Diagnosing Production Problems With Downhole Video Surveying At Prudhoe Bay
Authors: Ward Stephen L. , Allen Thomas T. , Chavers Raymond D. , Robertson Thomas N. , Schultz Philip K. 
Published: December 2007

This paper describes a real-time, fiber-optic downhole video (DHV) system and its use as a diagnostic tool in solving production problems in Prudhoe Bay wells. Recent developments in lens preparation technology and advancements in the application of electro-fiber-optic cable have proved the viability of DHV surveying in oilfield applications. This paper presents case histories of the first field applications in which the fiber-optic DHV system was used to perform downhole visual inspections of tubulars and casing. These operations determined fluid entry under flowing conditions, verified tubing and casing integrity, and facilitated wireline fishing operations. The fiber-optic system provided more comprehensible identification capabilities and other operational advantages than obtainable with alternative methods of downhole diagnostic techniques or the DHV coaxial-cable systems. This paper also overviews the enhancements made since the first applications of fiber-optic cable.

Introduction

Until recently, real-time DHV systems relied on specialized, large-diameter, 9/16-in. coaxial cable to transmit to surface the high data rates required for real-time video during surveys in wells of significant depth. These coaxial systems had to be used in wells with little or no surface pressure because of operational difficulties in maintaining pressure control with the large-diameter cable required for data transmission. A new electro-fiber-optic cable provides DHV systems with greater surveying capabilities. The new cable design consists of a 50/125u multimode fiber encapsulated in a 0.046-in.-diameter stainless-steel tube. Copper conductor wires over the tube provide a DC resistance of 8 ohms/1,000 ft. Although the cable appears to be identical to other 7/32-in.-diameter well surveying cables, its optical telemetry provides about a 50% increase in bandwidth compared with 9/16-in.-diameter coaxial cable. The cable has a breaking strength of 4,700 lbf, a working load rating of 1,200 lbf, a minimum radial bend of 12 in., and a conductor voltage rating of 300 V DC. As Fig. 1 shows, the sinker weight required for a 7/32-in.-diameter cable is 80% less than that required for a 9/16-in.-diameter cable. The 60% size reduction in cable diameter was instrumental in making DHV surveying possible in high-surface-pressure environments.

The second factor that had compromised earlier DHV systems was their limited capability to obtain quality surveys in opaque fluids. This problem has been solved with the development of a unique, highly specialized surfactant that, when properly applied, repels oil and inhibits condensation. The surfactant is now being used on the camera lens and light domes to allow viewing of wells on production. Before the development of the new surfactant, many other products (detergents, phosphates, petroleum-based coatings, acidified ethanol/isopropanol polish, and myriad other wetting agents) had been tried but were only marginally successful. The new surfactant, which is polished into the glass surface to prohibit oil adherence to the camera lens and light domes, has permitted viewing for more than 8 hours in wells with high oil concentration without the oil adhering to the camera lens and obstructing the video picture. Even after the system had traversed thousands of feet through a column of oil, visual clarity returned when a clear medium was encountered.