Maureen: A Faultless Performance? Symposium for Alain Gringarten – 13 Sept 2005

Maureen – a common heritage? 1971 –AG PhD thesis –JM LOCA studies 1974 – Maureen discovered 1977 – JM training in RE at IC – |Several Maureen studies – PhD thesis and MSc mentoring

Arp et al’s Formula, 1967 Correlation coefficient = 0.958

Fault: (as dictionary) 1) something which one is rightly open to blame 2) a mistake 3) a blemish 4) a failure to serve correctly (as in a game)

Reservoir Engineering Like a game –Make a move/shot, and wait for the response –Prepare next move/shot on that basis Or a court case –Ask a question –Try to sort out lies from facts –Focus on defending your client –Ask next question

Nature can lie?

Nature does not lie. Apparent lies are due to: Asking the wrong questions. Asking closed questions. Spending insufficient time and energy collecting data. Poor equipment.

Geological Faults Have the potential to disturb the hydraulic continuity between sections of the reservoir The traditional view is that this makes development of a reservoir more problematical –(extra wells needed to achieve same recovery) But can faults improve recovery?

Detecting Faults Seismic Drilling Logs Well tests Pressure regimes Fluid regimes OWC regimes

Map from Cutts, 1991

Well pressure and OWC data WellOWC feet FWL feet Pressure psi 2Xdatum-- 3X+80-- A10+25datum A A A

So, does the reservoir consist of many small compartments, bounded by sealing faults? This possibility seems to have been ignored in the early development decision to pre-drill development wells (though it was not until the A- series wells were drilled that this view of the reservoir was available).

Seismic sections (1995)

Mapping the faults (1995)

Can we reconcile? The static data –OWCs, FWLs and pressures The faults seen on seismic The subsequent reservoir pressure and flow performance?

1) The Area around Wells A10 and 3X The oil:water contact in A10 is shallower by 55 feet than in 3X –We could have a sealing fault (Allen, Gringarten) –We could have a perched contact (as Fulmar or Pickering) –The well depths are in error (Allen)

The Sealing Fault Option – T J Allen, 1992

Accuracy in Assessing Well Depths 3X is a straight hole. The errors in the depth measurement are due to cable effects (~1 in 1000, Sollie and Rodgers, 1994) A10 is a deviated well. The errors in the depth measurement are due to cable effects (~1 in 1000), plus errors in giro surveys (~30 feet, Wolff and de Wardt, 1981). Thus, the two OWCs lie within 2σ of each other.

Errors in Well Depths – T J Allen, 1992

Although the probability is low that the two OWCs are common, this is the most favoured option. Pressure behaviour of reservoir –Good communication across whole of reservoir Faults are seen on seismic, –but their extents are limited –and the shale/gouge ratio is small –And there is no other evidence of hydrothermal precipitation

2) The Anomalous OWCs Once we have adjusted the depths of data from well A10, the oil:water contact data (and FWLs) seems to show deeper contacts in the south and north than in the centre. Are these compartments –Fully sealing, –Stepped sealing, –Or what?

The Adjusted OWC data Deep in the SSE Shallow in the Centre Deep in the NNW

Asking if you sink or swim in the Dead Sea is a closed question.

The Thermal Behaviour of Maureen Maureen is an anticlinal structure overlaying chalk, which itself over-lays a salt dome. Salt is a good conductor of heat. Temperature measurements suggest that the centre of the reservoir is ~ 20 ºF hotter than the extremities. Steady-state temperature calculations support this assessment (S Goodyear).

Effects of Temperature Oil Water Hotter, 20 deg FCooler 40 feet

So good hydraulic continuity appears to have been a feature of Maureen. There were no sealing faults. But there may have been bonus points arising from the faults.

Effect of Faults One of those counter-intuitive results to come from the SAIGUP project was that short or partially sealing faults perpendicular to injector/producer directions can improve recovery.

Effect of Anisotropy In a five spot pattern, and line-drive, the best recovery is obtained when the maximum permeability is in the direction parallel to a line joining injectors. –Craig, 1971, Mortada and Nabor, 1961

So was Maureen’s performance faultless? The geological faults were either not significant hydraulically, or Assisted recovery. But with hindsight, there were things that we did not fully understand. It encourages us to keep active research going.

And keeping mentally active (such as by research) is one way of remaining youthful.