Single-well Injection-withdrawal Tests Research Articles

Single-well injection-withdrawal tests as a contaminant transport characterisation tool for landfilled waste.

A new single well injection withdrawal (SWIW) test was trialled at four landfills using the tracers lithium and deuterium, and by injecting clean water and measuring electrical conductivity. The aim of the research was to develop a practical test for measuring lateral contaminant transport to aid in the design of landfill flushing. Borehole dilution tests using dyes were undertaken prior to each SWIW test to determine background flow velocities. SWIW tests were performed at different scales by varying the volume of tracer injected (1 to 5,800m3) and the test duration (2 to 266days). Tracers were used individually, simultaneously or sequentially to examine repeatability and scaling. Mobile porosities, estimated from first arrival times in observation wells and from model fitting ranged from 0.02 to 0.14. The low mobile porosities measured rule out a purely advective-dispersive system and support a conceptual model of a highly preferential dual-porosity flow system with localised heterogeneity. A dual-porosity model was used to interpret the results. The model gave a good fit to the test data in 7 out of 11 tests (where R2≥0.98), and the parameters derived are compatible with previous experiments in MSW. Block diffusion times were estimated to range from 12 to 6,630h, with a scaling relationship apparent between the size of the test (volume of tracer used and/or the duration) and the observed block diffusion time. This scaling relationship means affordable small-scale tests can inform larger-scale flushing operations.

Understanding the effect of single-fracture heterogeneity from single-well injection-withdrawal (SWIW) tests

The single-well injection-withdrawal (SWIW) tracer test is a method used to estimate the tracer retardation properties of a fracture or fracture zone. The effects of single-fracture aperture heterogeneity on SWIW-test tracer breakthrough curves are examined by numerical modelling. The effects of the matrix diffusion and sorption are accounted for by using a particle tracking method through the addition of a time delay added to the advective transport time. For a given diffusion and sorption property (Pm) value and for a heterogeneous fracture, the peak concentration is larger compared to a homogeneous fracture. The cumulative breakthrough curve for a heterogeneous fracture is similar to that for a homogeneous fracture and a less sorptive/diffusive tracer. It is demonstrated that the fracture area that meets the flowing water, the specific flow-wetted surface (sFWS) of the fracture, can be determined by matching the observed breakthrough curve for a heterogeneous fracture to that for a homogeneous fracture with an equivalent property parameter. SWIW tests are also simulated with a regional pressure gradient present. The results point to the possibility of distinguishing the effect of the regional pressure gradient from that of diffusion through the use of multiple tracers with different Pm values.

Reply to comment by Maier and Kocabas on “A closed‐form analytical solution for thermal single‐well injection‐withdrawal tests”

Reply to comment by Maier and Kocabas on “A closed-form analytical solution for thermal single-well injection-withdrawal tests” Yoojin Jung and Karsten Pruess Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 1. Introduction We appreciate the comment by Maier and Kocabas (2012) on our research article (Jung and Pruess, 2012). In their comment, they raise the following issues: (1) their solutions presented in Kocabas (2010) and Maier and Kocabas (2012) are mathematically simpler and computationally more efficient than our analytical solutions and (2) the insensitivity of thermal breakthrough curve to the flow velocity, which is one of the important conclusions of our study, only holds for the special case where the injection and the withdrawal flow rate is identical. We address each of these comments, along with a few other relatively minor comments and suggestions, below. 2.1. Efficiency of the Analytical Solution Regarding the computational efficiency of the analytical solutions, we agree that the solutions developed by Kocabas (2010) and Maier and Kocabas (2012) using the iterated Laplace transform have a simpler form than our solutions and therefore need a shorter

A closed‐form analytical solution for thermal single‐well injection‐withdrawal tests

Thermal single‐well injection‐withdrawal (SWIW) tests entail pumping cold water into a hot and usually fractured reservoir, and monitoring the temperature recovery during subsequent backflow. Such tests have been proposed as a potential means to characterize properties of enhanced geothermal systems (EGS), such as fracture spacing, connectivity, and porosity. In this paper we develop an analytical solution for thermal SWIW tests, using an idealized model of a single vertical fracture with linear flow geometry embedded in impermeable conductive wall rocks. The analytical solution shows that the time dependence of temperature recovery is dominated by the heat exchange between fracture and matrix rock, but strong thermal diffusivities of rocks as compared to typical solute diffusivities are not necessarily advantageous for characterizing fracture‐matrix interactions. The effect of fracture aperture on temperature recovery during backflow is weak, particularly when the fracture aperture is smaller than 0.1 cm. The solution also shows that temperature recovery during backflow is independent of the applied injection and backflow rates. This surprising result implies that temperature recovery is independent of the height of the fracture, or the specific fracture‐matrix interface areas per unit fracture length, suggesting that thermal SWIW tests will not be able to characterize fracture growth that may be achieved by stimulation treatments.

Development and Evaluation of Micro Push–Pull Tests to Investigate Micro-Scale Processes in Porous Media

Soils and sediments are porous media characterized by heterogeneities across a wide range of spatial scales. Physical, chemical, and biological properties have been found to show great variation even at subcentimeter scales. Here we present a new micro technique for the in situ study of chemical and microbiological reactions in water-saturated porous media at the mm-scale. This technique combines micro suction cups with the principle of single-well injection-withdrawal tests ("push-pull" tests). Push-pull tests have been used extensively on larger scales in groundwater research to obtain quantitative information of physical, chemical, and microbiological characteristics of an aquifer. The micro push-pull technique presented here was developed and validated using a thin-slab chamber filled with sand. A porous micro cup was used to inject about 250 μL of a test solution into the water-saturated sand pack and then to slowly extract about 850 μL water from the same point. The extraction-phase breakthrough curves of the solutes were modeled considering advection, dispersion, and molecular diffusion without fitting any parameters. As an example we quantified the degradation of citrate injected into the water-saturated sand pack inoculated with denitrifying bacteria. The results show that the new technique can be used to assess local microbial degradation processes under in situ conditions on the micro scale.

Evaluation of single‐well injection‐withdrawal tests in Swedish crystalline rock using the Lagrangian travel time approach

A series of 10 single‐well injection‐withdrawal (SWIW) tests are evaluated with two tracers each: uranine and cesium (Cs). An evaluation tool for SWIW tests in crystalline rock is presented on the basis of the Lagrangian travel time approach, whereby probabilities of tracer particle residence times are computed for key stages of the test cycle. Calibration results for three transport parameters and each breakthrough curve are presented. We show that estimates of the controlling retention parameter group are robust for Cs but highly uncertain for uranine. The estimated retention for Cs is larger for the Laxemar‐Simpevarp site compared to the Forsmark site. Deviations from the −3/2 asymptotic breakthrough curve slope observed in a few of the tests at Forsmark are possibly due to a thin fracture coating that has been identified in mineralogical studies at some locations of the site.

Controls of mass recovery rates in single‐well injection‐withdrawal tracer tests with a single‐porosity, heterogeneous conceptualization

A single‐well injection‐withdrawal (SWIW) test is evaluated as a tool to assess a single‐porosity conceptualization for a transport system. We have shown that there are cases in which matrix diffusion cannot be proven definitively from the results of SWIW tests owing to uncertainties of aquifer properties. Drift during the resting phase of the test can carry a plume to areas where the flux is lower during the withdrawal phase, leading to decreased mass recovery rates. Results of sensitivity studies on multiple, stochastically generated, heterogeneous transmissivity fields indicate that variance in transmissivity has a strong effect on drift and consequently mass recovery rates. In addition to impacting drift directly, the regional gradient has the potential to influence mass recovery rates by interfering with the pumping‐induced fluxes toward the well during withdrawal. The Kolmogorov‐Smirnov test is used to evaluate whether simulated data using a single‐porosity conceptualization with plume drift significantly differ from SWIW tracer test data from a fractured dolomite unit at the Waste Isolation Pilot Plant. Given the uncertainty in the variance of transmissivity, it is unlikely that a single‐porosity conceptualization can match the field data. Matrix diffusion is a likely explanation for the observed gradual mass recovery rates.