Are your Permanent Downhole Pressure Gauges (PDHG) functioning? If not, DRC may have a solution.

By Adam Swartley | Tue, 26 Feb 2008

Producing a deepwater oil or gas well without bottomhole pressure measurements is akin to flying an airplane at night without an altimeter.  There is no reference point from which to make informed decisions. 

Most (not all) subsea completed wells have some type of permanent downhole pressure gauge (PDHG) that allows the operator to determine reservoir properties from shut-in and drawdown pressure transients that naturally occur over the course of the producing life of the well.  Data Retrieval Corporation has been directly involved with customers where PDHG’s have failed or were never installed to begin with.  In those occasions the driving reason to obtain downhole pressure information was to limit drawdown or DeltaP across the completion in order to avoid premature completion failure.

It has been estimated that more than 10,000 wells around the world have PDGH’s installed.  The vast majority are Subsea completions but many are deepwater, dry tree installations (Spar, TLP, etc.).  Published information concerning the functional life of PDHG’s varies depending on the company making the study.  A major service company states that their PDHG systems have an 88% survival rating after 4 years of use.  While a major oil & gas operator claims their experience and statistics indicate only a 68% survival rate after 5 years of use.  Even if we take the mid-point number between these two we still arrive with more than 2000 wells where there is no functioning PDHG, a very substantial number.

However, there are options for the owner of a failed PDHG. Typically there is a wellhead tree gauge (WHTG) installed and functional.  Prior to the failure of the PDHG, the wise operator will have developed a relationship between the PDHG and the WHTG for a variety of flowing and shut-in conditions.  This relationship will allow a qualitative judgment as to the changes in bottom hole pressure as the reservoir depletes.  However, if a WHTG to PDHG relationship does not exist or is no longer considered valid, the operator may rely on Data Retrieval Corporation (DRC) to convert WHTG to downhole conditions.  DRC has 23 years of experience in converting well head pressures to Downhole pressures for wells that are continuously self-unloading.  As an additional benefit, DRC will analyze the converted data to determine skin and the pressure drop or drawdown across the completion and determine skin, permeability and reservoir pressure.

The ability of DRC to analyze converted tree gauge data is dependent on the quality of the tree gauge data and the permeability of the reservoir.  WHTG data quality is a function of gauge resolution (the smallest pressure change the gauge can reliable detect) and the sampling frequency of the gauge.  When a reservoir has low permeability (<10 md.), gauge quality is less important than when the reservoir is relatively high permeability (> 100 md.).  The following plot shows a typical WHTG with, what we would consider, low resolution (+/- 5 psi) data file superimposed on a high resolution (+/- 0.01 psi) gauge data from gauges connected at the same point on a Subsea Tree.  Although the low resolution tree gauge allowed DRC to determine the quantitative skin on this completion, without the high resolution data it would not have been possible to determine the qualitative skin or the reservoir permeability or an accurate reservoir pressure.

If tree gauge resolution is inadequate, or if the both the tree gauge and the DHG are either non-functional or non-existent, it is possible to temporarily install a SubSea SPIDR gauge to acquire the pressure data necessary to determine skin and reservoir properties.  The SubSea SPIDR pictured below is shown installed into a hotstab port on a stab plate.

The SubSea SPIDR is certified for 10,000 feet of water and can operate for up to 2 years on its internal power.  The unit is installed and retrieved by an ROV.  Use of the SubSea SPIDR should be considered whenever maintenance is scheduled for the subsea tree or if it becomes necessary to shut-in the well.