Design of Well Barriers To Combat Circulation Losses

Summary The paper summarizes a 10-year research program at the University of Stavanger in borehole fracturing and mud design. Novel fracturing cells and mud cells were built to better understand the mechanisms that lead to circulation losses. Numerous experiments were conducted using both oil- and water-based drilling fluids. The paper presents a new mechanistic model for fracturing called "the elastoplastic-barrier model." It is different from other recent models, and it is verified with laboratory experiments. In simple terms, it defines optimal barrier filtrate loss to place particles in the loss zone, and the mechanical strength of the particles required to resist losses. Selected laboratory experiments are presented demonstrating that borehole fracturing resistance can be improved significantly by changing the mud composition. While testing commercial lost-circulation-material (LCM) products, it was found that some worked well, some were poor, and some worked only in synergy with others. On the basis of these findings, the compostion of an optimal LCM pill will be presented. Nonpetroleum products also have been tested to search for improvements in mud design. One result is that calcium carbonate can be replaced with more-efficient materials. We also have shown that adding small amounts of carbon fiber has a positive effect. This research has been conducted in close cooperation with major mud companies and operators. A field case is presented from a shallow field. The mud was designed and tested during operation at the laboratory of the University of Stavanger. The result was a clear increase in fracture pressure, resulting in a successful operation.