Abstract

Lost circulation costs operators millions of dollars annually. Generally speaking, the drilling industry handles lost circulation much as it did 40 years ago. A significant effort has taken place to understand the mechanics behind lost circulation, what new tools can be developed to help locate the thief zone and what new steps can be taken to minimize or eliminate this problem.

Controlling circulation loss during well construction is more than just selecting the proper type of lost circulation material (LCM). A fully engineered approach is recommended. This approach incorporates borehole stability analysis, equivalent circulating density (ECD) modeling, leak-off flow-path geometry considerations, drilling fluid and LCM material selection to help minimize effects on ECD, on-site monitoring using pressure while drilling (PWD), connection flow monitoring techniques, and timely application of LCM and treatments. In many cases, the success or failure of LCM will be largely dependent on the decisions taken and methods employed when using the LCM.

A major service company has engineered solutions designed to reduce drilling non-productive time (NPT) due to lost circulation. A multi-disciplinary menu of services and tools is available to support the planning and application stages. Specialty software is used during the planning stage leading to the application of unique products and systems during the delivery stage. A feasibility study by technology specialists may include a wide range of tools during the planning stage to support data acquisition, modeling and design of the application stage.

Planning services include hydraulics and fracture characterization modeling to assess expected ECD and their effect on potential lost circulation intervals. The fracture characterization modeling for borehole stress treatments uses a module added to a hydraulics modeling software package that estimates both the expected fracture width and the particle size distribution (PSD) required to plug the induced fracture and prevent propagation. Application services may include delivery of specialized lost circulation materials and systems in either a preventive (pretreatment and/or borehole stress treatments to strengthen the wellbore) or corrective mode (mitigating lost circulation and, where possible, providing additional wellbore strength).

Bridging Particle Size Distribution and Fracture Modeling

The modeling software and materials can be applied in support of wellbore pressure containment and techniques to more accurately predict and optimize LCM selection, concentration and target particle size distribution. Based on pore size, or estimated fracture widths, the model can utilize the proper types and sizes of materials to plug the pores and/or an initiated fracture. For pore bridging in the reservoir these materials are selected from a full range of ground marble products with d50's ranging from 5 to 150 microns. For borehole stress treatments, these materials generally are selected from a full range of specialized resilient graphitic carbon and ground marble products, with d50's ranging between 5 and 1200 microns.

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