During the drilling process with OB (oil-based) and SB (synthetic-based) mud systems, mud solids form internal and external filter cake along the wellbore through a filtration process (due to filtrate leakoff to the formation). After drilling is complete, a cleanup treatment to remove the OB/SB mud filter cake is needed to minimize skin and formation damage and increase production flow area. The cleanup treatment is necessary to break down the external water-in-oil emulsion filter cake, wash the damaged zone of the wellbore, and restore the formation fluid transfer properties.
Historically, solvent- or aqueous-based systems have been widely used to remove OB and SBM filter cakes. Solvent treatments are typically very expensive and aqueous systems may cause additional formation damage by forming emulsions and/or creating water saturation in the wellbore, thereby blocking oil production.
A successful OB/SBM filter cake treatment must break the mud's interfacial rheological properties, act as a demulsifier to break the water-in-oil emulsion, and break the filter cake's bond with the wellbore wall.
A synthetic oil/surfactant-based fluid system has been developed to cleanup OB and SB mud filter cakes. This paper discusses laboratory evaluation studies of the treatment fluid's effectiveness in breaking the emulsion inside the filter cake across a wide range of mud types to reduce cake cohesion and wellbore adherence. Studies with 10 field mud compositions from several mud suppliers have been conducted to evaluate the treatment fluid's filter cake removal efficiency. Laboratory data is presented showing the physical properties of each drilling mud's filter cake before and after treatment as well as the treatment's filter cake removal efficiency. Photos of the initial and post-treatment mud filter cakes are also provided for review. Finally, the paper will discuss the possible advantages of using a synthetic oil-based cleanup system over traditional aqueous-based treatments.