Abstract
Abstract
Tapping heavy-oil from fractured carbonates is a real challenge due to unfavorable rock and reservoir characteristics. We introduced a new technique called Steam-Over-Solvent in Fractured Reservoirs (SOS-FR) for efficient heavy-oil recovery from fractured reservoirs, more specifically carbonates. The process consists of cyclical injection of steam and solvent in the following manner: Phase-1: Steam injection to heat up the matrix and recover oil mainly by thermal expansion, Phase-2: Solvent injection to produce matrix oil through diffusion-imbibition-drainage processes, and Phase-3: Steam injection to retrieve injected solvent and recover more heavy-oil. Our preliminary experiments under static (SPE 117626) and dynamic (SPE 123568) conditions showed that, under very unfavorable conditions (oil-wet carbonate, ~4,500 cp crude), oil recovery at the end of Phase-3 could be as high as 85–90% OOIP with 80–85% solvent retrieval.
This paper presents numerical modeling of the dynamic experiments and an upscaling study for reservoir size matrix. Heptane was selected as the solvent to inject through a single-matrix/single-fracture oil-wet Berea sandstones saturated with ~4,500 cp oil. The experimental results were matched to a single matrix/single fracture numerical model and parameters needed for larger scale simulation (matrix-fracture interaction parameters such as thermal diffusion, solvent diffusion and dispersion coefficients) were obtained. The main focus was the matrix size and first an up-scaling study to field conditions was performed.
Specific observations and conclusions as to the applicability of this technique in the field effectively were reported. It is hoped this new technique will be an alternative for tapping heavy matrix oil from oil-wet, fractured, deep, carbonate fields.
Introduction
Finding an efficient way to produce heavy oil / bitumen production from tight naturally fractured carbonate reservoirs is one of the most challenging tasks the petroleum industry. Due to geological constraints, i.e., discontinuous structure of reservoir caused by fractures, steam injection can only be applied to heat matrix rather than conventional steam displacement (Briggs et al., 1988, 1992; Reis, 1990; Babadagli, 1996a-b, 2002). Therefore, steam injection applications in fractured reservoirs are not abundant and have been limited to a few pilot applications (Sahuquet and Ferrier, 1982; Nakamura et al., 1995; Snell and Close, 1999; Macaulay et al., 1995; Al-Shizawi et al., 1997; Babadagli, et al., 2008). The common observation in all these pilot attempts was the inefficiency of the process that was caused by the rapid movement of steam resulting in early breakthrough without effectively heating the matrix.
Out of these pilot tests, only the Qarn Alam project has been switched to a full field scale application (Rawnsley et al., 2005; Penney et al., 2005; Shahin et al., 2006). Babadagli and Bemani (2007) reported that the matrix drainage at relatively favorable laboratory conditions do not exceed 42% for challenging Qarn Alam cores containing ~3,000 cp dead oil. It is expected that this number will be reduced to lower values at field conditions. Shahin et al (2006) reported that the recovery only increases to 26% by steam injection through the process called thermally assisted gas oil gravity drainage (TAGOGD), which was expected to be only 4% without thermal assistance. Note that the drainage process is rather slow as pointed out by Macaulay (1995).
With huge amounts of oil locked in fracture reservoirs, the need for a novel approach for efficient recovery is eminent. Very recently, Al Bahlani and Babadagli (2008; 2009) proposed a new technique called steam-over-solvent injection in fractured reservoirs (SOS-FR). They showed that starting with hot-water injection followed by solvent injection and completing the process with how-water injection, up to 85–90% OOIP recovery for static conditions and 70% recovery under dynamic conditions with 80–85% solvent retrieval can be achieved. In this paper, we present numerical modeling of the dynamic experiments and an upscaling exercise for field scale matrix conditions