Hydraulic Fracturing Optimization Restores Development Prospectives in the Challenging Environment of Rostashinskoe Oilfield, Orenburg Region

Abstract

Orenburg region is a part of the Volga-Urals basin and, because of geological complexity, it poses problems for field development and reservoir stimulation. The Rostashinskoe field is one of the fields of the Pervomaysky group, and it has been under development since 1980. The main producing horizons of the field are Devonian: Afoninsky (DV), Vorobievsky (DIV), and Ardatovsky (DIII) formations. The Pervomaysky reservoir is unique in having very light oil.Oil traps in the Pervomaysky fields are formed by faulting. The entire area of these oilfields is now covered by 3D seismic surveys, which indicate that traps are related to a series of regional faults scattered from east to the west. Apart from the main regional faults, Pervomaisky fields have a large number of smaller low-amplitude faults, both conductive and sealed. The presence of these faults has significantly affected the field development strategy. The DIII formation of Rostashinskoe field is represented by two sandstone sublayers, DIII-1 and DIII-2. Large numbers of faults and pinchout zones have imparted a high degree of formation heterogeneity both vertically and laterally across the field. As a consequence of heterogeneity, Rostashinskoe presents difficulties with proper waterflood and fracturing modelling in the field development cycle. As a result, reserves are being recovered unequally across the field.The first trials of hydraulic fracturing in 2004 were unsuccessful because of high fracturing-treatment pressure, a large number of screenouts, and difficulties with fracture fluid flowback and well kickoff. To investigate flowback problems and fast production decline, detailed core analysis has been performed, including mineralogical composition and core flow testing with fracturing fluid to determine the fluid's influence on core permeability. It was determined that Rostashinskoe cores have clay migration problems and changes in wettability. Based on core analysis data, a new clay stabilizer and surfactant were recommended for fracturing fluid. Optimization of the fracturing fluid has led to significant improvement in final productivity results after fracturing and basically has proven the efficiency of hydraulic fracturing treatments in the Rostashinskoe field. From 2010 to 2013, over 60 fracturing jobs were performed in the Rostashinskoe oilfield. Proper candidate selection, optimized fracturing design, and best practice led to additional oil recovery from the DIII formation.Success of the modified fracturing design has rejuvenated the infill drilling program in the Rostashinskoe oilfield to achieve increased production, reduced decline rate, and better reserves recovery.