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Glacio-Eustatic Controls from Reservoir Architecture and Flow Units within Serpukhovian Carbonates of the Karachaganak Field (Kazakhstan): Implications for Optimisation of the Phased Gas Injection Strategy

Beavington-Penney, Simon J.1; Covill, Mark 2; Wright, Paul 3
1 Kazakhstan Asset, BG Group, Reading, United Kingdom.
2 KPO, London, United Kingdom.
3 BG Advance, BG Group, Reading, United Kingdom.

The Karachaganak gas condensate ‘super giant’ field, in north-west Kazakhstan, has been on production since 1984. Over the period 1984 - 2003 development of the field was based on a depletion-only strategy. Gas injection commenced in 2003 into the upper parts of the gas condensate leg, with the objective of maintaining reservoir pressure above dew point to increase liquid recovery in all hydrocarbon legs.

The current target gas re-injection rate is 40% of production, and this is attained by injection into central and crestal portions of the carbonate platform. This interval comprises Serpukhovian (Upper Mississippian, early Carboniferous) shallow platform interior sediments (characterised by relatively high porosity and matrix permeability) which have a marked cyclicity caused by glacio-eustatic, high amplitude, high frequency, sea-level fluctuations. This cyclicity exerts a strong control on gas migration pathways, with flowing intervals confined to thick (20-30 m) 4th order cyclothems within 3rd order Trangressive Systems Tracts and early Highstand Systems Tracts (HST). Thinner, discontinuous 4th order cycles within the 3rd order HST do not contribute significantly to horizontal flow. Flow units can be correlated for up to 10 Km across the platform interior, and are separated by vertical permeability baffles at 3rd order sequence boundaries.

In the next phase of field development, a further 18 gas injection wells are planned for the geological and geographical field margin - an area dominated by largely acyclic, biohermal facies, where porosity distribution, and potentially well deliverability, is less predictable. A key subsurface challenge is, thus, to understand the transition between the flow units within the cyclic platform interior and the markedly less cyclic biohermal boundstone facies at the platform margin, to ensure that gas injection is feasible, that an even pressure support is maintained, and to reduce the magnitude of gas recycling.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009