Study on the Rule of Change of Reservoir Rocks under Subcritical Steam Effect

Abstract

This paper examines the lithology, pore throat, and fluid characteristics of the reservoir in Liaohe Oilfield Block 3624, using group V rocks as an example, combining a high-temperature and high-pressure reaction still with other equipment such as a rock mechanics tester and a scanning electron microscope. This study also designs and develops three controlled variable experiments, including the subcritical steam reservoir rock dissolution experiment, the subcritical steam reservoir rock mineral composition transformation experiment, and the subcritical steam reservoir rock mineral mechanical property experiment, also making clear the rule of change on the part of the rocks in the deep and heavy oil reservoirs after the injection of subcritical steam. Experimental results reveal the following: (1) Steam causes the dissolution of rocks, and when the steam temperature is in the subcritical region, dissolution is visible. After a 350 °C subcritical steam treatment, the relative melting temperatures of the components of rock materials are substantially greater than the melting point of the cement holding them together, causing the cement to significantly dissolve and a secondary crack network to emerge. (2) The mineral composition of the rocks changes as a result of elevated temperatures, with various mineral transformation trajectories being recorded after various steam treatments. Montmorillonite in reservoir rocks is converted into minerals like illite and chlorite in the subcritical steam temperature range. Another element influencing the creation of secondary cracks on rock surfaces is the reciprocal transformation of minerals, which alters the cohesiveness among mineral components. (3) Rocks suffer thermal damage and changes to their mechanical characteristics as a result of high-temperature steam dissolution and mineral transformations; the severity of these changes increases with the steam temperature.

1. Introduction

Scholars have steadily increased their studies on the thermal degradation of rocks in the process of steam injection thermal recovery since the establishment of steam injection recovery technology in heavy oil reservoirs. McCorriston et al. [1] analyzed the damage mechanism of steam-flooding reservoirs under the influence of unstable clay minerals, condensate pH and other factors. Amaefule [2] et al. experimentally found that serious scale precipitation formed near the production wells during steam the injection process and clogged the pore space, etc. Amaefule et al. [3] pointed out that the damage caused by temperature on reservoir pore permeability could not be ignored, and high injection temperatures could cause serious damage to the reservoir. Lei [4] studied solid-phase particle transport in the process of steam injection and found that the dissolution of feldspar and quartz under the action of high-temperature steam destroyed the rock skeleton, and in severe cases, the phenomenon of sand and gas flurry occurred in the oil well. Scholars found that [5,6] under high-temperature conditions, compared with clay minerals, the mineral composition in the rock is more stable, and experiments found that [7] after steam flooding, the wettability of the reservoir was altered, and the reservoir was transformed from oleophilic to hydrophilic. Fan et al. [8] believe that after high-temperature, high-pH steam injection into the formation, the main dissolved component is quartz, while the rock in the reservoir becomes loose, and the permeability of the reservoir is reduced by 70% or more. Pang et al. [9] found that the reservoir permeability of a heavy oil reservoir was negatively correlated with temperature and pH during steam injection. Su et al. [10] explored the mechanism of temperature-sensitive damage in the thermal recovery of extra super heavy oil reservoirs, and pointed out that the mineralization of the injector should be strictly controlled during steam thermal recovery. Cheng et al. [11] found that the long-term scouring of the formation by high-temperature steam can lead to changes in the physical parameters of the reservoir; meanwhile, the porosity and permeability within the range of the steam wave in each small layer increase with the increase in the throughput rounds, and the oleophilicity of the reservoir rock is gradually weakened, while the hydrophilicity is gradually strengthened. Huang et al. [12] found that after steam flooding, the heavy components of oil were mapped to be retained in the formation and strengthened the cementation degree of the rock, thus affecting further development. Guo et al. [13] found that the use of CO₂+steam+sodium alpha olefin sulphonates stabilized the skeleton structure of the reservoir rock and prevented the deterioration of non-homogeneity in the subsequent development of heavy oil reservoirs for thermal recovery.

The mineral components of reservoir rocks will change to varying degrees when exposed to steam, and the temperature and steam state will dictate the course of this transition. Vinsin et al. [14] mixed formation minerals according to a certain proportion of high-temperature reactions and finally came up with the reaction equation for the transformation of dolomite to calcite under high-temperature conditions: dolomite + quartz + kaolinite→montmorillonite + calcite + CO₂ + H₂O. AHayatdavoudi et al. [15] found that under high-temperature conditions, kaolinite is converted to montmorillonite and at the same time may generate clastic rocks through other reaction paths, leading to a reduction in the pore space of the reservoir and causing a decrease in permeability. Wang et al. [16] carried out a study on the water–rock reaction in the reservoir after steam injection, pointing out the transformation conditions between montmorillonite, illite, and kaolinite. Ma [17] studied the water–rock reaction of the Liaohe Oilfield reservoir cores at temperatures ranging from 150 to 300 °C by means of a water–rock reaction device and found that the total clay minerals as well as the content of potassium feldspar increased with the increase in temperature. Wang [18] pointed out that under the conditions of 250 °C–400 °C superheated steam, the main reaction path of the core is that montmorillonite, kaolinite, and quartz react to produce illite; kaolinite, potassium feldspar, and quartz react to produce chlorite and illite; and, at the same time, the superheated steam will also change the thermal conductivity of the rock.

In order to compute and assess the parameters of the mechanical properties of rocks after thermal damage, researchers have sequentially created experimental models of rock thermal damage ontology based on the thermal damage of rocks by steam. Liu [19] proposed the concept of thermal damage and derived the thermal damage evolution equation of elastic modulus based on the three-point assumption, which can predict the elastic modulus of different temperature nodes in the thermal damage state:

$$D\left(T\right)=1-E_T∕E_0$$

(1)

$$D\left(T\right)=b_0+b_{1}T+b_2{T}^2$$

(2)

$$E\left(T\right)=E_0\left[1-D\left(T\right)\right]$$

(3)

$D\left(T\right)$ is thermal damage; $E\left(T\right)$ is the elastic modulus of rock specimen when the temperature is T; $E_{0 }$ is the elasticity of rock specimen at 20 °C; $b_0,b_1,b_2$ for material parameters.

Acoustic emission and other auxiliary tools were used by Zhang Zhizhen [20] to demonstrate the link between mechanical force and temperature in rock materials. He also proposed the thermal–mechanical coupling factor, which is a function of temperature t and strain and has a Gaussian distribution. This variable exposes a portion of the nonlinear interaction between temperature and force.

$$\mu ={\mu }_0+\frac{Aexp\left[\frac{-2{\left(\epsilon -{\epsilon }_c\right)}^2}{{\omega }^2}\right]}{\omega \sqrt{\pi /2}}$$

(4)

${\mu }_0$ characterizes the lower limit of thermal–mechanical coupling; ${\epsilon }_c$ is the peak strain of rock; $\omega$ is the standard deviation of the function, indicating the degree of set of the thermal–mechanical coupling of rock materials.

Experimental studies on thermal damage to hard rocks under thermal–mechanical interaction were undertaken by Gao [21]. He used the assumption that the strength of rock microelements follows a Weibull distribution and used the Drucker–Prager criterion as the failure criterion. The evolution equation for thermal damage in hard rocks was developed by Li et al. [22], who introduced the Drucker–Prager yield criterion and residual strength correction coefficient, the Weibull distribution of thermal damage, and three covariates based on the existing rock deterioration coupling model; established a damage coupling model considering the rock initiation stresses; and analyzed the results of the mechanical tests on hard and brittle mica granite to verify the damage coupling model.

The following challenges arise when the oil field develops ultra-deep heavy oil reserves and uses ordinary saturated steam or superheated steam for deep heavy oil development: ① the necessity for increased steam injection pressure, particularly near the block’s edge in areas with poor physical well properties; the use of conventional steam throughput has a poor development effect in these areas; ② injected steam because of heat loss along the extremely long wellbore, which causes the latter steam to reach the bottom of the wells with insufficient dryness. Only 29.9 percent of the injected steam met the required dryness, falling short of the threshold across 33 wells, or 26.2%.

Liaohe Oilfield uses the injection of subcritical/supercritical steam throughput methods to address the aforementioned issues. During several rounds of throughput, the wellhead steam of most wells was in a subcritical state, with a temperature of 350–374.5 °C and a pressure of 15.7–19.6 MPa (according to the boiler), while the wellhead steam of a few wells during the throughput period was in a supercritical state, with the temperature of the steam at the outlet end of the boiler >374.5 °C and the pressure >22.1 MPa. However, the measured temperature of the steam at the bottom of the wells could only reach 350 °C on average. When the four indexes of peak production, the oil increase in the same period, the oil production in the cycle, and the oil increase in the cycle of the wells after the conventional steam injection and this steam injection method were compared, it was discovered that the measures were effective in a total of 15 wells, or 41.7 percent of the wells measured. A total of 36 wells implemented subcritical/supercritical high-temperature and high-pressure steam injection.

In contrast to saturated-steam or superheated-steam heavy oil recovery technologies, the absence of a theoretical study on subcritical-steam deep oil recovery has hindered its widespread popularization and implementation in oil fields, despite field test results confirming the technology’s viability. In order to investigate mechanisms of subcritical steam on reservoir rocks, to clarify the effect of subcritical steam on the pore-permeability structure and mineral composition of reservoir rocks, and to provide theoretical support for the screening of technologically adapted blocks and the optimization of the development strategy (e.g., fracturing method and plugging design) in oil field sites, the authors analyzed the characteristics of the reservoir rocks in the GAO 3624 block of the Liaohe Oilfield and evaluated the fluid properties of the reservoir rocks in this block as the object of the research; the authors also designed and carried out three experiments, including the subcritical steam reservoir rock dissolution experiment, the subcritical steam reservoir rock mineral composition transformation experiment, and the subcritical steam reservoir rock mineral mechanical property experiment.

2. Experimental Set-up and Methodology

2.1. Reservoir Characteristics

The surface crude oil density of Block 3624, a pure heavy crude oil resource, ranges from 0.9449 to 0.9653 g/cm³, with an average of 0.954 g/cm³. The 50 °C degassed crude oil has a viscosity of 3150 to 4000 mP·s and a solidification point of 8 to 20 °C. It has a wax concentration of 4.15% to 4.46%, a colloidal and asphaltene content of 43.74% to 46.09%, and a sulfur content of 0.424% to 0.573%. The reservoir is situated between 1600 and 1950 m below the surface. It is a lithologic structural reservoir because it ends in the west and is impeded by faults in the east. The reservoir is distributed northeasterly along the long axis, with the main portion of the block being thicker and thinning down toward the east and west edges. Both the V and VI sandstone strata in this block include comparatively well-developed sand bodies. This study’s rock core samples are all from the V sandstone unit. Sandstone, pebbly sandstone, sandy conglomerate interbedded with siltstone, and mudstone make up the majority of the V sand body’s lithology. Sandstone conglomerate predominates among them, and the pebbles often have sub-rounded to sub-angular forms and sizes between 3 and 5 mm. Locally, the rocks include calcium and mud gravel, which give them their characteristic clayey cementation. The feldspar and quartz pebbles mostly consist of rock fragments and quartz, with some feldspar displaying weathering processes. This sand body’s effective thickness ranges from 10 to 95 m, and the sand body is most developed in the southwest direction and in the center of the block; the sand body gradually becomes thinner from the center to the two flanks, and from the southwest to the northeast direction, and the thickness of the sand body is the greatest around the wells of Gao 3-6-0211, Gao 3-6-0215, and Gao 3-6-252.The V sandstone unit’s oil layers are well developed, with usually good oil-bearing characteristics and oil saturation levels between 50% and 60%. Around wells like H3-6-0211, H3-6-0215, and H3-6-252 in the center, the thickest oil layers with the finest oil-bearing characteristics are visible. Due to the impact of the dispersion of sand bodies, oil saturation and layer thickness progressively diminish in other places. This sandstone unit has a total porosity range of 13.5% to 23%, an effective porosity range of 11% to 20%, and a permeability range of 200 to 1500 mD. The best physical characteristics may be found in the area around wells like H3-6-0222, H3-6-0232, and H3-6-023C in the center, where porosity typically ranges from 18% to 23%, and permeability ranges from 700 to 1500 mD. The physical characteristics increasingly deteriorate as one moves from east to west. The western peripheral characteristics of well H3-6-23C are subpar, with a permeability of only 100 to 200 mD and a porosity of around 12%.

2.2. Subcritical Steam Reservoir Rock Dissolution Experiment

The purpose of the subcritical steam dissolution experiment was to examine the characteristics of erosion and cementation destruction of reservoir rocks under various steam types. The Block V Sandstone Formation of the Liaohe Oilfield is the source of all experimental core samples; thus, we will not go into great depth about its characteristics here. A focused ion/electron dual-beam electron microscope (15120116), a vacuum pump, a nitrogen cylinder, and a high-temperature, high-pressure reaction still was all employed in the experiment. The main piece of the experimental apparatus was a reaction vessel with high pressure and temperature. The four main parts of the experimental system were as follows: injection, reaction vessel, temperature–pressure monitoring and control, and liquid production sampling (Figure 1). The separately designed reaction vessel has a pressure resistance of 40 MPa and a temperature tolerance of up to 370 °C to enable safe testing at subcritical conditions. Heating jackets were also fitted to the intermediate containers and tubes to prevent temperature loss throughout the experiments. Six experimental groups were set up in line with the actual bottomhole steam temperatures (350–360 °C) that were observed throughout production and development in the target region. These groups included room temperature (25 °C), wet steam at 150 °C, 200 °C, 250 °C, 300 °C, and 350 °C subcritical steam. The goal was to investigate how subcritical steam, as opposed to ordinary steam, impacts the dissolution and cementation damage of reservoir rocks. The reaction period was set at 4 days for each group of studies to guarantee that these reactions progressed fully.

The experimental steps are as follows:

(1)

After cleaning the oil-stained geological core, put it inside the reaction vessel, put the equipment together, and check the reaction vessel’s airtightness.

(2)

Use a vacuum pump to remove the empty reaction vessel after making sure it is airtight.

(3)

Determine the reaction’s steam and reaction vessel heating temperatures.

(4)

After the reaction, remove the geological core and dry it in a box with a continuous temperature of 50 °C.

(5)

Examine the dried geological core under SEM (10 mm).

2.3. Subcritical Steam Reservoir Rock Mineral Composition Transformation Experiment

To learn more about the microscopic effects of different types of steam on reservoir rocks, conduct experiments on subcritical steam-induced mineral transformations in reservoirs. You should specifically aim to clarify changes in the mineral and clay components of rocks after exposure to subcritical steam. The experimental core samples were again taken from the Liaohe Oilfield’s Block V Sandstone Formation. The experimental set-up included an XRD, a nitrogen cylinder, a vacuum pump, and a high-temperature and high-pressure reaction still. The current experiment was separated into five groups: room temperature (25 °C); 200 °C wet steam group; 250 °C wet steam group; 300 °C wet steam group; and 350 °C subcritical steam group. Our research examined the effects of subcritical and ordinary steam on rocks, with a particular focus on the mineral and clay components found in reservoir rocks. Each study group’s reaction duration was set at four days to ensure that the experimental reactions progressed fully.

The experimental steps are as follows:

(1)

Oil and grind the geological core that you took out of the stratum, weigh a certain amount of rock core powder, and then combine it equally with deionized water before adding it to the reaction vessel. Put the reaction apparatus together and check the airtightness.

(2)

After excellent airtightness has been established, use a vacuum pump to drain the reaction vessel.

(3)

Start the reaction vessel’s heating process and keep it going for 4 days while keeping the temperature and pressure configuration.

(4)

Place the reacted geological core and water in a beaker, and air dry the combination in a box with a constant temperature at 50 °C.

(5)

Use a mortar and pestle to pulverize the air-dried geological core to a fineness of 100 mesh, and then conduct XRD examination to determine the composition of the rock components.

2.4. Subcritical Steam Reservoir Rock Mineral Mechanical Property Experiment

SEM scanning and X-ray composition testing of minerals have shown that high-temperature steam may induce rock disintegration and mutual transition between high-temperature minerals. Rocks may generate microcracks as a result of thermal expansion incompatibility across grain boundaries caused by anisotropic particles with varying crystal orientations exhibiting a range of thermal elastic characteristics [23]. The rock’s mineral particles’ coefficient of thermal expansion is altered as a result. As the temperature of the steam increases, fractures may enlarge and form a network of fissures, resulting in the formation of new microcracks. This process may result in changes to the mechanical properties and elastic-plastic behavior of the rock. Based on these results, an experiment was designed and carried out to examine the effects of high steam temperatures on the mechanical properties of reservoir rocks under subcritical steam.

Three sets of rocks with different permeabilities made up the experimental rock cores, which were taken from the Liaohe Oilfield’s Gao 3624 Block V sandstone deposit. A high-temperature and high-pressure reaction vessel, a vacuum pump, a nitrogen cylinder, and a GCTS rock mechanics testing device were all parts of the experimental set-up. The experiment was split into three main groups, each with nine subgroups: room temperature (25 °C); wet steam groups operating at 250 °C; and subcritical steam groups operating at 350 °C. The reaction period was consistently fixed at 4 days for each experimental group to guarantee a thorough response.

The experimental steps are as follows:

(1)

After washing the obtained formation cores with oil, place it in the reaction kettle, assemble the reaction steam generator, reaction kettle, and other equipment, and check the airtightness of the reaction kettle.

(2)

After confirming good airtightness, use a vacuum pump to evacuate the empty reaction kettle.

(3)

Set the steam and reaction kettle heating temperatures for the reaction, which lasts for 4 days.

(4)

Remove the reacted core and place it in a constant temperature box at 50 °C for air drying;

(5)

Perform triaxial mechanical test experiments on the air-dried core by using the GCTS rock mechanics testing equipment with a confining pressure of 19.2 MPa.

3. Results and Discussion

3.1. Subcritical Steam Dissolves the Rock and Induces Secondary Cracks

Figure 2a shows how the granite is often extremely compacted with a significant amount of cementation, and bigger pores are rarely seen locally. However, partial breakdown of the rock particle cementation starts to happen following treatment with 150 °C wet steam in Figure 2b; however, the level of dissolution is restricted, and the matrix remains mostly compacted. The rock surface shows obvious signs of spalling after being treated with 200 °C wet steam, as shown in Figure 2c for comparison, which lowers the rock’s cementation degree. The rock is still mostly compressed overall, though. The rock’s surface and contact regions show more obvious symptoms of dissolving following exposure to 250 °C wet steam treatment, which further reduces the degree of cementation and causes the rock to start becoming porous in Figure 3a. The rock’s surface and contact regions experience substantial dissolving by high-temperature steam after treatment with 300 °C wet steam, which also causes the emergence of a few noticeable secondary fractures in Figure 3b. The unconsolidated particles on the surface of the rock vanish after 350 °C subcritical steam treatment, while secondary fractures widen, and the cementation on the rock’s surface and contact regions is largely destroyed, resulting in a highly porous and loose structure of the rock in Figure 3c.

Therefore, compared to normal wet steam, subcritical steam causes a certain level of permanent damage to both the degree of cementation of reservoir rocks and the particles adhering to the rock surfaces. Furthermore, a sizable number of secondary cracks are produced by rocks under the action of high-temperature subcritical steam because the internal cementation inside the material breaks first and causes cracks to form since the internal cementation within the material is quite high and significantly exceeds the melting temperature of the inter-particle cementation in the rock material. Additionally, minute interior fissures have a propensity to grow and spread outward. Under subcritical circumstances, the rock’s pore structure will be slightly altered by the dissolution damage phenomenon caused by reservoir rocks, increasing the rock’s porosity and permeability after dissolution. However, additional experimental investigation and verification are necessary to determine whether the abrasion effect might result in the obstruction of rock pore channels during dynamic core displacement, potentially affecting the dynamic development efficiency of subcritical steam. This is due to the intense abrasion effect of subcritical steam.

3.2. The Content of Montmorillonite Decreased after Subcritical Steam

Table 1. Table of changes in mineral content.

Number of Samples	Quartz (%)	Potassium Feldspar (%)	Plagioclase (%)	Calcite (%)	Dolomite (%)	Clay Minerals (%)
25 °C	49.8	22.8	20.5	1.5	1.3	4.1
200 °C	48.9	18.6	20.4	1.4	6.4	4.3
250 °C	48.0	17.5	20.3	1.5	8.2	4.5
300 °C	47.4	16.9	20.3	1.6	8.8	5.0
350 °C	47.1	18.9	20.4	1.5	1.4	10.7

and

Table 2. Table of changes in clay mineral content.

Number of Samples	Chlorite (%)	Illite (%)	Kaolinite (%)	Montmorillonite (%)
25 °C	4.2	14.6	28.3	52.9
200 °C	4.5	11.9	14.9	68.7
250 °C	4.7	10.8	14.3	70.2
300 °C	4.7	10.3	12.3	72.7
350 °C	18.4	22.3	8.7	49.6

indicate how the mineral and clay components of the rocks experience a variety of changes as a result of the action of different types of steam.

According to a researcher’s study findings [24], montmorillonite content increases and the mineral composition of the rocks changes to favor the production of montmorillonite under wet steam conditions. The soft, extremely expanding clay known as montmorillonite is pliable to movement and fluid flow. When petroleum is extracted, it frequently displays modest sand production characteristics, which blocks reservoir voids and reduces reservoir permeability, making it difficult to recover heavy oil. In contrast, montmorillonite content decreases in subcritical steam conditions, and the mineral is depleted within the rocks as a result of reactions. We can conclude the following about the interactions between reservoir rocks and superheated steam by looking at the changes in mineral composition:

montmorillonite + kaolinite + quartz–illite

(5)

kaolinite + potassium feldspar + quartz–illite + chlorite

(6)

When water vapor in a subcritical condition interacts with the clay minerals in reservoir rocks, it can evaporate water molecules present in the mineral layers as well as absorb water adsorbed on the clay surface. When the surface charge cannot be balanced due to this disruption of the diffusion double electron layer on the crystal surface, the crystal lattice is likely to reorganize. Clay minerals start to change when the interlayer spacing of montmorillonite keeps getting smaller. Montmorillonite initially goes through dehydration in order to become interstratified illite–montmorillonite minerals. Illite can develop further if kaolinite and quartz are present. Temperature, pressure, and the medium’s environment are the key determinants of kaolinite’s stability. Potassium feldspar crystals provide K+ ions at subcritical steam conditions, which causes kaolinite, potassium feldspar, and quartz to change into illite and chlorite.

According to several studies [25,26,27], reservoir characteristics can be adversely affect-ed by an overabundance of illite. Authigenic illite is frequently found in intergranular pores, where it creates pore-bridging structures that diminish the pore-throat radius and, by slicing several holes and throats, create small, restricted pores. Additionally, the degree to which authigenic illite bends within sandstone pores and throats increases with the length of the fibrous character of the material, showing a negative association with porosity and permeability. According to Cao Jian et al. [28], the influence of illite on reservoir characteristics is dynamic and necessitates research into processes like fluid flow. Detrital illite, for example, demonstrates malleable plasticity. A rise in formation pressure following medium injection during the development process may cause particles to separate from the rock’s surface and then be transported by the medium, causing blockages when they clog pores and throats.

The underlying crystal structure of rocks, the intermolecular cohesiveness between various mineral components, and the thermal expansion and contraction of the rock’s component particles have all been changed as a result of mineral transformations brought on by steam. Such conditions result in an increase in the thermal motion of molecules, a decrease in the contacts between molecules, and sub-lattice dislocations. Rocks produce thermal stress simultaneously as a result of the thermal deformation mismatch, which causes thermal cracking in the rocks and the development of secondary cracks on the rock surface.

3.3. Elastoplastic Changes of Rock after Subcritical Steam

Figure 4, Figure 5 and Figure 6 shows the plotted axial/radial stress–strain curves of various permeability rocks following varying steam effects, based on the subcritical steam reservoir rock mineral mechanical property experiment.

The primary stress value at the last peak point on the stress–strain curve is referred to as compressive strength, which represents the ultimate strength of the rock under compressive stress circumstances. A higher elastic modulus shows more stress during elastic deformation of the rock, signifying stronger material stiffness. Elastic modulus is a physical term that quantifies a material’s resistance to deformation. Poisson’s ratio measures a material’s capacity for lateral deformation.

These three physical variables jointly characterize the mechanical characteristics of the rock to some extent. In order to compare the trends of compressive strength, modulus of elasticity, and Poisson’s ratio of rocks with different levels of permeability after steam effect at different temperatures, the three mechanical property parameters are calculated by combining axial/radial stress–strain curves with the calculation formula. The compressive strength can be derived directly from curves.

For elastic modulus and Poisson’s ratio, the current work uses the formulae for calculating elastic modulus and Poisson’s ratio under confining pressure [29] in order to remove the interference of confining pressure:

$$E_c=\frac{{\left({\sigma }_a\right)}_{B1}-{\left({\sigma }_a\right)}_{A1}}{{\left({\epsilon }_a\right)}_{B1}-{\left({\epsilon }_a\right)}_{A1}}=\frac{\left(q_{B1}+{\sigma }_c\right)-\left(q_{A1}+{\sigma }_c\right)}{{\left({\epsilon }_a\right)}_{B1}-{\left({\epsilon }_a\right)}_{A1}}=\frac{q_{B1}-q_{A1}}{{\left({\epsilon }_a\right)}_{B1}-{\left({\epsilon }_a\right)}_{A1}}$$

(7)

$$\begin{gathered} v_c=\frac{{\left({\epsilon }_r\right)}_{B2}-{\left({\epsilon }_r\right)}_{A2}}{{\left({\epsilon }_a\right)}_{B2}-{\left({\epsilon }_a\right)}_{A2}}=\frac{1}{2}\frac{\left[{\left({\epsilon }_a\right)}_{B2}-{\left({\epsilon }_v\right)}_{B2}\right]-\left[{\left({\epsilon }_a\right)}_{A2}-{\left({\epsilon }_v\right)}_{A2}\right]}{{\left({\epsilon }_a\right)}_{B2}-{\left({\epsilon }_a\right)}_{A2}} \\ =\frac{1}{2}\left[1-\frac{{\left({\epsilon }_v\right)}_{B2}-{\left({\epsilon }_v\right)}_{A2}}{{\left({\epsilon }_a\right)}_{B2}-{\left({\epsilon }_a\right)}_{A2}}\right] \end{gathered}$$

(8)

$E_c$ is elastic modulus; $v_c$ is Poisson’s ratio;${\left({\sigma }_a\right)}_{B1} \mathrm{and} {\left({\sigma }_a\right)}_{A1}$ are the effective axial stress (MPa) at points A1 and B1, respectively;$q_{A1} \mathrm{and} q_{B1}$ are the effective shear stress (MPa) at points A1 and B1, respectively;${\left({\epsilon }_a\right)}_A \mathrm{and} {\left({\epsilon }_a\right)}_B$ are the axial strain at points A1, A2 and B1, B2, respectively; ${\left({\epsilon }_r\right)}_{A2} \mathrm{and} {\left({\epsilon }_r\right)}_{B2}$ are the radial strain at points A2 and B2, respectively; volumetric strain at points A and B in this paper, where positive volumetric strain represents compression; thus, the volumetric strain can be expressed as ${\epsilon }_v=2{\epsilon }_a-2{\epsilon }_r$.

The calculated results are shown in tables.

Table 3. Compressive strength of rock.

	500 mD	1000 mD	1500 mD
25 °C	90 MPa	84 MPa	68 MPa
250 °C	85.5MPa	75 MPa	59.5 MPa
350 °C	80 MPa	65.5 MPa	50 MPa

shows the compressive strength of rocks with different levels of permeability.

Table 4. Elastic modulus of rock.

	500 mD	1000 mD	1500 mD
25 °C	23 GPa	16 GPa	12 GPa
250 °C	17.5 GPa	15 GPa	11.7 GPa
350 °C	15.7 GPa	11 GPa	7.5 GPa

shows the modulus of elasticity of rocks with different levels of permeability.

Table 5. Poisson’s ratio of rock.

	500 mD	1000 mD	1500 mD
25 °C	0.16 f	0.20 f	0.24 f
250 °C	0.21 f	0.24 f	0.28 f
350 °C	0.26 f	0.32 f	0.37 f

shows Poisson’s ratio of rocks with different levels of permeability.

The rock curve passes through four major stages, which may be identified by examining the axial strain/radial strain–effective deviatoric stress plot [30,31,32]: the compaction stage, the linear elastic stage, the weakening stage, and the failure stage. When the stress reaches its peak, the rock sample cracks quickly and displays brittle failure. Analyzing axial/radial stress–strain curves at 25 °C, the rock’s non-elastic deformation process is rather brief. When the steam temperature rises to 250 °C and 350 °C as shown in Figure 4 and Figure 5, however, the slope of the linear elastic segment declines, suggesting a reduction in elastic modulus with rising temperature, and the peak strength is dramatically reduced. As a result of the weakening of rock brittleness and a rise in ductility, the axial strain also exhibits an increasing tendency. Strength thus declines, while plasticity is increased. In rock cores with various levels of permeability, this pattern of curve variation is often constant.

Low permeability causes rocks to generally become denser, have a lower elastic modulus, and have reduced compressive strength. Calculations based on Table 3, Table 4 and Table 5 show that rock cores with different levels of permeability exhibit better compressive strength and elastic modulus together with a lower Poisson’s ratio at a temperature of 25 °C. This implies that, under certain conditions, rocks become stiffer and more brittle. While, from a microscopic perspective, the bonding between atoms, ions, molecules, crystal structures, and mineral components inside the rocks remain reasonably stable, rocks in this condition have a stronger resistance to elastic deformation [33]. The compressive strength and elastic modulus of rock cores with various levels of permeability drop after treatment with 250 °C wet steam, but Poisson’s ratio rises as a result of the high-temperature steam. The changes in compressive strength, elastic modulus, and Poisson’s ratio within the temperature range of 25–250 °C are relatively small at this point, with the lowest change in compressive strength, showing that the rocks still maintain good mechanical properties. The effect of temperature on internal cementation damage in rocks is also insignificant. A comparison of the magnitude of change in the mechanical parameters of Table 3, Table 4 and Table 5 shows that after treatment with 350 °C subcritical steam, there have been noticeable changes in the temperature ranges of 250–350 °C for compressive strength, elastic modulus, and Poisson’s ratio compared to the prior temperature range. Under the influence of high-temperature steam, rock cores with varying levels of permeability show clear thermal damage, including a significant reduction in compressive strength and elastic modulus, an increase in Poisson’s ratio, and a transition of the rocks from a strongly brittle state to an elastic-plastic state.

Rocks are thermally damaged to varying degrees after being exposed to varying steam temperatures. Thermal damage in sandstone is divided into four stages by Zuo [34]: the unstable thermal damage stage, the early thermal damage stage, the steady thermal damage stage, and the fast thermal damage stage. The sandstone’s clay components still hold water during the unstable thermal degradation stage. The elastic modulus of the sandstone decreases as the temperature rises because of the strengthening of free water’s ability to dissolve cementing substances and clays between minerals. When the temperature rises above the water’s boiling point during the initial thermal damage stage, some of the free water evaporation causes the sandstone to reharden and become denser, which causes an increase in stiffness and a modest elevation in the sandstone’s elastic modulus. During the steady thermal damage stage, the sandstone begins to experience thermal cracking, which causes a decline in its rigidity. In the fast thermal damage stage, thermal cracking in the sandstone becomes more prominent as the temperature rises. The number, length, and breadth of the thermal cracks also grow, which causes the sandstone’s elastic modulus to drop rapidly.

4. Conclusions

To examine the static impact mechanisms of steam on reservoir rocks in the high-temperature subcritical steam injection development of deep heavy oil reservoirs, three tests were carried out utilizing rock samples from Block 3624 in the Liaohe Oilfield. The results of the experiment show that subcritical steam dramatically modifies the reservoir rocks’ degree of cementation, mineral content, and mechanical characteristics. The results of the experiments are as follows:

(1)

Subcritical steam at 350 °C has a more pronounced dissolution effect compared to conventional saturated steam. In reservoir rocks, this activity causes permanent secondary cracks that subsequently affect the rock’s porosity and permeability properties. The once-dense rock core surfaces and contact surfaces no longer exhibit obvious bonding after dissolution treatment. Images taken under a scanning electron microscope show how the granite core incurs free and secondary cracks form at the same time. On the one hand, the impact of hot, high-pressure steam causes the creation of tiny cracks on the rock’s surface, increasing the rock’s ability to flow fluids. The dislodged solid particles, on the other hand, may move with medium transfer following disintegration, causing pore-throat obstruction and unfavorable effects on porosity and permeability.

(2)

With different kinds of steam, different rock minerals and clay constituents change in different ways. Montmorillonite in reservoir rocks changes into other mineral components, such as illite and chlorite, within the temperature range that corresponds to subcritical steam. According to the results of X-ray composition testing, the amount of montmorillonite in the rock rises by 30% when wet steam interacts with mineral components in the rock. The degree of this change also increases with temperature. However, in the presence of subcritical steam, montmorillonite in the rock undergoes chemical reactions and is destroyed, ultimately generating compositions like montmorillonite + kaolinite + quartz–illite and kaolinite + potassium feldspar + quartz–illite + chlorite, among others. These mineral changes alter the cohesiveness between the components, which encourages the development of secondary cracks on the rock’s surface and gives rise to an explanation for their genesis.

(3)

Subcritical steam at 350 °C has an adverse effect on the mechanical qualities of reservoir rocks. Due to the thermal damage brought on by high-temperature subcritical steam, including mineral change and dissolution, the mechanical characteristics of the rock are altered. There are fewer secondary fractures and minimal variations in compressive strength, elastic modulus, and Poisson’s ratio as the temperature rises from room temperature to 250 °C because high-temperature steam has little impact on the internal bonding of the rock. This is because there is little transformation between clay minerals. However, as the temperature increases from 250 °C to the subcritical state at 350 °C, the rock suffers from severe thermal damage that causes bonding to be significantly disrupted, porosity to significantly increase, secondary fractures to significantly increase, illite and montmorillonite to mutually transform, and calcite to change into aragonite. Numerous factors have an effect on the rock’s mechanical properties, causing decreases in the rock’s compressive strength, elastic modulus, and Poisson’s ratio. High-temperature subcritical steam also causes the rock to gradually change from being a rigid body to an elastic-plastic body.

Following the subcritical steam effect, the rock dissolves readily, and the formation of microcracks alters the rock’s and the mineral components’ elasticity and plasticity. Thus, by using subcritical steam to optimize the fracturing and development process, deep heavy oil reservoirs with stratigraphic conditions similar to Block 3624 may potentially benefit from increased pore flow capacity and increased microfractures. Additionally, subcritical steam may lessen the occurrence of water sensitivity in the reservoir by altering the mineral composition of the rocks, which is more favorable for the development of reservoirs with continuous steam injection.

The current study provides evidence for a portion of the mechanism underlying the subcritical steam development of heavy oil reservoirs. Further research can focus on the degree of dissolution effect and its two-sided influence on the porosity and permeability of reservoir rocks, the connection between the formation’s rapid sensitivity and large amounts of ilmenite generation, and the plugging and unplugging process. Simultaneously, studies on subcritical steam reforming heavy oil may be conducted to elucidate the workings of subcritical steam on heavy oil and enhance the process of subcritical steam development of heavy oil reservoirs.