A Field Comparison of Methods of Evaluating Remedial Work on Water Injection Wells
Authors: Ouzts J.M. 
Published: December 2007

Since van Everdingen introduced the skin effect concept of determining permeability damage in oil wells, several authors have refined and transformed the analysis for use in water input wells. This paper presents a review of the skin factor determination and its assumptions by the pressure fall-oft and variable rate method of calculation. A comparison with a new evaluation method published by Pan American is offered. Pressure fall-oft data were recorded on 21 dolomite water injection wells prior to and after remedial treatment in the Morse area of the Panhandle field of Texas. Skin factors and effective permeabilities were calculated from these data and from injection rate-time curves. The calculated values of skin in most cases are negative both before and after treatment, indicating increased permeability around the wellbore; however, values of skin effect correlate rather well with injection rates before and after treatment. Values of effective permeability to water calculated from slopes of the pressure fall-off data reflect the low injection rates prior to treatment. Permeabilities after treatment check very well with core analysis permeability data and relative permeability curves for cases examined.

Introduction

In waterflood injection wells, one of the controlling factors of water injection rates is skin resistance in the area adjacent to the wellbore. This skin may exist for a variety of reasons. Among these are damage caused by drilling mud and filtrates or cement when the well was drilled, paraffin or asphalt deposition in wells converted from production to injection, unstable and incompatible injection water, and swelling clays.van Everdingen recognized the existence of this damaged area in producing wells and presented equations to evaluate its detrimental effect on a well's productive capacity and the effectiveness of remedial work performed on the well. Joers and Smith adapted this work to water injection wells. Others have refined and transformed this original work to other forms such as the variable rate procedure. The application of skin factor determinations as a water flood approaches fill-up and thereafter should be helpful in evaluating the quality of the injection water and point out the needs for improvement in water quality or remedial work on injection wells. Skin calculations which had been run before and after remedial jobs on wells discussed in this paper were used in part to determine the effectiveness of several variables and mechanical changes in order to determine the best remedial treatment for future jobs. Other methods of evaluation were also used since pressure fall-off tests were not available on all wells. These methods are also discussed in the paper. A total of 58 injection well remedial jobs have been performed in the Morse area of the Texas Panhandle field since Jan., 1962. Of this total. skin factors and effective permeabilities to water were calculated before and after treatment on 21 wells. Single skin factor tests were available on nine additional wells. Injection rate-time and pressure-time data were available on all wells. The Pan American evaluation method was calculated where sufficient history after treatment was available for comparison with pressure fall-off data on 12 wells. Effective permeabilities to water prior to and after treatment are presented.

Injection Rate-Time and Pressure-Time Curves

Injection rate and pressure-time curves or data from which they can be plotted are available on most water floods. These curves such as shown in Fig. 1 are characterized generally with high injection capacity at low surface injection pressures at commencement of injection. They indicate that, as the effective radius of the water bank grows, the injection pressure increases, the injection rate decreases logarithmically and that pressure usually stabilizes before fill-up at the injection equipment limitation. These curves can be used in evaluating remedial treatments in cases such as that shown in Fig. 1. Here, injection pressures before and after treatment were essentially unchanged and injection rates stabilized shortly after the treatment was completed. Where pressures are not stable. a correlation of BWPD injected per psi at the sand face or injectivity index can be used. In most cases, however. more sophisticated evaluation methods are needed.

JPT

P. 1121ˆ