Summary
Water influx is an important factor influencing production of gas reservoirs
with an active aquifer. However, aquifer properties such as size, porosity, and
permeability are typically uncertain and make predictions of field performance
challenging. The observed pressure decline is inherently nonunique with respect
to water influx, and large uncertainties in the actual reservoir state are
common. Time-lapse (4D) gravimetry, which is a direct measure of a subsurface
mass redistribution, has the potential to provide valuable information in this
context.
Recent improvements in instrumentation and data-acquisition and -processing
procedures have made time-lapse gravimetry a mature monitoring technique, both
for land and offshore applications. However, despite an increasing number of
gas fields in which gravimetric monitoring has been applied, little has been
published on the added value of gravity data in a broader context of modern
reservoir management on the basis of the closed-loop concept. The way in which
gravity data can contribute to improved reservoir characterization,
production-forecast accuracy, and hydrocarbon-reserves estimation is still to
be addressed in many respects.
In this paper, we investigate the added value of gravimetric observations
for gasfield-production monitoring and aquifer-support estimation. We perform a
numerical study with a realistic 3D gasfield model that contains a large and
complex aquifer system. The aquifer support and other reservoir parameters
(i.e., porosity, permeability, reservoir top and bottom horizons) are estimated
simultaneously using the ensemble smoother (ES). We consider three cases in
which gravity only is assimilated, pressure only is assimilated, and gravity
and pressure data are assimilated jointly. We show that a combined estimation
of the aquifer support with the permeability field, porosity field, and
reservoir structure is a very challenging and nonunique history-matching
problem, in which gravity certainly has an added value. Pressure data alone may
not discriminate between different reservoir scenarios. Combining pressure and
gravity data may help to reduce the nonuniqueness problem and provide not only
an improved gas- and water-production forecast and gas-in-place evaluation, but
also a more-accurate reservoir-state description.
© 2012. Society of Petroleum Engineers
View full textPDF
(
3,050 KB
)
History
- Original manuscript received:
1 November 2011
- Revised manuscript received:
29 February 2012
- Manuscript approved:
29 March 2012
- Published online:
12 September 2012
- Version of record:
6 December 2012
View Cart
