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Question 1Reservoir Engineering

Total porosity of a reservoir rock is defined as which of the following ratios?

  1. pore volume divided by bulk volume Correct answer.
  2. bulk volume divided by pore volume
  3. grain volume divided by bulk volume
  4. pore volume divided by grain volume

Explanation

Porosity is the fraction of the bulk volume that is void space, so phi = Vp/Vb where Vb includes both pores and grains. Because pore volume is always smaller than bulk volume, porosity must lie between 0 and 1.

Question 2Reservoir Engineering

A porous medium has a permeability of one darcy when it transmits which of the following?

  1. 1 cP fluid flowing at 1 bbl/day through 1 ft2 under a pressure gradient of 1 psi/ft
  2. 1 cP fluid flowing at 1 cm3/s through 1 cm2 under a pressure gradient of 1 atm/cm Correct answer.
  3. 1 cP fluid flowing at 1 cm3/s through 1 cm2 under a pressure gradient of 1 psi/ft
  4. 1 Pa-s fluid flowing at 1 m3/s through 1 m2 under a pressure gradient of 1 Pa/m

Explanation

The darcy is defined in centimetre-gram-second-atmosphere units: 1 D permits 1 cm3/s of a 1 cP fluid through 1 cm2 under a gradient of 1 atm/cm. The SI permeability unit is the square metre, and 1 D is about 9.87 x 10-13 m2.

Question 3Reservoir Engineering

Which primary drive mechanism normally gives the highest ultimate oil recovery factor?

  1. solution gas drive
  2. gas cap expansion drive
  3. strong water drive Correct answer.
  4. rock and fluid expansion drive

Explanation

An active water drive both maintains reservoir pressure and sweeps oil efficiently, typically recovering about 35 to 60 percent of the oil in place. Solution gas drive usually recovers only about 5 to 30 percent, and rock and fluid expansion recovers very little.

Question 4Reservoir Engineering

The oil formation volume factor Bo is best described as which of the following?

  1. the ratio of dissolved gas volume to oil volume at standard conditions
  2. the volume of oil at the bubble point divided by its volume at standard conditions
  3. the volume of dissolved gas released from one barrel of oil at standard conditions
  4. the reservoir volume of one stock-tank barrel of oil plus its dissolved gas Correct answer.

Explanation

Bo converts stock-tank oil volume to reservoir volume at the prevailing temperature and pressure, and therefore includes both the dissolved gas still in solution and thermal expansion. Values are normally greater than 1.0 reservoir barrel per stock-tank barrel.

Question 5Reservoir Engineering

The effective permeability to oil in a core that also contains water is which of the following?

  1. the permeability measured when the core is fully saturated with a single fluid
  2. the ratio of a phase permeability to the absolute permeability of the rock
  3. the permeability to oil at the existing saturation with other phases present Correct answer.
  4. the permeability measured with air on a dry core at atmospheric pressure

Explanation

Effective permeability is the phase permeability measured at the saturation that actually exists, with the other phases present and possibly flowing. Dividing it by the absolute permeability gives the relative permeability.

Question 6Reservoir Engineering

Which porosity range is most typical of a productive sandstone reservoir?

  1. 1 to 5 percent
  2. 45 to 55 percent
  3. 10 to 25 percent Correct answer.
  4. 30 to 40 percent

Explanation

Most commercial sandstone reservoirs fall in the 10 to 25 percent porosity range, with lower values in tight or cemented sands. Carbonates and shaly sands commonly plot below this band.

Question 7Reservoir Engineering

The bubble point pressure of a reservoir oil is the pressure at which which of the following occurs?

  1. the first bubble of gas comes out of solution as the pressure is lowered Correct answer.
  2. the last trace of free gas disappears as the pressure is raised at reservoir temperature
  3. the first droplet of liquid condenses from the gas phase at reservoir temperature
  4. the oil viscosity reaches its minimum value at reservoir temperature

Explanation

The bubble point is the saturation pressure of a black-oil system: below it free gas evolves, above it all gas is dissolved and the oil is undersaturated. For a gas condensate the analogous pressure is the dew point.

Question 8Reservoir Engineering

The oil left behind in the swept portion of a reservoir after a waterflood is called which of the following?

  1. the residual oil saturation of the swept region Correct answer.
  2. the critical gas saturation of the reservoir
  3. the movable oil saturation of the swept region
  4. the initial oil saturation of the reservoir

Explanation

Residual oil saturation is the immobile oil remaining in the water-swept rock, where it represents the endpoint of the relative permeability curve at high water saturation. Oil that has never been contacted by the displacing water is bypassed oil, not residual oil.

Question 9Reservoir Engineering

In the steady-state radial flow equation for a vertical oil well, which of the following quantities appears in the denominator?

  1. the formation thickness
  2. the logarithm of the drainage-radius-to-wellbore-radius ratio Correct answer.
  3. the formation permeability
  4. the pressure drawdown between the boundary and the wellbore

Explanation

For steady-state radial flow, q = kh(Pe - Pwf) / [141.2 B mu ln(re/rw)] in field units. Productivity therefore rises with kh and drawdown and falls as the drainage radius becomes large relative to the wellbore.

Question 10Reservoir Engineering

A well test yields a positive skin factor. What does this indicate?

  1. the reservoir pressure is above the bubble point
  2. near-wellbore damage adds an extra pressure drop Correct answer.
  3. the test was run in a gas well rather than an oil well
  4. the well is stimulated and has a flow efficiency above one

Explanation

Skin is a dimensionless additional pressure loss concentrated at the sandface, so a positive value means the near-well region is impaired and the well produces less than an undamaged well would. Stimulation such as fracturing or acidizing produces negative skin.

Question 11Reservoir Engineering

A waterflood has endpoint relative permeabilities of krw = 0.30 and kro = 0.60, with viscosities of 0.6 cP for water and 3.0 cP for oil. What is the endpoint mobility ratio?

  1. 1.25
  2. 2.50 Correct answer.
  3. 0.40
  4. 5.00

Explanation

The mobility ratio is M = (krw/muw)/(kro/muo), giving (0.30/0.6)/(0.60/3.0) = 0.50/0.20 = 2.5. A value greater than one means the water is more mobile than the oil and the displacement is unfavourable.

Question 12Reservoir Engineering

For a water-wet rock at constant interfacial tension and contact angle, how does capillary entry pressure change as the pore-throat radius decreases?

  1. it increases in proportion to the square of the radius
  2. it increases in inverse proportion to the radius Correct answer.
  3. it remains constant because the wettability is unchanged
  4. it decreases in proportion to the radius

Explanation

Capillary pressure is given by Pc = 2 sigma cos(theta)/r, so for fixed interfacial tension and contact angle it scales as 1/r. Small pore throats therefore require high entry pressures and hold water strongly in a water-wet rock.

Question 13Reservoir Engineering

A reservoir has 80 acres of area, 20 ft of net pay, 20 percent porosity, 25 percent initial water saturation and an oil formation volume factor of 1.2 RB/STB. What is the original oil in place?

  1. 1.24 million STB
  2. 2.07 million STB
  3. 1.86 million STB
  4. 1.55 million STB Correct answer.

Explanation

Volumetric OOIP is N = 7758 A h phi (1 - Swi)/Bo. Substituting gives 7758 x 80 x 20 x 0.20 x 0.75 / 1.2, which is about 1.55 million stock-tank barrels.

Question 14Reservoir Engineering

A solution-gas-drive well has a static reservoir pressure of 2500 psia and a maximum rate of 1000 STB/day at zero flowing pressure. Using Vogel's equation, what rate is expected at a flowing pressure of 1250 psia?

  1. 250 STB/day
  2. 800 STB/day
  3. 700 STB/day Correct answer.
  4. 500 STB/day

Explanation

Vogel's relationship is q/qmax = 1 - 0.2(Pwf/Pr) - 0.8(Pwf/Pr)2. With Pwf/Pr = 0.5 this gives 1 - 0.1 - 0.2 = 0.70, so the rate is 700 STB/day.

Question 15Reservoir Engineering

On a Horner buildup plot of shut-in pressure against the logarithm of Horner time, what is the semilog straight-line slope used to determine?

  1. the wellbore storage coefficient
  2. the formation permeability Correct answer.
  3. the distance to a nearby fault
  4. the fluid viscosity at reservoir conditions

Explanation

The Horner slope m satisfies m = 162.6 q B mu/(k h), so measuring the slope with known rate, thickness and fluid properties yields permeability. Wellbore storage appears early in the test and boundaries appear as late-time departures from the line.

Question 16Reservoir Engineering

Which statement about the gas deviation factor z is correct?

  1. z is always less than 1 for reservoir gases
  2. z equals 1 for an ideal gas at all conditions Correct answer.
  3. z is numerically equal to the gas formation volume factor
  4. z is independent of pressure at constant temperature

Explanation

The deviation factor measures departure from ideal behaviour through pV = znRT, so an ideal gas has z = 1 everywhere. Real gases show z below unity at moderate pressure and z above unity at high pressure.

Question 17Reservoir Engineering

At a given water saturation, krw = 0.10, kro = 0.50, water viscosity is 0.5 cP and oil viscosity is 1.0 cP. What is the water fractional flow?

  1. 0.71
  2. 0.29 Correct answer.
  3. 0.50
  4. 0.20

Explanation

Fractional flow is fw = 1/[1 + (kro muw)/(krw muo)]. The mobility group is (0.50 x 0.5)/(0.10 x 1.0) = 2.5, so fw = 1/3.5, about 0.29.

Question 18Reservoir Engineering

A Dykstra-Parsons permeability variation coefficient of 0.85 describes which type of reservoir?

  1. a highly heterogeneous reservoir Correct answer.
  2. an essentially homogeneous reservoir
  3. a reservoir with a favourable mobility ratio
  4. a reservoir with no permeability variation

Explanation

The Dykstra-Parsons coefficient ranges from 0 for a uniform permeability distribution to nearly 1 for extreme variation, and it is computed as (k50 - k84.1)/k50. A value of 0.85 therefore indicates severe layer-to-layer heterogeneity that will cause early water breakthrough.

Question 19Reservoir Engineering

In the Havlena-Odeh form of the material balance for a reservoir with no gas cap and no water influx, a plot of the withdrawal term F against the expansion term Eo through the origin has a slope equal to which of the following?

  1. the water influx constant
  2. the rock compressibility
  3. the original oil in place N Correct answer.
  4. the initial gas cap ratio m

Explanation

The material balance reduces to F = N Eo when only oil expansion drives production, so plotting F versus Eo gives a straight line of slope N through the origin. Scatter or curvature indicates an incorrect drive model or an additional term such as water influx.

Question 20Reservoir Engineering

In the van Everdingen-Hurst unsteady-state aquifer model, the dimensionless time depends on which of the following?

  1. aquifer diffusivity and elapsed time Correct answer.
  2. the cumulative water influx alone
  3. the oil column thickness alone
  4. the aquifer outer-boundary pressure alone

Explanation

Dimensionless time is tD = 0.00634 k t/(phi mu ct r2), so it combines aquifer permeability, porosity, viscosity, total compressibility and real time into a single diffusivity-based group. The aquifer model then relates tD to dimensionless influx.

Question 21Reservoir Engineering

In Buckley-Leverett frontal advance theory, the velocity of a plane of constant water saturation is proportional to which of the following?

  1. the capillary pressure gradient measured along the flow direction
  2. the reciprocal of the local interstitial velocity of the water phase
  3. the derivative of fractional flow with respect to saturation Correct answer.
  4. the water saturation existing at the shock front at breakthrough

Explanation

The frontal advance equation states that v_Sw = (q/(phi A)) (dfw/dSw), so each saturation travels at a rate set by the slope of the fractional flow curve. The Welge tangent construction uses this same slope to locate the breakthrough front.

Question 22Reservoir Engineering

In the general material balance equation, the dimensionless group m represents which of the following?

  1. the mobility ratio of the displacing to the displaced phase
  2. the ratio of cumulative gas produced to cumulative oil produced
  3. the gas deviation factor evaluated at initial reservoir conditions
  4. the ratio of initial gas cap volume to initial oil zone volume Correct answer.

Explanation

The gas cap term is written m N Boi (Bg/Bgi - 1), where m is the initial gas cap volume divided by the initial oil zone volume, both in reservoir barrels. A value of zero corresponds to a reservoir with no primary gas cap.

Question 23Reservoir Engineering

In the raw gas backpressure equation qg = C(Pr2 - Pwf2)n, an exponent n close to 0.5 indicates which of the following?

  1. a well producing at the critical coning rate
  2. turbulent non-Darcy flow near the wellbore Correct answer.
  3. the presence of a hydraulic fracture
  4. laminar flow throughout the drainage area

Explanation

The exponent ranges from 1 for fully laminar Darcy flow to 0.5 when inertial, non-Darcy effects dominate near the sandface. A value near 0.5 therefore signals high-velocity turbulent losses in the vicinity of the wellbore.

Question 24Reservoir Engineering

Raising the capillary number during a waterflood, for example by reducing interfacial tension, generally has which effect?

  1. it increases the mobility ratio of the waterflood
  2. it reduces residual oil saturation Correct answer.
  3. it has no effect on residual oil saturation
  4. it increases residual oil saturation

Explanation

The capillary number expresses the ratio of viscous to capillary forces, and the desaturation curve shows residual oil falling sharply once it is raised by several orders of magnitude. This is the physical basis of surfactant and other chemical enhanced oil recovery processes.

Question 25Reservoir Engineering

The radius of investigation reached during a transient well test grows approximately in proportion to which of the following?

  1. the square root of k t divided by (phi mu ct) Correct answer.
  2. the square of the sandface flow rate
  3. the logarithm of the drainage radius
  4. the shut-in time itself

Explanation

Pressure diffusion makes the investigation radius scale as the square root of hydraulic diffusivity times time, r ~ sqrt(k t/(phi mu ct)). Doubling the shut-in time therefore extends the investigated region by only about 40 percent.

Question 26Reservoir Engineering

The darcy is the unit used to express which reservoir rock property?

  1. Permeability Correct answer.
  2. Porosity
  3. Compressibility
  4. Capillary pressure

Explanation

Permeability measures the ability of a rock to transmit fluid and is defined by Darcy's law: one darcy passes 1 cm3/s of a 1 cP fluid through 1 cm2 under a 1 atm/cm gradient. Porosity, compressibility and capillary pressure are separate rock or rock-fluid properties with their own units.

Question 27Reservoir Engineering

Which of the following is an example of secondary porosity in a carbonate reservoir?

  1. Pore space between sand grains deposited at the time of sedimentation
  2. Intergranular pore space between deposited ooids
  3. Microporosity within the original lime mud matrix
  4. Vugs created by dissolution of limestone Correct answer.

Explanation

Secondary porosity develops after deposition through processes such as dissolution, dolomitization and fracturing. Intergranular, matrix and depositional pore spaces are primary porosity that formed at the time the sediment was laid down.

Question 28Reservoir Engineering

In the volumetric equation N = 7758 A h phi (1 - Sw)/Bo, what does the constant 7758 represent?

  1. Barrels of bulk volume in one acre-foot Correct answer.
  2. Pounds of oil contained in one cubic foot of rock
  3. Square feet contained in one acre
  4. Cubic feet of gas dissolved per barrel of oil

Explanation

One acre-foot equals 43,560 ft3, and dividing by 5.615 ft3 per barrel gives about 7758 barrels of bulk volume per acre-foot. With A in acres, h in feet and phi and Sw as fractions, the constant converts bulk volume directly to barrels.

Question 29Reservoir Engineering

Recovery factor in a reservoir is defined as which ratio?

  1. Water saturation divided by the average oil saturation in place
  2. Cumulative oil produced divided by original oil in place Correct answer.
  3. Original oil in place divided by the estimated ultimate recovery
  4. Produced volume divided by the bulk rock volume of the drainage area

Explanation

Recovery factor is the fraction of the original hydrocarbon in place that is ultimately produced, normally expressed as a percentage. It is used to translate volumetrically estimated oil in place into recoverable reserves.

Question 30Reservoir Engineering

A reservoir rock is described as strongly water-wet. Which contact angle measured through the water phase is consistent with this description?

  1. About 150 degrees
  2. About 90 degrees
  3. About 30 degrees Correct answer.
  4. About 120 degrees

Explanation

Wettability is characterised by the contact angle measured through the water phase; values well below 90 degrees indicate water-wet conditions, and values approaching zero indicate strongly water-wet rock. Angles above 90 degrees indicate oil-wet behaviour.

Question 31Reservoir Engineering

A normally pressured freshwater aquifer has a hydrostatic gradient of approximately 0.433 psi/ft. What is the expected pressure at a depth of 4000 ft?

  1. 1232 psia
  2. 2232 psia
  3. 1732 psia Correct answer.
  4. 4320 psia

Explanation

Hydrostatic pressure equals the gradient multiplied by the true vertical depth: 0.433 psi/ft times 4000 ft equals about 1732 psi. This normal gradient corresponds to the density of fresh water at standard conditions.

Question 32Reservoir Engineering

The dew point pressure of a gas condensate reservoir is the pressure at which which of the following first occurs during isothermal depletion?

  1. The gas deviation factor reaches unity at reservoir temperature
  2. The gas becomes immiscible with the formation water
  3. The first droplet of liquid condensate appears Correct answer.
  4. The last bubble of gas disappears from the liquid phase

Explanation

For a gas condensate, the dew point is the pressure at which the first liquid drop condenses out of the gas phase as pressure is lowered isothermally. Below that pressure the reservoir fluid is a two-phase mixture of gas and retrograde liquid.

Question 33Reservoir Engineering

In exponential decline analysis, the production rate plotted against time on semilog paper is which of the following?

  1. A concave upward curve
  2. A parabola
  3. A straight line Correct answer.
  4. A horizontal line

Explanation

Exponential decline follows q = qi exp(-Dt), so the logarithm of rate is linear in time. That straight-line behaviour on semilog paper is the diagnostic feature used to determine the nominal decline rate D.

Question 34Reservoir Engineering

A clean sandstone has porosity 0.20, formation water resistivity 0.05 ohm-m and true formation resistivity 5.0 ohm-m. Using Archie's law with a = 1, m = 2 and n = 2, what is the water saturation?

  1. 0.35
  2. 0.50 Correct answer.
  3. 0.71
  4. 0.25

Explanation

Archie's law gives Sw^n = aRw/(phi^m Rt) = (1 x 0.05)/(0.04 x 5.0) = 0.25, so Sw equals the square root of 0.25, or 0.50. The water saturation is therefore 50 percent of pore volume.

Question 35Reservoir Engineering

At a given saturation, krw = 0.30, kro = 0.15, water viscosity is 1.0 cP and oil viscosity is 2.0 cP. What is the water fractional flow?

  1. 0.60
  2. 0.80 Correct answer.
  3. 0.33
  4. 0.20

Explanation

Fractional flow of water is fw = 1/[1 + (kro/krw)(muw/muo)]. Here (0.15/0.30) x (1.0/2.0) equals 0.25, so fw = 1/1.25 = 0.80, meaning 80 percent of the flowing stream is water.

Question 36Reservoir Engineering

A p/z plot for a gas reservoir is a straight line that extrapolates to a positive intercept on the cumulative production axis. Which interpretation is correct?

  1. Strong water influx, making the intercept meaningless
  2. The gas deviation factor at initial conditions
  3. Original gas in place, if the reservoir is volumetric Correct answer.
  4. The reservoir is still above its dew point pressure

Explanation

For a volumetric gas reservoir, p/z is linear in cumulative gas produced and extrapolating to p/z = 0 gives the original gas in place. Departures from a straight line indicate water influx or abnormally pressured behaviour.

Question 37Reservoir Engineering

During boundary-dominated flow in a closed reservoir, which statement about pressure is correct?

  1. Pressure continues to decline at an ever-increasing rate at every point
  2. The flow rate declines to zero while the bottomhole pressure stays constant
  3. Pressure transients are still propagating radially outward from the wellbore
  4. Pressure declines at the same rate at every point in the drainage volume Correct answer.

Explanation

Once all boundaries are felt, the reservoir depletes in a semi-steady manner in which pressure falls at a constant rate with time at every location while a fixed spatial profile is maintained. Transient propagation has ended by that time.

Question 38Reservoir Engineering

Which change would act to increase the critical rate for water coning beneath an oil well?

  1. A larger density contrast between water and oil Correct answer.
  2. Increasing the viscosity of the oil in the reservoir
  3. Completing the perforations closer to the water contact
  4. Producing the well at a higher drawdown pressure

Explanation

Critical coning rate rises with greater density difference between the phases, higher vertical permeability, and greater distance between the perforations and the contact. It falls as oil viscosity increases.

Question 39Reservoir Engineering

The hydraulic diffusivity of a reservoir, which governs how fast pressure transients propagate, is defined as which ratio?

  1. Permeability divided by porosity, viscosity and total compressibility Correct answer.
  2. Porosity divided by the permeability of the rock
  3. Viscosity multiplied by total compressibility and formation volume factor
  4. Compressibility divided by the permeability of the formation

Explanation

Hydraulic diffusivity is k/(phi mu ct), the group that appears in the diffusivity equation governing pressure transients. Larger permeability speeds propagation, while larger porosity, viscosity or total compressibility slows it.

Question 40Reservoir Engineering

In a Corey relative permeability model, raising the oil relative permeability exponent from 2 to 4 at fixed endpoints has which effect?

  1. The curve is unchanged because only the endpoints matter
  2. The curve becomes a straight line between the endpoints
  3. The curve shifts upward at every water saturation
  4. The curve becomes more concave at intermediate oil saturations Correct answer.

Explanation

A larger Corey exponent makes the relative permeability curve more concave, so the oil relative permeability is lower at intermediate saturations while the endpoints stay fixed. This changes the shape of the fractional flow curve and the waterflood performance.

Question 41Reservoir Engineering

In a waterflood, an unfavourable mobility ratio primarily reduces which quantity?

  1. Vertical sweep efficiency only
  2. Areal sweep efficiency at breakthrough Correct answer.
  3. Displacement efficiency within the swept region
  4. The endpoint water saturation of the rock

Explanation

A mobility ratio above one promotes viscous fingering and early water breakthrough, lowering the fraction of the pattern area that has been swept when water arrives. Displacement efficiency at a given saturation and endpoint saturations are governed by relative permeability and rock-fluid properties.

Question 42Reservoir Engineering

Adding a high-molecular-weight polymer to injected water improves oil recovery mainly through which mechanism?

  1. Reducing interfacial tension to near zero
  2. Lowering water mobility and improving the mobility ratio Correct answer.
  3. Dissolving carbonate rock to create wormholes
  4. Raising the reservoir temperature near the wellbore

Explanation

Polymer increases the viscosity of the injected water, lowering its mobility so the mobility ratio becomes more favourable and volumetric sweep improves. Polymers do not significantly reduce interfacial tension or generate heat.

Question 43Reservoir Engineering

In a reservoir with a thick transition zone above the free water level, which factor most strongly controls how fast water saturation decreases with height above the contact?

  1. The lithostatic overburden gradient
  2. The capillary pressure curve of the rock Correct answer.
  3. The reservoir temperature
  4. The oil formation volume factor

Explanation

Saturation versus height is obtained by converting height to capillary pressure through the density difference and then reading saturation from the capillary pressure curve. A rock with a broad pore-size distribution therefore has a long transition zone.

Question 44Reservoir Engineering

Among the natural primary drive mechanisms, which typically gives the highest oil recovery factor in a light-oil reservoir?

  1. Rock and fluid expansion drive
  2. Gravity drainage acting alone
  3. Solution gas drive
  4. Water drive Correct answer.

Explanation

Water drive maintains reservoir pressure and displaces oil efficiently, typically recovering on the order of 35 to 60 percent of oil in place. Solution gas drive and rock/fluid expansion usually recover much less, and gravity drainage alone is generally weak.

Question 45Reservoir Engineering

For an undersaturated oil reservoir containing no free gas, which quantity is used as the total compressibility in the diffusivity equation?

  1. Gas compressibility only, neglecting oil and rock
  2. Rock plus oil compressibility Correct answer.
  3. Oil compressibility only, neglecting the rock
  4. Rock compressibility only, neglecting oil compressibility

Explanation

When only oil and rock are present, total compressibility is the pore-volume weighted rock compressibility plus the oil compressibility. Ignoring either term underestimates the storativity and misinterprets the pressure transient response.

Question 46Reservoir Engineering

A well produces under hyperbolic decline with b = 0.8 and an initial nominal decline rate of 0.6 per year. Using the Arps relations, what is the instantaneous decline rate at the time the rate has fallen to half its initial value?

  1. 0.34 per year Correct answer.
  2. 0.30 per year
  3. 0.17 per year
  4. 0.60 per year

Explanation

For hyperbolic decline, the instantaneous decline rate is D = Di (q/qi)^b. With q/qi = 0.5 and b = 0.8, D = 0.6 x 0.5^0.8 = 0.6 x 0.574, which is about 0.34 per year.

Question 47Reservoir Engineering

A pressure transient test on a hydraulically fractured well shows a half-slope straight line on the log-log diagnostic plot of pressure change versus time. Which flow regime does this indicate?

  1. Radial flow in the matrix
  2. Spherical flow near the perforations
  3. Boundary-dominated flow in a closed cell
  4. Linear flow into the fracture Correct answer.

Explanation

A half-slope on a log-log pressure-change plot is the signature of linear flow, in which the drainage area grows as parallel streamlines perpendicular to the fracture face. Radial flow shows a flat derivative, and boundary-dominated flow shows unit slope in pressure change.

Question 48Reservoir Engineering

During isothermal depletion of a gas condensate reservoir below the dew point, how does the volume of retrograde liquid in the pore space normally behave?

  1. It peaks well above abandonment pressure and then partly revaporizes Correct answer.
  2. It stays constant once condensate first forms
  3. It increases monotonically all the way to abandonment pressure
  4. It stays at zero until abandonment pressure is reached

Explanation

Retrograde condensate reaches a maximum liquid dropout at a pressure considerably above abandonment and then partially revaporizes as pressure is lowered further. This behaviour is read from a constant-composition or constant-volume depletion experiment.

Question 49Reservoir Engineering

A drawdown test gives a semilog straight line with a slope of 25 psi per log cycle. The test data are q = 500 STB/day, Bo = 1.2 res bbl/STB, oil viscosity = 1.0 cP and net pay = 50 ft. What is the permeability?

  1. 78 md Correct answer.
  2. 39 md
  3. 15.6 md
  4. 7.8 md

Explanation

For oil, k = 162.6 q Bo mu / (m h). Substituting gives 162.6 x 500 x 1.2 x 1.0 divided by (25 x 50), which equals 97,560/1250, or about 78 md.

Question 50Reservoir Engineering

In a black-oil reservoir simulator using the IMPES formulation, which set of variables is treated implicitly in a timestep?

  1. Saturations implicitly, with pressure updated explicitly
  2. Only the well rates, with all gridblock variables lagged
  3. Both pressure and saturations fully implicitly at each timestep
  4. Pressure implicitly, with saturations updated explicitly Correct answer.

Explanation

IMPES stands for implicit pressure, explicit saturation: a single pressure equation is solved implicitly and the saturations are then updated explicitly from the saturation equations. This is cheaper per timestep than fully implicit formulations but has stricter stability limits.