Abstract
An impermeable layer “filter cake” usually forms during the overbalanced drilling technique. Even though it helps in protecting the formation from a further invasion of drilling fluids, the removal of this layer is essential for a proper cement job and to avoid any reduction in wellbore deliverability. The design of the removal process is complicated and depends on the filter cake composition and homogeneity. This paper presents an experimental evaluation on the usage of a novel cake washer (NCW) in the removal of a filter cake formed by an invert emulsion oil-based drilling fluid that contains calcium carbonate as a weighting material while drilling a horizontal reservoir. The proposed NCW is a mixture of organic acid, mutual solvent and nonionic surfactant. It is designed to enable restored wellbore permeability for a sustainable production. Since the filter cake mainly consists of the weighting material, the solubility of calcium carbonate in NCW at different ranges of temperature, duration and concentration was investigated. An actual casing joint was used to test the corrosion possibility of the treating solution. High-pressure and high-temperature (HPHT) filtration tests on ceramic discs and Berea sandstone core samples were conducted to measure the efficiency of the filter cake removal and the retained permeability. Ethylene glycol mono butyl ether (EGMBE) was used as a mutual solvent and the solubility was higher compared to when the mutual solvent was not used in the washer formulation. A significant increase in calcium carbonate dissolution with time was observed for a duration of 24 h. The solubility was found to be proportional to the concentration of NCW with optimum results of 99% removal at a temperature of around 212 °F. At those conditions, no major corrosion problems were detected. Permeability of the core retained its pristine value after the treatment.
Highlights
The drilling fluids have many operational objectives, such as drillstring and drill bit lubrication [1], cuttings lifting and suspension [2], the prevention of formation fluids invasion, and the creation of a thin filter cake that protects formations from being contaminated by the drilling fluid additives [3,4,5].Due to a pressure difference between the formation and the wellbore during overbalanced drilling, fluid particles tend to invade the near-wellbore vicinity which could result in the rock plugging, and permeability reduction [6]
These weighting additives have a considerable effect on the created filter cake structure as they composed 70 to 80 wt% of the formed filter cake [13,14,15,16]
The objective of this study is to develop a new cake washer that can be used at a wide range of temperatures to remove the oil-based filter cake in horizontal wells with high efficiency and a low corrosion rate
Summary
Due to a pressure difference between the formation and the wellbore during overbalanced drilling, fluid particles tend to invade the near-wellbore vicinity which could result in the rock plugging, and permeability reduction [6]. This invasion is significantly reduced when a thin impermeable layer called a filter cake is built at the surfaces of the wellbore [7]. Various chemical solutions (oxidizers, enzymes, acids and acid precursor) are available as solvents for the calcium carbonate contained in the filter cake [22,23,24]
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