A Viewing Lens For High Pressure Applications
Authors: Appl F.C. 
Published: December 2007

This paper describes the design, development and testing of a large lucite viewing lens for use in high-pressure applications. Four of these lenses have been installed in a pressure chamber to permit visual study of the mechanics of rock failure under subsurface pressure conditions up to 15,000 psi, through the medium of high-speed photography.

Development of a lucite lens was undertaken when a commercially manufactured glass lens failed at 10,000 psi. The original lens was a 3-inch diameter cylinder formed of four laminated one-inch thick glass discs. It was pressure tested in a special cell and failure occurred when the outer lamina parted from the remainder of the lens.

Theoretical analysis indicated that large radial tensile stresses were present in the original design. Therefore, a prototype lens with a tapered end was machined from a 4-inch diameter lucite bar. A matching steel seat provided lateral support for the lens, thus reducing tensile stresses at the tapered end. The prototype lens and "0" ring seal withstood a test pressure of 30,000 psi with slight extrusion and cracking at the tapered end and no leakage around the seal. Minor yielding was also observed at 15,000 psi, the maximum working pressure, but no cracking occurred. Except for some edge distortion, optical properties remained consistently good at all pressures.

Successful development of a lens with a relatively large viewing area has made possible for the first time high-speed motion picture studies of rock failure and chip formation under pressure. The low yield characteristics and good optical properties of this design make it of potential value in many high-pressure applications.

Introduction

Photographic studies of oil well drilling at depths up to 15,000 feet have not previously been possible for two reasons. First, the subsurface drilling environment must be simulated in the laboratory where high-speed photographic equipment can be employed. Secondly, the camera must be able to view the rock failure process with adequate illumination and minimum distortion.

Such studies are being undertaken by Jersey Production Research Company in a high-pressure impact chamber equipped with special viewing lenses. A schematic of the chamber is shown in Fig. 1. Investigations of rock failure under simulated bottom-hole conditions are conducted by dropping a chisel on the surface of an 8-inch diameter rock specimen. Four viewing ports (lens cavities) are located in the wall of the chamber at 900 intervals, allowing the rock surface to be viewed at a 250 angle.

This paper describes the design, development and testing of the successful viewing lens that was used for photographic studies of rock failure under high pressures. Four of these lenses were to be used in the high-pressure chamber, three to transmit light to the rock surface, and the fourth as a viewing lens for a high-speed camera (Fig. 2).

It was apparent from the outset that a satisfactory lens would have to fulfill five requirements:Possess a viewing area large enough both for photographic coverage and for adequate illumination (from external spotlights) at camera speeds up to 7500 pictures per second.Withstand a test pressure of 22,500 psi, to insure safe operating pressures up to 15,000psi.