Abstract
Strains occur at shallow depths in response to pressure changes during
well tests in an underlying aquifer, and recent developments in
instrumentation have made it feasible to measure essentially the full
strain tensor. Simulations using poroelastic analyses indicate that
shallow normal strains are approximately proportional to the logarithm
of time when a well is injecting into or pumping from a uniform aquifer
or reservoir. The drawdown is also a function of log time, as shown by
the classic Cooper-Jacob type-curve analysis. The time when the semi-log
straight line intercepts the zero-strain axis is similar to the time
determined from the Cooper-Jacob pressure analysis, and it can be used
to estimate hydraulic diffusivity, suggesting that horizontal strain
data can be used directly to estimate aquifer properties. This approach
is applied to data measured with shallow (30 m) borehole strainmeters
during an injection test at a 530-m-deep sandstone aquifer/reservoir in
Oklahoma. The results show intercept times for the shallow normal strain
data are essentially the same as for deep pressure data from an
equivalent radial distance. The slopes of the semi-log plots of the
pressure and the strain increase at the same time, suggesting that they
both respond to a lateral aquifer boundary. These results confirm the
type-curve approach for interpreting strain data. Significantly, though,
strain was measured at shallow depths while the pressure data was
measured at 530 m depth. This suggests that strain data from shallow
depths could be an effective way to improve the characterization of an
underlying aquifer.