Summary
This paper presents a practical method to estimate the storativity ratio of
a dual-permeability layered reservoir with crossflow from pressure-transient
data. The method uses an analytical formula for the storativity ratio in terms
of the separation between the two semilog straight lines on the pressure vs.
log-time plot, similar to the method used for dual-porosity systems. Knowing
the storativity ratio from a well test allows individual-layer properties to be
estimated if the layer flow rates are available from production logs.
Demonstrations of the method to estimate the storativity ratio and
individual-layer properties are presented by examples. Comparison of the
results with those obtained from the existing techniques is also provided to
highlight the accuracy of the proposed technique.
Introduction
Depletion characteristics of commingled multilayer reservoirs are determined
by the characteristics of interlayer-fluid transfer, which is dictated by the
properties of the individual layers. In such systems, to obtain the individual
layer properties from pressure-transient tests, two parameters are required:
the storativity ratio, ω, defined as the ratio of the storativity of the
layer with higher flow capacity to the total system storativity, and the
transmissivity ratio, κ, which is the ratio of the higher of the layer
flow capacities to the total system flow capacity. If the layer skin factors
are equal, the transmissivity ratio is equal to the ratio of the flow rate of
the layer with higher flow capacity to the total flow rate and may be obtained
from production logs. The storativity ratio, on the other hand, needs to be
determined from the pressure-transient data or by independent means.
In the literature, dual-porosity and dual-permeability system definitions
are usually associated with naturally- fractured and layered systems,
respectively. In principle, dual-porosity systems constitute a subset (a
limiting case) of the dual-permeability systems and, as such, possess many
characteristics that resemble those of dual-permeability systems (Bourdet
1985).
For dual-porosity systems, such as naturally fractured reservoirs, ω may be
determined from the vertical separation,δp, between the two parallel
straight lines on the pressure vs. log-time plot (Pollard 1959; Warren and Root
1963). However, for dual-permeability systems, as in layered reservoirs with
crossflow, the separation between the two parallel semilog straight lines is
not only a function of ω but also a function of κ. Therefore, the
objective of this study is to obtain a practical relation for the storativity
ratio of layered systems with crossflow in terms of the separation between the
two semilog straight lines on pressure vs. log-time plot and the transmissivity
ratio. We demonstrate that having an initial estimate of ω is crucial
for the estimation of the other layer properties from straight-line or
regression-analysis techniques.
© 2008. Society of Petroleum Engineers
View full textPDF
(
2,592 KB
)
History
- Original manuscript received:
10 May 2004
- Meeting paper published:
5 October 2003
- Revised manuscript received:
21 May 2007
- Manuscript approved:
24 November 2007
- Version of record:
25 April 2008
View Cart
