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In analyzing the complex interaction between the wellbore and the reservoir formation, the hydromechanical properties of the region proximal to the wellbore, referred to as the "wellbore skin zone", play a pivotal role in determining flow dynamics and the resulting formation deformation. Existing models of the wellbore skin zone generally assume a constant permeability throughout, resulting in a sharp permeability discontinuity at the skin-reservoir interface. This paper introduces a model for a wellbore with a continuously graded skin zone of finite thickness within a poroelastic medium. Analytical solutions are derived using the Laplace transform method, addressing both positive and negative skin zones. Numerical results are presented to illustrate the effects of graded permeability/skin zone thickness on pore pressures and stresses around a wellbore. The results highlight a distinct divergence in stress and pore pressure fields when comparing wellbores with negative skin zones to those with positive skin zones or no skin at all. [ABSTRACT FROM AUTHOR]

This study presents an approximate analytical solution for predicting drawdown temperature transient behaviors of a fully penetrating vertical well in a two-zone radial composite reservoir system. The inner zone may represent a damaged (skin) zone, and the outer (non-skin) zone represents an infinitely extended reservoir. The analytical solution is obtained by solving the decoupled isothermal pressure diffusivity equation and temperature equation for the inner and outer zones with the Boltzmann transformation. The convection, transient adiabatic expansion and Joule–Thomson heating effects are all accounted for in the solution. The developed solution compares well with the results of a thermal numerical simulator. The analytical solution is used as a forward model for estimating the parameters of interest by nonlinear regression built on a gradient-based maximum likelihood estimation (MLE) method. A methodology, based on semilog analyses of pressure and temperature data as well as log–log diagnostic plots of pressure- and temperature-derivative data, is proposed to obtain good initial guesses of parameters, which derive the MLE objective function to have reliable optimized estimates. The statistical measures such as estimated standard deviation of noise in pressure and temperature data, confidence intervals for parameters and correlation coefficients between parameter pairs are used to evaluate the goodness of fit and reliability of the estimated parameters from history matching pressure and/or temperature data sets. The results show that the rock, fluid and thermal properties of the skin zone and non-skin zone can be reliably estimated by regressing on temperature transient data jointly with pressure transient data in the presence of noise. [ABSTRACT FROM AUTHOR]

• An analytical model for radially convergent tracer test in a two-zone aquifer is built. • The validity of using the model to actual asymmetrical tracer transport is assessed. • The effects of vertical dispersion and skin zone on breakthrough curve are examined. Radially convergent tracer test (RCT) with an extraction well and a tracer injection well is commonly conducted for determining aquifer dispersivity associated with the spreading of contamination plume. A variety of analytical models for RCT have been proposed, but the effects of skin zone and vertical dispersion are rarely explored. In addition, little attention is paid to the validity of those analytical models assuming asymmetrical tracer transport (ATT) as radially symmetrical transport (RST) toward the pumping well. This study develops a new analytical model for RCT subject to the above-mentioned effects in a confined aquifer with a skin zone around the wellbore. The Laplace-domain solution of the model for a continuous or instantaneous input is developed. A finite element solution (FES) for ATT is also developed to verify the Laplace-domain solution based on RST. Results suggest ATT can be regarded as RST in predicting the breakthrough curve (BTC) at the pumping well when four quantitative conditions are met. A lumped dimensionless parameter dominates those two effects on the BTC. Both the FES and Laplace-domain solution agree with monitored concentration data from a field RCT. [ABSTRACT FROM AUTHOR]

The paper examines the possibility of hydraulic characterization of radial well laterals in a manner that does not require prior hydrodynamic analysis by simulating groundwater extraction conditions on a numerical model. The first step of the proposed approach is to assess the groundwater level regime formed in the capture zone when the well is operating, which is indicative of the functional condition of the laterals and the aquifer potential in terms of groundwater availability. Then the efficiency of the laterals is examined through the hydraulic function of the skin zone, depending on its filtration properties. An expression for groundwater flow from the aquifer to the laterals (commonly used to simulate laterals on a hydrodynamic model) is employed to define representative values of the conductance coefficient of the skin zone (as an indicator of colmation), and then applied to several wells at Belgrade Groundwater Source. The present research shows that a conductance coefficient of [Ks/ds]=1.0×10-4 s-1 can be considered as the threshold value in relation to which the effects of colmation of well laterals are exhibited, which is consistent with the results reported by researchers who studied the effect of skin zone conductance on the occurrence and nature of the so-called early drawdown at radial collector wells. In addition to gaining insight into the present condition of the laterals, the proposed approach can be used to study the progress of colmation at different points in time and to quantify the effectiveness of regeneration of laterals. [Projekat Ministarstva nauke Republike Srbije, br. OI176022, br. TR33039 i br. III43004]

Summary This study develops a mathematical model for contaminant transport due to well injection in a radial two-zone confined aquifer system, which is composed of a wellbore skin zone and a formation zone. The model contains two transient equations describing the contaminant concentration distributions; one is for contaminant transport in the skin zone while the other is for transport in the formation zone. The contaminants are injected into the well with given dispersive and advective fluxes; therefore, the well boundary is treated as a third-type (Robin) condition. The solution of the model derived by the method of Laplace transforms can reduce to a single-zone solution in the absence of the skin zone. In addition, an approximate solution in the time domain is also developed by neglecting dispersion for the case that the contaminants move away from the injection well. Analysis of the semi-analytical solution showed that the influence of the skin zone on the concentration distribution decreases as time elapses. The distribution will be over-estimated near the wellbore if the constant concentration (Dirichlet) condition is adopted at the well boundary. The approximate solution has advantages of easy computing and yield reasonable predictions for Peclet numbers larger than 50, and thus is a practical extension to existing methods for designing aquifer remediation systems or performing risk assessments. [ABSTRACT FROM AUTHOR]

Summary: We analyze numerically the impact of key assumptions which are usually adopted to interpret borehole flowmeter test data. We base our work on a set of detailed numerical simulations of transient and pseudo-steady state convergent flow to a fully penetrating well that pumps at a constant total flow rate in a confined three-dimensional system formed by well demarcated facies. Our results allow identifying the effects of (i) the presence of a skin zone around the well, (ii) the contrast of hydraulic conductivities of the aquifer facies and the gravel pack, and (iii) the dynamics of the flow regime on the ability to characterize the local aquifer conductivities of a stratified system through the typical interpretation of downhole flowmeter tests. Vertical intervals along the wellbore, where reliable estimates of the aquifer conductivities can be obtained through classical formulations are identified in terms of a set of dimensionless parameters. These parameters involve the thickness of the facies constituting the stratified aquifer and the contrast of conductivities amongst different facies and between them and the skin zone. [ABSTRACT FROM AUTHOR]

Mitigation and control of borehole pressure at the bottom of an injection well is directly related to the effective management of well injectivity during geologic carbon sequestration activity. Researchers have generally accepted the idea that high rates of CO injection into low permeability strata results in increased bottom-hole pressure in a well. However, the results of this study suggested that this is not always the case, due to the occurrence of localized salt precipitation adjacent to the injection well. A series of numerical simulations indicated that in some cases, a low rate of CO injection into high permeability formation induced greater pressure build-up. This occurred because of the different types of salt precipitation pattern controlled by buoyancy-driven CO plume migration. The first type is non-localized salt precipitation, which is characterized by uniform salt precipitation within the dry-out zone. The second type, localized salt precipitation, is characterized by an abnormally high level of salt precipitation at the dry-out front. This localized salt precipitation acts as a barrier that hampers the propagation of both CO and pressure to the far field as well as counter-flowing brine migration toward the injection well. These dynamic processes caused a drastic pressure build-up in the well, which decreased injectivity. By modeling a series of test cases, it was found that low-rate CO injection into high permeability formation was likely to cause localized salt precipitation. Sensitivity studies revealed that brine salinity linearly affected the level of salt precipitation, and that vertical permeability enhanced the buoyancy effect which increased the growth of the salt barrier. The porosity also affected both the level of localized salt precipitation and dry-out zone extension depending on injection rates. High temperature injected CO promoted the vertical movement of the CO plume, which accelerated localized salt precipitation, but at the same time caused a decrease in the density of the injected CO. The combination of these two effects eventually decreased bottomhole pressure. Considering the injectivity degradation, a method is proposed for decreasing the pressure build-up and increasing injectivity by assigning a 'skin zone' that represents a local region with a transmissivity different from that of the surrounding aquifer. [ABSTRACT FROM AUTHOR]

Abstract: As a consequence of drilling and completion operations most wells have reduced permeability near the borehole (skin zone). This may result in a corresponding change of thermal conductivity of formations near the borehole. At the same time the mathematical models, which describe the process of thermal recovery of formations are based on the assumption that the thickness of the radius of heated zone (during circulation of the drilling mud) is significantly larger than the well radius (or skin zone). It is shown below that the radius of the heated zone (radius of investigation or radius of thermal influence) can be approximated by simple formulas. It is shown that for six field bottom-hole temperature surveys the radius of thermal influence (RTI) is too small, and for this reason, the duration of the drilling mud circulation period should increased. [Copyright &y& Elsevier]

Abstract: Radon concentration was monitored during the flow tests of well IC-09 at the Chingshui geothermal field. The radon concentration was found to increase from 54 ± 29 to 983 ± 65 Bq/m3 as a step function of production time, or cumulative production. The observed radon behavior can be explained by a radial composite model with the carbonate scales deposited in the skin zone near the well. The radius of skin zone near well IC-09 can be estimated with radon data at about 20 m using a plug flow model. Monitoring natural radon during the well flow tests is a helpful tracer to diagnose the formation damage near the well. [ABSTRACT FROM AUTHOR]

A mathematical model describing the transient hydraulic head distribution induced by constant-head pumping/injection at a partially penetrating well in a radial two-zone confined aquifer is a mixed-type boundary value problem. The analytical solution of the model is in terms of an improper integral with an integrand having a singularity at the origin. The solution should rely on numerical methods to evaluate the integral and handle the problems of convergence and singularity. This study aims at developing a new approximate solution describing the transient hydraulic head distribution for a constant-head test (CHT) at a partially penetrating well in the aquifer. This approximate solution is acquired based on a time-dependent diffusion layer approximation proposed in the field of electrochemistry. The diffusion layer can be analogous to the radius of influence in the area of well hydraulics. The approximate solution is in terms of modified Bessel functions for aquifers with a partially penetrating well and can reduce to a simpler form in terms of a natural logarithmic function for the case of well full penetration. The predicted hydraulic heads from the present approximate solution are compared with those estimated by the Laplace-domain solution of the model. The result shows that the predicted spatial head distributions are accurate in the formation zone and fairly good in the skin zone. In addition, the present solution gives an accurate temporal head distribution at a specific location when the radius of influence is far away from the observation wells. This newly developed approximate solution has advantages of easy computing and good accuracy from practical viewpoint, and thus is a handy tool to evaluate temporal and spatial hydraulic head distributions for the CHT. [ABSTRACT FROM AUTHOR]