Wellbore instability poses a significant problem for many wells drilled on the Northwest Shelf of Australia. With the discovery of the Wanaea and Cossack oilfields, research work has focused on an investigation involving the measurement of shale properties, analysis of operations and well logs and the application of this information to the analysis of wellbore stress and instability.
The drilling of wells in the Dampier Sub-basin has frequently experienced difficulties which are associated with wellbore instability. The Wanaea and Cossack oilfields have been discovered along the Madeleine Trend of this Sub-basin (see Figure 1). Tight hole conditions have required excessive and repeated reaming and caliper logs have shown large breakouts, particularly in the Cretaceous shale sections. Mud weights at or near the fracture gradient have been required to stabilise Cretaceous shale sections in vertical wells. Development options for these fields include directional wells which require detailed knowledge of wellbore stability to plan and predict possible well trajectories.
The various analytical tools which provide insight into well bore stability and which are needed to help directional well, mud and casing designs, require knowledge of the natural in-situ stresses and the response of the shales to the stress changes imposed by drilling. This paper draws on the data and experience obtained from wells in the Cossack and Wanaea fields and on laboratory measurements on shale cores, to develop an understanding of the natural in-situ stress field and the mechanical properties of the shales encountered.
The four distinct pressure regimes encountered in the Wanaea and Cossack fields are shown in Figure 2 with the general lithology and casing programs. Only the first three pressure regimes are of interest for the development of the Wanaea and Cossack fields completed in the normally pressured Tithonian sandstones. Surface casing has been designed to set in the Tertiary claystones to isolate carbonates where loss of returns commonly occurs if mud weights are allowed to rise above 1.10 SG. This setting depth was later deepened to near the base of the Tertiary claystones (â??2000m) to improve fracture gradients at the shoe as higher mud weights in the intermediate hole section were found to be necessary for hole stability in the Cretaceous claystones. The intermediate casing depth was first selected as close to the top of the reservoir as possible to isolate any possible shale stability problems before drilling the reservoir and to minimise mud overbalance in the reservoir. Later experience has shown that the more silty early Cretaceous formations below approximately 2600m have a much lower pore pressure gradient and lower fracture gradient and are not troublesome.