GATE Petroleum Engineering Oil and Gas Well Drilling Technology Syllabus

How is the hydrostatic pressure exerted by a mud column calculated (field units, psi)?

$$P = 0.052 \times \rho \times TVD$$ where $\rho$ is mud weight in $\text{lb/gal}$ (ppg) and $TVD$ is true vertical depth in feet; $P$ is in psi.

Define the pressure (mud-weight) gradient and its typical value for fresh water.

The pressure gradient is the hydrostatic pressure increase per unit depth: $$G = 0.052 \times \rho$$ in psi/ft. For fresh water ($\rho = 8.33$ ppg), $G \approx 0.433$ psi/ft.

What is plastic viscosity (PV) and how is it obtained from a Fann viscometer?

Plastic viscosity is the resistance to flow caused by mechanical friction of solids and liquid in the mud (Bingham model). $$PV = \theta_{600} - \theta_{300}$$ in centipoise, where $\theta$ are the dial readings at 600 and 300 rpm.

What is yield point (YP) and how is it calculated from Fann viscometer readings?

Yield point is the resistance to initial flow due to electrochemical attractive forces between particles. $$YP = \theta_{300} - PV = 2\,\theta_{300} - \theta_{600}$$ in $\text{lb}/100\,\text{ft}^{2}$.

Differentiate between gel strength and yield point of a drilling fluid.

Yield point is the stress needed to initiate flow under dynamic (sheared) conditions; gel strength is the shear stress needed to start flow after the mud has been static (10 s and 10 min gels), indicating thixotropy and the mud's ability to suspend cuttings when circulation stops.

Which rheological model uses the equation $\tau = \tau_{y} + \mu_{p}\gamma$, and what does it describe?

The Bingham plastic model, where $\tau$ is shear stress, $\tau_{y}$ is yield point, $\mu_{p}$ is plastic viscosity and $\gamma$ is shear rate. It describes fluids that need a minimum stress (yield) before flowing, then flow linearly.

State the Power-law (Ostwald-de Waele) rheological model and identify its parameters.

$$\tau = K\,\gamma^{\,n}$$ where $K$ is the consistency index and $n$ is the flow-behavior index. For $n<1$ the fluid is shear-thinning (pseudoplastic), typical of drilling muds.

What instrument measures mud density, and what is a common field unit and its conversion?

The mud balance measures density. Common units: $\text{lb/gal}$ (ppg), $\text{lb/ft}^{3}$, or specific gravity. Conversion: $1\ \text{ppg} = 0.052\ \text{psi/ft}$ gradient and $\text{ppg} \times 7.48 = \text{lb/ft}^{3}$.

What is the function of the Marsh funnel, and what is the funnel viscosity of water?

The Marsh funnel gives a quick field measure of relative mud viscosity (time in seconds for 1 quart to drain). Fresh water at $70\,^{\circ}\text{F}$ has a Marsh funnel viscosity of about $26 \pm 0.5$ seconds per quart.

What is filtration (fluid) loss and what device measures it?

Filtration loss is the volume of mud filtrate that passes into a permeable formation, leaving a filter cake. It is measured with the API filter press (low-pressure/low-temperature, $100\ \text{psi}$, 30 min) or the HPHT filter press; reported in cm$^3$.

What is the primary weighting material used to increase mud density, and a common viscosifier?

Barite ($\ce{BaSO4}$, SG $\approx 4.2$) is the primary weighting agent; bentonite clay is the common viscosifier/filtration-control additive. Hematite is used for very high densities.

Classify drilling fluids by their continuous phase.

(1) Water-based muds (WBM) — fresh, salt, or inhibitive; (2) Oil-based muds (OBM) / synthetic-based muds (SBM); and (3) Gaseous/pneumatic fluids — air, mist, foam, and aerated mud.

Give the formula to calculate the weight of barite (in lb/bbl) needed to increase mud density.

$$W_{barite} = 1470 \times \frac{\rho_2 - \rho_1}{35 - \rho_2}$$ in lb/bbl, where $\rho_1$ is initial and $\rho_2$ is desired mud weight in ppg (35 ppg = density of barite, 1470 = $42 \times 35$).

What is the main function of solids-control (maintenance) equipment in the circulating system?

To remove drilled solids and gas from the mud so that desirable mud properties are maintained, reducing dilution costs, equipment wear, and downhole problems. It includes shakers, hydrocyclones, mud cleaners, centrifuges and degassers.

List the solids-control equipment in the order mud passes through them and the particle sizes they target.

(1) Shale shaker (>74 μm, coarse), (2) degasser (removes entrained gas), (3) desander (45–74 μm), (4) desilter (15–45 μm), (5) mud cleaner (combined screen + hydrocyclone), (6) decanting centrifuge (2–7 μm, finest).

What is the function of the shale shaker and how does it work?

The shale shaker is the primary (first) solids-removal device. It uses vibrating screens over which mud flows; cuttings larger than the screen mesh are retained and discarded while the mud passes through, removing the coarsest solids.

How do desanders and desilters (hydrocyclones) separate solids?

They use centrifugal force: mud enters tangentially into a cone, spinning so heavier/larger solids move to the wall and exit the bottom (underflow) as a slurry, while cleaned mud exits the top (overflow). Desanders use larger cones (sand), desilters smaller cones (silt).

What is the function of a degasser in the circulating system?

To remove entrained/dissolved formation gas from the returning mud, restoring correct mud density and preventing gas-cut mud from reducing hydrostatic pressure, pump cavitation and potential kicks.

What is the role of a decanting centrifuge in solids control?

It removes the finest (colloidal/low-gravity) solids by high-speed rotation; commonly used to recover expensive barite (heavy solids returned to mud) while discarding fine drilled solids and excess water, controlling mud weight and viscosity.

What is the function of the well control (blowout prevention) system?

To detect, contain and circulate out formation fluid influxes (kicks) and prevent uncontrolled flow (blowout). Core equipment: the BOP stack, choke and kill lines, choke manifold, accumulator (Koomey) unit, and mud-gas separator.

Differentiate between an annular BOP and a ram-type BOP.

An annular (bag) preventer uses an elastomer element that closes around any pipe size or open hole; ram preventers use steel rams: pipe rams (seal a specific pipe size), blind rams (seal open hole) and shear (blind-shear) rams that cut the pipe and seal.

What is the function of the accumulator (Koomey) unit in well control?

It stores hydraulic energy (nitrogen-precharged accumulator bottles) to provide rapid, reliable closing/opening of the BOP rams and annular preventer even if rig power fails.

What is the primary function of a drill bit, and what are the two broad classes of bits?

A bit cuts/crushes rock to advance the wellbore. The two broad classes are (1) roller-cone (rock) bits — milled-tooth and tungsten-carbide insert (TCI), and (2) fixed-cutter bits — PDC and natural/TSP diamond bits.

Compare roller-cone bits with PDC bits in terms of rock-removal mechanism.

Roller-cone bits remove rock by crushing/gouging as the cones roll (best in hard, abrasive formations); PDC (fixed-cutter) bits shear the rock with polycrystalline diamond cutters (best in soft to medium formations, with no moving parts and longer life).

What does the IADC dull-grading or bit-classification code describe for roller-cone bits?

The IADC bit code (three digits + letter) classifies roller-cone bits: 1st digit = formation series (1–3 milled tooth, 4–8 insert), 2nd digit = formation hardness (1–4), 3rd digit = bearing/gauge features, and a letter for additional features.

State the formula for bit hydraulic horsepower (HHP) across the bit.

$$HHP_{bit} = \frac{\Delta P_{bit} \times Q}{1714}$$ where $\Delta P_{bit}$ is pressure drop across the bit (psi) and $Q$ is flow rate (gpm).

How is the jet (nozzle) velocity through bit nozzles calculated, and why is it important?

$$v_{n} = \frac{0.32086\,Q}{A_{t}}$$ (ft/s) where $Q$ is in gpm and $A_{t}$ is total nozzle area in in$^2$. High jet velocity improves bottom-hole cleaning and cuttings removal beneath the bit.

Define specific energy in drilling and its significance for bit/parameter selection.

Specific energy is the mechanical energy required to remove a unit volume of rock: $$E_{s} = \frac{WOB}{A_{b}} + \frac{120\,\pi\,N\,T}{A_{b}\,ROP}$$ (where $A_b$ is bit area, $N$ rotary speed, $T$ torque). Minimizing $E_s$ indicates efficient drilling and proper bit/parameter selection.

What is weight on bit (WOB) and how is it determined from the weight indicator?

WOB is the downward axial force applied to the bit, supplied by drill collars. It is found as the difference between the free-rotating string weight (off bottom) and the indicated hook load while drilling: $WOB = W_{string} - W_{indicated}$ (buoyed weight of collars in compression).