Oil-continuous emulsions are often termed slop muds.
Oil-based drilling fluids can become contaminated with significant quantities of water as a result of low efficiency well bore displacements to water or brine and from operations such as rig and pit cleaning. The presence of excess emulsifiers and oil-wet solids in typical oil-based drilling fluids allows large quantities of water to be emulsified. Oil-continuous emulsions are often termed slop muds.
Slop muds can be generated during the drilling process itself if significant quantities of formation water combine with the drilling fluid. Rig and boat cleaning operations can also generate slops where storage tanks, shale shakers and the rig floor are hosed with water (and possibly surfactant) between operations and the oil-based drilling fluid, sediments and wash water mix together to form a slop emulsion. An average of 500 bbl of slop is produced on a daily basis during normal drilling activities.
Oil-based muds are generally invert-emulsion systems, consisting of a continuous hydrocarbon phase and an emulsified aqueous phase. Fluids also typically contain emulsifier systems, weighting agents, fluid-loss additives, viscosity regulators, and other chemicals as needed for stabilizing the system and to establish the desired performance properties. Typically, the oil/water ratio (OWR) of a drilling fluid is in the range 60:40 to 90:10. After contamination however, the slop mud may contain 50 to 90 vol% water and 10 to 50% by vol of the original drilling fluid.2 The effect of this contamination is a lowering of the OWR, an increase in viscosity and a decrease in emulsion stability which ultimately renders the fluid unusable.
Batch treatment processes were previously developed as a method to treat the slop and have been utilized in large landscale operations and as smaller offshore systems.2 In such systems, the slop mud is pumped into stirred treatment tanks to which demulsifying chemicals are added causing the water droplets to coalesce. When sufficiently mixed, the slop mud is allowed to separate under gravity for 8 to 24 hours during which time a lighter, water-continuous layer, migrates to the surface.
The denser oil-continuous layer separates to the bottom, containing almost all of the solids and weighting agents of the original mud. Interface detection with pumps and/or gravity drains allows the separation of the upper water phase and the lower oil-continuous phase. The aqueous phase may be clarified either by addition of flocculants (bentonite clay, iron or aluminum salts, and polymers), the use of oil-water separators, filters and other technologies to remove the residual hydrocarbon before discharge to sea or sewer. The recovered mud typically has an OWR between 60:40 and 70:30 which is suitable for reconditioning to acceptable properties or blending with new mud stock.
The batch treatment system met an immediate market requirement. There are however, identifiable issues with the batch process. The demulsifying surfactant can be difficult to disperse in the treatment tanks due to high slop mud viscosity. Further, water droplets that have coalesced may be required to travel up to a meter through the viscous oil-continuous phase to reach the interface between the two layers. Consequently, the separation process takes considerable time. In addition, approximately half an hour is required to empty and fill the tanks. If the separation period required is 8 hours, for example, approximately 10% of the process time per batch is used for filling and emptying.
Faster and more efficient processes for separating excess water from slop mud were desired to reduce the capital costs and footprint of the process. To achieve this, an increased understanding of the process was required and laboratory research was initiated to define slop mud properties and structure, along with an investigation of fluid shear rates and quantification of the critical factors controlling phase separation and chemical dose.
From these studies, a continuous treatment process was developed on the laboratory scale and then as a full-scale equipment offering.
Slop Mud Properties and Structure
The presence of excess emulsifiers and oil-wet solids in a typical OBM makes it very easy to emulsify large quantities of excess water. The resulting emulsions remain oil-continuous despite containing much more water than oil. The added water and OBM brine phase become intimately mixed within the slop to form a high internal phase emulsion (HIPE), wherein the dispersed phase volume fraction is greater than the maximum packing fraction of monodisperse spheres.
Unlike more typical HIPEs, slop muds also contain a large proportion of solids which, depending upon their size may either increase or decrease the effective dispersed phase volume. Solids-stabilized emulsions form a class of their own known as Pickering emulsions. Solid particles with an intermediate contact angle (i.e., those that are not perfectly wetted by either phase) are more energetically stable at the interface than in either of the two phases of the emulsion, and so sterically stabilize the emulsion droplets.
Particles that are predominantly oil-wet can stabilize an invert emulsion. To perform this role the particles must pack densely at the interface and thus must be significantly smaller than the emulsion droplets, at least by an order of magnitude. OBM slops contain a wide variety of oil-wet solid particles, some of which may be small enough to occupy space between the water droplets and may help stabilize the emulsion. However, standard barite weighting agent is likely to be too coarse to have a significant stabilizing effect.
The Effect of Shear History
A significant amount of shear and mixing energy is required to emulsify contaminant water into an invert drilling fluid to produce slop. The shear history therefore may determine the slop structure and its behavior, particularly the dose of demulsifier required to break it. Slop muds containing 75 vol% added water and 25 vol% OBM were prepared in the laboratory under low and high shear conditions. Batches of 500ml were mixed at 6000rpm for several minutes using the Silverson L4R mixer fitted with a high-shear square-holed screen. A Kenwood Chef Planetary Mixer was used as a low shear mixing device to prepare slop muds of the same composition. After mixing, the viscosity of the slop was measured using a Fann 35 viscometer as per API RP 13B-2 procedures.3 Demulsifier was added in concentrations of 0.1 to 1.5 vol% and the water recovery under static conditions measured to determine the effect of shear history on water separation.
Factors Controlling Chemical Dose
The critical dose of demulsifier required to separate water from the slop mud may depend on competitive adsorption, transfer of emulsifier (from the OBM) or demulsifier (added to the slop for phase separation) to the water or oil phase and possibly a combination of all of these. If competitive adsorption at the interface is the main operating mechanism, the critical dose required will rise with water content. However, the presence of emulsifiers in the oil could potentially cause the solubilization and transfer of demulsifier from the water to the oil phase and vice-versa. If this occurs a reduced concentration of demulsifier in the water phase will result.
A matrix of separation experiments was conducted in the laboratory on slop muds prepared with varying water content and variable emulsifier content. A standard OBM was sheared using a Silverson L4R laboratory mixer with water at doses of 33 to 75 vol%. Samples of slop (40 cm3) were then shaken vigorously with varying doses of demulsifier in 50-cm3 centrifuge tubes. These were then left standing for 1 hour, and the volumes of oil and separated water phases noted. After standing overnight, the phase volumes were again noted and the samples were then centrifuged at approx 1,000 g for 30 minutes. Water recoveries were determined as a percentage of the total water in the slop. The clear-water phase was extracted for Attenuated Total Reflection Fourier Transform Infra-Red (ATR FTIR) analysis of the demulsifier content.