Environmental Engineering Reference
In-Depth Information
7.5. Subsidence due to the exploitation of gas reservoirs
Problems linked to the exploitation of gas reservoirs are of a similar nature to
those experienced with oil reservoirs. In fact, at reservoir level we generally have a
two-phase flow system, i.e. gas and water. We may define a capillary pressure at the
thermodynamic equilibrium condition given by the difference between gas and
water pressure, as in the case of oil reservoirs. During the exploitation of a gas
reservoir, which generally causes a decrease of the gas pressure, two mechanical
effects appear simultaneously:
- the decrease in gas pressure produces compaction in the reservoir formation
and acts as pressure boundary condition on the overburden;
- the capillary pressure in the reservoir diminishes and, after a small initial
elastic expansion, causes an irreversible compaction (also called structural collapse)
in most reservoir rocks. Reservoir compaction acts as a displacement boundary
condition for the overburden.
Both effects produce surface subsidence. The first effect may be modeled by
means of the modified effective stress concept, in this case the generalized Bishop
stress [LEW 82]. The second effect, as explained above, requires an appropriate
constitutive model, such as the BBM for partially saturated soils based on the net
stress [ALO 90], or similar models based on the generalized Bishop stress [MEN
08]. Both approaches use capillary pressure as the second stress variable.
Due to structural collapse there is a clear phenomenological distinction between
the exploitation of a gas reservoir and that of an aquifer. This distinction is
confirmed by field observation: in the case of an aquifer, if the pressure is kept
constant subsidence stops, and with increasing pressure a small rebound may be
observed (see Chapter 5).
In the case of a gas reservoir, subsidence continues after the end of production
even with increasing formation pressure. This fact may be explained by the capillary
pressure, which diminishes slowly over long time spans due to the encroaching
water. The need to use partially saturated soil mechanics concepts in the case of gas
reservoirs is confirmed by the suction-dependent behavior of the reservoir rock.
Figure 7.10 shows the volumetric deformation of a lime stone with varying
hydrostatic pressure for different degrees of saturation, represented through different
values of the relative humidity,
Rh
. It can clearly be seen that the higher the relative
humidity, the higher the volumetric compressibility. This is what we would expect
following partially saturated soil mechanics. A second example for sandstone will be
shown in the case study below.
