ABSTRACT:

In recent years there has been a growing awareness of the importance of Depleted Zone Drilling (DZD) and the many challenges it poses. One area of interest focuses on the role that drilling fluids have on sealing drilling-induced fractures in permeable zones. This paper details the development and application of a unique high-pressure testing device designed specifically for determining the sealing characteristics of drilling fluids in permeable formations. The equipment design provides the capability for testing the fracture characteristics of a porous substrate, high pressure operation, active precision control of fracture opening size and the discrete collection of filtrate loss to the formation and to the fracture tip. In addition, an indirect measurement of the seal location can be made from in-situ pressures as a radial distance from the point of fluid injection into the fracture. A comparison between the experimental results with permeable and impermeable plates was used to determine the effect of leak-off through the fracture faces on the sealing properties of the fracture. To better understand the sealing process, independent spurt-loss tests were performed with all testing fluids. A comparison is made between aqueous and non-aqueous invert-emulsion drilling fluids and the relative effects of different lost circulation and filtration control additives to the respective systems. Scanning electron microscopy techniques were employed in the final analysis in order to determine particle size distributions present within formed seals as well as the chemical composition of seal components. From the experimental results, conclusions are drawn regarding the influence of fluid loss through the fracture walls on the sealing capacity of the tested drilling fluids under downhole pressure and temperature conditions

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