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Petrology and formation damage control, Upper Cretaceous sandstone, offshore Gabon

Published online by Cambridge University Press:  09 July 2018

E. D. Pittman  and
G. E. King
E. D. Pittman
Affiliation:
Amoco Production Company, Research Center, PO Box 3385, Tulsa, Oklahoma 74102, USA
G. E. King
Affiliation:
Amoco Production Company, Research Center, PO Box 3385, Tulsa, Oklahoma 74102, USA
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Abstract

The subarkosic-sublitharenitic Upper Cretaceous sandstone, which has up to 30% porosity and 585 md permeability, produces on salt diapir structures in the Oguendjo West Block, offshore Gabon. The porosity consists of 68% intergranular porosity of primary and secondary origin, 17% secondary intragranular and moldic porosity, and 15% microporosity. A microcrystalline quartz druse, which was derived from alteration of lithic fragments, coats framework grains and retards the development of syntaxial quartz overgrowths. Other cements are patchy ankerite (0·3–13·7 vol%) and kaolinite, which also occurs as a replacement of framework grains. The total kaolinite content ranges from 1·8 to 8·2 vol%. Kaolinite and remnants of altered and partially dissolved lithic fragments are susceptible to movement with fluid flow. Formation sensitivity tests showed that the kaolinite-rich sandstone was stable to 2% NaCl water, but introduction of freshwater caused permeability impairment. Acid treatment to remove damage produced sporadic results. Injection of HCl raised the permeability temporarily, indicating that fines were still being liberated within the pore network. Injection of HCl/HF immediately reduced permeability through partial disaggregation of the rock. Plugging of the formation face by drill mud also damages the reservoir. Reversal of flow at high pressure differentials will remove formation damage produced by face plugging. A guideline for this process established in the laboratory is to backflow with a pressure differential at least as high as the overbalance used in drilling. The combination of oil-based drilling fluids and underbalanced perforating with filtered diesel in the wellbore should eliminate most formation damage in this reservoir.

Type
Research Article
Information
Clay Minerals , Volume 21 , Issue 4 , October 1986 , pp. 781 - 790
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1986

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