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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Modeling Pore Pressure Profiles in Carbonates

Stephen O’Connor1; Richard Swarbrick1; Steve Jenkins1; Sam Green1; Phill Clegg1

(1) GeoPressure Technology Ltd, Durham, United Kingdom.

Carbonate reservoirs are the targets of many drilling programs around the world. In other cases, carbonate rocks need to be drilled through to reach deeper reservoirs. Understanding the pressure regimes in these carbonates is vital both for safe drilling and for reducing uncertainty in actual reservoir pressures. As there is no relationship between effective stress and porosity/velocity in carbonates, approaches based on changes in porosity using seismic velocity and/or log data such as sonic and resistivity measurements will give false magnitudes of overpressure in these carbonate units. Therefore another approach is required, one based on understanding the mechanisms of pressure generation and build-up in a basin (a geological approach), “calibrated” using available (although often rare) direct pressure measurements in permeable horizons within these units, coupled with shale-based prediction techniques in any clastic intervals above and below the carbonates.

A geological approach based on lithology can be used to predict pressure in carbonates. Data needed includes porosity and permeability characteristics of the carbonates, where low permeability marls and wackestones produce different pressure profiles in comparison with high-energy, more permeable, reefal carbonates such as grainstones and packstones. The latter group of carbonates may be sufficiently well plumbed to allow hydrodynamic flow, leading to hydrocarbon/water contacts, a feature of some of the larger Middle East oil and gas fields. A significant control on the internal pressure regime of carbonates are the pressures of any associated clastics, both above and below the carbonates, i.e. carbonates themselves do normally generate overpressure but have pressure transition zones that reflect the pressures above and below. The shape of the transition zone relates to the carbonate permeability whereby high permeability pressures are hydrostat parallel and low permeability carbonates have pressure transition zones coupling top and base pressures. Using case study material from the North Sea Chalks and SE Asia Limestones, as well as from Middle East analogues, we will illustrate how a combination of these techniques can be used to model the pore pressure profiles better through and within carbonates.