As the most frontiers in petroleum geology, the study of dissolution-based rock formation in deep carbonate reservoirs provides insight into pore development mechanism of petroleum reservoir space, while predicting reservoir distribution in deep-ultradeep layers. In this study, we conducted dissolution-precipitation experiments simulating surface to deep burial environments (open and semiopen systems). The effects of temperature, pressure, and dissolved ions on carbonate dissolution-precipitation were investigated under high temperature and pressure (~200°C; ~70 Mpa) with a series of petrographic and geochemical analytical methods. The results showed that the window-shape dissolution curve appeared in 75~150°C in the open system and 120~175°C in the semiopen system. Furthermore, the dissolution weight loss of carbonate rocks in the open system was higher than that of semiopen system, making it more favorable for gaining porosity. The type of fluid and rock largely determines the reservoir quality. In the open system, the dissolution weight loss of calcite was higher than that of dolomite with 0.3% CO2 as the reaction fluid. In the semiopen system, the weight loss from dolomitic limestone prevailed with 0.3% CO2 as the reaction fluid. Our study could provide theoretical basis for the prediction of high quality carbonate reservoirs in deep and ultradeep layers.
Oil and gas formed in carbonate formations take up to 60% of the total oil and gas resources worldwide [
After the deposition of carbonate sediments, it went through the penecontemporaneous stage, the early, the middle, and the late diagenetic stage, and the epigenetic stage. The valuable reservoir porosity of carbonate rocks is largely related to the fluid transformation process in the diagenetic process. Pore was destructed and developed in early meteoric zone diagenesis. In late stage of dissolution, porosity developed due to hydrocarbon maturation and destruction [
With the simulation experiment and numerical simulation, it is possible to clarify the relationship between the dissolution process of limestone and dolostone and temperature, pressure, and fluid. Plummer et al. [
The carbonate and fluid interaction can be regarded as a solid-gas-liquid three-phase coexisting reaction system. In the system, carbonate rock served as the solid phase for dissolution or precipitation; the liquid and gas filled in the porosity served as the fluid. The main factors influencing the interaction between solid and liquid are temperature, pressure, pH, fluid/solid ratio, mineral surface structure, reaction surface area, and so on [
M represents alkali metal element (Ca, Mg, etc.). During reaction, CaCO3 or CaMg (CO3)2 dissolved and Ca2+ and Mg2+ entered into the fluid. The reaction progress can be monitored quantitatively by the change of metal cation concentration or the mineral weight loss before and after the reaction.
This experiment is carried out on carbonate reservoir dissolution rate instrument which is designed and manufactured by our lab (Figure
Carbonate reservoir dissolution simulation instrument (Type XYD-II).
During the experiment, the temperature and pressure of the reactor were gradually raised to the setting values. The fluid was pressurized in the reservoir tank and then flowed into the high-temperature and high-pressure reactor through the pipeline (Hastelloy). Six parallel quartz reaction tubes could be placed in the reactor. The fluid first flowed through the tube and then react with the carbonate sample. Following (
Following a geothermal gradient of 2.5°C/100 m, the temperature and pressure from earth surface to the formation depth (7000 m) ranged from 25~200°C and 1~70 Mpa, respectively. The temperature of the reactor was heated gradually around 5°C/min. The fluid flow rates were set at 1 ml/min and 0 ml/min for the open and the semiopen geological fluid system, respectively (Table
Experimental conditions.
Reaction fluid | Temperature |
Water/rock ratio | Flow rate (ml/min) | |
---|---|---|---|---|
Open system | 0.3% CO2 solution | 35°C to 200°C | 1 : 30 | 1 |
Semiopen system | 0.3% CO2 solution | 35°C to 200°C | 1 : 1 | 0 |
At the end of the experiment, the dissolution characteristics of the two kinds of fluid systems and various carbonate samples with temperature and pressure were investigated by measuring the change of dissolution weight loss and the change of ion concentrations.
In this study, samples were standard calcite mineral, standard dolomite mineral, and micritic limestone samples from Ordovician Pingliang Formation, dolomitic limestone samples, and fine crystalline dolostone samples from Ordovician Majiagou Formation. The microscopic characteristics of the samples are shown in Figure
Collecting location of the samples.
Sample number | Sample name | Lithology | Formation |
---|---|---|---|
C1 | Calcite standard mineral | Calcite | — |
D1 | Dolomite standard mineral | Dolomite | — |
A1 | XLG-O2P-16 | Micritic limestone | Ordovician Pingliang Formation |
A2 | TWD-O2P-7 | Dolomite limestone | Ordovician Tiewadian Formation |
A3 | Mixed standard mineral | Half calcite, half dolomite | — |
A4 | XF3 well | Fine crystalline dolostone | Ordovician Majiagou Formation |
Components and contents of the minerals.
Sample number | Calcite | Dolomite | Quartz | Pyrite | Clay mineral |
---|---|---|---|---|---|
C1 | 100% | — | — | — | — |
D1 | — | 100% | — | — | — |
A1 | 95.8% | — | 2.3% | 1.9% | — |
A2 | 68.2% | 26.6% | 2.3% | — | 2.9% |
A3 | 50% | 50% | — | — | — |
A4 | — | 90.1% | 4.2% | 2.5% | 3.2% |
Carbonate rock sample types: (a) micritic limestone; (b) dolomitic limestone; (c) fine crystalline dolostone.
The samples were crushed into particles of 2.8 mm to 4.2 mm in diameter to fit the sample tube. The samples were ultrasonically cleaned with deionized water and dried in the oven at 105°C for 24 hours. CO2 aqueous solution was selected as the acidic fluid medium, to simulate the in situ acidic fluid environment [
In the open system simulation, the dissolution of calcite generally exceeded that of dolomite, so as limestone and dolostone (Figure
The relationship between carbonate dissolution weight loss and temperature in the open system.
In the semiopen system simulation, the dissolution weight loss of dolomitic limestone was the largest among all the samples. The dissolution weight loss of calcite was higher than that of dolomite for the whole temperature range. Limestone and dolomite shared similar dissolution weight losses between 35°C~125°C, and the dissolution rate of the former exceeded the latter when temperature was higher than 125°C. The dissolution weight loss of all the samples came to the peak around 150°C. The highest dissolution weight loss of calcite is about 1.5 times that of dolomite. The temperature range for the dissolution window was from 120°C to 175°C. The dissolution window in the semiopen system appeared at higher temperature/pressure than that in the open system (Figure
The relationship between carbonate dissolution weight loss and temperature in the semiopen system.
In the semiopen system, dolomitic limestone (with a dolomite content < 30%) had a higher weight loss than other carbonate samples. Taylor et al. [
The simulation experiment of the semiopen system is close to the actual deep-ultradeep carbonate stratum. The opening of the fault systems associated with episodic tectonics or the specific fluid developed from adjacent formations such as CO2, organic acids which form hydrocarbon generation, or H2S from TSR could make it favorable for the fluid entering the fault or fractures to react with carbonate rock. The water/rock ratio is relatively small due to the small amount of the liquid. In that case, the fluid became supersaturation in a short time during the reaction, following the equilibrium of the dissolution/precipitation process. Such short period of reactions affected little on creating porosity but could maintain the preexisting pores formed in the open system and prevent the porosity loss through cementation.
In this study, the average weight loss of the standard calcite sample in the open system was 14 times larger than that of the semiopen system. For the standard dolomite samples, the average weight loss in the open system was 11 times of that in the semiopen system. As a result, the dissolution weight loss of carbonate rocks in the open system was higher under the same temperature and pressure conditions (corresponding to the burial depth), which is also favorable for porosity creation (Figure
The comparison of dissolution weight loss of carbonate rock samples in the open systems and semiopen systems.
In the dissolution process of carbonate minerals, H+ in the solution diffused into the diffusion boundary layer between the mineral and the solution and reacted with the surface of the mineral. Ca2+ and Mg2+, as the reaction product, diffused from the mineral surface into the solution. The thickness of the boundary layer determines the diffusion time and the reaction rate. The dissolution rate could be limited. When the flow rate of the fluid was low, a relatively thicker diffusion boundary layer was made, and the slow mass transfer velocity became the limiting factor of a higher dissolution rate and vice versa [
When the pH of fluid was below 4, the mineral dissolution rate was proportional to H+ concentration [
In addition, simulation experiments of water-rock interaction of carbonate rocks in the closed system showed that the whole carbonate rock tended to precipitation and cementation as temperature and pressure increased [
The carbonate samples were observed by scanning electron microscopy (SEM) before and after the reaction in the open system. The characteristics of carbonate dissolution are as follows: the calcite surface was flat before the reaction (Figure
Surface morphology of the carbonate samples before and after the dissolution experiment: (a) calcite sample before dissolution; (b) dissolution fractures on the calcite surface; (c) expanded dissolution fracture after dissolution; (d) dolomite before the dissolution; (e) dolomite dissolved on the edge; (f) etch pits on the dolomite surface.
When we applied the experimental results to the corresponding actual geological fluid system of which deep-ultradeep carbonate reservoir was formed, (1) open environment with a higher water/rock ratio would lead to a fluid with unsaturated CaCO3. In that case, the water-rock reaction kept moving towards the dissolution direction. New pores were formed, and the original pores or fractures were expanded. The new formed pores offered a larger surface area for the dissolution reaction. Carbonate strata continue to dissolve into scaled reservoir, along with the development of large pore and fracture system.
In the geological history, open geological environment mainly existed in the penecontemporaneous stage of long-term exposure sedimentary diagenesis such as reef, beach, and tidal flat with frequent exposures or the epigenesis stage of tectonic uplift such as near-surface karst environment and deep cycle fresh water dissolution environment. The long term and large scale of such dissolution could form considerable storage space. Despite the filling effect from mechanical, chemical, or biochemical processes, open environment is the main forming environment for high quality carbonate reservoirs. In addition, the scale and quality of the reservoir were further determined by the composition and structure of the carbonate rock, the interaction intensity and time of fluid, and the filling patterns and magnitude. For example, the quality of late karst reservoir may be degraded due to the effects of over dissolution and filling.
The water/rock ratio in the semiopen geological fluid system was relatively small. The fluid was prone to rapidly reach saturation or supersaturation state during the reaction. There was a dynamic equilibrium of dissolution/precipitation process between the fluid and the rock. Mineral dissolves and precipitates along the fluid flow pathway simultaneously. Some preexisting porosity may be expanded while some porosity may get cementation by calcite and dolomite.
In geological history, such environment is characterized by periodic fault, active fold, and special fluid event during basin evolution, which change the original fluid environment and break the chemical balance inside the formation to form a new fluid environment. As a result, episodic fluid-rock interaction happened. The exchange of material and energy in the formation would cause a series of processes including dissolution, metasomatism, dolomitization, recrystallization, and cementation and lead to the increase/decrease of porosity. Along with burial process, hydrocarbon generation, BSR, and TSR occurred in the strata, and some acid/alkali fluid may also enter the carbonate formation along with magmatic activity to form a new fluid environment and alter the storage space. Chemical aggressive fluid during fluid-rock interaction, charged with CO2, H2S, organic acids, mainly came from organic matter maturation, hydrocarbon degradation, TSR, and BSR [
The remaining pore water in the closed geological fluid system was saturated, and the fluid-rock interaction reached an equilibrium state. There was no scaled dissolution or precipitation, and the preexisting porosity was maintained. Although fluid flow was limited, changes in burial depth may break the reaction equilibrium by temperature or pressure change. Minor precipitation, dissolution, or recrystallization would appear along with the change of burial depth, but porosity was barely altered in this environment. The main function of a closed environment is the maintenance of reservoir space. The closed fluid environment requires a stable tectonic background and good cap conditions, which are the foundation and prerequisite of oil and gas accumulation, transformation, adjustment, and preservation.
In summary, open and semiopen fluid environment are the key to the formation of reservoir space, and closed fluid environment is essential to the maintenance of reservoir space.
In this study, we reached the following understandings through the comprehensive analysis of the experiments results: Open is the key. Almost all the high quality reservoirs had experienced one or more open or semiopen geologic fluid environments in the geological evolution history. Long-term precipitation leaching during penecontemporaneous stage and epigenesis stage and acid dissolution in the middle and late stages of deep burial diagenesis could significantly improve the physical properties of the reservoirs. The type of the fluid and rock determines the dissolution rate and reservoir quality. The flow pattern, intensity, and reaction time of the fluid determine the dissolution strength and the dissolution rate. The structure, composition, and contact surface of rock affect the quality of the reservoir. The various channel types of the fluid flow could form reservoir space such as dissolution pores, fractures, and holes. Closed geological fluid environment is essential for the preservation of preexisting reservoir space. If the preexisting pores of the carbonate reservoirs were well formed in the early diagenesis stage, the deep-ultradeep closed environment could provide effective preservation conditions. The burial or uplifting process of the formation will change the system temperature and pressure and lead to microcirculation of the fluid in the reservoir. Although the migration and minor adjustment of the materials may occur, the porosity barely changed along with the obvious variation of permeability and heterogeneity.
The authors declare that they have no conflicts of interest.
The assistance of Dr. Ming Xue is acknowledged. The help on the experiment and reference material from Dr. Donghua You, Dr. Shoutao Peng, and Dr. Lingjie Yu is acknowledged. This work is supported by Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDA14010201) and National Natural Science Foundation of China (Grants no. 41702134 and no. U1663209).