The Conversion of Wellhead Pressures on Subsea Wells

By Mike Redman and Chris Floyd | Thu, 7 Aug 2008

Frequent, accurate bottomhole pressures (BHP) are critical for a reservoir engineer. They enable the engineer to forecast reserves, evaluate reservoir properties, optimize production or identify production problems. For subsea wells, because of the large capital investment and high productivity, the importance of bottomhole pressures is even greater. Because of this, almost all subsea wells have both downhole and wellhead permanent gauges installed. Unfortunately, oftentimes downhole permanent gauges do fail, and until recently this left the engineer with little recourse in gathering the BHP’s necessary for reservoir analysis.

To solve this problem, DRC has introduced a new service: the conversion of non-SPIDR wellhead pressures from subsea wellheads to bottomhole conditions. This service has two limitations, the well must be a valid candidate for surface testing, and the wellhead gauge data must be of sufficient quality for interpretation. A brief phone call with a DRC engineer can determine if the well is a good candidate for surface testing. In general, the following types of wells are valid candidates:

• Dry Gas Wells
• Gas Wells in Mist or Annular Flow producing above the critical unloading velocity
• Undersaturated Oil Wells (above the bubble point)

If the well is a good candidate for surface testing, then a data set can be submitted to DRC for evaluation. DRC will evaluate the data quality based on a number of factors. Among these are:

• Accuracy
• Repeatability
• Resolution
• Sampling Frequency

If the data is interpretable, DRC will process the file to bottomhole conditions. There are many advantages gained by this service. First, the engineer has access to DRC’s 23+ years of experience converting and analyzing pressure-transient data from wellhead measurements. In this time, DRC has developed proprietary frictional loss models for calculating pressure drop and a proprietary wellbore cooling model to account for changing fluid density during a pressure build-up. These models were developed in conjunction with hundreds of downhole data sets on a wide range of wells.

By introducing this service, DRC hopes to provide value to its customers by enabling them to continue to make informed decisions to best optimize productivity and reservoir performance from their subsea wells even after their downhole permanents have failed. Additionally, subsea wells provide a unique opportunity for fine-tuning DRC’s models. Concurrent data files of subsea wellhead and downhole permanent pressures can be used to calibrate DRC’s model for the specific well. This calibration can be applied going forward to data sets recorded after the downhole permanent fails as well as to other similar wells in the same field.