Field Specialist Course

1- Porosity (∅):-

  1. Definition of Porosity Porosity is the fracrion of the rock volume that consists of pore spaces.
  2. Mathematecally:

    ∅ = V P V P

    Porosity = Pore Volume / Bulk Volume

    If a rock has 25% porosity, that means 25% of its volume can store fluids.
  3. Types of porodity: . Total Porosity VS Effective porosity
    • Total porosity = all pore spaces
    • Effective porosity = interconnected pore spaces that allow flow
  4. Why Porosity is important in well testing:-
    • How much fluid is stored in the reservoir
    • How fast pressure changes propagate
    • Tre reservoir's storage Capacity
    So porosity influences the speed of pressure response. Lower porosity ➡️ Less storage ➡️ faster pressure drop near the well Higher porosity ➡️ more storage ➡️ Slower pressure response

2- Permeability (K)

  1. Definition of Permeability Permeability measures how easily fluids can move throug the rock. If porosity is the size of the tank, permeability is the size of the pipe connected to that tank. If porosity is the pores that fluid stores inside, permeability is the ID of size of these pores that are interconnetced together
  2. What Controls production Rate:

    Q (Rate) ∝ K x ▲P

    Darcy's Law (simplified idea)

    Lower permeability ➡️ restricted flow Rate Higher permeability ➡️ High flow Rae Even if porosity is high , low permeability means fluids cannot move easily.
  3. Unit of Permeability Permeability is measured in:-
    • Darcy
    • miliDarcy
    Typical Ranges:-
    • Tight rock: < 1 mD
    • Average reservoir: 10-500 mD
    • Very good reservoir > 1 Darcy
  4. Why permeability is important in well testing:- Permeability controls:
    • How fast pressure responds
    • How fast reservoir delievers fluids
    • How quickly pressure disturbance travels
    Lower permeability ➡️ slower response and sharper drawdown Higher permeability ➡️ faster pressure propagation

👨‍🏭Now think like an Engineer: Two reservoir have the same porosity, but:

Which one will show a smoother pressure drawdown curve

reservoir pressure

  1. Definition of Reservoir pressure reservoir pressure is the natural pressure inside the reservoir before production begins. It is the force that pushes fluids (gas, oil & water) towards the wellbore
  2. Where Does it Come from?
    • the weight of the fluids above (hydrostatic pressure)
    • Depth of the reservoir

      • P (Pressure) ≈ Fluid denisity (ρ) x Gravity (g) x Depth (h)

      Deeper reservoir ➡️ higher pressure

  3. Why is it important? Reservoir pressure is your reference point. From it, we calculate:
    • Drawdown = Pi - Pwf
    • Productivity index
    • Depletion
    • Skin effect impact
    If Pi is wrong, everything else becomes wrong
  4. How do we meassure it? we estimate reservoir pressure from Buildup test
    1. produce the well
    2. Shut the well
    3. Let pressure recover
    4. Use analysis (like Horner method) to estimate Pi
  5. Important concept Reservoir pressure changes over time (It is not Constant)
    • Before production ➡️ Initial reservoir pressure
    • After years of production ➡️ Average reservoir pressure (Lower due to depletion)

👨‍🏭Now think Carefully: If reservoir pressure is very high, but permeability is very low:

Will production rate be high or low - and why

Flow Regimes

    When a Well starts producing (or after shut-in), pressure response behave is not same for all time, a Reservoir goes through differemt flow regimes:
  1. Wellbore Storage What happens?

    the fluid being produced is mainly coming from the wellbore itself, not reservoir yet.

    So:
    • pressure drops very quickly
    • the reservoir has not fully responded
    • Data here does not represent reservoir properties

    On a log-log derivative plot: log-log derivative plot

    Derivative slope ≈ 1

    You never calculate permeability here
  2. Transition Zone

    Now the reservoir begins to feel the production.

    pressure disturbance starts moving outward

    the flow pattern still developing

    the derivative still unstable here

    Still not safe for reservoir calculation
  3. Radial Flow this is the key regimes
      Here:
    • Pressure moves outward uniformaly in a circular pattern
    • Flow is purely radial towards the well
    • Behaviour becomes stable
      • On Semi-Log Plot:
    • Straight line
      • On Derivative plot
    • Horizontal flat line
      • This is where we Calculate:
    • Permeability
    • Skin
    • Reservoir pressure
      • If Radial Flow does not appear - analysis is incomplete
  4. Boundary Effect

    After enough time, pressure wave reaches reservoir boundaries

      Example:
    • Faults
    • Sealing boundaries
    • Aquifer Support
    • Closed Reservoir
      • when this happens:
    • pressure behaviour changes again
    • Derivative rises or falls depending on boundary type

productivity index

    :

Basic Drive Mechanisms

    :

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