Oil base muds (OBM) and their formulation can have a pronounced impact on cement performance in wellbores. This occurs when cement and OBM become mixed during normal well cementing operations. Increased mud contamination can ultimately compromise cement integrity. To minimize these mixing effects, the ability to improve compatibility of the mud system becomes paramount to completion success. The OBM base oil and the type and concentration of emulsifier can be especially crucial.
The most common type of OBM primary emulsifier is a fatty acid which reacts with calcium hydroxide to form a calcium soap. A partially water soluble emulsifier (an alkanolamide) is used as a secondary emulsifier in most OBMs because it keeps the emulsion stable even if the calcium source is depleted, but it is rarely used as a primary emulsifier. When employed as a primary emulsifier, the alkanolamide has shown the ability to reduce the effects of OBM contamination on cement. In a correlative work similar benefits have also been realized by mixing a water soluble surfactant with cement.
Proper design and management of an OBM system provides better cement performance. Although using an alkanolamide as the primary emulsifier costs slightly more than using a fatty acid, reducing the impact of OBM/cement contamination can more than offset the increased cost.
OBM is used in drilling critical or difficult wells. OBM provides certain advantages such as borehole stability, temperature stability, resistance to contamination, lubricity, and superior penetration rates in certain formations.
Oil muds are a water-in-oil emulsion consisting of three phases. The internal phase exists as water droplets emulsified within the external oil phase. This emulsion, sometimes called an "invert emulsion", is maintained by surfactants or emulsifiers to remain stable under downhole conditions. The third phase consists of drilled solids and commercial additives used to maintain required drilling properties. The solids phase is usually water wet, but the addition of surfactants which promote oil wetting help keep the solids adequately oil wet and in the external phase.
Although the emulsion provides some properties of viscosity, suspension, and filtrate control, most drilling situations require augmenting these properties.