Advanced Technologies of Acid Fracturing - a Great Advantage in the Development of Carbonate Reservoirs of the Timan-Pechora Region

Abstract

Abstract About 70% of the reserves of the Timan-Pechora oil province are classified as hard-to-recovery. Most of the production comes from large reservoirs during the late stage of development, which makes us look for reliable methods to increase production while reducing capital expenditures. Traditionally carbonate reservoir acidizing is considered to be an effective method to increase oil recovery. However, this is not always true for the Devonian carbonate formations of Timan-Pechora region. In conditions of low reservoir temperature (60-70 C), dissection cut, high tectonic stress and the tendency to water cut increasing classic acidizing and acid fracturing are ineffective without the use of advanced technologies. Acid Fracturing has been selected by JC RUSVIETPETRO as a basic method for the pilot stimulation campaign in 2015. 15% HCl was used as the main fluid for reservoir with a temperature of 65C. Heavy oil tendency to deposit paraffins and create sludge promoted the use of an acid emulsion with the solvent. Low permeability and dismembered reservoir structure required formations merging into a single system of fractures and leveling inflow profile. Viscoelastic surfactant was applied to this end, having the properties of both the classic diverter in matrix acidizing and as a "buffer stage" for acid fracturing, helping to increase the fracture half-length. Also in 2016 in addition to the described systems, fiber diverter was applied for acid fracturing the first time in Russia in the fields of the Timan-Pechora. It allows to achieve even greater results in stimulation by natural fractures plugging in reservoir and redirecting acid fractures flows. The oil fields are located in the Nenets Autonomous District, which automatically imposes a large number of restrictions on the methods of stimulation because of complex logistics and low temperatures. In 2015 winter season, when winter roads were availble from February to April, the first stimulation campaign was performed. Most of the wells produce from two clusters of Famennian stage at a depth of 3200 m. The main inflow of the hydrocarbons comes from the lower, more permeable interval, while the top remains untapped. Thus, after water cut increasing of the lower cluster, wells keep sufficient production capacity at the top cluster if stimulated by selective acid fracturing. Hydraulic acid fracturing was performed after mechanical isolation of the lower cluster and with application of new modern technologies, which increased the productivity of wells on average 4.3 times, with an average increase in production by 20.4 tonnes of oil/day. Taking into account the results of the first stimulation campaign in 2016 stimulation jobs continued with using of fiber diverter, the uniqueness of which is the ability to temporarily block fractures and redirect their development in heterogeneous reservoir. Whereby a multi-fractures structure is created and increases the contact area of the fracture and the formation, and as a result the efficiency of acid fracturing increases. According to the results of the second winter stimulation project the average increase of production rate was 21.9 tons/day/well.