Permanent downhole pressure gauges are becoming a standard completion component in many production wells. However, their use in exploration wells to date has been limited. Wireline-deployed memory gauges are more likely to be used to obtain downhole pressure data during testing of exploration wells. Unfortunately, pressure data from memory gauges does little to help the operator manage and optimize the completion and testing of a well where uncertainties in rock strength, pore pressures, and production rates can be high. Installation of a permanent downhole gauge in an exploration well can provide valuable data for real-time management of the production testing process. In addition, it can continue to monitor reservoir pressure after the well is shut-in.
This paper discusses the installation, application, and value of a permanent optical sensing pressure and temperature gauge system installed in an exploration well (Carbon #1) in the National Petroleum Reserve, Alaska (NPRA) during April 2004. The well was a delineation/confirmation well in a large hydrocarbon accumulation discovered during previous winter drilling seasons. All exploration work in the NPRA is done from ice roads and pads and therefore the season is limited by weather. Challenges and unique aspects of the installation included arctic winter operations, flawless integration of the data into surface well testing data acquisition systems and the ability to monitor real time bottom hole treating pressures during an EOB (extreme overbalanced) perforating treatment.
The real time data proved to be invaluable for determining the effectiveness of the stimulation treatment as well as optimizing the duration of the production test and final pressure build-up test. The ability to monitor pressure data real time rather than having to run and retrieve multiple memory pressure gauges provided nearly "textbook" data quality during multi-rate testing. The well is now temporarily abandoned and is available for use as a pressure monitoring well during future development. The results of the stimulation and well testing operation were very successful and will lead to a future commercial development in the NPRA.
The Carbon #1 well was drilled as part of the 2004 NPRA exploration drilling program (Fig. 1 & 2). The well was considered to be an exploration and/or delineation well for a hydrocarbon accumulation in Jurassic aged sandstone that had been discovered in previous winter drilling seasons. Production testing and determination of fluid type were critical portions of the evaluation.
Winter drilling operations on the North Slope of Alaska are carried out on ice roads and pads and thus have a limited time frame for completion. Conducting an efficient as well as thorough production test was critical to optimize the limited time available on the ice. Including the optical sensing pressure/temperature gauge system was viewed as a time saving option and therefore a cost saving option compared with running and retrieving multiple electronic pressure gauges during the stimulation and production testing operations.
The well was a near vertical wellbore with a 4–1/2″ monobore completion (Fig. 3). The optical sensing mandrel was placed just above the seal assembly which stung into the 4–1/2″ liner and allowed pressure measurements within approximately 300' of the top planned perforations. An EOB perforation stimulation treatment was planned in order to overcome near wellbore drilling damage and hopefully prevent the need to fracture stimulate the well prior to testing. Fracture stimulation would have added significant cost and time to the testing operations.