A New Way to Diagnose Injectivity Decline During Fractured Water Injection By Modifying Conventional Hall Analysis

摘要

For many years, unfiltered produced water has been used inthe flooding of heavy oil reservoir in Southern Oman forpressure maintenance. Initially it was thought that the waterinjection was under matrix conditions. However, it is wellaccepted in the industry that water injection and particularlyproduced water injection almost, always takes place underfracturing condition. Significant decline in injectivity wasrepeatedly seen due to downhole blockages. Routine clean-upsand acidizations were carried out without success. Early 2003,a dedicated study was conducted to identify the root cause ofthis problem and recommend a remedial action. The purposewas to determine if the decline is due to poor water quality,reservoir compartmentalization, matrix/fracture or wellboreplugging.An unconventional approach was used to conduct acomprehensive study using historical data by modifying theHall-Analysis technique. It was concluded that poor waterquality was the problem. The interpretation of the ModifiedHall Plot together with Bottomhole Pressure (BHP) vs. Rateplot also provided evidence of wellbore and/or fractureplugging. Both qualitative and quantitative analysescharacterized the initial matrix injection period and fracturinginjection regime that followed. Fracture plugging, routine wellstimulations and fracture propagation trends were interpretedfrom the plots. A 3D in-house simulation package forwaterflood under fracturing conditions was used,independently, to simulate injection conditions and confirmthe results. Simulated injection pressure was much lower thanactual field data confirming severe plugging. Further analysisshowed that the plugging was due to unconventionalparticulates different from solids and oil contaminants inwater. Recommendation was made to identify the contaminantand maintain a maximum suspended solid concentration of 100ppm to contain fracture propagation. A recent filter trialconfirmed the presence of FeS contaminant.Successful remedial action was taken followed by recommissioningand water treatment facility design to realizeestimated 150m3/d/well and improving ultimate recovery.Recently, another field study has successfully completedadopting same technique. By modifying Hall Analysistechnique, it is possible to use field data for diagnosingmatrix/fracturing regimes, assessing degree of damage anddetermining the location of the damage.