Heavy and extra-heavy crude oil reservoirs hold physicochemical characteristics that can frequently turn their operation into a technical and economic challenge. Typically, heating techniques are used to decrease oil viscosity. Areas with steam injection are susceptible to developing formation damage mechanisms such as scale precipitation that gradually restricts the flow of fluid towards the wellbore and ultimately decreases overall well productivity and/or injectivity. Acidizing treatments to either remove obstructing scale or to further increase near wellbore permeability are handled with caution: heavy oil is very sensitive and interaction with such conventional acids include asphaltene precipitation, sludge and emulsions. Suplacu de Barcau is a shallow, heavy oil reservoir (16°API average) located in the northwestern part of Romania. It has been successfully operated with aid of both in-situ combustion and steam injection since 1960. Scaling tendency of condensed water from steam and its incompatibility when mixed with formation water frequently ends in scale build-up in injector and producer wells respectively. Identifying a fluid able to clean-out the scale deposits while being fully compatible with sensitive heavy oil, involved extensive screening a compatibility testing protocols. A formulation based on the chelating agents N,N, Glutamic Acid Diacetic Acid (GLDA) and Diethylene Triamine Pentaacetic Acid (DTPA) were found not only to effectively dissolve the plugging materials, but remarkably it was also noticed that it reduced significantly the oil viscosity, which made this formulation the most appropriate treatment for field application. A number of 10 producer wells treated with the GLDA and DTPA based fluids delivered promising results by increasing oil rates by 3- 6 times of increase, significant improving steam coverage and penetration, decreasing drawdown and skin and ultimately enhancing the mobility of asphaltic oil. This paper describes the stimulation approach followed from diagnosis, fluid screening and selection, treatment design, job execution and results. Furthermore, the outcome of this stimulation campaign has shattered the myth that this type of stimulation does not work in hard oil.

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.