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|Title||An Effective Fluid Formulation to Remove Drilling Fluid Mud Cake in Horizontal and Multi-lateral Wells|
|Authors||H.A. Nasr-El-Din, M.B. Al-Otaibi, A.A. Al-Qahtani, Saudi Aramco; M. Samuel, Schlumberger|
IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition, 13-15 September 2004, Kuala Lumpur, Malaysia
|Copyright||2004. IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition|
Cleanup of drilling fluid filter-cake in long horizontal and multilateral wells is a difficult task. Both mechanical (water jetting) and chemical means (acids, oxidizers, and enzymes) have been used in the field. However, these methods have limitations, which can adversely affect well performance. Water jetting introduces large amounts of water into the formation, and can cause loss of well productivity through sanding and water blockage. Acids and oxidizers are very reactive and non-specific species. Enzymes can be used to degrade starch and xathan polymers. However, they are not effective in heterogeneous formations where there are high permeability streaks, which will require using large volumes of the treatment fluids, but with poor performance.
This paper examines using a combination of enzymes and a new class of viscoelastic surfactants. In this system, enzymes are used to degrade starch, whereas the visocelastic surfactant is used to increase the solution viscosity.
Extensive lab studies were conducted to examine compatibility of visocelastic surfactants with enzymes. The apparent viscosity of the enzyme/viscoleastic surfactant solutions was measured as a function of shear rate (57 to 1,740 s −1) and temperature (77 to 212°F). Surface tension was measured at various temperatures up to 284°F. Static (a HPHT fluid loss cell) tests were performed to assess the effectiveness of the combined system in cleaning filter cake formed by water-based drilling mud.
Experimental results indicated that the viscoelastic surfactant is compatible with the enzyme system. The low shear viscosity of the combined system is high enough to lift suspended solution during well flowback following the treatment. The addition of the viscoelastic surfactant to the enzyme solutions significantly reduced the surface tension of the enzyme solutions. This will reduce the time required to lift treatment fluids from the formation. The efficiency of the enzymes in degrading the polymers did not significantly change due to the presence of the surfactant.
Colloidal starch and its derivatives are generally used in water-based reservoir drilling fluids. This is due to the ability of starch to rapidly reduce the mud cake permeability, thereby reducing the invasion of the filtrate containing damaging water-soluble polymers (such as xanthan gum or scleroglucan), bridging agents and drilled solids into the reservoir. However, starch and other filter-cake components can cause significant impairment of well productivity. Formation damage treatments need to be performed to reestablish the native permeability of the near wellbore area.
Typical constituents of water-based drilling fluids are sized calcium carbonate, XC-polymer, starch, biocide, potassium chloride, potassium hydroxide, sodium sulfite, defoamer, and lubricant. The drilling mud filter cake can be removed by acids, oxidizers or enzymes. Lab studies, field applications, advantages and disadvantages of these systems were discussed in previous publications.1-14
An acid wash can dissolve most of the materials present in mud cake. Fast reaction of the acid with calcium carbonate results in the penetration the filter cake where the acid first contacted and can result in uneven removal of the mud cake. Some minerals present in sandstone formations (e.g., illites) are sensitive to acids. Acid leak-off into the formation can cause fines migration and formation damage.