CONTACT DETERMINATION

 

  • CHAT(Cased Hole Analysis Tool)
  • CASE STUDY/EXAMPLE
  • GLAUCONITE OIL WELL
  • APPLICATION: Current Formation Contact Determination

BACKGROUND

This example illustrates how the CHAT service is used for reservoir contact determination and reservoir surveillance.

This particular well was a Glauconite oil well in Southern Alberta that initially produced oil, then experienced excessive GOR, followed by water production. Based on the timing of these events, and other E.O.R. initiatives, a CHAT log was run to determine current reservoir fluid status. A more detailed description follows.

 

GLAUCONITE EXAMPLE
Application: fluid contact(s) identification
Low salinity formation water
Geology: Lower Cretaceous
Glauconite formation
Southern plains (Alberta)
Lithology: sandstone
Salinity: 30-35k ppm
CASE STUDY DESCRIPTION:
This was an oilwell being produced from a reservoir under water-flood.
Well initially produced oil from perforations at xx87.5 m – xx88.5 m. Well was shut-in due to high GOR. Production was resumed and a very high water cut resulted. Well was shut-in for logging.
xx76.5 m – top of reservoir porosity
– minor shale effects present down to xx79.5 m
xx82.5 m – bottom of original gas cap suggested by open hole porosity log (OHNPHI)
– gas effect on OHNPHI reduced in upper section (xx76.5 m – xx79.5 m) due to shale effect and deeper invasion of drilling fluids in lower porosity rock
xx82.5 – xx87.5 m – cased hole & open hole logs suggest displacement of formation liquids by free gas
xx87.5 – xx88.5 m – high water saturation indicated across perforations by SWE curve

During oil production, free gas was coned down to the open perforations. High SWE values at the top of the porosity interval suggest that the injected water has swept through the upper interval of the reservoir resulting in sections of bypassed oil (xx80.0 – xx83.5 m). The original gas cap within this well has been produced or moved up structure. The CRTN/CRTF overlay shows a limited gas effect from the top of the reservoir down to the perforations. This effect is consistent with residual gas being left behind in the formation pore space.

 

Fig. 1 Open Hole Log section across the Glauconite indicating the original gas cap at approximately 4210.0 Ft. KB.

Fig. 2 CHAT Raw Data Pass

Fig. 3 CHAT Raw Data merged with Open Hole Neutron Porosity

Fig. 4 Full Interpretation of the CHAT Neutron Data merged with the Open Hole Neutron Porosity.
click on image to zoom in

COMMENTS/OBSERVATIONS:

  • Comparison of the Open and CHAT neutron porosity suggests that the formation gas saturation below the original contact has increased significantly extending downwards to the top of the existing perforations.
  • Additionally, from the depth of the original gas cap, and extending upwards to the top of the porosity, the overlay suggests that the formation gas saturation has been reduced significantly

SUMMARY/COMMENTS:

This Gluaconite oil well initially produced oil from the open perforations, but during the production life of the well had to be shut in due to excessive GOR.

During the production life of the well, a water-flood was active within this pool.

Following an extensive ‘shut-in’ period due to high GOR, the well was put back on production, resulting in water production.

The well was then shut in, and the operator chose to run the CHAT service to determine current status of reservoir contacts.

Based on the results of the open hole logs and the CHAT data the following conclusions were reached:

  1. During the original production phase, gas coning occurred from the original gas cap down to the top of the perforations.
  2. During the shut-in period, the waterfront intersected this borehole and any remaining free gas had been forced up-structure.
  3. It was contemplated that movable oil may exist within the interval between the open perforations, extending up to the depth of the original gas cap. This however, was never recompleted to confirm this.