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Rock Compressibility

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Rock Compressibility

Reservoir rock properties

Professor Abdulla Ali Al-Dombi

Petroleum Production Engineer

مدونة بالعربية

https://aldambi.blogspot.com/

In English

https://aldambi2.blogspot.com/

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Introduction

  • A reservoir thousands of feet underground is subjected to an overburden pressure caused by the weight of the overlying formations.
  • Overburden pressures vary from area to area depending on factors such as
  • depth,
  • nature of the structure,
  • consolidation of the formation, and
  • the geologic age and history of the rocks.

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Introduction(cont.)

  • Compaction is the process of volume reduction due to an externally applied pressure. For extreme compaction pressures, all materials show some irreversible change in porosity. This is due to distortion and crushing of the grain or matrix elements of the materials, and in some cases, recrystallization.

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Source of Stress

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Platy Grains

(e.g., clays)

Non-Platy Grains

(e.g., qtz., feldspar)

Rotation and Closer

Packing

Ductile Grain

Deformation

Breakage of

Brittle Grains

Pressure Solution

At Grain

Contacts

Ductile Framework

Grain, e.g., Shale Rock

Fragment

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Source of Stress(cont.)

  • Rock can be subject to several different kinds of stress:
  • Lithostatic stress:
  • Rock beneath the Earth's surface experiences equal pressure exerted on it from all directions because of the weight of the overlying rock. It is like the hydrostatic stress (water pressure) that a person feels pressing all around their body when diving down deep in water.

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Source of Stress(cont.)

  • differential stress:
  • In many cases, rock may experience an additional, unequal stress due to tectonic forces. There are three basic kinds.
  • tensional stress (stretching)
  • compressional stress (squeezing)
  • shearing stress (side to side shearing)

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Source of Stress(cont.)

  • The rock subjected to the following type stress:
  • External stress: the burial depth induces stress by vertical loading from the weight of solid material above.
  • Internal stress: the fluids inside the pore space of the rock exert a counter acting hydrostatic force due to the hydrostatic head and any over pressuring.
  • Lateral stress: stress induced laterally as a result of geological deformation and lateral confine.

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Compaction of Rocks

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Compaction of Rocks(cont.)

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Compaction of Rocks(cont.)

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  • During production operations, the internal pore pressure decreases and, therefore, the effective overburden pressure increases. This increase causes the following effects:
  • The bulk volume of the reservoir rock is reduced.
  • Sand grains within the pore spaces expand.
  • These two volume changes tend to reduce the pore space and, there fore, the porosity of the rock.

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Compaction of Rocks(cont.)

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Effective overburden pressure= overburden pressure – pore pressure

Pressure depletion decreases pore pressure

Increasing the effective overburden pressure

  • Decreasing in Bulk volume
  • Sand grain volume expand
  • Reducing in pore space
  • Reducing in porosity

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Porosity and Depth Relationship

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  • Above figure shows the relationship between porosity and depth (or stress).
  • As the depth (and stress) increases, the porosity declines.

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Isothermal Compressibility

  • For a hydrocarbon reservoir, the reservoir rock grains are subjected to external pressure (stress) due to overburden and internal pressure represented by the fluid pressure.
  • As reservoir fluids are produced, the rock grains may expand causing a reduction in porosity which causes fluid to be expelled. (expansion of rock grains).
  • At the same time, the overburden may cause a decrease in the bulk volume causing fluid to be expelled (rock compaction). In the extreme case, we get subsidence.

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Isothermal Compressibility(cont.)

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Types of Compressibility

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Rock Matrix Compressibility

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Rock Bulk Compressibility

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Pore Compressibility

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Formation Compressibility

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Formation Compressibility(cont.)

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Formation Compressibility(cont.)

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Formation Compressibility(cont.)

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Formation Compressibility(cont.)

  • Formation compressibility is a complex function of:
  • Rock type (consolidated, unconsolidated).
  • Pore volume.
  • Rock matrix.
  • Pore pressure.
  • Overburden pressure.
  • The stress in different direction in the formation.
  • Maximum depth.

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Formation Compressibility

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Total Compressibility

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Subsurface Pressure

  • In general the elevated pressures encountered with depth are due to one or two causes:
  • Hydrostatic pressure

Is imposed by the weight of fluid which fills the voids of the rocks.

  1. Formation pressure

Is the pressure of the fluids in the pores of subsurface formations.

  1. Overburden pressure

Due to the weight of the rocks and their fluid content existing above the reservoir.

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Hydrostatic Pressure

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Formation Pressure

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Overburden Pressure

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Subsurface Temperature

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Factors Affecting rock Compressibility

  • Factors Affecting petroleum Reservoir rock Compressibility:
  • The type of rock: The compressibility of a rock is affected by its mineral composition, grain size, and cementation.
  • The porosity of the rock: more porous the rock = more compressible because of more space

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Factors Affecting rock Compressibility (cont.)

  1. The pore fluid: Oil is less compressible than water, so a rock with oil will be less compressible than a rock with water
  2. The temperature of the rock: As the temperature increases, the rock becomes more compressible
  3. The pressure of the rock: because the weight of the overlying rock presses down on the rock, forcing the molecules closer together. This makes the rock less compressible.

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Examples

  • What is the formation pore pressure at a depth of 4000 ft and the formation water density is of 9 ppg?
  • What is the pressure at a depth of 13000 ft with a mud weight of 14 ppg?
  • Find the temperature at a depth of 5000 ft when the average surface temperature is 75 oF and the temperature gradient is 1.5 oF/100 ft?
  • Estimate the compressibility coefficient of a sandstone formation that is characterized by a porosity of 0.2, using Newman’s correlation?

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Rock Compressibility

Reservoir rock properties

Professor Abdulla Ali Al-Dombi

Petroleum Production Engineer

مدونة بالعربية

https://aldambi.blogspot.com/

In English

https://aldambi2.blogspot.com/

​