Cementing of an Offshore Disposal Well Using a Novel Sealant That Withstands Pressure and Temperature Cycles

Abstract

A dedicated injector well, located in the Grane field in the Norwegian North Sea, was chosen as a candidate for application of a novel new sealant. The injector well, drilled for disposal of cuttings and produced water, was selected because of expected temperature and pressure cycling during the injection operation, and the risk of cement sheath integrity failure. This paper will discuss the design, execution, and evaluation of this injector well. Stress modeling indicated that as wellbore temperature decreased from bottomhole static to injection temperature and as bottomhole pressure cycled between static and dynamic conditions, conventional cement would fail in tension and create a microannulus. In view of these challenges, a redesigned the cement program, which incorporated stress analysis calculations and mechanical-properties testing provided a sealant material that would resist failure due to excessive wellbore stresses. The redesign required three steps;. Firstly, it was necessary to calculate temperature and pressure variations expected in the wellbore during injection. Next, temperature and pressure variations in the wellbore were evaluated using numerical analysis. Finally, a sealant was engineered so that its integrity would be maintained when exposed to repeated temperature and pressure cycles. The chosen sealant material incorporated flexible and expanding materials within an optimized particle size distribution blend. The sealant's mechanical properties were engineered in line with expected stresses to minimize the Young's modulus and improve bonding properties. The cuttings-disposal injector well was drilled, cased, and cemented using the new sealant material. Since placement, the well has been logged with sonic and ultrasonic tools and these data indicated an excellent bonding response. The well has now been used as an injector for more than two years with no indication of fluid communication in the annulus or loss of cement sheath integrity. Significantly, since this first application, which incorporated the successful of modeling of the expected stresses and the engineering of a sealant to manage these stresses, further injector wells have been cemented utilizing the same engineering approach. Introduction and History of Grane Grane field was discovered in 1991, located in 127 m water depth and approximately 100 km off the coast of Norway (Fig. 1). A template with 40 slots, of which 27 were allocated as oil producer wells, was installed on the seabed. 12 of the wells were pre-drilled on the template with a semi-submersible rig. The Grane platform was then installed in 2002 and the pre-drilled wells were tied back and completed prior to continued drilling from the platform (Fig. 2). With some of the strictest emission and environmental controls in the world in place in the Norwegian sector of the North Sea, the need to find new and alternative disposal methods led the operator to dedicate specific wells for the injection and re-injection duties (referred to only as "injection" for the remaining sections of the paper). Owing to the zero-discharge-to-the-sea policy, the operator dedicated one of the predrilled wells as a disposal well. The Grane injection well was designed with the following operational limitations:An average cuttings re-injection rate of 100 m3/d [630 bbls/d]An operating wellhead injection pressure of 6.0–8.0 MPa [870–1160 psi]

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