Aplicação combinada de fraturamento hidraúlico e acidificação para estimulação de depósitos de gás não convencionais da Bacia do São Francisco

Abstract

The production of oil and gas reservoirs is not something so simple as it appears. Simply drilling a well through the reservoir is not a sufficient condition to put the well into production. Most reservoirs need to be stimulated after the drilling phase, especially in the case of low permeability reservoirs also known as unconventional reservoirs. Brazil, besides, China, USA and Argentina, has one of the largest unconventional gas reservoirs in the world; and this is of significative importance since the gas is the transition element between oil and renewable energy sources in the world’s energy matrix. There are two possible ways to stimulate a hydrocarbon reservoir: hydraulic fracturing (mechanical stimulation) and acidizing (chemical stimulation). In hydraulic fracturing a fluid carries proppant (solid beads) with enough hydraulic pressure to crack open the rock in the reservoir. The proppant grains fill the void of the open fracture preventing it from closing, and the permeability of the proppant pack creates a permanent access channel between the hydrocarbon formation and the well. Acidizing methods are less known than hydraulic fracturing and they are mainly used in carbonate formations. Acidizing is divided into matrix acidizing and acid fracturing. Matrix acidizing is performed when the reservoirs permeability is sufficiently high to allow fluid injection into the rock matrix - the acid percolates through the pores increasing the overall pore communication and permeability. On the other hand, acid fracturing is performed when the permeability is so low (unconventional) that in order to inject acid into the reservoir it is necessary to fracture the rock applying hydraulic pressure. Unconventional carbonate reservoirs can be stimulated by hydraulic fracturing or acid fracturing (but not matrix acidizing as they do not have sufficient permeability to allow acid injection below the frac gradient). In hydraulic fracturing the proppant grains fill the fracture preventing it from closing, and in acid fracturing the acid reaction creates uneven patterns or asperities on the walls of the fracture that also prevent the fracture closure. Even though it is easier to achieve long fracture extensions with acid fracturing, the asperities created in the rock by the acid reaction cannot withstand high closure pressures as well as proppants do, and the acidized fractures are often short lived when compared to the propped fractures. Someone may ask then: why not combine the two methods (hydraulic fracturing and acid fracturing) into a single method to maximize the stimulated reservoir’s volume? The combination of proppant (hydraulic fracturing) with acid is still a taboo in the oil and gas industry. The reason for this is the assumption that the acid reaction would produce a neo-layer in the walls of the fracture soft and thick enough to cause proppant embedment and fracture closure. The primary objective of this work is to investigate the presence of neolayers resulting from the acidizing process and provide evidence that a combined fracturing method (proppant + acid) is feasible for specific types of unconventional reservoirs. In order to do so, acidized samples of a unconventional carbonate formation in Brazil, the Sete Lagoas Formation which is considered by the Brazilian Petroleum Agency (ANP) one of the main unconventional gas reservoirs in Brazil, were characterized. And for a secondary objective, an original acid system appropriate for deep carbonate stimulation was proposed and developed.