Fracture Hits Mitigated Successfully in High-Pressure Stimulation Offshore Black Sea
Authors: Feder Judy 
Published: August 2021

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 201422, “Successful Mitigation of Fracture Hits in High-Pressure Stimulation Using Bottomhole Gauges and an Optimized Engineering Design, Offshore Black Sea,” by Gabrijel Grubac, SPE, Radu Patrascu, and Peter Janiczek, SPE, OMV, et al., prepared for the 2020 SPE Annual Technical Conference and Exhibition, originally scheduled to be held in Denver, Colorado, 5–7 October. The paper has not been peer reviewed.
This paper presents a case study of fracture interaction mitigation in a multistage horizontal stimulation of an offshore Black Sea well. The authors discuss a multifaceted approach in applying lessons learned and pre-job geomechanical analysis of depletion-induced stress differential and its effects on fracture interactions. Intrastage fracture interference presents unique challenges that typically are managed on a case-by-case basis. This study aims to present critical analyses that are paramount to planning stimulation treatments in highly depleted segments and reservoirs with close-proximity wells.

Project Background
The operating company began a field redevelopment project in 2013 for a field in the Black Sea that was already producing from horizontal wells with multi-staged fractured wells. The project comprises three treatment wells (A, B, and C) and seven offset wells (1 through 7). Because of time-critical operations and related costs, the original treatment wells and sidetracks (5,000-m-long measured depth horizontals) were completed with multistage stimulation sleeves and were operated by a ball-drop system from the surface.
Infill drilling was implemented with the newly added sidetracks because of the maturity of the field and the desire to optimize the hydrocarbon drainage process. Well C was the first infill well. A multi stage fracturing campaign for Well C began in 2015.
The complete paper presents a detailed discussion of issues—including strong fracture communication—identified, and mitigation steps attempted, during the infill project. Various simulation methods and depletion models that were tried and later rejected are also discussed.
Several investigations were begun into how to predict, and subsequently avoid, fracture-driven interactions. Industry practice with huge quantities of water injection in the offset wells was not feasible because of the already-low economics of the treatment wells.
A half-iterative process between reservoir modeling and fracture modeling was begun, and an actual reservoir pressure-depletion map was created on a sector basis. Similarly, the fractures were simulated using a grid-oriented fracture simulator in full 3D. Using this approach, it was also possible to match previously identified communication between wells and highly nonsymmetric fracture growth.
The initial plan for the recent multi-stage fracturing campaign was to drill all wells closer in time to reduce localized depletion and to allow initial fractures from offset wells to act as a stress barrier to mitigate fracture growth in the direction of those wells. A matched reservoir and geomechanical model was used for the fracture simulation, and the actual depletion map for different time regimes was introduced. It was decided to redesign the fracture treatments and to shut in the offset wells 36 hours before the treatment, and to keep them shut in during the entire stimulation operation. At that point, it became clear that reservoir pressure depletion was one of the main causes of communication and also impeded optimal fracture geometries and hydrocarbon recovery.