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What are Magnetic Toolface and Gravity Toolface?

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Magnetic Toolface: It is the direction, in the horizontal plane, the bent sub scribe line is pointing with regard to the north reference (Grid, Mag, or True). Magnetic orientation is used when the inclination of the wellbore is less than 5° to 8°. When the inclination is below this amount, the survey instrument cannot accurately determine the highside of the instrument for orientation purposes. The toolface will be presented in azimuth or quadrant form, referenced to magnetic north. The magnetic toolface reading is whatever magnetic direction the toolface is pointed. If the orientation lug on the survey instrument is pointed towards S 20 E, then the magnetic toolface would be 160°.

Gravity Toolface : It is the angular distance that a bent sub scribe line is turned, about the tool axis, relative to the high side of the hole. If the inclination of the wellbore is above 5° to 8°, then the gravity toolface can be used. The toolface will be referenced to the highside of the survey instrument, no matter what the hole direction of the survey instrument is at the time. The toolface will be presented in a number of degrees either right or left of the highside. For example, a toolface pointed to the highside of the survey instrument would have a gravity toolface of 0°. A toolface pointed to the lowside of the survey instrument would have a gravity toolface of 180°. If the orientation lug was rotated to the right of highside, the gravity toolface would be 70° to the right.

Reference books: Directional Drilling Books

Lag Time for Drilling Business and How to Calculate Theoretical Lag Time

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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
2. Calculate annular volume at certain depth of hole
3. Calculate the theoretical lag time

Oilfield Unit

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
Triplex pump output = 0.102 bbl/stroke

Solution;

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

Metric Unit

Bit depth = 3,300 m
Pump rate = 1,200 liter/min
Annular volume at 3,300 m = 40 m3
Triplex pump output = 0.01622 m3/stroke

Solution;

Lag time in minutes = 40 m3 ÷ (1,200 ÷ 1,000 m3/ min) = 33.3 minutes
Lag time in strokes = 40 m3 ÷ 0.01622 m3/stroke = 2466 strokes

Download Lag Time Calculation Spreadsheet

Ref books: 

Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.

What are gas show, connection gas and trip gas?

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You may have heard about gas show, connection gas and trip gas. What are differences of those term?

Gas show: It is gas that appears in drilling mud and it indicates the presence of gas zones or soluble gas in oil.

Connection gas: It is relatively small amount of gas that enters well when mud pump is stopped because ECD (equivalent circulating density) decreases when stop pumping. Connection gas indicates that mud weight in hole is less than formation pressure.

The difference of gas show and connection gas:If there is connection gas, gas shown in a mud logger screen increases and decreases sharply like small gas peak; however, gas show demonstrates the smoother curve than connection gas. Lag stroke, lag time are used to  find where the location of connection gas or gas show comes from. Especially, when the connection gas presents, lag time of gas peak will be relative to the time when pumps are off.

Trip gas: It is gas entering to well bore when tripping out of hole. There are several factors as balled-up bit,  fast pulling out rate, bigger diameter BHA, etc can create swab effect that can reduce hydrostatic pressure while tripping out of hole. If too much reduction in hydrostatic pressure is present, formation gas can influx into the well bore, called “Trip gas”.

Reference book: Well Control Books

Positive Kick (Wellbore Influx) Indications

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Positive kick (wellbore influx) indications mean signs showing almost 100% kick (wellbore influx) into wellbore.

wellbore influx

Positive Kick Indicators While Drilling

1. Increase in flow show

Without any increasing in flow rate in, increase in return flow indicates something coming into wellbore while drilling. Therefore, flow show instrument provided by the rigs or service companies must be checked and calibrated frequently.

2. Increase of active pit system (Pit gain)

Because drilling fluid system on the rig is a closed system, increasing in flow show without adjusting flow rate in will cause pit gain in a pit system. Nowadays, with high technology sensors, detecting change in pit level is easily accomplished at the rig site. However, visually check the pit level is importance as well for double checking figure from the sensors. Sometimes, change in pit level may be detected after the increase in flow show because it takes more time to accumulate volume enough to be able to detect by pit sensors.

3. Continue flowing while the pumps are off

When pumps are turned off, bottom hole pressure will decrease due to loss of equivalent circulating density (ECD). If there is any flow coming after pumps off, it indicates formation influx into wellbore.

Positive Kick Indicators While Tripping

There are 2 positive kick indicators while tripping as listed below;

1. Trip log deviation such as short fill up while tripping out and excess pit gain while tripping in

For tripping operation, it is very important to have a filling system via trip tank that provides continuous hole fill all time. With utilizing that system, we can compare fluid that is filled in or returned from wellbore with steel volume of tubular (drill pipe, drill collar, BHA, tubing, casing, etc). If drilling fluid volume is less than theoretical pipe displacement while tripping out or more return fluid while running in, you need to flow check and monitor the well.

  • If flow check indicates wellbore influx, crew must quickly shut the well in.
  • If flow check does not show any influx, drill string must be run back to bottom in order to circulate at least bottom up to ensure hole condition.

2. Positive flow when pipe is static

Every time that pipe in static condition. Trip tank with correct filling system must be monitored all time by both rig personnel and mud logger. If volume in trip tank increases, personnel must confirm flow check and prepare to shut the well in.

References

Coleman, S. (2018). Well Control Quiz Online. [online] Well Control Quiz Online – Test Your Well Control Knowledge for Free. Available at: http://wellcontrolquiz.com/ [Accessed 2 Aug. 2018].

Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.

Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.

Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.

Grace, R. and Cudd, B. (1994). Advanced blowout & well control. 1st ed. Houston: Gulf Publishing Company.

Watson, D., Brittenham, T. and Moore, P. (2003). Advanced well control. 1st ed. Richardson, Tex.: Society of Petroleum Engineers.

Ton-miles while making short trip

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The ton-miles of work performed in short trip operation is also in term of round trips.

The ton-miles of work done in making a short trip is equal to the difference between round ton-miles of end depth and starting depth. The formula for short trip ton-miles is listed below;

Tst = T6 – T5

Where; Tst = ton-miles for short trip
T6 = ton-miles for one round trip at the deeper depth
T5 = ton-miles for one round trip at the shallower depth

Example;

Please determine short trip ton-miles from 8000 ft to 8050 ft
Ton-miles @ 8050 ft = 200
Ton-miles @ 8000 ft = 190
Tst = (200 – 190)
Tst = 10 ton-miles
Ref book: Formulas and Calculations for Drilling, Production and Workover, Second Edition