Predicting Long-Term Well Performance from Short-Term Well Tests in the Piedmont

A reliable estimate of the physically sustainable discharge of a well is a fundamental aspect affecting management of water resources, but there are surprisingly few analyses describing on how to make such an estimate. Current available methods include either an empirical or a quantitative approach. The empirical method is based on holding the head or flow rate constant in order to maintain a target drawdown for as long as possible. The second method involves conducting a constant rate test to calculate the properties of the aquifer, T and S, and extrapolate the drawdown using a type curve (i.e. Theis analysis). To improve well performance prediction, we have been using the effects of streams on short-term hydraulic well tests to predict long-term performance during pumping. An analysis was developed to calculate the long-term steady state specific capacity of a well using early-time information from a constant-rate test. The analysis first considers a homogeneous confined aquifer with a well fully penetrating the aquifer. A more detailed analysis considers a variable strength of interaction between a stream and a well extends the versatility of this method to a wide range of conditions. The analysis is evaluated numerically to explore effects from typical Piedmont geometries not included in the analysis. Evaluating the analytical solution with numerical models allowed the characterization of different Piedmont geometries to determine the effectiveness of the analysis. Numerical models were allowed to reach steady state conditions, and the analysis was compared to the numerical results. The analysis was then evaluated with two field examples from well tests in the Piedmont of South Carolina. The results show that the analysis successfully predicts the long term performance of wells within a few percent of the actual observed steady state specific capacities

Inferring field-scale properties of a fractured aquifer from ground surface deformation during a well test

International audienceFractured aquifers which bear valuable water resources are often difficult to characterize with classical hydrogeological tools due to their intrinsic heterogeneities. Here, we implement ground surface deformation tools (tiltmetry and optical leveling) to monitor groundwater pressure changes induced by a classical hydraulic test at the Ploemeur observatory. By jointly analyzing complementary time constraining data (tilt) and spatially constraining data (vertical displacement), our results strongly suggest that the use of these surface deformation observations allows for estimating storativity and structural properties (dip, root depth, lateral extension) of a large hydraulically active fracture, in good agreement with previous studies. Hence, we demonstrate that ground surface deformation is a useful addition to traditional hydrogeological techniques and opens possibilities for characterizing important large-scale properties of fractured aquifers with short-term well tests as a controlled forcing

Combining periodic hydraulic tests and surface tilt measurements to explore in situ fracture hydromechanics

International audienceFractured bedrock reservoirs are of socio-economical importance, as they may be used for storage or retrieval of fluids and energy. In particular, the hydromechanical behavior of fractures needs to be understood as it has implications on flow and governs stability issues (e.g., microseismicity). Laboratory, numerical, or field experiments have brought considerable insights to this topic. Nevertheless, in situ hydromechanical experiments are relatively uncommon, mainly because of technical and instrumental limitations. Here we present the early stage development and validation of a novel approach aiming at capturing the integrated hydromechanical behavior of natural fractures. It combines the use of surface tiltmeters to monitor the deformation associated with the periodic pressurization of fractures at depth in crystalline rocks. Periodic injection and withdrawal advantageously avoids mobilizing or extracting significant amounts of fluid, and it hinders any risk of reservoir failure. The oscillatory perturbation is intended to (1) facilitate the recognition of its signature in tilt measurements and (2) vary the hydraulic penetration depth in order to sample different volumes of the fractured bedrock around the inlet and thereby assess scale effects typical of fractured systems. By stacking tilt signals, we managed to recover small tilt amplitudes associated with pressure-derived fracture deformation. Therewith, we distinguish differences in mechanical properties between the three tested fractures, but we show that tilt amplitudes are weakly dependent on pressure penetration depth. Using an elastic model, we obtain fracture stiffness estimates that are consistent with published data. Our results should encourage further improvement of the method

Understanding the Hydromechanical Behavior of a Fault Zone From Transient Surface Tilt and Fluid Pressure Observations at Hourly Time Scales

International audienceFlow through reservoirs such as fractured media is powered by head gradients which also generate measurable poroelastic deformation of the rock body. The combined analysis of surface deformation and subsurface pressure provides valuable insights of a reservoir's structure and hydromechan-ical properties, which are of interest for deep-seated CO 2 or nuclear waste storage for instance. Among all surveying tools, surface tiltmeters offer the possibility to grasp hydraulically induced deformations over a broad range of time scales with a remarkable precision. Here we investigate the information content of transient surface tilt generated by the pressurization a kilometer scale subvertical fault zone. Our approach involves the combination of field data and results of a fully coupled poromechanical model. The signature of pressure changes in the fault zone due to pumping cycles is clearly recognizable in field tilt data and we aim to explain the peculiar features that appear in (1) tilt time series alone from a set of four instruments and 2) the ratio of tilt over pressure. We evidence that the shape of tilt measurements on both sides of a fault zone is sensitive to its diffusivity and its elastic modulus. The ratio of tilt over pressure predominantly encompasses information about the system's dynamic behavior and extent of the fault zone and allows separating contributions of flow in the different compartments. Hence, tiltmeters are well suited to characterize hydromechanical processes associated with fault zone hydrogeology at short time scales, where spaceborne surveying methods fail to recognize any deformation signal