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Compaction and Subsidence

Competency Statement:

Analyze, calculate and validate compaction, subsidence and settling parameters to predict their influence on the behavior of the project and their impact on recovery, using the results of this analysis to optimize exploitation projects.

Learning Objective:

Upon completing this Learning Module assignment, the participant should be able to
  • measure compaction, subsidence parameters.
  • determine the significanc of these parrmeters with re determining drive mechanisms, etc.

Assignment Instruction:

A sedimentary formation is the product of rock particle deposition, an ongoing process in which an underlying layer of particles is buried and forced downward by succeeding layers. As deposition and burial proceed, the underlying layer is subjected to a progressively increasing overburden pressure. This increased overburden results in compaction--a reduction in the thickness and bulk volume of the underlying layer.

Compaction also takes place when fluids are withdrawn from a subsurface formation. Fluid withdrawal causes a decrease in pore pressure, which in turn increases the grain-to-grain, or matrix pressure of the rock particles and causes them to compact. The amount of compaction that occurs in a given formation depends only upon the difference between the vertically applied overburden pressure and the fluid, or pore pressure. Thus, the uniaxial compaction of a rock sample of thickness h can be expressed as

    DVb/ Vb = Dh/h

Depending on geological conditions, subsurface compaction may be associated with varying degrees of subsidence, or sinking of the ground at the surface.

Compaction and its associated effect of surface subsidence can, in certain situations, have significant effects on reservoir management strategies and field operations. In this Assignment, you will build your general understanding of these phenomena and see how they can influence reservoir engineering decisions. You will also look at a system that has been developed by PDVSA to describe and predict compaction and subsidence in areas where they are of particular concern.

By the time you complete this assignment, you should be able to identify situations where compaction and subsidence may be important considerations in a reservoir management strategy, and you should be able to begin using computer-based tools for predicting compaction and subsidence under various conditions.