RIASSUNTO
Abstract
Efficient mud removal, filter cake removal and formation clean-up are critically required for well cementing, production and injection wells drilled with non-aqueous mud. Conventional surfactant clean-up system requires a turbulent flow and large volume to solubilize residue efficiently. Unsuitable surfactant system may cause emulsion blockage and oil-contamination in the completion brine, which consequently leads to a reduction in oil production rate. A new potential method to overcome these problems with higher efficiency to remove non-aqueous mud and filter cake is NanoEmulsion technology. This technology able to reduce the oil-water interfacial tension (IFT) down to < 0.001 mN/m, solubilize much larger amount of oily hydrocarbons without any mechanical stirring needed. This paper elucidates the potential of NanoEmulsion technology in removing non-aqueous based mud and filter cake during wellbore cleaning through laboratory scale set-up at ambient and HTHP conditions. The impact of temperature, pressure, mono- and divalent electrolytes, bridging agent such as calcium carbonate, acidity and polymers (Xanthan Gum and HEC) have been studied. It was found that the NanoEmulsion is effective in removing non-aqueous mud in all studied conditions at tested duration of < 10 minutes. The cleaning mechanism proposed are: (1) instantaneously reducing interfacial tension between oil-water and oil-solid surfaces, (2) emulsify and solubilized oil content, (3) facilitate wettability and allowing water molecules to penetrate in-between oil and solids surfaces , causing the oil to spontaneously repel from the hydrophilic solid surfaces, and (4) separation of oil from water and solids. This enables the recycle of the oil content which is free from solids. NanoEmulsion technology can be incorporate with acid to simultaneously dissolve calcium carbonate bridging agent and solubilize the oil in the filter cake. In addition, it is free from benzene, toluene and xylene (BTX), synthesis from bio-degradable natural sources, non-toxic, easy to handle, and 100% biodegradable.
Introduction
Wellbore Cleaning
In the search for oil and gas, boreholes are drilled deep into the earth. These holes are lined with casing, which are usually heavy steel pipes, and cement is forced down the casing and up the annulus between the outer wall of the casing and the borehole wall. The cement is forced to the bottom of the casing with a displacement fluid, typically drilling mud. Normally, a variety of drilling mud can be used for drilling. The drilling muds are generally solid containing water-based or non-aqueous based muds and can be weighted with weighing material such as barite or non-weighted. The main functions of drilling fluids include providing hydrostatic pressure to prevent formation fluids from entering into the well bore, keeping the drill bit cool, clean during drilling, carrying out drill cuttings and suspending the drill cuttings. The drilling fluids used for a particular job is selected to avoid formation damage and to limit corrosion. Beside viscosifiers that are added to improve the rheology, bridging agents, such as calcium carbonate and ground cellulose, are added to enhance the filter cake formation to prevent the invasion of fluids to the formation.
Filter cake formation has to be well control in order to prevent some catastrophic events. If the thickness of filter cake is too thin, there will be in-balance pressure across the filter cakes and excessive fluids exchange may occur across the wall of the wellbore. If the thickness is too much, then it may lead to tight hole conditions, poor log quality, stuck pipe, lost circulation and formation damage. These issues consequently induce in a major problem of dramatic reduction in the oil production rate and wellbore stability.