Abstract For many years, various types of simple nozzles have been regularly used for wellbore cleaning with coiled tubing. This paper describes the advances made in fluid jetting technology, associated tools and the parameters that are crucial to the efficiency of these techniques. A laboratory and field testing programme has been completed and data is included. By detailed design of the internal fluid path, orifice geometry, jet stand off and pump rates, tool effectiveness has now been improved to a level several times more efficient than previously available for the same feed pressures and flow rates. Pre-engineering using computer simulation to optimise the job design parameters has proven essential in designing clean out jobs and sizing the appropriate coiled tubing and pumping equipment. Three distinct categories of tools have emerged; conventional drilled nozzles, non-rotating high-pressure nozzles and high pressure rotating tools. All three types have gained a common advantage from the development of a Downhole Phase Separator. The high-energy losses previously associated with at wo-phase fluid necessary in low hydrostatic wells can now be eliminated. A step change in operational efficiency has resulted through combining the major improvements in jetting tool design with the ability to separate gas and liquid phases downhole. With this development, the full benefits of using single phasefluid jetting can be realised in low hydrostatic wells. Introduction A number of techniques exist for the removal of downhole deposits. These range from chemical soaking to aggressive mechanical techniques such as broaching and milling. Coiled tubing (CT) jetting offers a number of advantages, including simplicity, the ability to clean varying diameters, speed and cost. Under field conditions, barium sulphate, in its weaker forms is the current proven limit of the technology. The principle mechanisms utilised for cleaning; either in isolation or combination is as follows: * erosion, as the fluid strikes the deposit * stress cycling, as pressure is applied and removed * cavitation, when localised liquid vapourises to create a pocket, the pocket collapses and high pressure liquid rushes in The various tools available have been designed to take advantage of all these mechanisms to a greater or lesser degree with the exception of cavitation. Cavitation is an extremely effective mechanism in surface jetting, but is virtually non-existent downhole due to the relatively high ambient pressures. Cavitation effects tend to disappear above 300 psi.. Simple surface jetting nozzles are used where performance is non-critical and tend to have a conical tapered entry profile. Despite the totally different operating conditions, conventional CT jetting nozzles share the same basic design. The number of holes and their orientation was generally based on previous experience. Cleaning relies almost entirely on erosion as the accelerated fluid strikes the deposit. Although these basic nozzles continue to provide an economic means of removing produced sand or drilling solids their efficiency is very limited and the scope for improvement plainly evident. Addressing this need, resources were initially directed towards high-powered rotating jetting tools, which offer complete wellbore coverage. Some of these have proven to be extremely effective, although design and manufacturing costs, combined with their requirement for regular maintenance, mean they are relatively expensive. This left a gap between the simple, inexpensive swirlnozzles and the new generation of rotating tools. The non rotating, highpressure vortex nozzle was designed to satisfy the requirement for a simple, inexpensive but highly effective method of removing deposits, thus providing three distinct categories of tools, each with specific advantages/disadvantages dependant on wellbore parameters, deposit composition and economics.