Downhole Upgrading Athabasca Tar Sand Bitumen Using THAI - SARA Analysis

Abstract

Abstract Downhole upgrading of virgin Athabasca Tar Sand bitumen has been investigatede in a series of 3-D experiments using THAI - ‘Toe-to-Heel Air Injection’. The process uses direct line drive, or a staggered arrangement of horizontal producer wells; in combination with either horizontal or vertical wells for gas injection (HIHP or VI2HP). The 3-D tests involved THAI as a primary process, steam injection aslo as a primary method using a new steam flood process, THSF (Toe-to-Heel steamflood), and THAI on a secondary recovery mode, post-THSF. Except for THSF, very high oil recoveries were achieved: THAI primary: 80% OOIP, THSF: 23% OOIP and THAI secondary: 66.7% OOIP. The low THSF recovery is attributed to the low steam temperature, because of the low cell opersting pressure. Very significant upgrading of the Athabasca bitumen occurred during the air injection tests for the majority of the experimental period. The API gravity of the produced oil increased by 8 API points and the viscosity decreased to 50 to 1000 mPa.s, compared with the original bitumen viscosity of 1,000,000 mPa.s. There was also a substantial decrease in sulphur, nitrogen and heavy metals content of the produced oil. The quality of the in-situ upgraded oil was investigated using SARA analysis (Saturates, Aromatics, Resins, Asphaltenes). This revealed that there was a dramatic increase in the saturates content of the produced oil, up to 62% to 72%, compared with 15.5% for the original bitumen. Key words: Air injection, In situ combustion, Heavy oil recovery, Athabasca Tar Sand, THAI, Downhole upgrading, Horizontal well, SARA 1. Introduction There are extreme large resources of heavy oil and bitumen existed throughout the world, most in Venezuela and Canada. However, heavy oil presents many technical challenges at all stages of oil processing. During the oil production phase, heavy oils are much more difficult to recover from the reservoir. Cold production of heavy oil only achieves recovery of less than 20% OOIP (original oil in the place). Usually, thermal methods are required to recover the remaining oil after cold production. In situ combustion (ISC) is one of the thermal enhanced oil recovery (EOR) processes for the heavy oil production, which is achieved by burning a small fraction of the oil in the reservoir in order to enable flow of the unburned fraction. It has been extensively studied for many years as a method of heavy oil recovery1,2,3. Compared with other thermal methods of heavy oil recovery, the In situ combustion has certain potential advantages, such as high thermal efficiency, high oil recovery, less total environment impact and better economics. However, conventional ISC (using vertical-injection and vertical producer wells) has suffered from serious operational problems, due to gas overriding and channelling. Onsequently, the ISC process has not achieved the overall success in the field that theory predicts. In ...., many of the failures can be attributed to the poor selection of reservoir, and there were also some significant successes4. THAI - ‘Toe-to-Heel Air Injection’ THAI is a new EOR process, which integrates in situ combustion and advanced horizontal well concept5. It uses a horizontal producer well instead of a vertical producer well (conventional ISC). Furthermore, the injection well (vertical or horizontal) and horizontal producer well(s) are arranged in line drive. The combusion front propagates along the horizontal well, from the ‘toe’ to the ‘heel’ position (Fig. 1).