Interference Testing With Wellbore Storage And Skin Effect At The Produced Well
Authors: Sandal Hugo M. , Horne Roland N. , Ramey Henry J. , Williamson John Wright 
Published: December 2007

This study investigates the effect of wellbore storage and damage (skin effect) at a produced well on the pressure response (interference) in the reservoir, away from the produced well. The same problem had been investigated earlier by Jargon, and by Garcia-Rivera and Raghavan, using finite difference or approximate methods. In this work the problem was solved analytically by using the Laplace transformation method. When dimensionless time, tD, was greater than approximately 100, the results from this study were the same as those presented in ref. 4. But when tD was less than 100, the results did not agree. The correlation used in ref. 4, based on an effective wellbore radius, was also found to be poor for tD less than 100. A new correlation was found for tD less than unity.

In order to compute results, a numerical Laplace transform inversion method was used. The characteristics of the method were studied for two purposes:to determine the properties of the inversion method for negative values of the skin effect, andto determine whether the inversion method might provide an analytic tool to study the dimensionless provide an analytic tool to study the dimensionless groups which control a solution. Both studies were successful.

The Laplace transform inversion method was used to prepare original tables of dimensionless pressures for the subject problem for both positive and negative skin effects. No other such tables appear to exist.

Introduction

Interference testing is a multiple-well test which requires at least one active well and at least one observation well. The active well is either a producer or an injector, and the observation wells are shut-in wells in which pressure effects caused by the active well are measured. This kind of testing has the advantage of investigating more reservoir volume than a single-well test.

Usually both the mobility-thickness product, and the porosity-compressibility-thickness product, phi phi ct h, may be estimated from multiple-well product, phi phi ct h, may be estimated from multiple-well tests. The data from the test may be analyzed by type-curve matching.

Interference testing has become popular as more sensitive pressure gages have become available, thus making interference testing feasible in systems of high compressibility. As the accuracy and sensitivity of pressure data has improved, it has been recognized that the line-source solution neglects the finite radius of the active well, wellbore damage (skin effect), and wellbore storage. This can introduce significant errors. The effect of storage is to cause a changing sandface flow rate. Ideally, the skin effect does not have a transient effect (steady state or zero-storage skin). However, a skin effect can make wellbore storage-influenced sandface flow rate changes last longer. For the case of constant surface production rate, the combined effect of skin effect and wellbore storage causes the sandface flow rate to increase gradually from zero toward the constant surface flow rate, over a period of time. The time required for the sandface flow rate to essentially equal the surface flow rate is a function of the skin effect and wellbore storage. The higher the skin effect and wellbore storage, the longer the time will be.

In the case of the line source solution, the sandface flow rate is equal to the surface flow rate, instantaneously. Thus one should expect a smaller pressure drop at a particular time and radius in the pressure drop at a particular time and radius in the case of a well with a skin effect and wellbore storage than for the line source well solution.

The aim of this study was to solve the problem of constant surface production rate from a well with a skin effect and wellbore storage analytically, and to check the results with previously published data obtained by using others' methods. Another objective was to correlate the interference data if possible.