The Importance of PVT to the Steam Solvent Process

Fan, Shuanshi | Li, Sijia | Xin, Hua

OnePetro 


Steam based thermal processes are major technologies for bitumen and heavy oil recovery. They are energy intensive, high water demand and huge CO2 emission. When such technologies are applied to the reservoir with thief zones, the economics are even worse. In order to increase the efficiency, people are trying to add solvent into steam to reduce the operation pressure and temperature so the economics. The mechanism of adding solvent into is further reducing the viscosity of bitumen or heavy oil an addition to thermal effects. That means the solvent should be dissolved into oil as soon as they meet each other at the interface. The boiling point of the solvent should be near that of steam. When it is to far from steam, it stays either liquid being produced without reacting with oil or gas stopping the heat exchange of steam. In this paper, PVT of some pure and commercial solvents is reviewed and potential operation pressure and temperature are discussed. This can help engineers to select solvent and design the process properly.

1. Introduction
It is inescapable that global energy demand is in an increasing trend and will remain so for the coming decades. Heavy-oil and bitumen resources have a significant impact on meeting this demand because of their huge but almost untouched volume, it amounts to it amounts to approximately 70% of world's total oil resources (approximate 6.3-9.1×1012bbl) [1]. With around 7 trillion barrels of heavy oil available globally, the lack of an effcient, feasible, and environmentally friendly heavy-oil production technology is eminent. Steam injection is a proven thermal technique to be used for this purpose and it can be achieved through continuous or cyclic (huff-and-puff) injections. Field experience and simulations studies show that performing these techniques are associated with technical difficulties and usually low recovery factors. Steam Assisted Gravity Drainage (SAGD) is the most used commercial steam-based process being used in bitumen reservoirs, proposed by Butler more than 30 years ago [2,3,4]. The main idea of SAGD was to overcome the problems associated with the highly viscous bitumen by gravity drainages in steam chambers generated by displacement of heavy oil [5].

SAGD is not economic in cases where reservoirs are thin, because heat losses to confining strata become excessive compared to the resource. Another problem in SAGD is the cost involved for treating effluent water and the high energy requirements in order to have a continuous production of steam. On the other hand, huge quantities of heavy oil are trapped in tight but fractured carbonate reservoirs. Until recently, except for limited efforts in the applicability of steam injection at a field pilot scale, there was no method introduced to produce heavy oil from fractured carbonate reservoirs [6].

Considering the drawbacks of the conventional SAGD process, some modifications and even alternative recovery processes were proposed to enhance the overall performance of the SAGD process such as the use of solvent (noncondensable gas) along with the injected steam phase to reduce the amount of heat loss to the overburden and also low-pressure SAGD. Those techniques include miscible flooding (VAPEX) or modified versions of SAGD (ES-SAGD) through different configurations of wells or using additives to steam [7,8]. Although steam solvent process(SSP)is a highly promising technique, many uncertainties and unanswered questions still exist and they should be clarified for SSP to world wide applications. The boiling point of the solvent should be near that of steam. When it is to far from steam, it stays either liquid being produced without reacting with oil or gas stopping the heat exchange of steam. In this paper, PVT of some pure and commercial solvents is reviewed and potential operation pressure and temperature are discussed. This can help engineers to select solvent and design the process properly.