Reservoir Volume and Pressure Changes

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Questions and Answers

What is the formula for the compressibility of rock?

$\frac{\Delta V_{pr}}{\Delta p V_p}$

What is the effect of connate water on pore volume?

It decreases the pore volume

What is the formula for the compressibility of water?

$\frac{\Delta V_{pw}}{\Delta p V_{pw}}$

What is the total pore volume change due to rock and water?

$\Delta V_p = \Delta V_{pr} + \Delta V_{pw}$ Signup and view all the answers

What is the effect of net overburden stress on pore volume?

It decreases the pore volume Signup and view all the answers

What is the formula for the pore volume change due to rock and water in the absence of a gas cap?

$\Delta V_p = (c_f + c_w S_{wc}) \Delta p V_p$ Signup and view all the answers

What happens to the bulk volume of the rock when the net overburden stress increases?

It decreases Signup and view all the answers

What is the impact of the net overburden stress on hydrocarbon reserves?

It decreases the hydrocarbon reserves Signup and view all the answers

What is the effect of reducing the pressure from pi to p on the total available pore volume?

It decreases due to pore compressibility effects Signup and view all the answers

What happens to the volumes of the three phases (oil, gas, and water) when the pressure is reduced from pi to p?

They expand Signup and view all the answers

What is the consequence of the new fluid volumes, plus the water influx, not fitting in the available pore space?

There is a shortage of space equivalent to the sum of the shaded areas Signup and view all the answers

What is the term for the reduction of total pore volume due to pressure change?

Reduction of total pore volume Signup and view all the answers

What is the effect of the nett overburden stress on the rock?

It decreases the pore volume Signup and view all the answers

What is the result of the bulk volume reduction?

A decrease in hydrocarbon reserves Signup and view all the answers

What is the equation for the material balance, according to Dake?

Underground withdrawal = expansion of the system + cumulative water influx Signup and view all the answers

What is the subsurface volume at pressure p of oil, including an amount of dissolved gas?

Np Bo res.bbl.including an amount of dissolved gas Signup and view all the answers

What is the effect of decreasing pressure on rock bulk volume?

It reduces due to the increase in nett overburden stress. Signup and view all the answers

What happens to the volume of grains when pressure falls?

It increases due to the increase in nett overburden stress. Signup and view all the answers

What is the effect of pore volume changes on hydrocarbon reserves?

It reduces the hydrocarbon reserves due to the reduction in pore volume. Signup and view all the answers

What is the impact of connate water on pore volume changes?

It reduces the pore volume due to its high compressibility. Signup and view all the answers

What is the effect of nett overburden stress on the rock?

It decreases the rock's compressibility. Signup and view all the answers

What is the formula for the change in volume due to gas cap expansion?

(G - Gpc)Bg - GBgi Signup and view all the answers

What is the effect of water encroachment on the reservoir volume?

It reduces the reservoir volume due to the displacement of hydrocarbons. Signup and view all the answers

What is the formula for the total change in volume?

Original oil volume - (remaining oil volume + free solution gas) Signup and view all the answers

Study Notes

Material Balance Equation

Reservoir volume at pressure P of produced fluids = expansion of primary gas cap + expansion of oil plus originally dissolved gas + expansion of connate water + water influx + reduction of total pore volume

Components of Reservoir Volume

Production consists of:
- Np STB of oil
- Gp or Np RpSCF of gas
- Wp STB of water
Subsurface volume at pressure P of oil = Np Bo res.bbl, including an amount of dissolved gas equivalent to Np Rs SCF

Pore Volume Changes

Effect of pore volume changes due to connate water: expansion of water can contribute to reduction in pore volume for the hydrocarbons
Compressibility of water: cw = ΔVpw / (Vpw Δp)
Total pore volume change due to rock and water: ΔVp = ΔVpr + ΔVpw
ΔVp = (cf + cwSwc) ΔpVp, where cf is the compressibility of rock

Material Balance Equation with Gas Cap and Water Drive

N = Np Bo + (Gp - Np Rs) Bg - G (Bg - Bgi) - (We - Wp)
Gp = Gpc + Gps
N = Np Bo + (Gp - Np Rs) Bg - G (Bg - Bgi) - (We - Wp)

Effects of Pore Volume Changes

Water and rock pore compressibility can contribute to overall pore volume changes when pressure declines
Impact of pore volume changes due to rock: bulk volume reduces and increases in volume of grains as pressure falls