A Successful Field Experience: Shallow Water Flow—How to Avoid the Need for an Additional Casing String

Abstract

In areas with shallow water flow, additional casing strings are normally set to seal off the water-bearing zone. While drilling the first well in Fram East in the North Sea, the operator used a shallow 20-in. surface casing, but this measure increased well costs and caused a number of later limitations in the well construction process. To avoid these complications in other Fram East wells, the operator searched for alternative solutions.It selected a cement design with properties compensating for the loss of hydrostatic pressure and thereby preventing the water flowing. The service company carried out extensive designing and testing of the cement formulations. A major challenge was qualifying the final design for a full-scale field test. A third-party laboratory qualified the cement system, using a modified setup of the gas migration equipment. The results were promising.A dedicated cutting injection well was chosen for the field test. The successful test allowed planning the remaining wells using the new system with projected savings of more than USD 20 million. This paper will describe the development of the solution, the design, and testing of the cement formulation, and the full-scale field test performed. IntroductionThe main objective of the cement is to provide a hydraulic seal across the various permeable formations; zonal isolation is compromised when formation fluids such as water are allowed to enter the annulus. During and after the placement, the cement column and the other fluids in the wellbore or annulus exert a hydrostatic pressure that initially must be greater than the formation pore pressure to prevent invasion of formation fluids. However, as the cement hydrates, the cement slurry becomes self-supporting and the hydrostatic pressure exerted by the slurry decreases. When the hydrostatic pressure decreases below the formation pore pressure, formation fluids such as water can enter the annulus, potentially leading to water flow through the cement matrix, which leads to the loss of hydraulic isolation.

Cementación de revestidor en flujo de gas/agua presurizado superficial no esperado en un campo de desarrollo costa afuera: Rio de Janeiro, Brasil

“…El diseño del sistema cementante fue basado en: sistema estabilizado, baja pérdida de fluido, baja permeabilidad (con la utilización de microsílica alrededor de 0,15 micrón), tiempo de tránsito para desarrollar el esfuerzo de gel estático (cemento clase c, con alto contenido de calcio) y minimizando la reducción de volumen. Las causas de un flujo después de un trabajo de cementación pueden ser que la presión hidrostática en el anular disminuye con respecto a la presión de formación por deshidratación, agua libre o canales o durante la gelificación del sistema cementante [11].…”