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BOREHOLE HYDROGEOLOGY

FIGURE 2. Example of the 2D vertical cross-section model              FIGURE 3. Slice view of the 3D model output with hydraulic
output. Shown are the hydraulic head (color plot), deformation        headprofile (color plot) and deformation of the borehole.
and velocity field (arrow plot) within the aquifer.

	 “We used the parametric sweep               or ‘short-circuit’ within the process. All  several boreholes in one site.”	
feature in COMSOL Multiphysics to
perform an extensive study that focused       these situations are examined using our     	 Having proved that the DSI method
on different infiltration rates while the
abstraction rates were kept constant,”        modeling approach.”                         works, with some limitations, the
explains Jin. “This enabled us to make
a comparison with the classical method                                                    university team is now focusing on
where water is pumped out or abstracted
and there is no infiltration. We found that   èGENERATING SPECIFIC                        optimization. Jin concludes: “DSI has
when abstraction and infiltration rates       GUIDELINES                                  many advantages. Yet we know that
are equal, the DSI technology yields half                                                 even when experts with experience use
the drawdown rate of the conventional         One of the DSI application sites is in the
technique of pumping alone. While                                                         DSI, it can take several days of trial and
drawdown still occurs, it is more manageable  immediate vicinity of an abandoned
than if water was removed from the site.”                                                 error to work out where to drill and how
This is illustrated in Figure 4 in which      mining site. “We are dealing with a
the hydraulic head variation along the                                                    many boreholes are required. Through
groundwater table away from the well is       genuine problem as there has been
displayed for different infiltration rates.                                               simulation we will be able to produce
	 So far, numerical simulation results        flooding of basements in houses due
show the same trends as measured data                                                     specific guidelines that will save time and
from the field. “Simulation is really         to the closing of the old mining site,”
helping us to see why, how, and when                                                      effort on site and enable this method
DSI works,” says Jin. “For example, we        explains Jin. “Because it is an open
learned that it can only be applied to        aquifer, it is not suitable for conventional to be taught effectively and replicated
permeable aquifers, made up mainly of                                                     successfully.” v
sand and gravel. We have also discovered      dewatering methods anyway because
that a sufficient amount of regional
groundwater is critical as it positively      pumping would have to be continuous.”
affects the rate of infiltration. Moreover,
the anisotropy of hydraulic conductivity of   Jin is adding new parameters to the 3D
the aquifer plays an important role.
	 In particular, it must be considered        model as she receives
that as groundwater flow changes with
the seasons, it also changes according        data from the field
to the relative conductivity of different
layers within an aquifer. One problem is      experiments and
that water injected into the lower part
of a borehole may create a local bypass       this expands the

                                              ability of the model

                                              to reflect all of the

                                              real-life variables

                                              that may apply. “It is

                                              clear that the point

                                              at which injection

                                              takes place, the size

                                              of nozzle, flow rate,

                                              and corresponding

                                              pressure all have

                                              to be combined.

                                              We are therefore

                                              moving towards

                                              more complicated,

                                              heterogeneous

                                              conditions. We are

                                              also accounting for     FIGURE 4. Decreased drawdown with increased infiltration

                                                                      rates.

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