On the analysis of ‘slug test’ data

ASTM D4044/D4044M-15A, Standard Test Method (Field Procedure) for Instantaneous Change in Head (Slug) Tests for Determining Hydraulic Properties of Aquifers (Withdrawn), describes several methods to start a slug test, which all involve some quick slugging of the water column in the riser pipe. Having a standard for the slugging action seems important. However, the initial dynamic effects are mostly ignored in slug test theories. In the field, slug tests may have a shaky start instead of a rapid and clean start. The question is, does the slugging quality influence the test results? This can be studied using recent tests for which shaky starts were registered by a pressure transducer (PT) and an atmospheric pressure transducer (APT). The PT–APT pair must be calibrated and synchronized, as explained in previous articles, but this is rarely done and regrettably it is not requested in standards. This article uses the data collected by PT–APT pairs for a few shaky starts in aquifers. Theoretical and practical aspects are explained. Then, this article examines the graphs that are needed for a correct analysis and compares results obtained with either a clean start or a shaky start in the same monitoring well. The test data are fitted with a three-parameter model for overdamped (OD) tests and a four-parameter model for underdamped (UD) tests. For OD tests, the start quality slightly influences the results for hydraulic conductivity, K, and piezometric level, PL, for the test. For UD tests, the slugging action may influence markedly the K value derived with the ASTM D5785/D5785M-20, Standard Practice for (Analytical Procedure) for Determining Transmissivity of Confined Nonleaky Aquifers by Underdamped Well Response to Instantaneous Change in Head (Slug Test), method. The analysis of residuals between data and best fit is very useful in selecting the correct starting time and avoiding the influence of slug dynamic effects and outgassing effects. Several recommendations are made to improve standards.

As important parameters for characterizing heat transfer, thermal property parameters of aquifers and rock-soil skeletons have important research significance in the development and utilization of geothermal resources. The slug heat test is inspired by the slug test, and the heat is instantaneously excited in the test well so as to change the temperature of test section in the test well instantaneously. Based on the thermal radial convection-dispersion theory and the principle of heat conservation, the theoretical model of the slug heat test is established, and the model is solved by Laplace transform and inverse transform to obtain multiple sets of standard curves under different conditions. The slug heat tests were conducted in the indoor model, the slug heat test data under different hydrodynamic conditions were fitted with the standard curves and the thermal property parameters, including effective thermal conductivity, stagnant thermal conductivity, thermal mechanical dispersion coefficient, thermal dispersive degree, thermal diffusivity, heat capacity of aquifer, heat capacity and thermal conductivity of rock-soil skeletons, were accurately obtained. The test results are in good agreement with the empirical values. Meanwhile, the effective thermal conductivity of the aquifer also clearly increases with the increase of flow rate. The excitation temperature difference had little effect on the effective thermal conductivity of the aquifer. At the same time, numerical simulation methods are used to establish a numerical model consistent with the indoor test model, and the numerical model is assigned with the thermal property parameters obtained from the indoor slug heat test, and the measured values of temperature changes in the test well during the slug heat test under different hydrodynamic and excitation strength conditions are compared with the simulated values for verification. The research results show that the slug heat test has the characteristics of high applicability, simple operation and rapid testing, and can effectively determine the thermal properties parameters of aquifers and rock-soil skeletons.

In field data from slug test (decreasing flow rate) DSTS, an initial period of constant flow rate can often be observed. This may be a consequence of critical flow. "Critical flow" means that flow rate is independent of the pressure drop across the flow restriction. In the case of compressible fluid flow, this implies that the flow velocity is equal to the sonic velocity in the fluid. This was suggested by Ramey et al. in 1975. However, critical (constant-rate) flow was not included in their analysis of slug test DST flow period data. A new method which does consider an initial constant flow rate for a slug test was found. Graphs for the type-curve matching method are presented and field data are shown. Introduction Often in drill stem testing, the flow periods are characterized by a pressure trace which increases with increasing time, showing the accumulation of liquid in the drill string. In the expected case, the pressure-time trace is linear, showing constant-rate pressure-time trace is linear, showing constant-rate production. In other cases, the pressure-time trace production. In other cases, the pressure-time trace curves and is concave to the time axis. This shows a decreasing flow rate. In the case that formation pressure is too low to lift a column of reservoir pressure is too low to lift a column of reservoir liquid to the surface, the well may stop flowing before the tester valve is closed. This results because the head of liquid in the drill string becomes equal to initial formation pressure. This sort of test is similar to a pressure transient test called a "slug test" by Ferris and Knowles in 1954. The word "slug" refers to the maximum volume of liquid which may be produced by the time the well becomes static. produced by the time the well becomes static. A similar test involving the cooling of a batch of hot water was reported by Beck, Jaeger, and Newstead in 1956. Cooper et al., in 1967, reported the results of a field test in a static water well from which a float was suddenly removed, giving the appearance of the sudden removal of a quantity of water equal to that displaced by the float. Maier presented an approximate analysis of the equivalent DST presented an approximate analysis of the equivalent DST problem in 1970. problem in 1970.Van Poollen et al., in 1970, and Kohlhass, in 1972, applied the Cooper et al. solution to DST flow period data analysis. Although most current studies period data analysis. Although most current studies reference a study by Jaeger in 1956 which included a sandface resistance similar to the skin effect, most recent works do not include wellbore damage effects. A solution including the skin effect was presented by Ramey and Agarwal in 1972, although use of the solutions was not demonstrated. Papadopulos et al. presented extended results for the zero skin effect presented extended results for the zero skin effect case in 1973. The most complete discussion of DST applications of the slug test was presented in 1975. Three new slug test type-curves were developed for analysis of flow period data, and three field cases were analyzed. This study was included in the Earlougher monograph in 1977, and large-scale type-curves were printed with that monograph. printed with that monograph. Finally, Fenske showed that pressure buildup data for a well produced a short time (with wellbore storage) would follow the slug test type-curves. This is a potent observation. It means that the slug test type-curves may work for the initial shut-in, as well as for flow period data. This establishes the importance of slug test type-curves for DST work whether the test would normally be classified a "slug test" or not. Although not within the scope of this study, slug test type-curves may be used to match the initial shut-in for a DST following a very short initial flow to provide permeability, skin effect, and extrapolated initial pressure. One of the most important observations made from attempts to match field data with slug test type-curves is that there is often an initial constant-rate flow period before the rate begins to decline. Reference period before the rate begins to decline. Reference 1 pointed out this phenomena was related to conventional DST results for liquid flow wherein flow rate was constant for both flow periods: "Before proceeding with the presentation of the 'slug test', it should be pointed out that the conventional DST… contains the seed of a paradox.

A new method for determining hydraulic properties from slug test well data in a confined aquifer is presented. The new procedure is based on an exact equation which converts measured slug test head data to equivalent head data that would be obtained due to a constant discharge of a well with well bore storage and skin effects. A related procedure yields equivalent head derivative data (time rate of change of head) that would be obtained for the classical constant discharge with well bore storage and skin problem without applying numerical differentiation. After the constant discharge head data and its derivative data are generated by our procedure, these converted data can be analyzed by using existing well bore storage and skin type curves for the appropriate aquifer/well model. Therefore slug test type curves are no longer needed. For cases in which the relative change in the water level is not significant during the slug test, most of the converted data will fall on the unit slope line of the conventional well bore storage and skin type curves and it will be difficult to obtain a unique type curve match of converted head data. For such cases, it is shown that a discharge normalization procedure can be applied to improve the reliability of the analysis. A field case illustrating the reliability of the new method is presented.

Influence of seasonal hydraulic head changes on slug tests conducted in shallow low-permeability soils

Determination of skin and aquifer parameters for a slug test with wellbore-skin effect

The correct characterization of aquifer parameters is essential for water-supply and water-quality investigations. Slug tests are widely used for these purposes. While free software is available to interpret slug tests, some codes are not user-friendly, or do not include a wide range of methods to interpret the results, or do not include automatic, inverse solutions to the test data. The private sector has also generated several good programs to interpret slug test data, but they are not free of charge. The computer program SlugIn 1.0 is available online for free download, and is demonstrated to aid in the analysis of slug tests to estimate hydraulic parameters. The program provides an easy-to-use Graphical User Interface. SlugIn 1.0 incorporates automated parameter estimation and facilitates the visualization of several interpretations of the same test. It incorporates solutions for confined and unconfined aquifers, partially penetrating wells, skin effects, shape factor, anisotropy, high hydraulic conductivity formations and the Mace test for large-diameter wells. It is available in English and Spanish and can be downloaded from the web site of the Geological Survey of Spain. Two field examples are presented to illustrate how the software operates.

Increases in water pressure, if large enough, can be sufficient to open existing fractures in a fractured rock-water system. Such large water pressure increases are expected during earthquake activity and are also produced artificially in high-pressure hydraulic fracturing and slug injection tests. In an in-situ slug injection test, the slug of water is initially released into a packed-off interval of the borehole where the contained fluid is at ambient pressures. We recognize three separate stages in the behaviour of the fracture as the water pressure decreases due to drainage, namely opening, closing and, finally, completely closed. In the first stage, the water will initially drain out along conduits between the faces of the opening fracture. As the fracture opens and grows, it can become long enough to intersect any pre-existing network of open joints and fractures. If this connection occurs, the opened fracture will stop growing and there will be further rapid flow of fluid out along these open conduits. Even if connection to an open conduit does not occur, as the pressure drops the fracture will stop growing and eventually halt. At the pressure where the fracture tip begins to close, drainage into any open conduits will cease and drainage during fracture closing will thus be by flow into the porous walls of the fracture and borehole. This effect of the conduit being shut off may manifest as a kink in the observed height decay of the slug at this pressure due to the effect of the different drainage terms. As the water pressure decreases, the fracture will continue to close until the water pressure drops below the critical pressure and the fracture becomes completely closed. This total closure occurs at the same pressure as that necessary to re-open the fracture, and has been taken equal to the minimum principal stress. This total closure of the fracture also may manifest as a kink in the observed height decay of the slug, as below this closure pressure is the third, and final, stage where drainage is at the usual low pressures controlled by the permeability of the rock just around the packed-off interval of the borehole. We develop the most general mathematical model for this phenomenon using conservation of mass of the water in the borehole-fracture system for each separate pressure stage. This model of fracture behaviour, with separate flow stages and pressure regimes, explains the physical basis for the increasingly used technique in high-pressure injection tests of measuring the so-called re-opening pressure at the sharp change in slope of the rate of change of water height vs. height. Analytic and numerical solutions yield the rate at which the water height drops during the slug test and comparison with high-pressure test data confirms the accuracy of the model. Analysis of data from Yucca Mountain, Nevada, gives values for the fracture opening pressures that indicate the presence of extensional stresses of significant magnitude and of zones of pre-existing open fracture networks in the vicinity of the proposed repository site.

A single-porosity model is developed to deal with a fracture zone slug test in a large dip angle by assuming the fracture zone causes a downward regional flow. For the oscillatory response, a larger dip angle causes larger amplitude while introduces little impact on period. The effective water length, an important parameter necessary for analyzing the oscillatory response, is proven to be independent of the dip angle and can be evaluated using the available horizontal formation methods. The dip angle effect is more pronounced for a larger storage coefficient. An empirical relationship is developed to evaluate the limiting dip angle, below which the dip angle effect is negligible. Field data analysis of a slug test in a 47° dip angle fracture zone indicates that neglecting the dip angle can result in a 27% transmissivity over estimation and a 53% storage coefficient under estimation.

Hydrologie test analysis based on the derivative of pressure (i.e., rate of pressure change) with respect to the natural logarithm of time has been shown to significantly improve the diagnostic and quantitative analysis of slug and constant‐rate pumping tests. The improvement in test analysis is attributed to the sensitivity of the derivative to small variations in the pressure change that occurs during testing, which would otherwise be less obvious with standard pressure change versus time analysis. The sensitivity of the pressure derivative to pressure change facilitates its use in identifying the effects of wellbore storage, boundaries, and establishment of radial flow conditions on the test.DERIV is a program for converting slug and pumping test data and associated type curves to derivative format. The program has features that permit the smoothing of noisy test data, accounts for pressure derivative end‐effects, and can be used to convert slug test data to equivalent pumping test responses. Test examples that demonstrate the use of pressure derivatives also are provided.

Observation well response time and its effect upon aquifer test results

Sensitivity analysis of slug tests Part 2. Observation wells

The linear curve fitting methods such as the Hvorslev method and the Bouwer and Rice method provide a rapid and simple means to analyze slug test data for estimating in-situ hydraulic conductivity ( k ) of geologic material. However, when analyzing a slug test in a relatively compressible aquifer, these methods have difficulties to fit a straight line to the semi-logarithmic plot of the test data that shows a concave-upward curvature because the linear curve fitting methods ignore the role of the compressibility or specific storage ( S s ) of an aquifer. The comparison of the Hvorslev method and the Bouwer and Rice method is made for a partially-penetrating well geometry to show analytically that the Hvorslev method estimates higher hydraulic conductivity than the Bouwer and Rice method except that the well intake section locates very close to the bottom of the aquifer. The effect of fitting a straight line to the slug test data is evaluated along with the dimensionless compressibility parameter (α) ranging from 0.001 to 1.

An increasing need for surveillance of the quality of our ground water resources requires a comprehensive hydraulic modeling. A thorough knowledge of the hydraulic parameters is a necessity for reliable hydraulic modeling. Pumping and slug tests have usually been the main source of information about the hydraulic conductivity and the specific storage. Pumping tests result in conductivities and storage coefficients averaged over large volumes, and slug tests are often seriously affected by the well screens and filter packs. A new method for in situ determination of the hydraulic conductivity and the specific storage has been developed at the University of Aarhus. By making use of a hollow auger drill stem it is possible to inject water directly into the formation and measure the rise in hydraulic head at a short distance from the injection point. Other procedures for in situ determination of hydraulic parameters have been developed (Fejes and J6sa, 1990). The measured data (the transient hydraulic head variations) are interpreted using an inversion algorithm based on a least squares iterative formalism. To make the problem more linear the inversion is carried out in the log parameter space. The model response applied is the transient head variations due to a point source in a homogenous isotropic fullspace, where the source is considered as a step source. Since there are only three model parameters to be estimated and typically more than one hundred data, the inverse problem is strongly overdetermined.

This paper presents the first field scale measurements of in-situ stress effects on permeability of coal seams. The importance of these effects on a highly compressible reservoir such as coal is demonstrated by relating permeability and production to stress. Well testing complications and the implications of stress toward exploitation of existing reserves and exploration for new reserves are also discussed. Additionally, comparisons of this paper’s findings to prior theoretical work, core testing, and limited field data are presented.