Analysis Of Drawdown Data Research Articles

Forward Coupled Modeling and Assessment of Gravity Anomalies Caused by Pumping Tests in Unconfined Aquifers Under Unsteady-State Conditions

Analysis of drawdown data from pumping tests is the most common method for identification of aquifer properties and estimation of hydraulic parameters in hydrogeology. Limited hard hydraulic data from wells, expensive and intrusive, can be supplemented with geophysical spatially distributed information. Gravity, sensible to mass variations, can provide information about hydraulic parameters and storage coefficients of aquifers. A satisfactory use of gravity requires the computation of temporal evolution of gravity anomalies related with the dynamic hydraulic process. For simplified and ideal problems, the calculation can be accomplished with analytical solutions. However, the limitations of analytical solutions in capturing some relevant characteristics, like heterogeneities, that require the use of numerical models, are well known. This article shows how the time-dependent gravity anomaly caused by a pumping test can be calculated in a unique code, in a coupled manner and in a single run. The methodology avoids the use and transference of data between various codes and can be utilized in a coupled hydrogeophysical inversion procedure. Additionally, it is flexible enough to include a wide range of characteristics that can be encountered in any pumping test, and is an adequate framework for field survey planning via analysis and study of time-dependent gravity curves.

抽水和自然条件下水流在英国裂隙白垩岩中流动过程分析

An integrated set of different measurements has been used to study the behavior of groundwater in an observation well in a fractured rock formation, the UK Chalk, under pumped and ambient conditions. Under pumped conditions, the response of the open borehole was relatively straightforward with flow mainly concentrated along four discrete flow horizons. Furthermore, excellent correspondence was observed between the three methods of borehole flow velocity measurement: impeller flowmeter, heat-pulse flowmeter and dilution testing. Under ambient conditions, the system appeared more complicated. Specifically, in the upper half of the borehole, the impeller flowmeter exhibited substantial downward flow and the heat-pulse flowmeter exhibited almost negligible upward flow, whilst dilution testing indicated significant dilution. It was concluded that this was due to cross-flow occurring over the upper 29 m. Analysis of drawdown data, recovery data and a Drost analysis of the ambient cross-flow data yielded aquifer transmissivity estimates of 2,049, 2,928 and > 4,388 m2/day respectively. The discrepancy between the drawdown and recovery estimates was attributed to non-linear head-losses associated with turbulence and inertial effects. The difference between the pumping test and Drost results was explained by the flow during the pumping test bypassing this aforementioned 29 m region of rock.

Parameter Estimation/Sensitivity Analysis for an Aquifer Test with Skin Effect

Aquifer information carried by aquifer test data may be affected by the presence of a finite thickness skin around the wellbore. The mathematical treatment for an aquifer accounting for the skin zone can be characterized by five parameters, that is, the outer radius of the skin zone and the transmissivity and storativity for each of the skin and aquifer zones. Sensitivity analysis was performed to examine the ground water flow behavior in the skin and aquifer zones in terms of the constant-head test (CHT) data. The simulated annealing procedure was applied to simultaneously determine the skin and aquifer parameters from the analysis of CHT data. Toward the previously mentioned goals, four suites of CHT data were analyzed in this article. The analyses of wellbore flow rate at the test well and the specific drawdown at the observation well gave accurate estimates for the skin and aquifer parameters, respectively. Only the skin thickness and both the skin and the aquifer diffusivities could be accurately estimated from the analysis of drawdown data in the observation well. The estimates for all skin and aquifer parameters from the composite analysis of flow rate and drawdown data were the most accurate. The results of sensitivity analyses and parameter estimations provide instructive references in the analysis of the skin-affected CHT data.

Influence of heterogeneity on the interpretation of pumping test data in leaky aquifers

Pumping tests are routinely interpreted from the analysis of drawdown data and their derivatives. These interpretations result in a small number of apparent parameter values which lump the underlying heterogeneous structure of the aquifer. Key questions in such interpretations are (1) what is the physical meaning of those lumped parameters and (2) whether it is possible to infer some information about the spatial variability of the hydraulic parameters. The system analyzed in this paper consists of an aquifer separated from a second recharging aquifer by means of an aquitard. The natural log transforms of the transmissivity, ln T, and the vertical conductance of the aquitard, ln C, are modeled as two independent second‐order stationary spatial random functions (SRFs). The Monte Carlo approach is used to simulate the time‐dependent drawdown at a suite of observation points for different values of the statistical parameters defining the SRFs. Drawdown data at each observation point are independently used to estimate hydraulic parameters using three existing methods: (1) the inflection‐point method, (2) curve‐fitting, and (3) the double inflection‐point method. The resulting estimated parameters are shown to be space dependent and vary with the interpretation method since each method gives different emphasis to different parts of the time‐drawdown data. Moreover, the heterogeneity in the pumped aquifer or the aquitard influences the estimates in distinct manners. Finally, we show that, by combining the parameter estimates obtained from the different analysis procedures, information about the heterogeneity of the leaky aquifer system may be inferred.

Seeking Synergy Between Drawdown and Buildup Analyses

Summary Drawdown tests are rarely interpreted unless accompanied by instantaneous surface or downhole rate measurements. Rate variations prevent a constant-rate analysis in most situations. Unsteady wellbore dynamics precipitated by progressive formation cleanup, multiphase fluid flow, and wellbore thermal effects influencing compressible fluids, to name a few, may prevent one from obtaining a stable surface rate. This reality compels an analyst to fall back upon buildup interpretation. Thus, one typically presupposes that the rate was held constant before shut-in. Drillstem tests and long-duration exploratory well tests are cases in point. This conventional practice raises an important question: Can we calculate a variable-rate history such that the drawdown analysis will be consistent with the buildup? This paper presents a method to interpret drawdown data whenever variable rates are encountered, either inherent in a test or induced by design, such as in a gas-lift well test. The method entails a stepwise procedure. First, buildup data are interpreted assuming a constant rate during drawdown. Second, the estimated parameters are used to compute instantaneous rate corresponding to each pressure data point by superposing constant-pressure solution with an appropriate pD model. The reconstructed rate history is then used to perform the variable-rate (convolution) analysis of drawdown data. Third, the new rate history is used to reinterpret buildup data. Parameters obtained from the two analyses provide the desired consistency. The proposed procedure is entirely general, in that any appropriate reservoir pD model may be used to compute the instantaneous rate values. However, best results are obtained when the late-time data are free from outer-boundary effects. We present field examples from various scenarios to show the usefulness of this approach. These examples include a short-duration onshore drillstem test (DST), a gas-lift well test involving a gas-vent period, a long-duration production test with suspected pressure-sensitive rock properties, and a slug/closed chamber test. In all cases, we have shown the synergy between the results of the two tests.

Analysis of Short-Time Buildup Data for Finite-Conductivity Fractures

Summary A finite-conductivity vertical fracture intersecting a well is considered. The well is produced at a constant rate or at a constant pressure and then is shut in. The pressure response during the buildup period and the pressure or rate response during the production period are obtained from a numerical model. In this work we discuss the analysis of pressure buildup data following a short flow period. We present a buildup type curve to analyze pressure buildup data when the pressure (or rate) response is influenced by only the bilinear flow period. This type curve can be used to estimate the fracture flow capacity and a lower limit for the fracture half-length. It will be useful when the equivalent-time or the equivalent-drawdown concepts cannot be used. We discuss strengths and limitations of the equivalent time method proposed in the literature. The theoretical basis for this procedure is provided. Introduction Virtually all type curves available in the literature are intended for analysis of drawdown data and thus can be used to analyze buildup data only if the producing time before shut-in is significantly longer than the buildup time. Although this limitation is stated in the literature as far back as 1970, the consequences of using the drawdown type curves to analyze pressure buildup data measured after a short flow period were documented only recently. Buildup type curves that take into account the influence of producing time also are presented in Ref. 1. More recently, attempts have been made to bypass the buildup type curves and to use the drawdown curves directly. Raghavan et al. Suggests that buildup data can be converted to drawdown data and thus that the drawdown type curves can be used directly. This is the most accurate procedure available since it involves a minimum number of assumptions, but it does require that drawdown data be available unless Delta t/t less than 0.1 or Delta t/t >1. The principal advantage of the method is that it allows analysis of buildup data when Delta t >t. This is extremely important if the producing time before shut-in is small. An alternative procedure suggested by Agarwal involves a modification of the time scale and is known as the equivalent-time method. Theoretically, the method is valid only if plane radial flow prevails. However, Agarwal was able to demonstrate empirically that this technique applies to fractured wells. The principal objectives of this paper areto show that the normal form of the drawdown type curves for finite-conductivity fractured wells is inadequate to analyze buildup data following a short flow period,to present a modified version of these curves to analyze short-time data (drawdown or buildup),to present a buildup type curve suitable for analyzing data measured after a short production period,to discuss the strengths and limitations of the equivalent-time method, andto establish a theoretical basis for this procedure and to explain the empirical findings of Agarwal. 3 The buildup type curve presented here can be used to analyze tests in fractured wells when the response during the flow period is dominated by the bilinear flow regime. The type curve should prove useful when the other two methods suggested in the literature cannot (or should not) be used. Definitions All results given in this study are presented in terms of dimensionless variables. The dimensionless variables are defined in the following equations. JPT P. 2413^

Iterative Analysis: Bounded Leaky Artesian Aquifer

Application of the leaky artesian well function (Hantush and Jacob, 1955) to the analysis of aquifer-test data may be difficult if there are boundaries close enough for their effects to obscure those of leakage. If available information is adequate to establish boundary locations with reasonable accuracy, and if transmissibility and storage coefficient can be determined from early drawdown data, the leakage factor, B, may be determined by iterative adjustment of an initial rough estimate until observed and calculated drawdowns match within some specified range of accuracy. An algorithm for iterative adjustment is developed and a method of improving its efficiency discussed. Formulas are presented for generalized image-well positions for a strip aquifer and all results combined and applied to digital-computer analysis of drawdown data for a buried preglacial bedrock-valley aquifer near Stettler, Alberta. Three computed values of B agreed closely and calculated drawdowns based on an average B matched observed drawdowns satisfactorily.

Determining the Ratio of Horizontal to Vertical Permeability by Aquifer‐Test Analysis

The ratio of horizontal to vertical permeability may be determined by analysis of drawdown data from piezometers or partially penetrating observation wells near a partially penetrating production well. The analyses are made by comparing measured drawdowns in the piezometers to those predicted if the production well fully penetrated the aquifer. The differences between the measured and predicted drawdowns are determined, and the distances from the pumped well at which these differences would occur in an isotropic aquifer are determined from an equation. The permeability ratio is computed as the square of the ratio of actual distances to computed distances. The ratio of horizontal to vertical permeability in glacial outwash in central Wisconsin was determined from 5 aquifer tests to range from about 2 to about 20.