex pump output:
PO @ 100% = 0.000243 x 6² x 12
PO @ 100% = 0.104976 bbl/stk

Adjust the triplex pump output for 97% efficiency:
Decimal equivalent = 97 ÷ 100 = 0.97
PO @ 97% = 0.104976 bbl/stk x 0.97
PO @ 97% = 0.101827 bbl/stk

Duplex Pump Output Formula

Duplex Pump Output in bbl/stk = 0.000162 x S x [2(D)² - d²]

Whrere:
D = liner diameter in inch
S = stroke length in inch
d = rod diameter in inch

Example: Determine the duplex pump output in bbl/stk at 100% and 85% efficiency

Liner diameter = 6 inch
Stroke length = 12 inch
Rod diameter = 2.0 in.

Duplex pump efficiency = 100 %.
PO @ 100% = 0.000162 x 12 x [2 (6) ² -12² ]
PO @ 100% = 0.13219 bbl/stk

Adjust pump output for 85% efficiency:
PO @ 85% = 0.132192 bbl/stk x 0.85
PO @ 85% = 0.11236 bbl/stk

[2] Annular capacity is one of basic values that you really need to understand. This post demonstrates you how to calculate annular capacity between casing or hole and drill pipe, tubing, or casing. There are several formulas as shown below to calculate annular capacity depending on unit of annular capacity required.

Note: Dh is bigger ID and Dp is smaller OD. The examples below will show the Dh as hole size and Dp is drill pipe OD

a) Calculate annular capacity in bbl/ft

Annular capacity in bbl/ft =  (Dh² – Dp²) ÷1029.4

Example: Hole size (Dh) = 6-1/8 in.
Drill pipe OD (Dp) = 3.5  in.
Annular capacity in bbl/ft = (6.125² – 3.5 ²) ÷1029.4
Annular capacity = 0.0245 bbl/ft

b) Calculate annular capacity in ft/bbl

Annular capacity in ft/bbl = 1029.4 ÷ (Dh² – Dp²)

Example: Hole size (Dh) = 6-1/8 in.
Drill pipe OD (Dp) = 3.5  in.
Annular capacity in ft/bbl = 1029.4 ÷ (6.125 ² – 3.5 ²)
Annular capacity = 40.743 ft/bbl

c) Calculate annular capacity in gal/ft

Annular capacity in gal/ft = (Dh² – Dp²) ÷ 24.51

Example: Hole size (Dh) = 6-1/8 in.
Drill pipe OD (Dp) = 3.5  in.
Annular capacity in gal/ft = (6.125 ² – 3.5²) ÷24.51
Annular capacity = 1.031 gal/ft

d) Calculate annular capacity in ft/gal

Annular capacity, ft/gal = 24.51 ÷ (Dh² – Dp²)

Example: Hole size (Dh) = 6-1/8 in.
Drill pipe OD (Dp) = 3.5  in.
Annular capacity in ft/gal = 24.51 ÷  (6.125 ² – 3.5 ²)
Annular capacity in ft/gal = 0.97 ft/gal

After obtaining annular capacity, we can calculate annular volume by this following formula;

Annular volume in bbl  = annular capacity (bbl/ft) x length of annulus (ft)

Note: annular volume can be expressed in several unit depending on unit that you use in the calculation.

Example:

Annular capacity = 0.0245 bbl/ft
Length of annulus = 1000 ft

Annular volume = 1000 x 0.0245 = 24.5 bbl.

** Please remember that if you have several annular profile, you must calculate volume based on each annular profile in order to get total annular volume.