Effective Matrix Acidizing Based in Chelating Agents: A Case Study in Romanian Heavy Oil Reservoirs

Abstract

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.