Feldspar Geothermometer: A Novel Method Measuring Oilfield Fire-Flooding Temperature

0. INTRODUCTION

Fire flooding is a thermal oil recovery technique in which heating promotes high-temperature oxidation reactions between crude oil and injected air, causing heat and gas release as well as the migration of unburnt crude oil toward production wells. The method is advantageous due to its high recovery rate, low cost, and wide applicability as an effective replacement technique for heavy-oil reservoirs, and has been successfully practiced at the Liaohe Oilfield of China (Zhang et al., 2020; Aleksandrov and Hasçakir, 2015; Elbaz et al., 2015; Hasçakir et al., 2011; Huang et al., 2010).

Evaluation of the underground combustion temperature during fire flooding is essential for understanding the combustion state and its adjustment during hydrocarbon production (Shi et al., 2016). Traditional method for temperature monitoring involves thermocouples placed at specific depths in ignition or production wells (Jiang and Jin, 2014); however, this is costly, has limited monitoring scope, and cannot monitor temperatures in different production layers during fire flooding.

Temperature is a primary factor controlling mineral formation, and minerals formed at various temperatures exhibit distinct differences in composition and crystallinity. Mineral contents and microscopic characteristics of reservoirs thus vary when fire-flooding temperature is different (Zhao et al., 2018; Cheng et al., 2014), and the relationship between the type and content of feldspar minerals at different temperatures enables to determine combustion temperatures in the reservoir at various depths, as investigated here in Block D of the Liaohe Oilfield, northeastern China.

1. GEOLOGICAL SETTING

The Liaohe depression lies on the eastern edge of the North China Craton (Qiu et al., 2022a) and is divided into the western uplift and depression, central uplift, and eastern depression and uplift by the Tanlu and secondary faults (Fig. 1b). Block D lies in the middle segment of the western depression where the Shahejie Formation (Cui et al., 2021), specifically the Cenozoic fourth member (Es₄), is the primary oil-bearing stratum (Wang et al., 2023). The depositional environment is mainly characterized by fan delta front facies with predominant lithofacies including conglomerate, sandstone, and mudstone. Quartz (~62%) and feldspar (~26%) are the predominant detrital minerals, and others are interstitial materials (~12%).

Figure  1.  Sketch map (a) and diagram (b) showing the location and the structural characteristicsof the Liaohe depression (modified from Wang et al., 2023; Qiu et al., 2022b).

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2. ANALYTICAL METHOD

2.1 Sample Preparation

Eight core samples were obtained from well 47-K039 in Block D. Laboratory high-temperature physical simulation experiments were undertaken at the Liaohe Oilfield Exploration and Development Experimental Center (LOEDEC) to investigate variations in type, crystal structure, and content of feldspar minerals with temperature, with the aim of providing a means of determining underground combustion temperatures during fire flooding.

2.2 Analytical Procedures

Experiments employed a resistance-wire muffle furnace (SXJD-V16-10, Suzhou Jiangdong Precision Instrument Co., Ltd., China) comprising a control console and a furnace body. Proportional-integral-derivative (PID) digital control was used to regulate temperature, displaying real-time furnace cavity temperature with an accuracy of ±1 ℃. The furnace body operated at 220V/10A with a maximum power of 6 kW, allowing heating at 100–1 000 ℃ for 40 min.

Core samples were crushed and homogenized, and 500 g samples were heated in ceramic containers for 8 h, after which the furnace was turned off and the samples allowed to be cooled down to room temperature. Samples were subjected to calcination at temperatures of 150, 250, 350, 450, 550, 650, 750, and 850 ℃ before whole-rock, trace-element, and cathodoluminescence (CL) analyses (Wang et al., 1996).

Thin-section CL analyses were undertaken at LOEDEC using a Relion3 (Zeiss, Germany) instrument, and minerals were identified by their luminescence characteristics (Li and Zhang, 2011; Liu and Huangfu, 2000; Lai, 1995).

Major-element analyses were undertaken at the China Geological Survey, Shenyang Geological Survey Center, Shenyang, via electron-probe microanalysis (EPMA; JEOL8100, Japan) using an Inca spectrometer (Oxford Instruments, UK; Yang and Chen, 2003).

After oil extraction and grinding to 320 mesh (~40 μm), the mineral compositions of the samples were determined via X-ray diffraction (XRD) analysis at LOEDEC, using a D8 DiscoverX (Bruker Corporation, the USA) XRD system.

3. RESULTS AND DISCUSSION

3.1 Feldspar Variation

The CL characteristics of samples subjected to various temperatures (Fig. 2) indicate the transformation, kaolinization, and alteration of feldspar in three stages (montmorillonitization, kaolinization, and alteration), as follows.

Figure  2.  Microscopic features of rocks after calcination at different temperatures (cathodoluminescence, 100×). Qtz. Quartz; Kfs. potassium feldspar; Pl. plagioclase; Kln. kaolinite. The spots of electron probe analyses (100×) are shown in Fig. 2i.

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3.1.1   Feldspar montmorillonitization

At temperatures of < 350 ℃, feldspar underwent alkaline decomposition to form montmorillonite. The predominant detrital minerals include reddish-brown quartz, bright blue K- feldspar, and yellow-brown plagioclase (Figs. 2a–2d), and some particles also exhibit dissolution edges and etching pores (Luo et al., 2001). The Block D strata contain abundant interstitial calcites, together with NaHCO₃ in formation fluids, which decompose on heating from 50 to 270 ℃. Under high-temperature conditions, calcite and NaHCO₃ decompose to form NaOH, increasing the alkalinity of the reservoir fluids (Eqs. 1–3).

This leads to the decomposition of feldspar into montmorillonite, resulting in serrated or spotted dissolution pores. It seems that feldspar decomposition increased with temperature.

3.1.2   Feldspar kaolinization

At 350–650 ℃, feldspar underwent acidic decomposition to kaolinite. At these temperatures, NaHCO₃ decomposed completely and the reservoir fluid became weakly acidic as the decomposition of carboxylic acids accelerated. In this mid-stage of fire flooding, carboxylic acids decompose to short-chain acids and CO₂. In later stages, with more oxygen-bearing compounds present, longer-chain acids are formed (Sun, 2018; Fang, 2017; Zhao et al., 2015), rendering the reservoir fluid weakly acidic and causing feldspar to undergo kaolinization. Oxidation occurs during high-temperature (T > 350 ℃) fire flooding, and sulfur in minerals such as FeS₂ and FeS and crude oil reacts with oxygen to produce SO₂, which dissolves in water to form H₂SO₃, further increasing reservoir fluid acidity.

At temperatures of > 450 ℃, the presence of weak organic acids and H₂SO₃ causes kaolinization of plagioclase and K-feldspar from crystal edges (Figs. 2a–2d), resulting in the formation of kaolinite and CaCO₃ from plagioclase. At these temperatures, CaCO₃ decomposes to CaO and CO₂. The transformation of K-feldspar to kaolinite also produces K₂CO₃, which decomposes into K₂O and CO₂.

3.1.3   Feldspar alteration

At temperatures of > 650 ℃, feldspar kaolinization continued to increase with temperature, accompanied by element exchange. Feldspar alteration was initiated at 600–650 ℃, with marginal plagioclase undergoing limited alteration to produce yellow-green plagioclase bands and spotted yellow-green plagioclase (Figs. 2g–2j). The oxidation of Fe²⁺ in feldspar increased with temperature, leading to the formation of hematite, with minor exchange of Ca²⁺ with Mn²⁺. This caused brown feldspar to change to yellow-green feldspar (Fig. 2; Table 1), with this trend increasing with temperature.

Table  1.  Composition (wt.%) of feldspar samples

Major oxides	Sample 1	Sample 2	Sample 3
MgO	0.051	0.183	0.002
Al2O3	18.551	18.01	15.792
SiO2	69.328	65.1	64.392
K2O	7.097	10.7	12.792
CaO	0.009	0.15	0.033
TiO2	-	0.003	0.011
MnO	0.15	-	-
FeO	0.17	0.622	0.067
BaO	0.106	-	0.145
Na2O	4.333	5.242	4.5
P2O5	0.008	0.019	-

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3.2 Variations in Feldspar Content

Results of whole-rock XRD analyses are shown in Fig. 3 and Table 2, with feldspar content varying with temperature and reservoir fluid properties. During montmorillonitization at < 350 ℃, some feldspar underwent alkaline decomposition to form montmorillonite, with a reduction of 2.8%–4.0% in plagioclase content, whereas the K-feldspar content remained relatively stable. During kaolinization at 350–650 ℃, some feldspar underwent high-temperature acidic decomposition to form kaolinite, with K-feldspar and plagioclase contents decreasing by 4.4%–7.8% and 4.0%–10.0%, respectively. Decomposition of K-feldspar resulted in the formation of kaolinite, quartz, K₂O, and CO₂. During alteration at > 650 ℃, further acidic and high-temperature decomposition of feldspar occurred, leading to continued reduction in feldspar content and the generation of kaolinite. However, owing to the lower content of short-chain organic acids (formed by the breakdown of organic matter in the reservoir), the extent of feldspar decomposition remained relatively stable across different temperature intervals, with no widespread decomposition.

Figure  3.  Scatter plot of whole-rock quantitative X-ray diffraction analysis for fire-heated samples. Quartz refers to the left scale bar, and the others refer to the right scale bar.

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Table  2.  Mineral contents (wt.%) of the whole-rock sample for the fire flooding

Temperature (℃)	Clay minerals	Quartz	K-feldspar	Plagioclase	Calcite	Siderite	Dolomite	Feldspar	Carbonate	Total
50	7.00	62.00	9.00	17.10	2.70	0.70	1.40	26.10	4.90	100
150	7.10	63.00	8.70	16.40	2.60	0.75	1.40	25.10	4.75	100
250	6.60	63.00	8.70	17.40	2.00	0.80	1.50	26.10	4.30	100
350	6.70	63.02	8.60	17.85	1.85	0.75	1.40	26.45	4.50	100
450	6.10	66.05	8.30	15.80	1.60	0.50	1.45	23.50	3.55	100
550	5.80	67.85	8.15	14.80	1.40	0.20	1.45	22.95	3.40	100
650	4.00	66.80	9.70	16.10	1.20	-	1.35	25.80	2.55	99
750	3.50	69.90	9.90	14.60	0.65	-	1.00	24.50	1.65	100
850	2.10	69.00	11.20	16.05	0.45	-	0.50	27.25	0.95	99
'-', below the detection limit; data source Shi (2020).

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4. FIELD TESTS

Fire flooding was undertaken in Well 47-040C of Block D at Liaohe Oilfield. Rock samples were then obtained by core drilling, and their CL images were compared with those of the experiments to determine temperatures. Based on the alteration characteristics of clay minerals during high-temperature calcination, montmorillonitization of feldspar was evident (Fig. 4a), indicating a fire-flooding temperature of ~350 ℃ at a depth of 943.0 m. Spotted yellow-green plagioclase was observed at the edges of detrital particles together with feldspar kaolinization (Fig. 4b), indicating a temperature of < 650 ℃ at a depth of 944.8 m. No feldspar kaolinization or yellow-green plagioclase is evident in Fig. 4c, indicating a fire-flooding temperature of < 150 ℃ at a depth of 949.7 m.

Figure  4.  The CL images of borehole cores from Well 1-47-K039 (100×) showing effective of fire flooding. (a) 943.0 m; (b) 944.8 m; (c) 949.7 m.

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Based on this analysis, the fire-flooding temperature was estimated to be ~300 ℃ at 943.3 m depth (a coking zone) and 550–650 ℃ at 944.8 m (the burnt zone or leading edge of fire flooding), whereas at 949.7 m, the original oil zone was not affected by fire flooding. The upper part of the reservoir, influenced by air coning, had a higher air migration rate and strong gas adsorption, resulting in a rapid advance of the fire flooding front at an average of 2.5 cm d^-1. In contrast, the lower part of the reservoir had relatively low air migration with an average advance rate of 1.0 cm d^-1.

5. CONCLUSIONS

Through identification of different feldspar transformation patterns at various temperatures, the transformation of feldspar was divided into three stages: montmorillonitization at 150–350 ℃ with high-temperature alkaline decomposition leading to the formation of montmorillonite; kaolinization at 450– 550 ℃ with acidic decomposition; and alteration at 650–850 ℃ with element exchange and the formation of spotted or banded yellow-green feldspar. Based on this method of estimation and core sample analyses, fire-flooding temperatures in Well 47-040C were ~300 ℃ at 943.3 m depth and 550–650 ℃ at 944.8 m, indicating the burnt zone of fire flooding, and < 150 ℃ at 949.7 m, representing the original oil zone.