BHP from WHP in a Subsea environment
By Rod Kelly |
Wed, 6 Feb 2008
A major operator in SE Asia with a large offshore gas field is producing dry gas from 6 Subea Wells in relatively shallow water (~ 400 ft.). No Downhole Permanent Pressure gauges (DHPG) were installed or any means for individual well flow measurement. The operator contacted DRC because he was interested in knowing reservoir pressure, skin and permeability for his individual wells from pressure transient testing.
The six wells have been on line and producing large volumes of gas for several years. All wells produce from the same reservoir and are in direct communication with each other over the extent of the field. The wells exhibit extremely high permeability (>1 Darcy) and flow large volumes of gas (50 to 100 MMSCFD). The wells also all tie back to a common subsea header and flow to an offshore production facility where cumulative rate is measured.
The Subsea Wellheads are equipped with wellhead pressure gauges. They are sapphire strain gauges of low resolution (5-10 psi) and the data capture frequency (once per 30 minutes) makes any attempt at Pressure Transient Analysis difficult if not impossible under these ultra high permeability conditions.
DRC / Oceaneering deployed the SubSea SPIDR system via a Remote Operated Vehicle (ROV) on one well for the purpose of proving the system. If successful, the SubSea SPIDR would be used for a project later in the year when the field would be shut down for two weeks for installation of compression equipment.
The Subsea SPIDR was installed via the Hot Stab port on the SubSea Tree and accessed the annular pressure pathway. By manipulating valving at the wellhead, we were able to isolate annular pressure from the SubSea SPIDR and record production tubing pressure only. The well was then flowed at a steady rate for several hours before it was shut-in and allowed to build pressure. A minor problem encountered was the inability of the process controls to fully shut-in the well in less than 60 seconds. DRC would prefer the well to be shut-in as quick as possible in order to avoid a ìstagedî shut-in for this high permeability well. However, even with the less than optimum testing procedure, we were able to compare and contrast the high frequency, high resolution pressure data from the SPIDR to the low resolution and low frequency data collection from the Subsea Wellhead gauge and make recommendations going forward.
The primary value that DRC provides to operators in this type of deepwater environment, is the ability to process pre-existing subsea wellhead gauge data (when DHPGís have failed or were not initially installed) to bottomhole conditions. Our proprietary conversion algorithms for gas and gas condensate wells have been proven on thousands of wells that flow and unload produced liquids without slugging at the wellhead. DRC also provides the operator with the option to consider the use of the Subsea SPIDR for higher resolution, higher frequency pressure data capture if it is determined that the Subsea Wellhead gauge data is not acceptable for Pressure Transient Analysis.
