This paper will describe the development, prototype testing, full well system test and field trial of an evolutionary slender well construction system. The system is a method of constructing oil and gas wells from close-clearance flush-jointed cemented liners from top to bottom to form the entire well. Significant technical hurdles were overcome in the development of a system that is safe and practical to deploy whilst at the same time allowing drilling and completions engineers to plan the wells using accepted oil and gas well engineering methods. The paper will describe the technical hurdles to overcome to enable this method of well construction to become accepted. This evolutionary method of constructing oil and gas wells has the potential to compete favorably with conventional techniques and other new technology methods already being deployed and could be used in a wide variety of well types. In its full well construction format of four or more close-clearance liners, the cost of constructing the well can be reduced significantly, perhaps by more than 50%, whilst at the same time enhancing safety and reducing environmental impact. In its partial format of one to three close-clearance liners during a sidetrack or well extension, the available flow area across the zone of interest can be increased by 100% as a result of an increased tubular size deployed at total depth compared with convention. Figure 1 shows a comparison of the system to conventional techniques for full wells and Figure 2 shows the flow area increase of the system in its partial format. Introduction Slimming down the oil and gas well geometry specifically to reduce the cost of well construction has been an economic goal of operators for many years. The challenge faced has been to achieve the optimum size of pipe across the zone of interest whilst at the same time slimming down the rest of the well geometry, but still maintaining the desired size and configuration of the completion. Previous attempts to slim down wells (refered to as slim hole drilling) was predicated on the well design introducing smaller hole and casing sizes whilst maintaining industry standard clearances between casing or liner strings. This often resulted in small diameter holes across the reservoir and a subsequent compromise in well delivery characteristics. Operators have evaluated the application of slim hole to projects and often concluded that the economic or technical benefits gained do not stack up against the detailed additional work involved or that well productivity/functionality is significantly compromised. The result is to revert to convention. In the development of a slender well construction system it was therefore considered essential that the system enabled greater flexibility in the well architecture options whilst maintaining optimum pipe sizes at the zone of interest. The significant difference between a slim hole design and a slender well design is reduction in the annular clearance between consecutive cased sections. Significant reduction in the telescoping geometry of the well means that a wider variety of well architecture options exist. The new slender well system will allow annular clearances between consecutive cased liners to be as small as 1/8″ radially in the lower reaches of the well and as much as 1/4″ in the upper reaches. This annular clearance can be planned within the tolerances already allowable with API casing and the radial clearance will be a minimum radial clearance within these tolerances. Figure 3 shows a chart which checks the radial clearance of a slender well example based on the new system. The benefits of reducing the casing sizes from top to bottom utilising the close-clearance liner system while maintaining the optimum size across the zone of interest are significant.