Summary
Oil reserves from shallow-shelf carbonate reservoirs account for 22% of the
original oil in place (OOIP) of the entire U.S. oil resource. Many of these
reservoirs are naturally fractured. A pressure-pulsing technique has been used
in fractured fields to improve oil recovery. In some situations, imbibition of
water can be promoted by chemical stimulation to alter the reservoir
wettability toward water-wetness such that oil is expelled at an economic rate
from the rock matrix into fractures. Shallow-shelf (i.e., Class II) carbonate
reservoirs typically produce less than 10% OOIP during primary recovery and
respond poorly to water injection.
In this work, promotion of imbibition was determined for a cationic
surfactant, cocoalkyltrimethyl ammonium chloride (CAC), and a nonionic
surfactant, an ethoxylated alcohol (POA). Cores from three dolomitic Class
II reservoirs, Cottonwood Creek, Dagger Draw, and Lustre, were used in the
laboratory tests. After preparing core samples using the corresponding
reservoir crude oil and brine, spontaneous expulsion of oil was measured in
glass imbibition cells at elevated temperature for more than 50 core samples.
When reservoir brine was used as the imbibition fluid, oil recovery was in the
range of 0 to 35% OOIP. After imbibition of reservoir brine had ceased, the
cores were transferred into surfactant solutions at or somewhat above the
critical micelle concentration (CMC) to test for enhanced recovery by further
imbibition. Typically, immersion in the surfactant solution resulted in an
additional recovery of 5 to 10% OOIP. The increased recovery is mainly ascribed
to increased water--wetness. The effect of acidization before surfactant
treatment was also tested and found to be detrimental to oil
recovery.
Introduction
Approximately 22% of the OOIP of the entire U.S. oil reserves resides in
shallow-shelf carbonate reservoirs. Most of such reservoirs are heterogeneous
and naturally fractured. Attempts to displace oil by waterflooding pose
significant problems because the fractures lead to bypassing of the rock matrix
and early breakthrough. Spontaneous imbibition can be of special importance to
oil recovery from fractured reservoirs.1–3 However, spontaneous imbibition only
occurs when the pore surfaces are effectively water-wet so that water imbibes
into the rock matrix and oil is expelled into the fractures. The oil can then
be flushed along the fractures toward the production wellbore.
© 2005. Society of Petroleum Engineers
View full textPDF
(
1,084 KB
)
History
- Original manuscript received:
15 January 2004
- Revised manuscript received:
1 January 2005
- Manuscript approved:
9 May 2005
- Version of record:
15 September 2005
View Cart
