Multidisciplinary Workflow applied to Multiple Stage Hydraulic Fracturing of Horizontal Wellbores: Evolving the Process in the Lebada Field, Black Sea

Abstract

Oil production from offshore Lebada field has been improved significantly over the last two years through the use of multiple proppant fractures placed from horizontal completions. Following early successes the engineering effort now focuses on increasing value through engineering innovation and increased productivity. This paper describes, through case histories and field data, the advances made in the application of workflow processes, design and execution of these completions. Due to the offshore environment, emphasis is placed on high-efficiency operations based on specially tailored solutions incorporating new workflow processes and technologies. This resulted in a significant impact on operational efficiency by lowering costs and ultimately increasing hydrocarbon production. In the first well candidate, decision was made to complete the horizontal openhole well with a permanent completion system consisting of multiple mechanical packers and ball activated frac ports that would enable multiple fracturing treatments. It was determined that by using a novel viscoelastic polymer-free surfactant-based fluid, the desire fracture geometry could be optimized while being able to mix the fluid on-the-fly with sea water, therefore eliminating the need of re-loading water and materials after each frac job and allowing the excecution of the entire operation in a single trip, avoiding unnecessary vessel trips to shore to reload materials. This novel multistage fracturing system was introduced in Europe for the first time in 2008. Because of this initial success, additional wells were selected and treated using same engineered approach. Two horizontal wells were logged using Wireline Technology and logging while drilling technology, interpreted openhole images and advanced sonic logs were used to determine the optimum completion configuration and to select favorable fracture initiation points and treatment designs. Multistage frac was designed according to the Geological and petrophysical information from log data integration. A vertical well that completed with a limited set of logs and is, in this presentation our blind test well. The openhole packers were placed in sections of the wellbore free of open fractures, contacting shale streaks with higher stress acting as barriers for fracture propagation and providing hydraulic isolation. The frac ports on the other side were placed in the points where the fracture would most likely initiate. Those parts of the wellbore contacting open fracture systems, identified by image logs and sonic logs were expected to show lower stress and lower shale content. Fracturing ports were selectively spaced to provide the best reservoir drainage pattern. Productivity is the driver for economic success and processes and treatment design requirements have evolved through the new techniques implemented, that has now become the standard in Lebada field. Production data on these three wells indicate expected production and demonstrated the full success of the technology utilized. Multi disciplinary approach on all 3 wells has allowed a perfect optimization of completion techniques that has allowed PETROM to save rig time in an expensive offshore environment. Normalized well productivities continue to improve and the challenge is to maintain the rate of evolution through the improved application of both new and existing technologies.

Elemental geochemistry of the Upper Cretaceous reservoir and surrounding formations applied in geosteering of horizontal wells, Lebăda Field – Western Black Sea

“…The Upper Cretaceous reservoir is a calcareous, post-rift sequence also known as the Santonian-Coniacian-Turonian complex (St-Co-Tu complex). It is a micro-fractured reservoir complex with a mixed carbonate and sandstone lithology (Cațaraiani et al, 2010;Contreras and Sarmiento, 2016). Occasional conglomerates and calcareous-sandstone layers are present at the bottom of the section ranging from 10 to 20 cm thickness up to 5-10 m. At the middle of the section, calcareous-sands and marl intervals overlain by a 30-35 m thick micritic-biogenic-limestone cap-rock dominate the mixture.…”

“…Occasional conglomerates and calcareous-sandstone layers are present at the bottom of the section ranging from 10 to 20 cm thickness up to 5-10 m. At the middle of the section, calcareous-sands and marl intervals overlain by a 30-35 m thick micritic-biogenic-limestone cap-rock dominate the mixture. The targets consist of a thin laminated and tight reservoir with porosity values from 0% up to 25% while permeability ranges from 0.1 to 2.0 mD with an average of 0.8 mD (Bukovac et al, 2009;Cațaraiani et al, 2010;Sofonea et al, 1996).…”