A new analysis procedure for determining aquifer properties from slug test data

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.

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

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.

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

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.

Recent interest in ground‐water contamination problems has led to a renewed interest in the development of accurate procedures for the analysis of in situ permeability tests. The present study involves the derivation of analytical procedures for slug tests and constant head tests performed in wells with casings open at the bottom and installed flush with the upper confining boundary of a finite‐depth, anisotropic, homogeneous aquifer; the analysis takes into account the effects of a finite‐thickness skin located at the casing bottom. The Laplace transform is used on the governing ground‐water flow equations to derive the transform of time‐dependent water levels measured in the casing. The expression is numerically inverted back into the real time domain to obtain (1) a set of dimensionless type curves for analyzing slug test data, and (2) a relation between the inflow rate, the head maintained in the well, the hydraulic conductivity, and a shape factor expression for the analysis of constant head data. Existing methods of analysis are reviewed and compared with the presently derived analytical procedures.

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.

Uncertainty in the modelling of spatial and temporal patterns of shallow groundwater flow paths: The role of geological and hydrological site information

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.

Slug tests provide an efficient way to obtain information about the spatial heterogeneity of an investigated site. Combining with the imaging method of hydraulic tomography, they can present broad application prospects in aquifer reconstruction. However, unlike pumping tests, the water level responses of slug tests may be significantly affected by wellbore effects, especially for underdamped cases. This study proposes a new characterization framework for characterization of aquifer heterogeneity based on water level responses recorded during cross well slug tests with a tomographical arrangement. The peak head of each water level response and its corresponding time, i.e. the hydraulic travel time, are utilized in the inversion process, rather than the whole water level time series. To eliminate the influence of wellbore effects, two correction methods with respect to the hydraulic travel time and peak head amplitude are developed. Based on corrected data, the distributions of hydraulic diffusivity (D) and specific storage (Ss) are obtained through travel time-based and attenuation-based inversions, respectively. The new framework is verified by a series of simulated tomographic slug tests through a synthetic example based on an aquifer outcrop analog, where the hydraulic parameter distribution is known. The verification results indicate that the distributions of D and Ss are well estimated, and their mean values are much improved compared to the old framework. With the same method, a field application is employed at a fractured rock experimental site located in Göttingen, Germany. Three potential transmissive fractures are revealed with hydraulic conductivities varying from 6.9 × 10–5 to 3.4 × 10–4 m/s. The uncertainty of the inversion results is discussed for future studies and related applications.

A review of slug tests analysis on South African aquifers in potential yield and transmissivity estimations

Methods of analyzing ‘slug test’ data are reviewed, and additional type curves for the analysis of test data from formations with very low storage coefficients are presented.

The pressure or head response of an aquifer to a slug test in a fully penetrating well with wellbore storage and skin is presented. Three kinds of type curves are discussed: early time log‐log, intermediate time semilog, and late time log‐log. The early time response of the slug test depends on the presence of wellbore skin; hence two kinds of early time type curves are presented. When the active well has wellbore skin, the early time correlating parameter is the product of the skin and dimensionless storage. When wellbore skin is not present, the early time response is proportional to the square root of time, with dimensionless storage as the correlating parameter. The intermediate time semilog type curves are applicable when wellbore skin is not present. The late time type curves may be used when the dimensionless storage is greater than one and wellbore skin is positive or when the dimensionless storage is smaller than one and wellbore skin is negligible. The radius of investigation of the slug test is also examined with and without wellbore skin at the active well. Although the presence of wellbore skin reduces the magnitude of the dimensionless head responses at observation wells, the detectable radius of investigation is not significantly dependent on wellbore skin.

A mathematical formulation is presented in this paper which includes wellbore inertial and frictional effects during a slug test or the flow period of a drillstem test from naturally fractured reservoirs. This formulation is obtained by coupling a wellbore equation and the diffusivity equation for double-porosity reservoirs. The wellbore equation represents a transient momentum balance relating liquid level in the wellbore and bottomhole pressure. Wellbore storage and skin effect are considered in the coupling conditions. A simplified formulation resulting from the assumptions of a small slug and friction losses for laminar flow in the wellbore is solved by applying the Laplace transformation with numerical inversion. Solutions obtained in this study show a distinctive double-porosity behavior for some naturally fractured reservoirs. For those naturally fractured reservoirs with double-porosity behavior, an estimate of the interporosity flow parameter, λ, and of the storativity ratio, ω, can be obtained in addition to fissures transmissivity, kfh/µ, and skin effect factor, s. On the other hand, during the practical range of time, the response of other naturally fractured reservoirs follow either the response for a single-porosity reservoir with the properties of the fracture system or the response of a single-porosity reservoir with the combined properties of the fractures and matrix blocks. Criteria to estimate the magnitude of wellbore inertial and frictional effects for practical systems are developed from an analysis of results obtained in this study. Also, the solution method proposed in this paper can be used to forecast or match bottomhole flowing pressures during a drillstem test from estimates of the properties that describe a naturally fractured reservoir.