The Physics of Steam Injection in Fractured Carbonate Reservoirs: Engineering Development Options that Minimize Risk

Abstract

Naturally fractured carbonate reservoirs hold well over 100 billion barrels of heavy oil worldwide. Thermally Assisted Gas Oil Gravity Drainage (TAGOGD) is a new and novel thermal EOR technique, which has applicability in selected reservoirs. In conventional isothermal GOGD, vertical fractures cause the gas-oil contact in the fracture system to advance ahead of the gas-oil contact within the matrix blocks, causing the oil in these blocks to become mobile. The addition of heat in the fractures generates additional hydrocarbon gas cap, lowers the viscosity of the oil, and accelerates conventional GOGD, as seen in the 220 cp heavy-oil Qarn Alam field in Oman. Pilot results in the Qarn Alam field support the commerciality of this process, and a first-of-it's-kind steam injection project is being implemented. The economic success of the Qarn Alam project depends on the ability to credibly predict steam requirements and oil production. Two key oil production mechanisms are heat transport through the fractures and into the matrix, and subsequent gas cap generation due to thermal volatilization of the oil. The process mechanisms involved in TAGOGD were validated through laboratory experiments, while the field forecast model results were validated by history matching pilot performance data. A fully integrated workflow of fracture characterization, integrated reservoir physics, and static and dynamic modeling has enabled uncertainties and risks involved in developing the Qarn Alam field to be managed in a scenario based design approach. Introduction The Qarn Alam field is a highly fractured carbonate field that lies atop a salt diapir in Northern Oman. The 6 km long and 3 km wide field forms a relatively high-relief anticline with a N-NE by S-SW orientation. The reservoir is relatively compact dome-shaped structure, with a maximum oil column of 165 m. The main oil bearing reservoirs, the Shuaiba and Kharaib formations, are separated by a very low permeability oil bearing zone called the Hawar. The crest of the Shuaiba is located at 212 mss, and the original oil water contact is ∼375 mss. Fracturing occurs throughout all zones, and is believed to be contiguous and in hydraulic communication with a very active aquifer. The initial oil saturation is about 95% and initial water saturation is connate water. The matrix porosity is high (∼30%), while the matrix permeability ranges between 5 md-20 md. Under primary production, the reservoir produces on average about 100 m3/day of 16o API "heavy" oil, at a GOR of 10 m3/m3.

Wettability Alteration Mechanism for Oil Recovery from Fractured Carbonate Rocks

“…Oil-phase swelling and viscosity reduction also help in improving oil recovery. Shahin et al (2006) have studied thermal-assisted gas oil gravity drainage for Qurn Alam fractured carbonate reservoir in Oman. Al-Hadhrami and Blunt (2001) have discussed steam injection in fractured carbonates.…”

Impact of well configuration on performance of steam-based gravity drainage processes in naturally fractured reservoirs

“…Steam is injected in the crest of the reservoir and oil drains into horizontal producers. Oil recovery takes place in a process called thermally assisted gas oil gravity drainage (Shahin et al 2006).…”

Impact of Reservoir Heterogeneity on Steam Assisted Gravity Drainage in Heavy Oil Fractured Reservoirs

“…Steam is injected in the crest of the reservoir and oil drains into horizontal producers (Shahin et al, 2006). Some studies proposed the use of additives to improve the performance of this process in this field (Boukadi et al, 2007).…”