DU Yu-kun, WANG Rui-he, NI Hong-jian, LI Mu-kun, SONG Wei-qiang, SONG Hui-fang

College of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China, E-mail: duyukun_100@hotmail.com

(Received December 26, 2011, Revised April 13, 2012)

DETERMINATION OF ROCK-BREAKING PERFORMANCE OF HIGHPRESSURE SUPERCRITICAL CARBON DIOXIDE JET*

DU Yu-kun, WANG Rui-he, NI Hong-jian, LI Mu-kun, SONG Wei-qiang, SONG Hui-fang

College of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China, E-mail: duyukun_100@hotmail.com

(Received December 26, 2011, Revised April 13, 2012)

In this study, a well-designed experimental setup is used to determine the rock-breaking performance of a high-pressure supercritical carbon dioxide (SC-CO2) jet. Its rock-breaking performance is first compared with that of a high-pressure water jet under the same operation conditions. The effects of five major factors that affect the rock-breaking performance of the high-pressure SC-CO2jet, i.e., the nozzle diameter, the standoff distance, the jet pressure, the rock compressive strength and the jet temperature are experimentally determined. The experimental results indicate that the rock-breaking performance of the SC-CO2jet is significantly improved over the high-pressure water jet. It is also found that the rock-breaking performance of the SC-CO2jet is improved with the increase of the nozzle diameter or the standoff distance, until the nozzle diameter or the standoff distance reaches a certain critical value and after that it begins to deteriorate. The rock-breaking performance of the SC-CO2jet improves monotonically with the increase of the jet pressure, while it shows a monotonic deterioration with the increase of the rock compressive strength. In addition, it is found that, under the same working conditions, the SC-CO2jet can always provide a better rock-breaking performance than the subcritical liquid CO2jet.

supercritical carbon dioxide, drilling engineering, rock-breaking performance

Introduction

The current exploration and development of oil and gas resources gradually shifts from integrated homogeneous reservoirs with high pressure and high permeability to complex fault-block reservoirs with low permeability and depleted pressure, as well as shale gas, coalbed methane and other unconventional oil and gas resources[1-3]. With the current rising energy demand, it becomes very urgent to develop new technologies to improve the exploration and development efficiency of these difficult-to-recover hydrocarbon resources. High-pressure water or abrasive jet technology has seen successful applications in various sectors of the petroleum industry, for example, the abrasive jet drilling and perforation[4], the waterjet-assisted multilayer fracturing[5], and the offshorewellhead cutting with the abrasive water jet technique[6]. Nonetheless, it should be noted that the waterjet-assisted drilling process is not applicable for drilling some reservoirs vulnerable to formation damages, for which an underbalanced way is required. The supercritical carbon dioxide (SC-CO2) seems to be an ideal alternative to the water because it is shown that the SC-CO2can pierce the formation rock at a lower pressures than the water. Hence, a high rate of penetration (ROP) can be achieved with the use of SC-CO2as the drilling fluid[7-10]. In addition, SC-CO2possesses some advantageous properties including the low viscosity, the high density, and the large diffusivity, which are favorable for improving the hole-cleaning performance and reducing the formation damage[7].

Effective drilling or completion fluids are required to meet the requirement that the formation damage is minimized in contacting with the reservoirs, in particular, with unconventional oil and gas reservoirs. SC-CO2can provide a superior hole-cleaning performance because a turbulent flow can be readily formed due to its low viscosity and high density[7]. Another advantage of SC-CO2is that it is a highly diffusivefluid that does not induce the formation damage, mainly due to the following two facts. Firstly, unlike the water, SC-CO2does not react with clay and cause the swelling of clay. Secondly, SC-CO2works as an effective solvent in the wellbore. SC-CO2can dissolve the heavy hydrocarbons and other chemicals and remove them in the near-well formation[7], hence leading to a smaller skin factor.

Previous studies were mainly focused on the feasibility evaluation and simulation study of SC-CO2drilling technique[11-13]. Rather few experimental data with regard to the rock-breaking performance of SCCO2were available in the literature. In this study, a well-designed experimental setup is used to comprehensively examine the relations of the rock-breaking performance of the high-pressure SC-CO2jet with various influencing factors. The core of the whole experimental system is a simulated wellbore which is designed and manufactured by adopting the Euler similarity theory[14].

1. Theoretical formulation

1.1 Similarity criterion

It is desirable that the experimental findings obtained by using a lab-scale model can be used to predict the corresponding actual results for a fieldscale prototype. This can be achieved if the lab-scale model and the field-scale prototype conform to a certain similarity principle. As the main purpose of this study is to examine the effects of various dynamic parameters, including hydraulic parameters and working conditions, on the rock-breaking performance of SC-CO2, the Euler dynamic similarity is chosen as the similarity criterion to design the SC-CO2drilling experimental system. The Euler similarity criterion is as follows

where Epis the Euler number for the field-scale prototype, Emis the Euler number for the lab model, vpis the velocity inside the field-scale prototype, and vmis the velocity inside the lab model.

The physical quantities involved in the SC-CO2jet rock-breaking process include the jet velocity, the annulus flow velocity, the SC-CO2density and the stagnation pressure on the rock surface. Among these physical quantities, the fluid properties of SC-CO2should remain the same for both the experimental model and the prototype. Thus the density similarity ratio of the experimental model to the field-scale prototype is 1. The threshold SC-CO2jet pressure required to break the rock in the lab experiment is the same as that in the actual field applications. Therefore, the pressure similarity ratio is also 1.

1.2 Structure design of the experimental setup

1.2.1 SC-CO2jet nozzle

A package of hydraulic parameters typically used in oilfield drilling practices is employed in the analysis. The pump flow rate, the nozzle diameter, and the nozzle number are chosen to be 32 L/s, 0.01 m and 4, respectively. Therefore, the jet velocity can be calculated by the following formula

wherejv is the jet velocity, Q is the pump flow rate, A is the cross-sectional area of the nozzle exit, andeD is the equivalent nozzle diameter. It should be noted thateD is determined by

where Diis the nozzle diameter (i=1,2,…,z)and z is the number of nozzles.

The flow rate of the experimental pump is chosen as 2 L/s by taking into account the capacity of the SC-CO2pump available. In order to meet the requirement of the dynamic similarity, the nozzle diameter of the experimental SC-CO2nozzle is such as to achieve the same jet velocity as the prototype.

where0D is the laboratory nozzle diameter and0Q is the laboratory pump flow rate.

1.2.2 Design of the simulated wellbore

The dimensions of the wellbore should provide sufficient cutting-carrying capability of the SC-CO2fluid. The widely-used minimum-kinetic-energy standard is chosen as the criterion for designing the simulated wellbore in the SC-CO2jet rock-breaking experiments. In the minimum kinetic energy standard, the cutting-carrying fluid and the cuttings are regarded as homogeneous fluids with the same density and flow velocity, i.e., the slippage between the cuttings and the fluid is neglected. Angel model is currently a widely used minimum-kinetic-energy standard model[15]. Inthe Angel model, the minimum annular return velocity and density for air to effectively carrying solid cuttings are 15.25 m/s and 1.225 kg/m3at 0.1 MPa and 15.0oC. Because the cuttings in the SC-CO2drilling are smaller than those in the air drilling based on the lab experimental observations in this study, thus, the minimum cuttings-carrying standard in the air drilling process should also function well for the SC-CO2drilling process.

The cuttings-carrying capability of the air can be evaluated by the unit volume kinetic energy as follows

where E is the unit volume of the air kinetic energy, ρ is the fluid density, and v is the fluid velocity. Therefore, 142.44 J is the minimum unit volume kinetic energy for air to carry cuttings. The return velocity of the SC-CO2carrying cuttings can be determined as

where2cov is the return velocity of SC-CO2and2coρ represents the SC-CO2density at the supercritical point, i.e., 468 kg/m3. It is noted that a rather smaller minimum return velocity of SC-CO2is required due to the fact that the SC-CO2density is much higher than the air density.

The equivalent diameter of the wellbore annulus is determined as

where Dceis the equivalent diameter of the annulus, Dais the outer diameter of the wellbore, and Dpis the inner diameter of the drill pipe. The outer diameter of the simulated wellbore is designed as 0.1 m. Consequently, the inner diameter of the drill pipe is calculated to be 0.0821 m according to Eq.(7).

2. Experimental section

2.1 Materials

Three artificial cores and two field cores with compressive strengths (σ) of 25 MPa, 29 MPa, 35 MPa, 65 MPa and 95 MPa, respectively, are used in the rock-breaking experiments. CO2with purity of 99.9% is purchased from Qingdao Tianyuan Gas Company, China.

Fig.1 Schematic diagram of the experimental setup for SC-CO2rock-breaking experiments

2.2 Experimental setup

Figure 1 illustrates a schematic diagram of the whole system used to perform the rock-breaking experiments by using SC-CO2. The experimental setup includes the storage unit, the pressurization unit, the heating unit, the simulated wellbore unit, the separation unit and the cooling unit. The booster pumps can be used to further pressurize CO2to higher pressures. The pressurized SC-CO2flows into the simulated wellbore after being heated to the test temperature by using the heater unit. The mixture containing SC-CO2and cuttings resulted from the breaking of the core sample flows out of the simulation wellbore. The mixture then first passes through the solid separator where the solid cuttings are removed, and then flows through the liquid separator where the water vapor is removed. Finally, the pure CO2fluid is cooled down to the storage tank temperature by using the refrigerator and returns to the CO2storage tank.

Fig.2 Schematic diagram of the simulated wellbore

It is worth noting that the simulated wellbore shown in Fig.1 can be readily replaced by other equipments to conduct experiments for the SC-CO2jet perforation, the SC-CO2-based fracturing and any other processes that uses SC-CO2. Figure 2 shows the simulated wellbore designed for the SC-CO2jet rock-breaking experiments by adhering to the above-mentioned Euler similarity criterion. The simulated wellbore can be used to effectively simulate the actual drilling processes with the consideration of various factors, including the confining pressure and the formation temperature. The rock-breaking time by the SC-CO2jet can be precisely controlled by manipulating the baffleshown in Fig.2. The duration of the rock-breaking experiments is all set to be 120 s in this study. The pressure, the temperature and the flow rate of the SCCO2in the experimental process can be controlled and measured in real-time by the data acquisition and control system.

Fig.3 SC-CO2jet nozzle

The designed cone-shape SC-CO2jet nozzle is shown in Fig.3. The outlet nozzle diameter (0D) is 0.0025 m, while the length of the cylindrical section (0H) is two times of0D.

The data acquisition and control system developed in this study is capable of testing, storing and analyzing up to 86 parameters. It can also be used to control eleven equipments simultaneously. The data acquisition and control system provides the remote and automatic control of the experimental processes, as well as the real-time logging of the experimental data, to ensure that all the high-pressure experiments are conducted in a safe manner.

2.3 Experimental procedures

Prior to each test, the heater and the refrigerator are switched on to reach the preset temperatures; sufficient liquid CO2is charged into the CO2storage tank, and a core sample is loaded in the simulated wellbore as shown in Fig.2. It should be noted that the baffle is positioned against the core sample by using the baffle controller prior to the rock-breaking experiments to ensure that the SC-CO2jet comes into contact with the baffle, instead of the core sample, prior to the rockbreaking experiments. Subsequently, the booster pump unit is turned on to pump the pressurized SCCO2throughout the closed-loop circulation system. An adjustment of the rotation speed of the booster pumps is required so that the preset pressure is reached at the wellbore inlet. Then the baffle is pulled away by using the baffle controller, which indicates the beginning of the rock-breaking experiments. Once the test lasts for 120 s, it is terminated by pushing the baffler back into the original place. After the pressure of the close-loop circulation system is released, the core sample is unloaded from the simulated wellbore. The eroded volume and depth of the core sample can then be measured.

Fig.4 Comparison of rock-breaking depth obtained by the SCCO2jet and the high-pressure water jet under different jet pressures

3. Results and discussions

3.1 Comparison of rock-breaking depth obtained with SC-CO2jet and high-pressurewater jet

The high-pressure water jet technology is a widely used technique to improve the ROP. In this study, the SC-CO2jet is compared with the high-pressure water jet in terms of the rock-breaking depth. Figure 4 shows the rock-breaking performance of the high-pressure water jet and the SC-CO2jet. The comparative experiments are conducted under the following conditions: the jet temperature at the wellbore inlet is 70oC, the nozzle diameter is 0.0023 m and the standoff distance is 0.0046 m. It can be found from Fig.4 that the SC-CO2jet performs obviously better than the high-pressure water jet in terms of the rockbreaking depth. For example, the rock-breaking depth of SC-CO2jet is 3 times of that obtained by the water jet when the wellbore inlet pressure is 30 MPa. Nonetheless, this incremental effect decreases to 2 times under the pressure of 40 MPa and 1.6 times under 50 MPa. Compared to the water, it is easier for the SC-CO2to permeate through micro-pores due to its large diffusion coefficient, subsequently, to induce micro-cracks in the rock body and extend the existing cracks. Another beneficial effect is that the swelling of the SC-CO2after the pressure relief contributes to the generation of a large tensile stress inside the microcracks. All these mechanisms are favorable for inducing deeper and wider micro-cracks, consequently increasing the ROP. The gradual reduction in the incremental effect of the rock-breaking depth caused by the SC-CO2jet over the water jet may be due to the following fact. In the initial stage of a rock-breaking process, the stress wave due to the jet impact plays a dominant role in generating micro-cracks on the rock surface. Compared to the water jet, the significantly stronger stress wave induced by the SC-CO2jet gene-rates more micro-cracks on the rock surface, leading to a much deeper penetration. In the later stage of the rock-breaking process, the jet hydraulic pressure plays a more important role in extending the micro-cracks and penetrating deep into the rock body[16], which results in a lesser enhancement effect on the penetration depth caused by the SC-CO2compared to the water.

Fig.5 Effect of nozzle diameter on the rock-breaking performance

3.2 Rock-breaking performance of SC-CO2jet

3.2.1 Effect of nozzle diameter

Experiments are conducted to examine the effect of the nozzle diameter (0D) on the rock-breaking performance of SC-CO2under the following conditions: the jet pressure at the wellbore inlet is 30 MPa, the jet temperature at the wellbore inlet is 70oC, and the standoff distance is maintained as 2 times of the nozzle diameter used. The rock-breaking performance is described by two parameters, i.e., the erosion depth (h) and the erosion volume (V). Figure 5 shows the effect of the nozzle diameter on the rock-breaking performance. It can be seen that both the rock-breaking depth and the rock-breaking volume increase initially with the nozzle diameter, until the diameter reaches an optimal point and then begin to decline. The optimal nozzle diameter in this study is found to be 2.3×10-3m. The initial improvement of the rock-breaking performance is due to the larger volumetric flow rate resulted from the utilization of a larger diameter under a given jet pressure, leading to a larger energy impacting on the rock surface. However, if the diameter is too large, the jet velocity would be significantly decreased, which results in a smaller impacting energy and a reduced rock-breaking performance accordingly.

3.2.2 Effect of standoff distance

Experiments are carried out to examine the effect of the standoff distance (L) on the rock-breaking performance of SC-CO2under the following conditions: the jet pressure at the wellbore inlet is 30 MPa, the jet temperature at the wellbore inlet is 70oC and the nozzle diameter is 2.3×10-3m. Figure 6 shows the variation of the rock-breaking volume and depth against the standoff distance. It can be seen from Fig.6 that the standoff distance shows a similar effect on the rock-breaking performance as that of the nozzle diameter. The rock-breaking performance improves at first, then deteriorates with the increase of the standoff distance. The initial improvement of the rock-breaking performance with a small standoff distance can be attributed to two factors. First, the SC-CO2jet is not fully developed when the standoff distance is too small[6,17]. Second, the mutual interaction between the incoming jet and the returning flow is strong with a small standoff distance, leading to a large energy loss. When the standoff distance reaches a certain point, the further increase of the standoff distance leads to a significant reduction of the jet axial velocity, which results in a deteriorated rock-breaking performance. The optimal standoff distance is determined to be 0.0046 m in this study.

Fig.6 Effect of standoff distance on the rock-breaking performance

Fig.7 Effect of jet pressure on the rock-breaking performance

3.2.3 Effect of jet pressure

Figure 7 shows the variation of the rock-breaking performance against the jet pressure (p) at the wellbore inlet under the following conditions: the jet temperature at the wellbore inlet is 70oC, the nozzle diameter is 2.3×10-3m m and the standoff distance is 4.6×10-3m m. It can be seen from Fig.7 that both theerosion depth and the erosion volume are increased with an increase of the jet pressure. The improvement of the rock-breaking performance with the increase of the jet pressure is due to the increase of the jet energy, which leads to more material removal and larger cutting depth[6]. The threshold pressure is determined to be around 14 MPa, below which no erosion will occur.

Fig.8 Effect of rock compressive strength on the rock-breaking performance

3.2.4 Effect of rock compressive strength

Figure 8 shows the effect of the rock compressive strength on the rock-breaking depth caused by the SC-CO2jet under the following conditions: the jet pressure at the wellbore inlet is 30 MPa, the jet temperature at the wellbore inlet is 70oC, the nozzle diameter is 2.3×10-3m m and the standoff distance is 4.6×10-3m m. It can be seen from Fig.8 that the rockbreaking depth is decreased with the increase of the rock compressive strength. This observation is expected since it would become more difficult for the SCCO2jet to pierce the rock sample of a higher compressive strength. More kinetic energy is needed for breaking a harder rock sample.

Fig.9 Effect of bottom-hole temperature on the rock-breaking performance

3.2.5 Effect of jet temperature

In the SC-CO2drilling process, the liquid CO2is pressurized firstly at the wellhead, then its temperature gradually rises inside the drill pipe with the increase of the well depth due to the heat conduction between the wellbore fluid and the formation. CO2will reach a supercritical state at a certain depth where the wellbore temperature exceeds the critical point (31.1oC). The jet temperature should have a great effect on the rock-breaking performance of SC-CO2as it greatly affects the properties of SC-CO2. Figure 9 shows the variation of the rock-breaking performance against the jet temperature at the wellbore inlet under the following conditions: the jet pressure is 30 MPa, the nozzle diameter is 2.3×10-3m m and the standoff distance is 4.6×10-3m m.

It is interesting to discover from Fig.9 that there are two distinct regions on the rock-breaking depth curve and the rock-breaking volume curve, respectively. The two distinct regions for each curve are separated by an inflection point, i.e., the critical temperature of CO2. The increase of the jet temperature leads to a dramatic improvement of the rock-breaking performance in terms of either the erosion volume or the depth. This large enhancing effect lasts until the jet temperature reaches the supercritical temperature of CO2(31.1oC). When the temperature is lower than the supercritical temperature, the rock-breaking mechanism of the liquid CO2jet resembles that of the high-pressure water jet. The density of the liquid CO2plays a major role in the erosion of the core sample as it greatly affects the power of the CO2jet. The density of the liquid CO2will be greatly increased with the increase of the jet temperature as the pressure is kept constant. The large density increase accounts for the sharp improvement of the rock-breaking performance with the increase of the subcritical temperature. When the jet temperature surpasses the supercritical temperature of CO2, a further increase of the jet temperature can still improve the rock-breaking performance, however, in a less effective way.

It is apparent from Fig.9 that under the same working conditions, a supercritical temperature can always provide a better rock-breaking performance than a subcritical temperature. This is mainly due to the fact that the SC-CO2possesses a lower viscosity and a larger diffusivity, which makes the SC-CO2permeate easily into the pores or micro-cracks inside the core. The vigorous permeation of SC-CO2into the pores or micro-cracks enhances the propagation and the expansion of micro-cracks inside the core, leading to an improved rock-breaking performance in comparison to the subcritical liquid CO2.

4. Conclusion

A well-designed experimental setup is developed in this study to determine the rock-breaking performance with a high-pressure SC-CO2jet. It is found that the rock-breaking depth obtained by using SC-CO2jet is significantly increased as compared with the high-pressure water jet under the same experimental conditions. The rock-breaking performance of SC-CO2jet improves initially with the nozzle diameter or the standoff distance until the nozzle diameter or the standoff distance reaches a certain point, and then it starts to deteriorate. The rock-breaking performance of SC-CO2jet improves monotonically with the jet pressure, while it shows a monotonic deterioration with the increase of the rock compressive strength. A jet temperature above the supercritical temperature of CO2can dramatically enhance the rock-breaking performance as compared with a subcritical temperature at which the CO2is in the liquid state. The promising experimental findings in this study demonstrate that the SC-CO2jet deserves a further investigation in field scales.

Acknowledgement

This worked was supported by the Excellent Ph. D. Thesis Training Fund and Graduate Independent Innovation Project of China University of Petroleum (Grant No. 11CX06021A).

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10.1016/S1001-6058(11)60277-1

* Project supported by the National Natural Science Foundation of China (Grant Nos. 50974130, 51034007), the National Key Basic Research and Development Program of China (973 Program, 2010CB226700).

Biography: DU Yu-kun (1983-), Male, Ph. D., Lecturer