Oil-Base Coring For Connate Water Saturation Utikuma Keg River Sandstone
Authors: Hamp T. , Mayder A. , Vokey G. , Shaw J. , Cebuliak N. 
Published: March 2010

Abstract
A successful oil-base coring operation has resulted in identification of an average Connate Water Saturation (Swc) at Utikuma Lake oilfield supporting low Swc from recently completed Single Well Tracer (SWT) tests. As oil-base coring was intended to validate SWT results, careful design of the coring program was required.
Alteration of Swc by the coring procedure itself WAS eliminated by preserving wettability through appropriate coring fluid selection and surface handling of the core. Special attention to the drilling program design eliminated contamination by uphole formation and drilling fluids.
This paper presents details of the oil-base coring program, including drilling program design, equipment and coring fluid selection core handling and core analysis results.

Introduction
Utikuma Lake Field is located north of the town of Slave Lake, Alberta, between the Nipisi and Red Earth fields (Fig. 1). Field characteristics are summarized in Table 1.
Utikuma produces 1500 m3/d of light, sweet oil under primary water drive from 180 wells completed in the Keg River sandstone. Utikuma Keg River oil is paraffinic with low asphaltene content. Oil properties are listed in Table 2. Formation water is highly saline, with 275 000 mg/l total dissolved solids, as indicated in the water analysis data presented in Table 3.

Geology
Utikwna Keg River sands were derived from the Peace River Arch west of the field, being deposited in Utikuma as numerous separate pools in topographic lows of irregular highs in the eroded Precambrian granite. The porous Keg River sands were encased in impermeable shales that formed upper and lower seals.
Keg River sands consist largely of interbedded sand and shale, each being several centimetres to several decimetres thick. The sands are feldspar-rich quartzose with calcite and anhydrite cementation, and fine to very coarse grains. Clays are almost entirely illite, a non-swelling clay. Average porosities and permeabilities for the Keg River sands are 20% and 1000 mD. Average net pay is 3 m.

Need for Oil-base Coring
Oil-base coring was undertaken at Utikuma to confirm low connate water saturations (Swcs) identified by Single Well Tracer (SWT) tests conducted during 1986–1988. Swcs of 10% to 15% from SWT tests were significantly lower than the 29% Swc recognized for most Utikuma Keg River pools. Oil-base coring was justified by the significant impact on reserves estimates and field development plans that would be caused by adopting the lower Swcs.
Oil-base coring has been undertaken at two development well locations in Utikuma. The first attempt at well 5-30-81-9 WSM in December 1987, did not provide representative Swc data as the Keg River sand was determined to be entirely within a transition zone above an unexpected oil/water contact. Experience gained from well 5–30 was used in the subsequent successful oil-base coring project at well 7-33-81-9 WSM in July 1988. The 7–33 oil-base coring project is the subject of this paper.

Oil-base Coring Project Design
The Utikuma oil-base coring project began with selection of a project team comprising reservoir, production and drilling engineers, laboratory representatives and downhole equipment and drilling fluid suppliers.