Analysis of Water Injection and Heat Recovery Potential of Abandoned Oil Wells Transformed into Geothermal Wells in Northern Shaanxi

Abstract

The Chang 2 bottom water reservoir area in the western part of northern Shaanxi constitutes one of the key oil-producing regions within the Ordos Basin. A principal reservoir here is the Triassic Yanchang Formation’s Chang 2 reservoir, which is characterized by favorable physical properties, dynamic edge and bottom water activity, and a high geothermal gradient. This study employs the STARS module of the CMG reservoir numerical simulation software to model water intake and heat recovery processes in the target region. It analyzes the heat recovery rate and efficiency of three water production methods—direct water extraction, four-injection–one-production, and one-injection–four-production—under varying injection pressures. The results indicate that direct water extraction from the bottom water reservoir is challenging. However, the annual heat recovery per well for the four-injection–one-production and one-injection–four-production methods equates to a standard coal production ranging between 190 and 420 tons, suggesting that there is significant potential for water injection and heat recovery in the Chang 2 reservoir in the western part of northern Shaanxi.

1. Introduction

China’s proven geothermal resources rank second in the world. The reserves of geothermal resources represented by the hydrothermal type total about 1250 billion tons, but their current development level is only 2%. First of all, from the perspective of resource development, if we vigorously develop geothermal resource projects, the annual exploitation of geothermal resources will be equivalent to half of China’s coal consumption in 2015. Makauskas’ research on the potential development of geothermal energy in Lithuania also shows that geothermal energy development has both environmental and economic advantages [1]. As a renewable energy source, geothermal energy helps to reduce greenhouse gas emissions, improve air quality, reduce dependence on imported fossil fuels, save foreign exchange, increase energy security, and create jobs in multiple sectors. Therefore, the development potential of geothermal resources is beyond doubt. Secondly, from the perspective of the treatment of abandoned oil wells, the number of abandoned oil wells is increasing with the rapid development of China’s oil industry. Abandoned oil wells emit a large amount of gas and liquid that pollute the environment, and a lot of manpower and material resources are required to improve the strict sealing of wells. The use of structurally complete abandoned oil wells to develop geothermal energy for building heating or refrigeration, industrial production, power generation, etc., can not only boost energy utilization, but also reduce the drilling cost of new geothermal wells and the cost of sealing abandoned oil wells. In Barbier’s research, the cost of drilling a new geothermal well accounted for 50% of the total cost of geothermal projects [2]. Therefore, the development direction of abandoned oil wells to geothermal wells is worthy of further study. According to research conducted by Xu Fuqiang et al., repurposing abandoned oil wells as geothermal wells for heat extraction can significantly reduce the capital expenditure associated with geothermal projects, thereby enhancing their economic efficiency [3]. Additionally, reservoirs that have reached their economic water production limit can be utilized for heat extraction, facilitating the co-production of heat and residual oil resources.

In the process of injection and heat recovery, Zhang Jie et al. believed that when using abandoned oil wells to extract heat [4], attention can be paid to the well pattern distribution of oil fields. Choosing a reasonable injection–production ratio and well pattern distribution can improve geothermal development. They numerically simulated the heat production methods of the two systems of one-injection–one-production and two-injection–one-production. The simulation results showed that the heat production performance of the two-injection–one-production system was better than that of the one-injection–one-production system under the same system conditions. Guo Tao used the data of the Liubei buried hill reservoir in Huabei Oilfield to establish a thermal-water-mechanical multi-field coupling mathematical model [5], simulated the geothermal development of abandoned high-temperature oilfields, and analyzed the sensitivity of the geothermal development effect to well pattern, fracturing conditions, and working media. The results showed that the heat transfer area of the one-injection–four-production well network was the largest, and the hydraulic fracturing model led to an earlier thermal breakthrough, but generated more heat than the non-fracturing model.

In this paper, combined with the injection–production heat transfer effect of different well patterns in the research status and the existing well pattern structure in the western part of northern Shaanxi, the STARS module of the reservoir numerical simulation software CMG is used to analyze the production potential of direct water production, four-injection–one-production and one-injection–four-production in the Chang 2 bottom water reservoir in the western part of northern Shaanxi.

2. Heat Transfer Analysis

2.1. Geothermal Geological Conditions in the Study Area

(1)

Thermal reservoir temperature conditions

The terrestrial heat flow value is the heat flow value per unit area of the earth’s surface per unit time. This parameter eliminates the influence of time, and is currently the most representative parameter of the earth’s surface temperature. The terrestrial heat flow and temperature gradient can be used to estimate the temperature conditions of thermal reservoirs in a certain range of underground. The more active the tectonic activity is, the higher the terrestrial heat flow value is, and the more stable the structure is, the smaller the terrestrial heat flow value is. The average value of heat flow in the Ordos Basin in central China is 55~80 mW/m², which is smaller than that in Tibet, but larger than that in Xinjiang and other regions. In addition, the geothermal gradient in various regions of China is 1.5~4.0 °C/100 m, with an average of 3.2 °C/100 m. In general, the geothermal gradient in the north is larger than that in the south, and that in the east is larger than that in the west. The average geothermal gradient in the south is about 2.45 °C/100 m, and the average geothermal gradient in the north is about 3 °C/100 m.

(2)

Regional geological resource reserves

Geothermal resources in China are mainly derived from hydrothermal geothermal resources and dry hot rocks. The study area is located in the Ordos Basin. The resources in the Ordos Basin have the characteristics of good reservoir conditions, many reservoirs, large thickness, and wide distribution.

As shown in Figure 1, according to the understanding of geothermal resource reserves provided by Wang Guiling and others [6], the recoverable heat of geothermal fluid in the Ordos Basin is not the highest in the national basins, but the total amount still reaches 308 million tons/year, so it still has great geothermal development potential. In addition, Kang Jing et al. evaluated the geothermal resources in the Yanchang oil and gas area in the southeast of Ordos [7], and the Chang 2 bottom water reservoir in the west of northern Shaanxi also belongs to the geothermal resource enrichment area. In the research status, we mentioned that the area in which the reservoir is located can multiply its geothermal production effect. Chang 2 is a medium-porosity and low-permeability reservoir with active bottom and edge water, so it has the required conditions for water extraction and heat extraction after abandonment.

2.2. Direct Water Intake from Bottom Water Reservoir

The water cut of oil and gas fields is classed as high when the water cut is 70–98%. However, due to continuous injection and production, the formation and reservoirs of oil and gas fields have changed greatly. The current development focus has developed from a large range of connected remaining oil into a smaller range. When the aquifer exceeds 98%, these wells will be shut down, and some will even be permanently shut down and become abandoned wells. After the oil and gas resources in the oil and gas wells are exhausted, if there are hydrothermal geothermal resources, the oil layer can be blocked and converted into a water layer, so that the geothermal water is transported to the heating station through the geothermal pipe network, and then heat exchange is carried out by the heat exchanger to transfer the heat of the geothermal water to the circulating water. The heated circulating water is then transported to the user through the heating pipe network. After the reservoir is fully water-flooded, sometimes, a bottom water reservoir will form. A bottom water reservoir refers to a reservoir with large and sheet water at the bottom of the reservoir. This kind of reservoir stops oil production due to abandonment after water injection. However, due to the rich water resources in such reservoirs, it is possible to try to directly take water for heat recovery or to inject heat, and change abandoned oil wells into geothermal wells for heat recovery operations.

The northern Shaanxi oil region is a “three low” reservoir. Based on the basic data of the Chang 2 reservoir in a target block, the situation of water production and heat extraction is simulated. The reservoir’s physical parameters are shown in

Table 1. Physical parameters of reservoir.

Physical Parameters	Parameter Value
Porosity	18%
Permeability	20 × 10−3 μm2
Reservoir top depth	1200 m
Reservoir thickness	6 m
Reservoir bottom depth	1206 m
Reservoir temperature	45 °C
Reservoir pressure	7 MPa
Rock compression coefficient	0.27 × 10−4 MPa−1
Formation water salinity	28,870 mg/L

, and the formation heat recovery model is established on this basis. When constructing the three-dimensional geological model for direct water extraction in the study area, a region with both horizontal and vertical dimensions of 800 m was established, and the depth of the vertical range of the model was 1200~1206 m. A homogeneous model was utilized, meaning that the calculation area of the model in the study area was composed of a cube of 800 m × 800 m × 6 m. The model grid was divided using an isometric method. The initial formation conditions of the reservoir model for Block Chang 2 can be found in Table 1. Additionally, the model boundaries were defined as thermally insulated and impermeable.

As shown in Figure 2, the water layer is perforated, and then the water layer is extracted for heat extraction. During water production, the bottom hole flow pressure is controlled to be 0.2 MPa, and each layer is ejected.

The water production and heat extraction simulation is carried out using the model in Figure 3, and Figure 4 shows the change in water production and formation pressure without water injection. It can be seen from the simulated production data that the water production decreases rapidly after well opening, from a maximum of 28 m³/d in the initial stage to less than 10 m³/d within less than 100 days, and decreases to about 1 m³/d within 11 months. This production curve is very similar to the scenario of depletion development of the oilfield in this area. The reason for this is mainly related to the low-permeability physical properties of the reservoir. Therefore, in this low-permeability reservoir, relying on natural pressure to extract water and heat will lead to a serious decline in the average pressure of the formation, and it is unsustainable to exploit groundwater only by natural energy. There is a starting pressure gradient in low-permeability reservoirs, and there is often a low-speed, high-resistance and non-flowing area between oil and water wells, which is the main reason affecting the development effect of low-permeability oilfields.

2.3. Injection–Production Heat Exchange

In the analysis of well pattern distribution of the Chang 2 reservoir conducted by Liu Xue et al. [8], the well pattern distribution of the Chang 2 reservoir in the target area has a more flexible diamond-shaped inverted nine-point well pattern. Therefore, the existing abandoned well pattern is suitable for a variety of injection–production methods. However, it can be seen from the research status that the heat exchange of two-injection–one-production is greater than that of one-injection–one-production, and the heat exchange area of the one-injection–four-production well pattern is the largest. Therefore, two well pattern structures, four-injection–one-production and one-injection–four-production, were selected. Combined with different injection pressures, the heat recovery of the Chang 2 bottom water reservoir in western Shaanxi was analyzed. For the establishment of the thermal model for injection in the study area, a homogeneous model was also adopted. Drawing on the previous literature, a reservoir thickness of 50 m was selected, and the cubic model grid was divided into dimensions of 1000 m × 1000 m × 50 m using an isometric method. The initial formation conditions and boundary settings of the Chang 2 reservoir model in this study block are consistent with those of the direct water extraction model, the injection–production well spacing is set at 200 m, and layers 2 through 9 are shot.

2.3.1. Four-Injection–One-Production Scheme

As shown in Figure 5 and Figure 6, p1 represents the production well, i1~i4 represent the injection well, and the three-dimensional temperature distribution can be seen.

It can be seen that the water injection volume is declining in Figure 7. It should be the case that as the total injection volume increases, the formation pressure increases and the water injection rate slows down. In addition, it can be seen that the greater the injection pressure, the greater the daily water production rate. When the injection pressure is 15 MPa, the formation pressure can be replenished in time. When the injection pressure is 10 MPa, the injection pressure is too small to replenish the formation pressure in time, so the water production volume decreases first, and then remains unchanged. When the injection pressure is 20 MPa, the injection volume is large, the formation is replenished or even increased, and a high permeability channel may be formed [9]. In the case of sufficient formation pressure, the water production rate continues to rise, but, since it is limited by the low porosity and low permeability of the reservoir, the increase in water production rate becomes gradually more gentle.

It can be seen from Figure 8 that the daily heat recovery rate increases at first and then levels out, which is related to the water recovery rate. The cumulative heat recovery is related to the heat recovery rate and the number of years [10]. The larger the heat recovery rate is, the longer the heat recovery period is, and the greater the cumulative heat recovery is.

2.3.2. One-Injection–Four-Production Scheme

P1~P4 represent the production well, and i1 represents the water injection well. From Figure 9 and Figure 10, the three-dimensional temperature distribution can be seen.

It can be seen from Figure 11 that the daily water injection rate at 20 MPa and 15 MPa decreases, but at 15 MPa, it decreases slowly. This is because after a period of injection, the formation pressure around the injection well gradually increases [11], the pressure around the production well becomes smaller, the production pressure difference becomes smaller, and the injection rate becomes slower. In addition, it can be seen in (b) that the greater the pressure, the greater the daily water extraction rate.

From the heat extraction situation depicted in Figure 12, it can be seen that the water extraction rate decreases, the heat extraction rate decreases, and the cumulative heat extraction increases more and more slowly with time.

The statistics of heat recovery under different injection–production methods and injection pressures were calculated, as shown in

Table 2. Effects of different heat recovery models and injection pressure conditions on heat recovery.

Injection–Production Method	Injection Pressure (MPa)	Single Well Annual Heat Production (J)	Annual Cumulative Heat Recovery of System (J)
Four-injection–one-production	10	5.68 × 1012	5.68 × 1012
	15	7.71 × 1012	7.71 × 1012
	20	9.65 × 1012	9.65 × 1012
One-injection–four-production	10	2.33 × 1012	9.33 × 1012
	15	2.70 × 1012	1.08 × 1013
	20	3.06 × 1012	1.22 × 1013

.

It can be seen that the greater the injection pressure, the greater the annual heat production of a single well. Under the same injection pressure, the annual heat production of a single well with the one-injection–four-production method is greater than that of a single well with the four-injection–one-production method. However, there are four production wells with the one-injection-four-production method; therefore, under the same injection pressure, the annual heat production of the system with four-injection–one-production is less than that of the system with one-injection-four-production.

Using the “reference conversion coefficient of various energy sources and standard coal” issued by Guizhou Provincial Energy Bureau in November 2018, the coal price is 558.5 yuan/ton, and the conversion coefficient of thermal energy to standard coal is 0.0341 kgce/MJ. The heat obtained each year is converted into standard coal, which is converted into data such as carbon emissions and carbon dioxide emissions. It can be seen from

Table 3. Comparison of heat recovery between four-injection–one-production and one-injection-four-production methods.

Injection–Production Method	Injection Pressure (MPa)	Annual Heat Extraction (J)	Convert Standard Coal Quantity (t)	Reduced C Emissions (t)	Reduced CO2 Emissions (t)	Profits (Ten Thousand Yuan)
Four-injection–one-production	10	5.68 × 1012	193.8	129.8	476.1	38.8
	15	7.71 × 1012	263.0	176.2	646.2	52.6
	20	9.65 × 1012	329.2	220.6	808.8	65.8
One-injection-four-production	10	9.33 × 1012	318.2	213.2	781.7	63.6
	15	1.08 × 1013	368.6	247.0	905.5	73.7
	20	1.22 × 1013	417.0	279.4	1024.3	83.4

that the amount of standard coal converted from annual heat production has certain economic value, and reduces the carbon emissions corresponding to standard coal, supporting the energy development goal of “carbon neutralization and carbon peak” [12]. Therefore, the abandoned oil wells in the western part of northern Shaanxi have the potential for significant heat injection.

3. Conclusions

(1)

The edge and bottom water of the Chang 2 bottom water reservoir in the western part of northern Shaanxi are more active, and the geothermal temperature condition is better, giving it the potential to replace abandoned wells with geothermal wells for water and heat recovery.

(2)

If the direct water extraction of the reformed well is equivalent to the depletion production, the formation pressure drops seriously, and cannot be continuously mined.

(3)

CMG simulation shows that when the water injection pressure is between 10 and 20 MPa, the water injection pressure increases and the average annual heat recovery increases.

(4)

In the CMG model, the simulation time of 50 years is longer. In the early stage, there are four production wells with the one-injection–four-production method, so the total heat exchange is greater than that of the four-injection–one-production wells. However, with development, the water production rate of the one-injection–four-production wells decreases rapidly, while the water production rate of the four-injection–one-production wells is relatively stable. Therefore, four-injection–one-production wells have the potential for long-term development in well reform. However, in the simulated 50 years, the annual average heat production proves that one-injection–four-production wells are feasible in a short period of time.

(5)

The average annual heat production of the other two injection–production methods is converted into a standard coal production between 190 and 420 t. Therefore, whether using the one-injection–four-production method or the four-injection–one-production method, it is proven that the Chang 2 bottom water reservoir in the western part of northern Shaanxi has the ability to take water and heat.