Improving Cementing Practices for Top Hole Casing Strings at the Valhall Field.

Abstract

The main goal of a primary cement job is to provide zonal isolation in the well. Obtaining zonal isolation according to regulations at the Valhall field have been challenging. The complex overburden at Valhall contains several permeable zones that need to be isolated from each other and the surface. A new set of strict internal regulations for cementing operations has lead to a change in the basic casing design. This was done in an effort to increase the chances of obtaining an approved job according to the new regulations. This has, however, proven to be challenging as the cement jobs have suffered severe problems with lowside channeling and general bonding problems.This thesis investigates the recent cementing operations at Valhall for the top hole casing strings (18 5/8" and 13 5/8"). A total of eight case histories are investigated, where important aspects of the operations are presented. An evaluation of the case histories are performed, where the effect of important factors are discussed. This includes spacer volume, displacement rate, centralization, and fluid properties. The effect of the factors are discussed both from a theoretical perspective and with the basis in the case histories and results.One of the most significant improvement was seen when displacing the drilling fluid to a low rheology mud prior to performing the cement operation. The low rheology WARP OBM from M-I SWACO was found to give the best results. This is also consistent with the theory, as the WARP OBM had the lowest rheological properties of all the investigated drilling fluids.A big potential improvement was found to be the increase in displacement rate up towards turbulent flow for the spacer. This is not realistic to be achieved using todays cementing technique. Managed Pressure Drilling (MPD) equipment during cementing was found to reduce the Equivalent Circulating Density (ECD) by 0,9 ppg equivalent mud weight at target depth. This reduction in pressure can be used for a significant increase in displacement rate, possibly up towards turbulent flow.