A combination of small sample testing and numerical modelling offers a powerful tool for identification of mud chemistry effects on wellbore stability in shales. The tool allows for modelling of how the stability limits for the mud-weight varies with time, based on measured time scales and magnitudes for the various processes involved. It may also assist in estimation of the optimal amounts of chemical additives in the mud.
Shale instability remains a costly drilling problem. The industry usually estimates that 5–10 % of the drilling time is lost due to borehole stability problems. Although significant scientific progress has been made over the last decade, the practical borehole stability problems have not been reduced accordingly. This is of course a result of the complexity of the problem, difficulties in establishing a proper diagnosis once the problem occurs, and lack of implementation of research results into drilling operations.
The mudweight is the main parameter to control in order to counteract borehole instability. However, field experience shows that the drilling fluid as such may also have an impact. For instance, it is found that water-based mud usually creates more problems than oil-based mud. A possible explanantion for this is that capillarity prevents the oil phase from entering low permeability shale; hence the borehole wall is effectively impermeable for an oil-based mud. This allows the well pressure to be kept higher than the pore pressure so that the mud may effectively support the borehole wall. Water may on the other hand enter the shale, thus the pore pressure in the vicinity of the well will gradually approach the well pressure. This eliminates the effective support from the mud to the borehole wall, and thus reduces stability over time.
There is also the possibility of chemical interaction between the water phase and the shale. Various salts such as KCl and CaCl2 are frequently added to the mud in order to improve borehole stability. Although positive effects of these additives are reported, their presence in the mud does not always cure the problem1.
As indicated above, and confirmed by operators in the field, borehole stability is time dependent. That is, an initially stable hole may become unstable after some time. Wellbore instability is however a problem only for a limited period (until casing has been set). Hence it is of great importance when deciding drilling fluid composition to be able to quantify the impact of shale - mud interaction on rock mechanical behaviour, and in particular on the time scale involved.
The objective of this study has been to identify and quantify relevant parameters for shale-mud interaction, and to enable quantitative estimation of the consequences of KCl and CaCl2 additives on wellbore stability. To achieve this, several shale materials have been studied in the laboratory, by specially designed tests that reveal how the materials respond mechanically to exposure of water with various salt concentrations. The observed response is compared to mathematical models for the expected behaviour, and relevant values are obtained for the model parameters characterizing the shale properties. By transforming the mathematical models to borehole geometry, expected behaviour of the shales around a borehole can be estimated. The borehole model can then be used to predict and analyse how the mud chemistry may affect borehole stability problems in shales.
Effects of fluid exposure on shales
Borehole stability problems are related to mechanical instabilities of the rock around the hole, and the problems are therefore directly related to the mechanical properties of the material, and to the effective stresses. The problems may however have non-mechancial causes.
A shale material consists of clay minerals and water with dissolved ions in a state of mechanical, chemical and electrostatic equilibrium, specific for the actual condition of the material. If the material is cut or broken, such that a free surface is exposed, this state of equilibirium is disturbed. Several processes may then occur, which affect the material's mechanical properties and the effective stresses.