Abstract
An increase in effective stresses takes place in reservoirs as a consequence of
fluid production, a well-known phenomenon in both shallow and deep reservoirs.
It may seem reasonable to assume that permeability and porosity decrease as
pore pressure declines, since both effective radial and axial stresses become
intensified during reservoir depletion. However, laboratory results show that
this is not always the case. Porosity certainly decreases as a result of the
compaction process, which allows the breakage of grain-to-grain cement bonds.
Grain particles will become more compacted as both lateral and axial effective
stresses increase. On the other hand, permeability shows no definite
trend.
In this paper, a series of delicate experimental procedures were conducted to
reveal some of the most intriguing phenomena in pore collapse and their impact
on permeability. Sandstone samples were tested using a triaxial set-up. Based
on the experimental results of this study, in weak reservoir formations, pore
collapse does not occur suddenly. Rather, rocks gradually compact as
grain-to-grain cement bonds break down. It was found that permeability indeed
changes as effective stresses increase. However, the pathway to permeability
was found to be much more complex than previously stipulated. It was discovered
that enhancement or damage to permeability is not a function of pore collapse
alone. Other factors, such as stress path, initial porosity, particle size,
particle shape and particle distribution play a major role in determining what
type of alteration in permeability occurs and to what magnitude that alteration
would be.
Introduction
Traditionally, petrophysical properties data are very crucial for reserves
assessment and fluid flow characterization of petroleum reservoirs. A great
deal of money and effort are usually allocated in order to accurately estimate
these properties. Permeability and porosity are among those properties and are
by far the most important. Porosity is the key for reserves estimates while
permeability is the main parameter to predict flow rates, design drawdown and,
therefore, wellbore completion. Until recently, the common belief was that,
once determined, these properties remain constant throughout the production
life of the reservoir. Studies(1-3), including this paper, showed
that this is not a realistic assumption.
During the pressure depletion process, as production from the reservoir
continues, effective stresses within the reservoir increase. It is understood
that the effect of reservoir stresses on porosity and permeability of the
reservoir is more severe when porosity and permeability are high, although some
experimental studies like Hubbert and Willis(4), Voight and St.
Pierre(5) and Rosepiler(6) showed this effect is still
significant, even at low porosity and permeability. It is also understood that
stress paths have a large influence on horizontal and vertical permeability,
and also on porosity.
The elastic uniaxial strain model is mostly used in reservoir engineering to
describe production-induced changes in horizontal stress due to pore pressure
decline (pressure depletion). It predicts the total horizontal stress by using
overburden stress, reservoir pressure decrease and material mechanical
parameters. The principal assumption in this model is that there is no lateral
deformation (zero horizontal strain condition) during the depletion
process.
© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)
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History
- Original manuscript received:
25 May 2007
- Revised manuscript received:
30 December 2008
- Manuscript approved:
8 June 2009
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