A Laboratory Evaluation of Suitable Operating Strategies For Enhanced Heavy Oil Recovery By Gas Injection

Abstract

Abstract A majority of Saskatchewan's heavy oil reservoirs are thin and marginal, unsuited for thermal recovery methods. The immiscible CO2 or gas displacement process appears to be a very promising enhanced oil recovery technique for these heavy oil reservoirs. This process, if proven applicable, will access 90% of the initial-oil-in-place. Since an inexpensive source or CO2 is not available in Saskatchewan/Alberta, alternative gases containing CO2 can be used. This paper discusses results of a laboratory study carried out by Saskatchewan Research Council (SRC) to assess the suitability of a flue gas containing 85 mol% N2 and 15 mol% CO2, and to evaluate various operating strategies for heavy oil recovery. The test heavy oil (14 º API gravity) was collected from Senlac reservoir located in the Lloydminster area. The study investigated the effect on oil recovery of drive type (secondary vs. tertiary), injection mode (slug test vs. water- alternating-gas [WAG] flood), oil type (Senlac live oil vs. dead oil), and injection gas (flue gas vs. CO2). Results of one-dimensional linear coreflood tests (conducted at 28 ºC, 2.5–2.7 MPa) with Senlac oil-flue gas led to four major conclusions. (1) The secondary gas slug flood with live oil was the most suitable injection strategy for heavy oil recovery from among three different injection strategies (secondary slug, tertiary slug, and tertiary WAG) investigated because this flood had the highest displacement efficiency; (2) The tertiary WAG process recovered more oil than the tertiary slug process because the former predominantly improved mobility control; (3) Ultimate oil recovery was higher in runs conducted using live oilthan in those using dead oil. This is attributed to the relatively more favourable mobility of live oil and slightly higher operating pressure (2.7 MPa instead of 2.5 MPa) in live oil runs; (4) Flue gas appeared to be an effective flooding agent, since the oil recoveries were only 2 to 4 percentage points lower than those obtained with CO2, Comparable oil recoveries in flue gas runs were believed to be a combined result of two competing mechanisms -a free gas mechanism provided by N2 and a solubilization mechanism provided by CO2 in contrast to the solubilization mechanism (provided by CO2) which predominates in CO2 floods. Introduction In Canada, Alberta and Saskatchewan account for nearly all of the proven heavy oil resource. In Saskatchewan alone, l.5 billion m3 of proven and 3.4 billion m3 of probable heavy oil reserves exist,(2) over 62% of the total resource(1).. These reservoirs are, however, characterized by thin pay, shaly sand, heterogeneity, low roductivity, and bottomwater(3, 4). Of the province's proven initial heavy oil-in-place, 97% is contained in reservoirs less than 10 m pay zone, and 55% in reservoirs less than 5 m thick(5, 6). Primary and secondary methods combined recover only about 7% of the initial-oil-in-place (IOIP) of the proven resource(2). In comparison, such conventional techniques can access, on average, about 22% of the proven 0.43 billion m3 of medium oil. The remaining heavy and medium oil resources of Saskatchewan offer potential for exploitation by tertiary recovery techniques.