A Review of Zechstein Drilling Issues
Authors: Williamson M.D. , Murray S.J. , Hamilton T.A.P. , Copland M.A. 
Published: December 2007

A number of issues can arise when drilling through Zechstein formations that have a bearing on the health and safety of personnel, as well as on the time to complete operations and final well costs. Operational problems present some intrinsic hazard or safety risk. Additionally, the procedures applied to alleviate these problems also present a safety risk, as they are usually non-routine and unfamiliar, thus increasing the likelihood of errors.

In this paper, the operational issues, their causative mechanisms and alleviation procedures are introduced and categorised. Decision analysis is applied to each of the operational issues, both for the well planning stage and for the drill site, which relates the well responses to the causative mechanism and, therefore, to the selection of the most appropriate alleviation procedure.

Introduction

The Zechstein Supergroup is a sequence of formations found, offshore, beneath the North Sea and, onshore, beneath many coastal European countries, notably the United Kingdom, the Netherlands, Denmark, Germany and Poland (Fig. 1).

The Zechstein makes up the Upper Permian and is estimated to have been laid down during the late Palaeozoic period about 250 million years ago. During this period, what is now the Zechstein basin was a large coastal basin. Each sequence (Z1 - Z5) corresponds to a change in sea level which allowed a major influx of seawater into this basin. The precipitation of salts during the evaporation of these influxes resulted in the deposition of a sequence within the group. Evaporation caused the least soluble salts to precipitate first. The order of precipitation, and therefore deposition, is shown in Fig. 2. Whilst this is a useful framework, there can be many omissions and reversals of stages in these sequences.

Carbonates (calcite and dolomite) are the least soluble and, therefore, were the first to precipitate for any cycle. Dolomite forms the reservoir rock for the limited number of fields within the Zechstein because of its porosity and permeability. These properties also mean that dolomite sections can be the origin of kicks and losses within the Zechstein.

Sulphates (anhydrite and gypsum) overlie the carbonates. They have poor porosity and permeability, and thus, have efficient sealing properties. The anhydrite acts as a cement that not only seals the top of the dolomite but seeps down into the pores reducing permeability and porosity, and compartmentalising the hydrocarbons into commercially nonviable deposits. This explains why there are so few producing reservoirs within the Zechstein. The gas fields of the Southern North Sea (Rotliegends sandstone - Lower Permian), however, owe their existence to these sealing properties. These fields are the targets of many wells that are drilled through Zechstein.

Rock salts (halite) and Squeezing salts (carnalite, sylvite, kieserite, and bischofite) overlie the sulphates. These salts have two physical properties which are of consequence to drilling. Firstly, they have a high solubility in water, for example, bischofite is about ten times more soluble in hot water than is halite (NaCl, common table salt). Washouts beyond the range of calliper logs (22") have been reported after using water based muds (WBM). Secondly, their response to differential overburden stress can resemble that of a highly viscous fluid. The rate at which they will move is dependent on the depth of burial, formation temperature, mineralogical composition, water content, and the presence of impurities such as clay. Rates of 1" per hour have been reported, however, these can be highly variable. The minerals containing water (e.g. bischofite and carnalite) are the most mobile, and anhydrite and carbonates are essentially immobile.

Behavioural Mechanisms

The characteristics discussed above result in Zechstein formations being able to exhibit a number of behavioural mechanisms. P. 189^