Parent Command
> OBSERVATION

Subcommand
-

Description
This command selects points in time at which step changes are to be made to individual elements or element groups. At this specific point in time, the user can either change the element’s volume, porosity, permeability, or any of its primary variables. Such times are termed "restart times" because in standard TOUGH2 one would have to stop the simulation, and restart it after having changed the initial conditions of certain elements. This option is useful for modeling well tests, where the condition in the borehole is changed at certain points in time. For the simultaneous calibration of well tests consisting of many test events, it is imperative to model the entire sequence in a single simulation run, i.e. individual events must be connected. Recall, that complicated, time-varying boundary conditions can also be programmed into subroutine USERBC. The specification of a restart time is identical to specifying calibration points (see command >> TIME (o)). Instead of calibration times, a restart time is indicated. Following the line containing the restart time, elements are defined by a name, which is either an element name, an element number, or a material name. The name is followed by an integer ipv and a value. If ipv is negative, value is taken as a change from the conditions encountered at the restart time.

Variables ipv and value in Command RESTART
abs(ipv)	value
0	Element volume
1 <= ipv <= NEQ	Primary variable No. ipv
NEQ + 1	Porosity
NEQ + 2	(+)Permeability or (-) permeability modifier
NEQ + 1 + i	Variable USERX(i)

The general format is as follows:

>> RESTART TIME: 1 (time_unit)
restart_time
elem ipv value
elem ipv value
.... ... .....

A comprehensive example is given below. This option can also be used to periodically generate a SAVE file. This may be desirable for very long TOUGH2 simulations. In case of a computer failure, the run can be restarted at the time of the last restart time. If keyword NEW is present, a new SAVE file with unique file name is created for each restart time.

Example
This example demonstrates the use of restart times to connect four individual test events into a single simulation. The test sequence starts with a shut-in recovery period, i.e., in the TOUGH2 input file the actual interval volume is assigned to the element representing the borehole. At time tCP, a constant pressure pumping test is initiated, i.e. the element volume is increased to a very large number, and the prescribed interval pressure is specified as initial conditions. A shut-in pressure recovery period starts at time tREC, modeled by reassigning the actual interval volume. Finally, a pulse test is simulated, assuming that all the gas that potentially accumulates in the borehole interval has been released at time tPULSE, prior to applying a short pressure pulse. The pressure response in the injection well is schematically shown in the figure below, followed by the appropriate iTOUGH2 block with restart times and boundary condition specifications.

> OBSERVATION
>> RESTART TIME: 1 [HOURS]
2.56 (tCP)
BOR_1 0 1.0E+50 (large volume for const. pressure b.c.)
BOR_1 1 1.3E+05 (set constant interval pressure)

>> RESTART TIME: 1 [HOURS]
3.51 (tREC)
BOR_1 0 0.73 (actual volume for shut-in recovery)

>> RESTART TIME: 1 [HOURS]
5.44 (tPULSE)
BOR_1 2 0.00 (prescribe single-phase liquid cond.)
BOR_1 1 4.85E+05 (pressure pulse)

TOUGH2 stops at each restart time, writes a SAVE file, reads it as an INCON file, changes initial conditions, i.e. updates the primary variables or grid block volumes of the indicated elements, and continues the simulation.

See Also
>> TIME (o)