Article pubs.acs.org/EF
Laboratory Investigation on the Feasibility of Light-Oil Autoignition for Application of the High-Pressure Air Injection (HPAI) Process Hu Jia,*,† Jin-Zhou Zhao,*,† Wan-Fen Pu,† Yong-Ming Li,† Zhong-Tao Yuan,‡ and Cheng-Dong Yuan† †
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, People’s Republic of China ‡ Tarim Oilfield, China National Petroleum Company (CNPC), Korla, Xinjiang 841000, People’s Republic of China ABSTRACT: Whether thermal effects have a contribution on oil recovery mechanisms in the high-pressure air injection (HPAI) process in a light-oil reservoir has confused many people over several decades. The argument is due to the lack of convictive evidence from both direct laboratory and simulation studies. This paper is the prelusion for the subsequent study for a better understanding of the influence of thermal effects on oil production. In a previous study, thermogravimetry/differential thermogravimetry (TG/DTG) and differential thermal analysis (DTA) tests were employed to investigate the kinetic parameters and exothermic behavior of Keke Ya light crude oil (Tarim Basin, China) and oil + cuttings, showing that Keke Ya light crude oil as well as oil + cuttings exhibited very low activation energy and favorable exothermic behavior. In this study, we develop a thermal effect monitoring device, which is similar to the isothermal oxidation tube, and we have detected the obvious exothermic phenomenon of Keke Ya light crude oil in porous media in the isothermal aging test at reservoir conditions (80 °C and 16.7 MPa). The witness of the combustion (or high-temperature) process is exhibited, which is expressed in specific characteristics made up of the self-heating behavior, low hydrogen/carbon (H/C) ratio of 1.50 from gas composition analysis, and generated precipitation coverings (halite crystalloid) in the core section surface through scanning electron microscopy (SEM) analysis. This study provides new insight that thermal effects should exist during the light-oil isothermal aging process in porous media and proposes a laboratory analysis method to understand the feasibility of light-oil autoignition at the oil-bearing reservoir during application of HPAI.
1. INTRODUCTION
Over the years, HPAI has been considered a simple fuel-gas flood, giving little credit to the thermal drive as a production mechanism,14 where the flue gases provide the main driving mechanism for oil production and the thermal effects (i.e., bulldozing effect) have very little impact on the oil recovery mechanism. Montes et al. suggested that this combustion front (bulldozing effect) acts as a bulldozer to mobilize most of the oil immediately ahead of it that cannot be displaced by any of the other driving mechanisms present (i.e., flue gases, hot water/steam displacement, etc.).14 Although the presence of this front has been recognized for decades, its relevance to the air injection process has not always been interpreted properly.15 The pre-existing field evidence from the Buffalo Red River Unit (BRRU) and South Buffalo Red River Unit (SBRRU) in the Buffalo field indicates that oil is actually burning and suggests that the combustion front is having a favorable impact on the production performance of the field.16,17 However, with time gone, many people still keep a conservative viewpoint about oil recovery contributed by thermal effects. For instance, Sarma and Das expressed an uncertain viewpoint that thermal effects would be expected to contribute more to the oil production by way of oil-viscosity reduction.18 The crucial reason is due to the lack of laboratory studies. Thus, laboratory investigation on the feasibility of lightoil autoignition is needed. The goals of this paper include (1)
Most of the current world oil production comes from mature fields. Increasing oil recovery from the aging resources is a major concern for oil companies. Thermal methods have been tested since 1950s, and they are the most advanced among enhanced oil recovery (EOR) methods, as far as field experience and technology are concerned.1 The thermal method can be split into steam-assisted gravity drainage (SAGD),2 steam flooding,3 cyclic steam stimulation (CSS),4 and in situ combustion.5−7 Air injection in light-oil reservoirs [high-pressure air injection (HPAI)] has gained greater attention during the past decade. HPAI has been successfully applied in the Williston Basin, West Hackberry in the U.S for more than 20 years and is currently being considered by many operators for application in their assets.8,9 HPAI projects have been steadily increasing in recent years, especially in light-oil carbonate reservoirs in the U.S.10 The recovery mechanisms that may be attributed to air injection are summarized as follows:11−13 (1) improved sweep efficiency because of flue-gas sweeping, (2) rapid repressurization of the reservoir, (3) “stripping” of light volatiles by flue gas and subsequent natural gas liquids (NGL) recovery, (4) oil swelling by flue gas, (5) injection gas substitution because of the generation of CO2 in situ, (6) miscibility of flue gas with oil, and (7) operation above the critical point of water and, therefore, potential superextraction benefits associated with steam fingering ahead of the combustion front, resulting in viscosity reduction and oil mobilization. © 2012 American Chemical Society
Received: February 29, 2012 Revised: August 1, 2012 Published: August 1, 2012 5638
dx.doi.org/10.1021/ef301012d | Energy Fuels 2012, 26, 5638−5645
Energy & Fuels
Article
ment tools to intuitively verify the combustion (or hightemperature) experienced history.
on the basis of the kinetic parameters of Keke Ya light crude oil, preliminary estimating the feasibility of Keke Ya light-oil autoignition during the HPAI process at reservoir conditions and (2) discerning the evidence of combustion (or hightemperature) experienced history for Keke Ya light crude oil in porous media by macroscopic and microcosmic analysis. On the basis of this laboratory study, model assumptions and model parameters in a subsequent paper for understanding the influence of thermal effects on oil production will be wellfounded. It is well-known that the reservoir can provide a favorable adiabatic condition. There are available laboratory adiabatic systems, such as accelerating rate calorimetry (ARC). In addition, devices, such as ARC, differential scanning calorimetry (DSC), and derivative thermogravimetry (DTG), have been used to investigate thermal effects of oil. However, these devices are the qualitative analysis methods that are valuable “fingerprints” to the oxidation/combustion/cracking behavior of given oil under low-pressure and high heating rate conditions. The oxidation behavior and exothermic characteristic of crude oil in porous media at reservoir conditions are much different than that which would occur if the gas was passing through an oil-bearing reservoir. It should be noted that a small-batch, high-pressure isothermal reactor (SBR), such as the oxidation tube experiment, is very helpful for studying the low-temperature oxidation (LTO) process.19,20 In this study, we develop a thermal effect monitoring device (see Figure 1).
2. EXPERIMENTAL SECTION 2.1. Thermal Tests. Thermal studies have been conducted to obtain the kinetic parameters and exothermic behavior of test oil, and we briefly summarize the previous study first. Four representative actual consolidated sandstone core samples (numbered 66, 70, 71, and 74) were cut from the X52 pay zone in the Keke Ya oilfield (Tarim Basin, China), and light crude oil is obtained from the Ke 322 well in the X52 pay zone. This oil field is a condensate gas reservoir with formation temperature and pressure around 80 °C and 16.7 MPa, respectively, and oil viscosity and American Petroleum Institute (API) density at surface conditions (25 °C and 1 atm) are 3.7 mPa s and 44.3° (0.805 g/cm3), respectively. According to a previous study, the Keke Ya reservoir is expected to have favorable conditions for the HPAI project implementation based on the present reservoir screening criteria.13 Hydrocarbon distribution was determined by oil chromatography (see Figure 2). It is evident that more than 70.55 mol % of compounds fall in the C5−C16 group. X’Pert Pro MPD (Philips, Ltd., The Netherlands) is used to investigate the rock composition, and the calculated values of rock composition are summarized in Table 1. Netzsch STA 409 PC/PG (Netzsch, Ltd., Germany) with a thermogravimetry/differential thermogravimetry (TG/DTG) and differential thermal analysis (DTA) module was employed for thermal analysis. The actual reservoir cores are crushed to the same size of 50−60 mesh. It is assumed that the test cuttings can provide the same contribution for oil oxidation because of the surface effects. In the thermal study, a constant mass flow of air at the rate of 30 mL/min was set for each test. The normal TG/DTG and DTA tests were conducted at atmospheric pressure, and the temperature range was 25−650 °C with the heating rate of 10 °C/min. Prior to experiments, temperature calibration was performed. Sample weights of the oil-only and oil + cuttings combined tests were around 90 and 60 mg, respectively. For oil−cutting combined tests, oil and cutting weights are both around 30 mg. All experiments were performed twice to see the reproducibility. The oxidation of hydrocarbon is a complicated phenomenon because numerous components of crude oil are simultaneously oxidized. All of the models to determine the kinetic parameters are based on the Arrhenius kinetic theory. Kök and Acar developed a simplified model (Arrhenius method) that assumes that the rate of the mass loss of the total sample is only dependent upon the rate constant, the mass of the sample remaining, and the temperature with the reaction order of unity. The final form of the equation takes the following form:21−23
dw/dt = kw n k = A r exp(− E /RT ) Assuming first-order kinetics
dw/dt = A r exp(− E /RT )w [dw/dt /w] = A r exp(− E /RT ) Taking the logarithm of both sides
log(dw/dt /w) = log A r − E/2.303RT When log(dw/dt/w) is plotted versus 1/T, a straight line is obtained, which will have a slope equal to E/2.303R, and the values of E and Ar can be obtained from the slop and intercept of the linear fit line. k is the reaction rate. n is the reaction order. w and T are the mass of the sample remaining and the temperature at a certain time t. R is the universal gas constant. E and Ar represent the activation energy and Arrhenius constant. 2.2. Isothermal Oxidation Tube Experimental Study. 2.2.1. Thermal Effect Monitoring Device Description. Generally, controllable heating devices are used for monitoring heat produced by oxidation. However, these kinds of devices have a limited volume for
Figure 1. Schemes of the thermal effect monitoring device.
More details are given in the follow section. In addition, gas chromatography (GC) (HP 6890 series, Agilent Technologies, Inc.), Fourier transform infrared (FTIR) spectrometry (WQF520 series, Beijing Beifen-Ruili Analytical Instrument Co., Ltd.), scanning electron microscopy (SEM) (Quanta 450 series, FEI Co., Ltd.) microanalysis approachs are conducted as supple5639
dx.doi.org/10.1021/ef301012d | Energy Fuels 2012, 26, 5638−5645
Energy & Fuels
Article
Figure 2. Hydrocarbon distribution for test oil.
Table 1. X-ray Analysis Results of the Representative Sandstone Lithology in the Keke Ya Pay Zone clay minerals (wt %)
other minerals (wt %)
core number
illite
illite/smectite
smectite
kaolinite
chlorite
gypsum
quartz
orthoclase
plagioclase
calcite
dolomite
metallic salts
66 70 71 74
6.194 3.756 7.328 6.566
3.077 1.861 2.665 3.442
0.000 0.000 0.000 0.000
0.584 0.392 0.727 0.485
2.920 1.646 2.907 2.424
1.100 0.500 0.500 0.800
51.700 70.000 38.200 67.000
2.400 6.100 3.000 2.100
21.200 9.100 31.400 8.900
10.300 6.700 13.100 8.100
0.600 0.000 0.200 0.300
