A Type‐Curve Approach for Evaluating Aquifer Properties by Interpreting Shallow Strain Measured During Well Tests.

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 deep aquifer or reservoir. The drawdown is also a linear function of log time, as shown by the classic Cooper‐Jacob type‐curve analysis. The time when the semilog 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 was validated using measurements from shallow (30‐m deep) 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 semilog plots of the pressure and the strain increase at the same time, suggesting that they both respond to a lateral aquifer boundary. Significantly, though, strain was measured at shallow depths while the pressure data were 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. Plain Language Summary: Drilled wells are used to recover water, oil and gas, minerals, heat, and other valuable resources, and they are also used to dispose of carbon dioxide, sewage, and other unwanted wastes. Most tests to evaluate well performance involve measuring the pressure change in a monitoring boring that taps a permeable layer, such as an aquifer or reservoir. The pressure change is analyzed by fitting it to an appropriate mathematical analysis, called a "type curve." This is an effective approach, but monitoring borings can be expensive and they are almost always in short supply, so we are interested in evaluating alternatives. Dropping the pressure in an aquifer causes it to shrink by a tiny amount, and this in turn deforms the overlying rocks. We used high precision instruments called borehole strainmeters to measure the very small strains that occurred at shallow depths (30 m) when a well test was being conducted in a much deeper permeable sandstone at 530‐m depth. We also measured the pressure at three monitoring wells completed in the sandstone. Data from the tests show that the strain signal at shallow depths is similar to the pressure signal in the sandstone. Pressure measurements from the monitoring wells were analyzed using a type curve that is standard for the test that was conducted. We show in the paper that a similar type curve function can be used to analyze the strain signal and the results are similar to those from the pressure analysis. This finding is significant because it shows that strain measurements made at shallow depths can be analyzed using a straightforward approach to help characterize much deeper aquifers and reservoirs. This can reduce risk and cost, and improve the reliability of wells used recover resources or store wastes. Key Points: New method that uses strain measurements for interpreting well tests are suggestedStrain measured at shallow depths above the aquifer can be used instead of the pressure in the aquifer itself to estimate aquifer propertiesThe suggested method of strain measurements could lead to a new way for characterizing aquifers using pumping tests [ABSTRACT FROM AUTHOR]