Achieving the Fiber Optic Downhole Wet Connection
Authors: Stoesz Carl W. , Christiansen Matt 
Published: March 2015

A prototype downhole fiber optic wet mate has been designed and successfully deployed in a land-based test well. Special strain-sensing fiber using Fiber Bragg Gratings (FBG) were placed near the gravel pack screens at the sand/oil interface. The FBGs were read through the wet connector. This paper reviews the wet mate design specifications and prototype testing. The deployment is presented, covering the gravel pack string and completion string, including downhole fiber optic connectors. Results of the downhole fiber optic wet connection are presented.

Introduction

Fiber optic sensors such as pressure, temperature, distributed temperature, and flow rate have been run in wells for years. Fiber optic distributed temperature sensing (DTS) has been used for hydraulic fracturing diagnostics and well performance evaluation in unconventional well completions1. DTS has proven to provide quantitative well performance information, and inflow, outflow diagnostics.

More recently, fiber optic strain gauges using Fiber Bragg Gratings (FBG) have been developed to be able to provide real-time casing and sand screen monitoring2,3 to monitor reservoir compaction with the ability to convert the strain readings into a casing shape to better understand the extent and cause of damage. Advanced knowledge of the damage rate and location allows for adequate time for wellbore intervention techniques to be applied or workover efforts to be scheduled.

A major obstacle in expanding the use of optical sensors has been the inability to make up an optical connection down hole due to the harshness of the environment. The connector must be able to handle debris of various types and sizes in addition to downhole pressures and temperatures. Debris can be that which is pumped downhole, including completion sands, proppant, or gravel pack sands. A major source of debris comes from the tripping pipe that deposits pipe dope and mill scale4. Surface items such as nuts, bolts, and more can also end up in the wellbore.

A fiber optic downhole wet connector might also end up coping with various chemicals used downhole. Following the completion of drilling the well, a cleanup run is typically made to clean the casing and change from a drilling mud to a production fluid. A wide variety of cleaners (surfactants), rust inhibitors, and acid pickle treatments 4–6 can all be pumped downhole at various stages to prepare the wellbore. While the intent of these operations is to reduce the contaminants in the wellbore, in some circumstances the connector could be exposed to a high solids-laden fluid.

The ability to make up a fiber optic wet connect down hole opens up an amazing amount of opportunities for fiber sensing. One of the most desired applications of this new technology is to be able to monitor sensors such as FBG, DTS, and pressure/temperature gauges, at the sand/oil interface in gravel packed or hydraulically fractured well. To be able to accomplish this, one half of the connector must be run in hole with the gravel or frac packer. The packer is then set and the fracturing operation performed. The frac tools are then pulled out of hole, leaving behind the wet connector. At a later point in time, the completion string is run. The mating half of the wet connector is run on the completion tools and must stab into the mating fiber optic connector half mounted on the packer.