Analysis of Pressure and Pressure Derivatives Without Type-Curve Matching: I-Skin and Wellbore Storage

1993 ◽  
Author(s):  
Djebbar Tiab
Keyword(s):  
Curve Matching ◽  
Type Curve ◽  
Wellbore Storage

An Investigation of Wellbore Storage and Skin Effect in Unsteady Liquid Flow: II. Finite Difference Treatment

1970 ◽  
Vol 10 (03) ◽  
pp. 291-297 ◽  
Author(s):  
Robert A. Wattenbarger ◽  
H.J. Ramey
Keyword(s):  
Differential Equation ◽  
Skin Effect ◽  
Annular Region ◽  
Curve Matching ◽  
Type Curve ◽  
Flow Rates ◽  
Flow Capacity ◽  
Wellbore Storage ◽  
Straight Line ◽  
Short Time

Abstract An investigation of the effect of wellbore storage and skin effect on transient flow was conducted using a finite-difference solution to the basic partial differential equation. The concept of skin partial differential equation. The concept of skin effect was generalized to include a composite annular region adjacent to the wellbore (a composite reservoir). The numerical solutions were compared with analytical solutions for cases with the usual steady-state skin effect. It was found that the solutions for a finite-capacity skin effect compared closely with analytical solutions at short times (wellbore storage controlled) and at long times after the usual straight line was reached. For intermediate times, presence of a unite-capacity skin effect caused significant departures from the infinitesimal skin solutions. Two straight lines occurred on the drawdown plot for cases of large radius of damage. The first had a slope characteristic of the flow, capacity of the damaged region; the second straight line had a slope characteristic of the flow capacity of the undamaged region. Results are presented both in tabular form and as log-log plots of dimensionless pressures vs dimensionless times. The log-log pressures vs dimensionless times. The log-log plot may be used in a type-curve matching plot may be used in a type-curve matching procedure to analyze short-time (before normal procedure to analyze short-time (before normal straight line) well-test data. Introduction Skin effect was defined by van Everdingen and Hurst as being an impediment to flow that is caused by an infinitesimally thin damaged region around the wellbore. The additional pressure drop through this skin is proportional to the wellbore flow rate and behaves as though flow through the skin were steady-state. Wellbore storage is caused by having a moving liquid level in a wellbore, or by simply having a volume of compressible fluid stored in the wellbore. When surface flow rates change abruptly, wellbore storage causes a time lag in formation flow rates and a corresponding damped pressure response. A recent study was made to determine the combined effects of infinitesimally thin skin and wellbore storage. Analytical methods were used along with numerical integration of a Laplace transformation inversion integral. Tabular and graphical results were presented for various cases. It was recognized during the study that this representation of skin was oversimplified; that skin effect should be thought of as a result of formation damage or improvement to a finite region adjacent to the wellbore. It was suggested that a skin effect could arise physical in a number of ways. One simple example physical in a number of ways. One simple example would be to assume that an annular volume adjacent to the wellbore is reduced uniformly to a lower permeability than the original value. This would be similar to the composite reservoir problem. Perhaps a better example would be to problem. Perhaps a better example would be to assume that the permeability increases continuously from a low value at the wellbore to a constant value in the undamaged reservoir. In either case, the damaged region would have a finite storage capacity and would lead to transient behavior within the skin region. A negative skin effect could arise from an increase in permeability within an annular region adjacent to the wellbore. This might physically result from acidizing. But it is believed that cases of more practical importance are those in which negative skin effects are caused by hydraulic fracturing. A high-permeability fracture communicating with the wellbore gives the appearance of a negative skin effect. For the purposes of this study, it was decided to represent a skin effect, either positive or negative as an annular region adjacent to the wellbore with either decreased or increased permeability. permeability. SPEJ P. 291

Analysis of Short Transient Tests Affected by Changing Wellbore Storage

1998 ◽  
Vol 1 (03) ◽  
pp. 261-267 ◽  
Author(s):  
M. Vasquez-Cruz ◽  
R. Camacho-Velazquez
Keyword(s):  
Exponential Function ◽  
Radial Flow ◽  
Error Function ◽  
Early Time ◽  
Curve Matching ◽  
Well Test ◽  
Flow Conditions ◽  
Type Curve ◽  
Wellbore Storage ◽  
Matching Procedure

Abstract It is well known that wellbore storage and phase redistribution have a direct influence over well testing data, mainly on those recorded at early times of the test. After this early time period, such influence disappears, and the pressure response is dominated by the reservoir and the skin zone properties. However, sometimes the effects of wellbore dynamics last long enough as to completely disguise the reservoir response. The above situation frequently constrains the use of some analysis procedures such as type curve matching, especially if the test did not last long enough as to reach radial flow conditions. In this way, some tests are uninterpretable because of the duration of these wellbore effects. Using as a basis two classical models related to well test affected by changing wellbore storage, this work introduces a new method of analysis for these tests with insufficient duration to reach radial flow conditions. The use of the methodology proposed in this work is illustrated with synthetic examples and a field case. For some synthetic cases the type curve matching procedure may yield completely erroneous values of the parameters, while with the suggested method reasonable estimates are obtained. Introduction Wellbore storage is recognized as a phenomenom that affects the recorded pressure behavior at early times during a well test. Sometimes, the phenomena related to mass balance and fluid momentum that occur inside the wellbore, constrain the application of some analysis procedures, especially if the test does not show transient radial flow conditions, and can even disguise completely the pressure response. Phase redistribution occurs when a producing well (that contains more than one phase) is closed at the surface, and due to gravitational forces these phases segregate from each other, this causes a distorsion on early time data of the test. In 1981, Fair introduced a model which takes into account the phase redistribution phenomenom in the diffusivity equation solution as applied to buildup analysis. He assumed an exponential increase in wellbore storage. More recently, Hegeman et. al., proposed an extension to Fair's model, obtaining a general solution in Laplace space, which allows to add the changing wellbore storage effect for a variety of analytical well-reservoir models. These authors considered two models of changing wellbore storage, the exponential function and the error function. For the error function, the transition period is more abrupt than with the exponential function. In 1992, Fair presented another study on the influence of a wide range of wellbore phenomena including mass-balance and momentum phenomena. Also, in that work wellbore phenomena such as fluid temperature changes (represented by an exponential function as in the case of wellbore storage), phase changes inside the production tubing and inertial effects were considered. Currently, the only available method to estimate the reservoir permeability and skin factor from insufficient transient data (tests affected by changing wellbore storage and that have not reached radial flow period) is the type-curve matching technique. To apply this procedure it is necessary to identify the function (exponential or error function) that fits best the storage effect. Nevertheless, the absence of data in the infinite acting period difficults the matching procedure, resulting in a match with high uncertainty. With the purpose of modeling in a more rigorous way the reservoir-wellbore system, it is necessary an algorithm that couples the mechanistic behavior of the well with that of the reservoir, such as that of Hasan and Kabir, Winterfeld propose the use of a completely numerical scheme that simulates the transient conditions of the system.

Reserves Determination Using Type Curve Matching and Extended Material Balance Methods in the Medicine Hat Shallow Gas Field

1994 ◽  
Author(s):  
S. L. West ◽  
P. J. R. Cochrane
Keyword(s):  
Material Balance ◽  
Poor Quality ◽  
Curve Matching ◽  
Pressure Data ◽  
Gas Reservoirs ◽  
Shallow Gas ◽  
Gas Field ◽  
Type Curve ◽  
Balance Analysis ◽  
Diffusivity Equation

Tight shallow gas reservoirs in the Western Canada Basin present a number of unique challenges in accurately determining reserves. Traditional methods such as decline analysis and material balance are inaccurate due to the formations' low permeabilities and poor pressure data. The low permeabilities cause long transient periods not easily separable from production decline using conventional decline analysis. The result is lower confidence in selecting the appropriate decline characteristics (exponential or harmonic) which significantly impacts recovery factors and remaining reserves. Limited, poor quality pressure data and commingled production from the three producing zones results in non representative pressure data and hence inaccurate material balance analysis. This paper presents the merit of two new methods of reserve evaluation which address the problems described above for tight shallow gas in the Medicine Hat field. The first method applies type curve matching which combines the analytical pressure solutions of the diffusivity equation (transient) with the empirical decline equation. The second method is an extended material balance which incorporates the gas deliverability theory to allow the selection of appropriate p/z derivatives without relying on pressure data. Excellent results were obtained by applying these two methodologies to ten properties which gather gas from 2300 wells. The two independent techniques resulted in similar production forecasts and reserves, confirming their validity. They proved to be valuable, practical tools in overcoming the various challenges of tight shallow gas and in improving the accuracy in gas reserves determination in the Medicine Hat field.

Type-Curve Matching Using a Computer Spreadsheet

Ground Water ◽  
1996 ◽  
Vol 34 (3) ◽  
pp. 554-562 ◽  
Author(s):  
Theodore G. Cleveland
Keyword(s):  
Curve Matching ◽  
Type Curve
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