Wettability

Soolety of PetroleumEngineersI

SPE 29883 GEOLOGIC CONTROLS ON NETTABILITY RESERVOIRS, ABU DHABI, U.A.E.
I. Marzouk,

OF CARBONATE

Abu Dhabi National Oil Company; Company Ltd. (Japan).

H. Takezaki

and M. Miwa, Abu Dhabi Oil

Copyright 1995, Socmty of Petroleum Engineers, Inc. Thus paper was prepared for presentation at the SPE Middle East Oil Show held m Bahrain, 11-14 March 1995.Thts paper was selected for prasantatmn by an SPE Program Committee following rewew of mformat!on contained m an abstract submitted by the author(s). Contents of the paper, as presented, have not been rewewed by the Society 01 Petrolem E“gmeers a“d are subject to correction by the author(s). The material, as presented, dces not necessarily reflect any position of the So.aety of PetroleumE“gmeers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Edttorial Committees of the Society of Petroleum Engineers. Permlss!on to copy IS restrmted to an abstract of not more than 300 words. Illustrationsmay “ot be copied, The abstract should comam conspmuousacknowledgment of where and by whom the paper is presented. Write Librarian, SPE, P.O. Box 833S36,Richardson, TX 75083.3S36, U,S,A., Telex, 163245 SPEUT,

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Nettability is one of the most important rock properties that controls fluid saturation distribution and fluid flow behavior. h will have significant original oil-in-place, reservoir impact on performance, ultimate oil recovery and accordingly, field economics. Geologic controls on nettability is a subject of increasing interest,which will lead to better understanding of the origin, distribution and prediction of this important reek property. The Thamama carbonate reservoirs, which are the main producing reservoirs in Abu Dhabi, U.A.E., are the prime target of this paper. studies of core Geological and Petrophysical sampies indicated unique phenomena, which couid not be explained by the conventional theory of wettabllity.For example, review of the “Amott” nettability measurements generally indkated oil wet character in the oil zone, mixed wet in the transition zone and water wet in the water zone. Fresh core resistivity measurement showed variable saturation exponent “n” with changing water saturation of the same sample. Other examples are discussed in this paper. In order to explain these phenomena, careful ofgeological and pemophysical examination properties such as pore throat size distribution and petrographic image analysis, were conducted, which confirmed close relation between wettabllity and rock type especially the pore system. The pore system proposed in this study is based on the”pore throat size. Accordingly, the pore system of the Thamama reservoirs is classified into; Macropore throats (morethan 4 microns), Mesopore throats (4-0.3 microns) and Micropore throats (less than 0.3 microns). MicroPores are always filled with water (Swi). Mesopores are filled with either oil or water depending on the oii coiointi ‘diickness. MacroPore are easily filled with oil above the free water level (FWL). n --- J -- ~i= .. J&*”h:l ;*., ~as~u uli .I..- ,.I--.... -l..--,.,+: .,’. distribution model isproposed and the following new concepts are introduced:
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True Nettability, represents stabilized condition of grain surfaces in a water/oil system under conditions where oil can contact the rock True wettabllity of the Thamama surfaces. carbonate reservoirs is considered to be oil wet. Nettability Change, represents the accumulation process in a virgin watertilled oil

Reference at the end of paper 449

SPE29883

GEOLOGICAL RESERVOIRS,

CONTROLS ON NETTABILITY ABU DHABI, U.A.E.

OF CARBONATE

2

reservoir. Before oil accumulation, wettabllity was originally water wet, after oil accumulation nettability changed to oil wet if grain surfaces contacted with oil.
q

Apparent Nettability, which is the measured wettabllity, represents the...