Matrix Stimulation Treatments Using Non-Corrosive and Environmentally Friendly Systems: A Decade of Experience Across a Variety of Romanian Fields

Abstract

Because of the challenges commonly associated with matrix acidizing, chelating agents are increasingly reinforcing their good reputation as standalone alternative treatments in oil and gas wells worldwide. Systems based on GLDA and DTPA have been extensively used in tens of limestone and sandstone Romanian reservoirs over the past decade. This paper offers useful insights, design criteria and best practices based on substantial field experience that led to remarkable productivity boost in more than 50 wells. From deep and high temperature sour-wells, to shallower heavy oil plays. From inland oil producing assets to offshore gas condensate fields. Field applications included wells in sandstone, limestone and dolomite and placement involved foam and plain injection both via coiled tubing and bull-heading. The different stimulation campaigns involved a comprehensive laboratory evaluation, structured damage assessment followed by detailed treatment designs and execution. Experiments included both core-flood tests through limestone cores at 120 deg. C and solubility evaluation of mineral deposition at downhole conditions after scale characterization showed presence of sulphate-rich minerals like CaSO4 and BaSO4. Results of experimental evaluation showed creation of highly conductive wormholes without signs of face dissolution despite low injectivity. Solubility of challenging scales achieved dissolution from 43% to 78% in formulations containing DTPA and GLDA. Fully compatible with well completion components including sensitive equipment and jewelry like ESP was found. Bottle tests using challenging heavy crude oil demonstrated not only complete compatibility without signs of sludge, emulsions or precipitates but it also exhibited unexpected benefits in viscosity reduction during lab evaluation and field implementation. Experimental evaluations were followed by field execution that covered over 60 matrix stimulation treatments across 10 fields using chelating agent-based formulations of Glutamic-Acid-Diacetic-Acid (GLDA) and Diethylene-Triamine-Pentaacetic-Acid (DTPA). The outcome was a substantial increase in wells productivity with 2-3 times of improvement (TOI) in average and 30 % reduction of downtime without safety, environmental or asset integrity issues. Field results achieved and summarized in this paper demonstrate the efficacy of the methodology employed for productivity diagnosis. In addition, the numerous benefits of using chelating agents as standalone stimulation systems were corroborated. Described criteria and lessons learned represent a concise and useful tool to facilitate fluid selection and matrix treatment design in challenging field conditions with multi-functioning, non-corrosive, biodegradable and safe chemicals.