The oil-in-water emulsion drilling fluids, prepared by adding 5 12% mineral oil (or diesel) to water-based muds, have been widely used for stuck-pipe prevention in Shengli Oilfield. In some cases, the emulsion stability of this kind of mud systems is not strong enough to meet the requirements of drilling operations. To overcome this drawback, a solid emulsifier which is characterized by its very small particle size and the special wetting behavior (slightly water-wet), has been developed and successfully applied to improve the emulsion stability and other performance of these drilling fluids.
Prior to the development of this technology, an extensive study dealing with the influence of various kinds of finely divided insoluble solid particles on the emulsification of oil and water was carried out. The substances used include bentonite, organophilic clay, kaolinite, barite, two kinds of calcium carbonate with different particle size, two kinds of silica with different wetting behavior, and the new solid emulsifying agent. Both emulsion stability experiments and drop coalescence experiments were performed to evaluate the contribution of these particles to emulsification. The measured final emulsion volume left after 12 hours and the half-life for water and oil drops were used as the measures of emulsion stability. The experimental data show that the type, size, concentration and wettability of the particles, and the presence of some surfactants control the type and stability of emulsions for a given oil/water system, and the fine particles of the solid emulsifier provide the most effective stabilization to water-continuous emulsions compared with other kinds of particles. Meanwhile, it can be observed that these particles also stabilize oil-continuous emulsions effectively in some cases since their wetting behavior is close to neutral.
The newly developed oil-in-water emulsion mud stabilized by the solid emulsifier has become one of the major mud systems used in directional and horizontal well drilling in this oilfield. In addition to its improvement on emulsion stability, this solid emulsifier has proven to enhance wellbore lubrication and to be of benefit to the control of rheological parameters and filtration. The field applications of the emulsifier, and the typical formulation and properties of the mud system are reported in this paper.
It has been well known that the fine solid particles existing in oil/water interfaces can stabilize emulsions. Many investigators presented the results on the effect of various finely divided solids, and the combinations of these solids and surfactants, on the stability of water/oil emulsions. Some of them discussed the mechanism by which particles stabilize emulsions. For example, Gelot et al. observed that water-wet particles such as Ca-bentonite tended to stabilize oil-in-water emulsions, while oil-wet particles such as carbon black tended to stabilize water-in-oil emulsions. Tadros and Vincent concluded that particles will tend to remain at the interface when the three-phase contact angle equals to 90 and play a role in preventing drop-drop coalescence in emulsions. Tambe and Sharma presented the experimental data to show the influence of various colloidal particles, such as calcium carbonate, barium sulfate, bentonite and carbon graphite on the stability of oil/water emulsions and claimed that these solids stabilized emulsions both by providing steric hindrance to the coalescence of oil and water droplets and by modifying the rheological properties of the interfacial region. They also thoroughly summarized the results of previous investigations.
Most results described above were derived from fundamental work and did not involve with practical applications. Unlike the previous studies, the main objective of this study is to develop a solid emulsifier to improve the performance of drilling fluids.