3. New technology of gas recycling drilling

The current gas drilling practice of handling gas returned from the borehole is to discharge it to the atmosphere directly. If the gas can be recycled in the same way as drilling mud, the energy consumption and drilling cost can be greatly reduced. The recycling system can also allow the produced gas from the reservoir to be compressed and transported to the gas gathering system in the field. The overall efficiency of gas drilling will be improved significantly.

The gas recycling system (GRS) has been investigated at the China University of Petroleum, Beijing (CUPB), for several years. A systematic research and development group at the CUPB carried out a comprehensive study including an integration design, technological process investigation, cuttings transport analysis, separation and filter equipment selection, and control system design.

#### 3.1. System description

On the one hand, the latter phenomenon is due to the weak ability of cuttings transportation of gas, i.e., the cuttings have to be fine enough so as to be returned from the bottom. On the other hand, the excessive gas injection rate is non-commercial and may lead to the ice-balling of drill bit induced by Joule Thompson effect as we have mentioned in the previous text. Moreover, field practice and theoretical analysis have shown that the returned debris in gas drilling is extremely fine, regardless of strata types [34]. Utilizing this value in the new model expressed

Well no. 1 2

Angel's model 69 66 The new model 85 82

Hole section (m) 2840–3650 2550–3305

sections in Well no. 1 and Well no. 2, respectively. The maximum permissible gas injection rates

Table 2 shows a comparison of model-calculated data and field-applied gas injection rates. Using the new model and a design factor of 1.15, The designed gas injection rate was 1.1585,

cleaning and hole enlargement. Using the new model and the same design factor of 1.15, the

optimum range of gas injection rates and the field-observed problem-free gas injection rates.

The current gas drilling practice of handling gas returned from the borehole is to discharge it to the atmosphere directly. If the gas can be recycled in the same way as drilling mud, the energy consumption and drilling cost can be greatly reduced. The recycling system can also allow the produced gas from the reservoir to be compressed and transported to the gas gathering system

The gas recycling system (GRS) has been investigated at the China University of Petroleum, Beijing (CUPB), for several years. A systematic research and development group at the CUPB

in the field. The overall efficiency of gas drilling will be improved significantly.

/min. For the hole section in Well no. 1. The section was drilled with a fixed

/min. This comparison indicates a good consistency between the model-predicted

/min, which is between the minimum required gas rate of

/min) — —

/min) 144 134

/min) 120 95

/min for the two hole

/min, with no problem of hole

/min, which is between

/min for the two hole sections in Well no. 1

/min for the hole section in Well no. 2. The

/min and the maximum permissible gas rate of

by Eq. (1) gives the minimum required gas injection rate of 85 and 82 Nm<sup>3</sup>

/min and the maximum permissible gas rate of 144 Nm3

section was drilled smoothly with a fixed compressor capacity of 95 Nm3

were calculated by Eq. (5) to be 144 and 134 Nm<sup>3</sup>

Table 2. Comparison of model-calculated data and field observations.

designed gas injection rate was 1.1582, or 94.3 Nm3

3. New technology of gas recycling drilling

the minimum required gas rate of 82 Nm3

and Well no. 2, respectively.

compressor capacity of 120 Nm3

The minimum required gas injection rate (Nm<sup>3</sup>

The maximum permissible gas injection rate (Nm3

Field-applied gas injection rate (Nm3

168 Drilling

or 97.8 Nm3

85 Nm3

134 Nm3

The general idea of the GRS is to separate gas effectively from the gas-liquid-solid mixtures returned from the well and re-inject the gas back into the well. At the same time, cuttings and fluids are discharged after the separation. During the natural gas drilling process, the separated gas can be released to the gas gathering system in the field. The integrated design of the process is illustrated in Figure 1.

When nitrogen is used as the drilling fluid, a low-capacity nitrogen generator is employed to supply nitrogen gas and inject it into the well. If the well is deep, compressors or boosters may be used to provide the required injection pressure. When the gas pressure and volume reach the required value for recycling, the gas drilling process can be initiated. Because the nitrogen gas is recycled, only a low-capacity nitrogen generator is required for supplying a small amount of nitrogen gas to make up the losses due to leakage and to meet the requirement of additional gas volume in the wellbore as depth increases.

The major equipments in the GRS are described as follows:

Compressors and boosters: The compressors and boosters used in the gas recycling system are the standard equipment used in conventional gas drilling operations. After filtration, the clean nitrogen gas is introduced into the system through parallel connections.

Figure 1. A sketch of the gas recycling system.

Primary separator: The primary separator separates drill cuttings of larger than 0.1 mm equivalent diameter by centrifugal force. Water is introduced into the separator to dilute the cuttings in the separator. The solid particles and liquid are discharged at the bottom of the separator. The separated gas exits the separator at the top and flows into a cyclone separator for further purification. The discharge system was designed to prevent gas loss and liquid overflow by automatically maintaining the dynamic liquid level during the solids and liquids discharge process in the pressurized separator. It is expected that any liquid hydrocarbon/condensate from the drilled formation will drain out of the system at the bottom of the separator.

Treatment of drilling complications: In case of drilling complications such as borehole collapse and excessive formation liquid influx, the nitrogen gas flow rate should be increased immediately. The nitrogen generator should be turned on as soon as possible to provide additional nitrogen gas volume. For instance, if the normal gas circulation rate is 120 Nm3

shortage will not be a problem. Moreover, as time passes, the gas volumetric flow rate in the

System control: The process in the GRS is more complicated than that in a conventional gas drilling system. The gas supplement rate adjustment, the valve activation during pipe connection, the timely turning on/off of the nitrogen generator, etc., cannot be achieved by manual operations. An automatic control system was implemented in the developed gas recycling

To verify the feasibility of the GRS and the performance of the related equipment, the development group at the CUPB conducted a special test of the system on the Dayi101 well in 2010. An open loop mode was adopted to ensure the safety of the drilling operation. The assembled

The GRS was installed in the middle of the blooie line. This arrangement was based on two considerations. First, the setup location was not close to the drilling floor to prevent its direct influence on the drilling floor operations should complications occur. Second, the location was not close to the outlet of the blooie line, thereby avoiding the impact of the igniting device on the GRS. The safe distance is essential for preventing hazardous conditions when gas leaks

min and the capacity of the membrane nitrogen is 30 Nm3

generator will increase the gas rate to 150 Nm3

system to ensure operational safety.

system is shown in Figure 2.

from the separation system.

Figure 2. A gas recycling system installed in Sichuan province of China.

3.3. Pilot test

well will continue to increase to clean the borehole.

/

171

/min, turning on the nitrogen

/min. Because the gas is still in recycling, gas

New Development of Air and Gas Drilling Technology http://dx.doi.org/10.5772/intechopen.75785

Cyclone separation unit: Two cyclone separators are used to separate solid particles of a size larger than the 7 μm equivalent diameter. An air-lock and waste-discharge device is specially designed in the separator to guarantee the timely debris-discharging by using the recycled water and the gas tightness. Consequently, the separator can work continuously without deposition of debris. Use of the two cyclone separators in series guarantees that most, although not 100%, of the drill cuttings are removed from the gas phase. The gas phase with small particles (dust) is led to a fine filter for further purification.

Fine filter: The fine filter works on the principle of filtration and aggregation to remove the solid particles that are larger than the 3 μm equivalent diameter. The purity of the postseparation gas is superior to the atmospheric air in terms of particle concentration. The filtered gas is introduced to the compressors for injection into the well. Two fine filters are prepared for alternation. If the debris is overstocked in one of the fine filters, the other one is switched over duly. And the element of the overstocked filter is replaced.

#### 3.2. Operating procedure

The procedure of operating the GRS is different from that of the conventional gas drilling system. The procedure is outlined as follows:

Air displacement: If the upper well section is drilled with mud, one can follow the conventional air drilling procedure to lift the liquid and dry the hole with compressed air in order to save the cost of nitrogen generation. When the air lift is completed, one can replace the air in the well with nitrogen gas to ensure safe drilling.

Preparation of nitrogen gas: The nitrogen generator is started first. The generated nitrogen gas is injected into the well by the compressors. Drilling operation is initiated when the gas pressure and gas flow rate reach the desired levels for the well condition.

Nitrogen supplement: The fluid mixture returned from the well is led to the separation system to remove solids and liquids. The separated gas from the gas-liquid-solid mixture is fed into the compressors and boosters and reinjected into the well. The nitrogen generator runs intermittently to make up for the gas loss in the system and the increased borehole volume as the well deepens.

Drill pipe connection operation: Pipe connection will cause some gas loss. The loss in the annulus can be controlled by closing the rotating head. To minimize the gas loss, one can use the check valves in the drill string to prevent backflow of nitrogen gas during pipe connections. Treatment of drilling complications: In case of drilling complications such as borehole collapse and excessive formation liquid influx, the nitrogen gas flow rate should be increased immediately. The nitrogen generator should be turned on as soon as possible to provide additional nitrogen gas volume. For instance, if the normal gas circulation rate is 120 Nm3 / min and the capacity of the membrane nitrogen is 30 Nm3 /min, turning on the nitrogen generator will increase the gas rate to 150 Nm3 /min. Because the gas is still in recycling, gas shortage will not be a problem. Moreover, as time passes, the gas volumetric flow rate in the well will continue to increase to clean the borehole.

System control: The process in the GRS is more complicated than that in a conventional gas drilling system. The gas supplement rate adjustment, the valve activation during pipe connection, the timely turning on/off of the nitrogen generator, etc., cannot be achieved by manual operations. An automatic control system was implemented in the developed gas recycling system to ensure operational safety.

### 3.3. Pilot test

Primary separator: The primary separator separates drill cuttings of larger than 0.1 mm equivalent diameter by centrifugal force. Water is introduced into the separator to dilute the cuttings in the separator. The solid particles and liquid are discharged at the bottom of the separator. The separated gas exits the separator at the top and flows into a cyclone separator for further purification. The discharge system was designed to prevent gas loss and liquid overflow by automatically maintaining the dynamic liquid level during the solids and liquids discharge process in the pressurized separator. It is expected that any liquid hydrocarbon/condensate

from the drilled formation will drain out of the system at the bottom of the separator.

particles (dust) is led to a fine filter for further purification.

duly. And the element of the overstocked filter is replaced.

system. The procedure is outlined as follows:

the well with nitrogen gas to ensure safe drilling.

3.2. Operating procedure

170 Drilling

well deepens.

Cyclone separation unit: Two cyclone separators are used to separate solid particles of a size larger than the 7 μm equivalent diameter. An air-lock and waste-discharge device is specially designed in the separator to guarantee the timely debris-discharging by using the recycled water and the gas tightness. Consequently, the separator can work continuously without deposition of debris. Use of the two cyclone separators in series guarantees that most, although not 100%, of the drill cuttings are removed from the gas phase. The gas phase with small

Fine filter: The fine filter works on the principle of filtration and aggregation to remove the solid particles that are larger than the 3 μm equivalent diameter. The purity of the postseparation gas is superior to the atmospheric air in terms of particle concentration. The filtered gas is introduced to the compressors for injection into the well. Two fine filters are prepared for alternation. If the debris is overstocked in one of the fine filters, the other one is switched over

The procedure of operating the GRS is different from that of the conventional gas drilling

Air displacement: If the upper well section is drilled with mud, one can follow the conventional air drilling procedure to lift the liquid and dry the hole with compressed air in order to save the cost of nitrogen generation. When the air lift is completed, one can replace the air in

Preparation of nitrogen gas: The nitrogen generator is started first. The generated nitrogen gas is injected into the well by the compressors. Drilling operation is initiated when the gas

Nitrogen supplement: The fluid mixture returned from the well is led to the separation system to remove solids and liquids. The separated gas from the gas-liquid-solid mixture is fed into the compressors and boosters and reinjected into the well. The nitrogen generator runs intermittently to make up for the gas loss in the system and the increased borehole volume as the

Drill pipe connection operation: Pipe connection will cause some gas loss. The loss in the annulus can be controlled by closing the rotating head. To minimize the gas loss, one can use the check valves in the drill string to prevent backflow of nitrogen gas during pipe connections.

pressure and gas flow rate reach the desired levels for the well condition.

To verify the feasibility of the GRS and the performance of the related equipment, the development group at the CUPB conducted a special test of the system on the Dayi101 well in 2010. An open loop mode was adopted to ensure the safety of the drilling operation. The assembled system is shown in Figure 2.

The GRS was installed in the middle of the blooie line. This arrangement was based on two considerations. First, the setup location was not close to the drilling floor to prevent its direct influence on the drilling floor operations should complications occur. Second, the location was not close to the outlet of the blooie line, thereby avoiding the impact of the igniting device on the GRS. The safe distance is essential for preventing hazardous conditions when gas leaks from the separation system.

Figure 2. A gas recycling system installed in Sichuan province of China.

The first objective of the pilot test was to assess the performance of the separation and filtration equipment. This was achieved by evaluating the capacity of separation and filtration equipment, including separating performance of the first and second cyclone separators, stability of the separation system, and the purity of gas at the outlet of the fine filter. The second objective of the test was to assess the adaptability of separation and filtration equipment to the drilling conditions, including normal drilling condition and complication condition such as formation fluid influx. Three working conditions were created. The first condition was the closed-gas flow test to check the liability of the separation system. The second condition was the normal gas drilling test to examine the effectiveness of the separation system. The third condition was the formation fluid influx test to inspect the adaptability of the new system.

Gas tightness test: After the third openhole section of Dayi 101 had been drilled with water, a gas lift was conducted to blow off water and cuttings out of the hole. It took 8 hours to dry the hole completely. The gas circulation was normal. This was a favorable condition for the tightness test of the new system. During the tightness test, the gas injection pressure was 3.8 MPa and the gas injection rate was 90 Nm3 /min. A small leak was found at the outlet of the second cyclone separator. After an investigation, it was confirmed that a collision had occurred to the outlet of the separator during transportation, which damaged the gaskets and caused the leak. After the gaskets were replaced, no more leaks were found during a half-hour test, which indicated that the seal was effective. It provided a sound base for conducting the subsequent tests.

The drilling process started after the hole drying operation. To ensure operational safety, a driller was appointed to control the valve on the blooie line (V<sup>1</sup> in Figure 1). In case of an unforeseen

The output at the exit of the blooie line was normal after the drilling operation began. Therefore, the inlet valve (V<sup>2</sup> in Figure 1) to the separation system was opened and the V<sup>l</sup> was closed. Then the full stream of the gas-liquid-solid mixture returned from the well entered the separation system. To better observe the effect, the water pump for dust removal was closed temporarily. Dust appeared at the exit of the blooie line before separation. After resuming the water injection with the pump to initiate separation, no visible dust was observed at the exit of the blooie line.

As the separated gas was prepared for recycling, its purity must meet the requirement of the compressor. Therefore, a dust concentration test was conducted for the separated gas. Before the test,

tion monitoring instruments were installed at both the inlet and outlet of the filter. The quality parameters of the gas were recorded and analyzed automatically. The monitoring result showed that the dust concentration at the inlet of the filter was 50–80 mg/m3 and that at the outlet

. This means that the purity of the filtered gas reached the level of the

. The concentra-

New Development of Air and Gas Drilling Technology http://dx.doi.org/10.5772/intechopen.75785 173

situation, the V<sup>1</sup> should be immediately switched to the conventional work position.

Figure 4. No water or dust was seen at the outlet of the blooie line after separation.

The filtered gas was very clean. The test result showed that the separation was effective.

the dust concentration of the air at the well site was measured to be 0.03–0.1 mg/m3

Figure 5. The contrast curve of TSI AM510 measurements and PALAS 3000 measurements.

was 0.05–0.08 mg/m3

Separation test: Water zones were encountered during drilling in the fourth openhole section. The drilling operation was immediately stopped for discharging water. During this period, a separation system test was conducted. The injection gas pressure was about 4.6 MPa and injection gas rate was 90 Nm<sup>3</sup> /min. Visible dust and water were seen at the outlet of the blooie line in the beginning, as shown in Figure 3a and b. After switching to the separation system, however, the gas at the outlet was seen to be clean and no water droplets were observed, as shown in Figure 4a and b. This indicated that the formation water and cuttings had been separated effectively by the system. The white mist was caused not by the solid dust, but by the high velocity of gas. However, there was no evidence of any remaining liquid hydrocarbon/condensate in the gas stream.

Because the formation water influx was little and the amount of dust was small in the hole drying process, the system worked effectively. The next step was to test the separation system in normal drilling conditions in which a large amount of dust exists in the system.

Figure 3. Water and dust was seen at the outlet of the blooie line before separation.

Figure 4. No water or dust was seen at the outlet of the blooie line after separation.

The first objective of the pilot test was to assess the performance of the separation and filtration equipment. This was achieved by evaluating the capacity of separation and filtration equipment, including separating performance of the first and second cyclone separators, stability of the separation system, and the purity of gas at the outlet of the fine filter. The second objective of the test was to assess the adaptability of separation and filtration equipment to the drilling conditions, including normal drilling condition and complication condition such as formation fluid influx. Three working conditions were created. The first condition was the closed-gas flow test to check the liability of the separation system. The second condition was the normal gas drilling test to examine the effectiveness of the separation system. The third condition was

Gas tightness test: After the third openhole section of Dayi 101 had been drilled with water, a gas lift was conducted to blow off water and cuttings out of the hole. It took 8 hours to dry the hole completely. The gas circulation was normal. This was a favorable condition for the tightness test of the new system. During the tightness test, the gas injection pressure was 3.8 MPa and the

separator. After an investigation, it was confirmed that a collision had occurred to the outlet of the separator during transportation, which damaged the gaskets and caused the leak. After the gaskets were replaced, no more leaks were found during a half-hour test, which indicated that

Separation test: Water zones were encountered during drilling in the fourth openhole section. The drilling operation was immediately stopped for discharging water. During this period, a separation system test was conducted. The injection gas pressure was about 4.6 MPa and

line in the beginning, as shown in Figure 3a and b. After switching to the separation system, however, the gas at the outlet was seen to be clean and no water droplets were observed, as shown in Figure 4a and b. This indicated that the formation water and cuttings had been separated effectively by the system. The white mist was caused not by the solid dust, but by the high velocity of gas. However, there was no evidence of any remaining liquid hydrocar-

Because the formation water influx was little and the amount of dust was small in the hole drying process, the system worked effectively. The next step was to test the separation system

in normal drilling conditions in which a large amount of dust exists in the system.

Figure 3. Water and dust was seen at the outlet of the blooie line before separation.

the seal was effective. It provided a sound base for conducting the subsequent tests.

/min. A small leak was found at the outlet of the second cyclone

/min. Visible dust and water were seen at the outlet of the blooie

the formation fluid influx test to inspect the adaptability of the new system.

gas injection rate was 90 Nm3

172 Drilling

injection gas rate was 90 Nm<sup>3</sup>

bon/condensate in the gas stream.

The drilling process started after the hole drying operation. To ensure operational safety, a driller was appointed to control the valve on the blooie line (V<sup>1</sup> in Figure 1). In case of an unforeseen situation, the V<sup>1</sup> should be immediately switched to the conventional work position.

The output at the exit of the blooie line was normal after the drilling operation began. Therefore, the inlet valve (V<sup>2</sup> in Figure 1) to the separation system was opened and the V<sup>l</sup> was closed. Then the full stream of the gas-liquid-solid mixture returned from the well entered the separation system. To better observe the effect, the water pump for dust removal was closed temporarily. Dust appeared at the exit of the blooie line before separation. After resuming the water injection with the pump to initiate separation, no visible dust was observed at the exit of the blooie line. The filtered gas was very clean. The test result showed that the separation was effective.

As the separated gas was prepared for recycling, its purity must meet the requirement of the compressor. Therefore, a dust concentration test was conducted for the separated gas. Before the test, the dust concentration of the air at the well site was measured to be 0.03–0.1 mg/m3 . The concentration monitoring instruments were installed at both the inlet and outlet of the filter. The quality parameters of the gas were recorded and analyzed automatically. The monitoring result showed that the dust concentration at the inlet of the filter was 50–80 mg/m3 and that at the outlet was 0.05–0.08 mg/m3 . This means that the purity of the filtered gas reached the level of the

Figure 5. The contrast curve of TSI AM510 measurements and PALAS 3000 measurements.

atmospheric air. It is believed that the filter device completely blocked dust particles that are larger than 3 μm equivalent diameter and the purified gas met the requirement of the compressor.

used in this test. The automatic measurements and control systems should be emphasized in

New Development of Air and Gas Drilling Technology http://dx.doi.org/10.5772/intechopen.75785 175

The entrance of a conventional compressor is open to air. Because the separated gas needs to be introduced to the compressor through piping, a proper parallel gas distributing manifold should be designed to fit the compressor inlet. Currently, such a manifold has been conducted

Some operational risks still exist with the new technology. These risks include: (1) quick addition of gas volume into the borehole in an emergency, (2) oxygen rust corrosion, and (3) downhole

Whenever the hole cleaning raises a concern because of drill cuttings accumulation, borehole collapse, excessive formation liquid influx, and/or gas leakage, it is imperative to automatically switch on the membrane nitrogen generator for increasing gas input volume to the system. This step will minimize drilling complications and ensure smooth drilling. Since nitrogen gas is highly compressible, which does not cause an immediate pressure drop in the borehole, it may be a good practice to select between 25 and 35% of the capacity of the membrane nitrogen

Rust corrosion due to oxygen in a wet system is a concern in any nitrogen gas drilling if the oxygen filters do not perform well, whether the system is an open or a closed one. Fortunately, most membrane nitrogen generators remove oxygen to much lower than percent level and no significant risk is expected. Because CO2 and H2S corrosion occurs in wet systems, they should

Downhole fire/explosion can occur when drilling hydrocarbon-bearing zones in the presence of oxygen. For this to happen, the oxygen/hydrocarbon ratio has to be in a certain range. In systems containing natural gas and air only, the natural gas concentration needs to be between 5 and 15%, depending on pressure. Since air contains about 21% of oxygen while membranegenerated nitrogen contains less than 5% oxygen, it is uncommon to see a downhole fire/

Regarding the determination of the required gas injection rate and direct discharge of the returned gas in gas drilling, we derived a mathematical model for predicting the optimum range of gas injection rate, developed a new technology of gas recycling drilling, established a system of gas separation and filtration corresponding in the GRS, and performed a pilot test.

be minimized with inhibitors whenever these gases are encountered during drilling.

and tested with conventional compressors and the result is satisfactory.

generator using normal nitrogen drilling practice.

explosion in a nitrogen gas drilling operation.

This study allows for drawing the following conclusions:

4. Conclusions

subsequent development.

3.4.4. Compressor inlet design

3.5. Operational risks

fire/explosion.

In order to ensure the accuracy of the measurements, a calibration test for the on-site monitor TSI AM510 was further conducted. The calibration device PALAS 3000 is a more accurate instrument suitable for indoor test. A calibrated contrast curve is presented in Figure 5, which shows that the dust concentration was much lower than that of the field data. This again proves the reliability of the separation and filtration system.

It was known from the data collected by the mud logger that the stand pipe pressure increased by only 0.1 MPa because of the separation process. This is the total pressure drop in the separation system. Apparently, the separation system has negligible effect on the drilling pressure.

### 3.4. Problems and solutions

The first open-loop field test of the GRS was essentially successful. All the equipment was in working order and the separation efficiency was high. This work served as a solid base line for more closed-loop tests. The following problems were found during the test:

#### 3.4.1. Continuous discharge design

According to the original plan, the separated cuttings should be discharged continuously by circulating water. However, the electrical motor for suction pump was not explosion-proof. It did not meet the field security requirement. Therefore, the gas discharge mode had to be adopted. All electrical equipment must be explosion-proof in the future design. In the discharge process, the working condition of the second cyclone separator was normal. However, the blooie line of the first separator was blocked for a moment. Larger size blooie line should be adopted in the future design.

## 3.4.2. The height of the equipment and skid mounted design

The current separation system is about 7 m high. Collision may occur easily during the transportation and installation process. This height is also inconvenient for monitoring and maintenance of the system. Therefore, the equipment's height should be reduced in the subsequent design without affecting separation efficiency. At this time, the design of a new horizontal separation system has been completed. After further improvements, a skid mounted system will be fabricated for easy transportation and equipment integration.

#### 3.4.3. System measurement and control design

When a closed-circulation is achieved, the operating parameters such as pressure and gas flow rate should be monitored in real time for safety. Valve switching should be used with both manual and automatic modes. In addition, a real-time alarm system should be added for safe operation. Due to the constraints of time and field conditions, an onsite reading method was used in this test. The automatic measurements and control systems should be emphasized in subsequent development.

#### 3.4.4. Compressor inlet design

atmospheric air. It is believed that the filter device completely blocked dust particles that are larger than 3 μm equivalent diameter and the purified gas met the requirement of the

In order to ensure the accuracy of the measurements, a calibration test for the on-site monitor TSI AM510 was further conducted. The calibration device PALAS 3000 is a more accurate instrument suitable for indoor test. A calibrated contrast curve is presented in Figure 5, which shows that the dust concentration was much lower than that of the field data. This again

It was known from the data collected by the mud logger that the stand pipe pressure increased by only 0.1 MPa because of the separation process. This is the total pressure drop in the separation

The first open-loop field test of the GRS was essentially successful. All the equipment was in working order and the separation efficiency was high. This work served as a solid base line for

According to the original plan, the separated cuttings should be discharged continuously by circulating water. However, the electrical motor for suction pump was not explosion-proof. It did not meet the field security requirement. Therefore, the gas discharge mode had to be adopted. All electrical equipment must be explosion-proof in the future design. In the discharge process, the working condition of the second cyclone separator was normal. However, the blooie line of the first separator was blocked for a moment. Larger size blooie line should

The current separation system is about 7 m high. Collision may occur easily during the transportation and installation process. This height is also inconvenient for monitoring and maintenance of the system. Therefore, the equipment's height should be reduced in the subsequent design without affecting separation efficiency. At this time, the design of a new horizontal separation system has been completed. After further improvements, a skid mounted system

When a closed-circulation is achieved, the operating parameters such as pressure and gas flow rate should be monitored in real time for safety. Valve switching should be used with both manual and automatic modes. In addition, a real-time alarm system should be added for safe operation. Due to the constraints of time and field conditions, an onsite reading method was

system. Apparently, the separation system has negligible effect on the drilling pressure.

more closed-loop tests. The following problems were found during the test:

proves the reliability of the separation and filtration system.

compressor.

174 Drilling

3.4. Problems and solutions

3.4.1. Continuous discharge design

be adopted in the future design.

3.4.2. The height of the equipment and skid mounted design

3.4.3. System measurement and control design

will be fabricated for easy transportation and equipment integration.

The entrance of a conventional compressor is open to air. Because the separated gas needs to be introduced to the compressor through piping, a proper parallel gas distributing manifold should be designed to fit the compressor inlet. Currently, such a manifold has been conducted and tested with conventional compressors and the result is satisfactory.

#### 3.5. Operational risks

Some operational risks still exist with the new technology. These risks include: (1) quick addition of gas volume into the borehole in an emergency, (2) oxygen rust corrosion, and (3) downhole fire/explosion.

Whenever the hole cleaning raises a concern because of drill cuttings accumulation, borehole collapse, excessive formation liquid influx, and/or gas leakage, it is imperative to automatically switch on the membrane nitrogen generator for increasing gas input volume to the system. This step will minimize drilling complications and ensure smooth drilling. Since nitrogen gas is highly compressible, which does not cause an immediate pressure drop in the borehole, it may be a good practice to select between 25 and 35% of the capacity of the membrane nitrogen generator using normal nitrogen drilling practice.

Rust corrosion due to oxygen in a wet system is a concern in any nitrogen gas drilling if the oxygen filters do not perform well, whether the system is an open or a closed one. Fortunately, most membrane nitrogen generators remove oxygen to much lower than percent level and no significant risk is expected. Because CO2 and H2S corrosion occurs in wet systems, they should be minimized with inhibitors whenever these gases are encountered during drilling.

Downhole fire/explosion can occur when drilling hydrocarbon-bearing zones in the presence of oxygen. For this to happen, the oxygen/hydrocarbon ratio has to be in a certain range. In systems containing natural gas and air only, the natural gas concentration needs to be between 5 and 15%, depending on pressure. Since air contains about 21% of oxygen while membranegenerated nitrogen contains less than 5% oxygen, it is uncommon to see a downhole fire/ explosion in a nitrogen gas drilling operation.
