Steam-Over-Solvent Injection in Fractured Reservoirs (SOS-FR) for Heavy-Oil Recovery: Experimental Analysis of the Mechanism

Abstract

Abstract Heavy oil trapped in fractured carbonate reservoirs possesses a great challenge. Previous experience has shown that heating the matrix to drain heavy-oil by gravity is a slow and inefficient process due to low matrix recovery caused by unfavorable matrix properties (oil wetness and low permeability). Acceleration of matrix-fracture interaction by changing matrix and oil properties could be possible, however, by injecting proper hydrocarbon solvent alternately with steam as reported by our earlier work (SPE 117626). This process called Solvent-Over-Steam injection in Fractured Reservoirs (SOS-FR) consists of several cycles, each having three phases:steam/hot water injection,solvent injection, andsteam/hot water injection. This paper reports a detailed experimental analysis of this new method. Static and dynamic tests were conducted by exposing (or injecting) heavy-oil saturated fractured rocks into steam/hot water and solvent alternately. Oil-wet sandstone and carbonate samples were saturated with heavy-crude. Different solvents were tested including heptane, kerosene, decane and light crude. In addition to the core experiments, the same procedure was repeated on "Hele-Shaw like" glass models to visually identify the mechanics of the process. Specific observations and conclusions as to how to apply this technique in field conditions, which mechanisms control the rate of recovery and residual matrix oil saturation, ideal solvent type, and optimal operational conditions (optimal soaking time for huff-and-puff and injection rate for continuous injection) were reported. It is hoped the proposed new technique will be an alternative for tapping heavy matrix oil from oil-wet, fractured, deep, carbonate fields. Introduction Heavy oil/bitumen production from fractured reservoirs (especially oil-wet carbonates) is considered one of the greatest challenges to the petroleum industry. The combination of unfavourable flow and accumulative properties urged researchers to pursuit novel and innovative methods to overcome these challenges. Yet, an acceptable solution is far from reach, and intensive research efforts are underway where several suggested solutions are at the stage of development, but are not yet proven. The main problem in heavy oil recovery/bitumen recovery is the efficiency of the process. A huge amount of energy is required to heat the reservoir and mobilize oil. Steam injection is the only proven way to achieve this technically. Its cost, operational and environmental problems are the major impediments of this technique. The reduction of its amount and cost has been a challenge in the oil industry. Solvent injection was proposed as an alternative but its cost and solvent retrieval still remain the main obstacles. Recent experiences showed that neither technique provides a fully effective application with very low residual oil saturation and a high recovery rate. A hybrid application of these two materials, i.e., heat and solvent, has been proposed for oil sands but tests at the laboratory and field scale revealed that more research is still required for the sake of optimization and the selection of a proper combination of solvent type and operational conditions. In heavy oil/bitumen recovery from sands, chemicals (mainly surfactants) have been tested as additions to steam but yet technically and economically, they are still not viable. The current tendency is on solvent rather than the IFT reducing agent to improve steam recovery efficiency in heavy-oil/bitumen recovery. In carbonates which are generally oil wet, wettability alteration is a critical problem as matrix oil recovery through capillary imbibition and gravity drainage is controlled by mainly its wettability. Wettability alteration in such reservoirs was studied at elevated temperature (Anderson 1986; Zhou et al. 1996; Motealleh et al. 2005; Rao, 1999; Al-Hadrami and Blunt, 2001; Seethepalli et al., 2004; Schembre et al., 2006; Gupta and Mohanty, 2008; Zhou, 1993). High temperatures will break the bond between organic matters attached to the rock surface and alter the wettability to water-wet which will promote water imbibition and hence a sandstone-like thermal production.