1. Introduction

Air and gas drilling technology is the utilization of mainly (>97% in volume) compressed air or other gases (e.g., nitrogen or natural gas) as a rotary drilling circulating fluid to carry the rock cuttings to the surface. When the gases are injected into the well with incompressible fluids such as fresh water, oil, or drilling mud, the operations are called aerated drilling or stable foam drilling if foaming agents are added to create a continuous foam circulating fluid. Due to

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the strong capability of cutting transportation of the aerated drilling and foam drilling, the gas injection rate calculation is less significant compared to that in air and gas drilling. Therefore, discussion of aerated and foam drilling is beyond the scope of this chapter.

investigated the optimum range of nitrogen injection rate in shale gas well drilling. Chen et al. [20] present a method for determining the minimum gas injection rate required for hole

New Development of Air and Gas Drilling Technology http://dx.doi.org/10.5772/intechopen.75785 165

Recent developments in gas drilling include thermal failure of rock and gas temperature prediction. Zhang et al. [21] determined the effect of fluid temperature on rock failure in borehole drilling with gas. Li et al. [22] identified the complexity of thermal effect on rock failure in gas-drilling shale gas wells. Li et al. [23] developed a closed-form mathematical model for predicting gas temperature in gas-drilling unconventional tight reservoirs. Guo et al. [24] presented an analytical thermal-model for optimization of gas-drilling in unconventional tight-sand reservoirs. Guo et al. [25] published a mathematical modeling of heat transfer in counter-current multiphase flow found in gas-drilling systems with formation fluid influx. Other recent development in gas drilling includes distribution of the sizes of rock cuttings in

Another key issue of air and gas drilling to be solved is the environmental pollution and wasting of resources caused by direct discharging or combustion of the returned gas. To address this problem, our research group developed a new gas recycling system [28–30]. Unlike the conventional gas drilling process, the returned gas is re-injected into the wellbore after treatment by separators and fine filters, rather than being discharged or burned directly. The impurity content, humidity and other parameters of the treated gas can fully meet the requirements of the gas suction standard of a compressor. Therefore, the returned gas can be recycled through compressors, and consequently, the objectives of saving resources, lowering

In the current work, a modified mathematical model for predicting the minimum gas injection rate is derived, taking into account Charles' theory of particle grinding energy. The maximum required value of gas injection rate is estimated using the sonic flow criterion at a bit. The proposed model allows calculating the optimum range of gas injection rate more precisely. Also, we present our work in developing the gas recycling system, including the corresponding equipment, operating procedure, and results of a pilot test. The test results

The structure of the text is as follows. Section 2 presents the modified mathematical model for predicting the optimum gas injection rate and the comparison between the model prediction and the field experience. Section 3 demonstrates the newly developed gas recycling system.

It is shown in [18] that only the result given by Angel's minimum kinetic energy criterion has a trend that is consistent with field experience, although the minimum volumetric gas requirements are underestimated. We believe that this underestimation is partially because Angel's

cleaning in horizontal gas drilling.

gas drilling [26] and gas-lift drilling [27].

indicate a promising prospect of GRS.

Section 4 concludes this chapter.

the cost, and environmental protection are achieved.

2. Mathematical model for gas injection rate

2.1. The minimum required gas injection rate

Gas injection rate is one of the basic parameters during the design process of air and gas drilling. On the one hand, overestimated value of required gas injection rate may lead to high equipment investment, high cost, and ice balling of drill bit. On the other hand, underestimated value of required gas injection rate may cause cuttings transport and pipesticking problems. However, how to find the optimum gas injection rate accurately remains a question.

Several criteria and methods for determining the minimum gas volume requirement have been used in the gas drilling industry. They fall into two categories: (1) the minimum velocity criterion and (2) the minimum kinetic energy criterion. The minimum velocity criterion considers the interactions between solid particles, fluids, and the boundary of flow domain (borehole wall). The concept of terminal velocity is used to determine the minimum required gas velocity at the deepest large annulus. The terminal velocity of a solid particle can be influenced by many factors, including size, shape, and density of the particle; density and viscosity of the fluid and flow regime. Among many mathematical models proposed to account for the effects of these factors, Gray's model has been widely accepted for small-size hole drilling because it considers particle-wall interaction [1, 2].

The minimum kinetic energy criterion was established in 1950s based on Angel's pioneering work [3]. The mixture of gas and solid is treated as one homogeneous phase with mixture density and velocity, i.e., interactions between particles and fluids are not considered. Several models have been presented, for example, see [3–6]. Although McCray and Cole's model permits a constant-percentage slip velocity of solid particles, it uses the same particle lift criterion as Angel's model. The criterion for the minimum volume requirement is based on the experience gained from quarry drilling with air. The minimum annular velocity to effectively remove solid particles from the borehole is usually assumed to be 15 m/s, or 50 ft./sec (ft/s), under atmospheric conditions. This velocity was believed to be high enough to remove dustlike particles in air drilling. Although big cuttings not removed from the vicinity of the bit by the circulating air are reground by the bit teeth, it would be uneconomical to lift large cuttings without first trying to control their initial size at the bit. It is reported in [7] that the gas flow rate values obtained from Angel's method were at least 25% below the actual field's needs. This motivated numerous investigators to develop more accurate models to determine the minimum required gas injection rate for gas drilling, for example, see [8–17].

Guo et al. performed a comparison of results from the model calculation and the field experience [18]. The comparison shows that, among those existing models, only the result given by Angel's is mostly consistent with actual needs. Guo et al. found that the assumption of Weymouth friction-model is the reason for the underestimation. The Weymouth friction-model is suitable for smooth pipe walls, but not for the borehole walls which are rather rough. Then Guo et al. introduced Nikuradse's friction factor into Angel's model so that the modified model become more reasonable and practical. However, due to the difficulty in determining the friction factor, the application of Guo's model is limited to some extent. The latter motivates us to develop a new mathematical model to determine the gas injection rate. Li et al. [19] investigated the optimum range of nitrogen injection rate in shale gas well drilling. Chen et al. [20] present a method for determining the minimum gas injection rate required for hole cleaning in horizontal gas drilling.

the strong capability of cutting transportation of the aerated drilling and foam drilling, the gas injection rate calculation is less significant compared to that in air and gas drilling. Therefore,

Gas injection rate is one of the basic parameters during the design process of air and gas drilling. On the one hand, overestimated value of required gas injection rate may lead to high equipment investment, high cost, and ice balling of drill bit. On the other hand, underestimated value of required gas injection rate may cause cuttings transport and pipesticking problems. However, how to find the optimum gas injection rate accurately remains

Several criteria and methods for determining the minimum gas volume requirement have been used in the gas drilling industry. They fall into two categories: (1) the minimum velocity criterion and (2) the minimum kinetic energy criterion. The minimum velocity criterion considers the interactions between solid particles, fluids, and the boundary of flow domain (borehole wall). The concept of terminal velocity is used to determine the minimum required gas velocity at the deepest large annulus. The terminal velocity of a solid particle can be influenced by many factors, including size, shape, and density of the particle; density and viscosity of the fluid and flow regime. Among many mathematical models proposed to account for the effects of these factors, Gray's model has been widely accepted for small-size

The minimum kinetic energy criterion was established in 1950s based on Angel's pioneering work [3]. The mixture of gas and solid is treated as one homogeneous phase with mixture density and velocity, i.e., interactions between particles and fluids are not considered. Several models have been presented, for example, see [3–6]. Although McCray and Cole's model permits a constant-percentage slip velocity of solid particles, it uses the same particle lift criterion as Angel's model. The criterion for the minimum volume requirement is based on the experience gained from quarry drilling with air. The minimum annular velocity to effectively remove solid particles from the borehole is usually assumed to be 15 m/s, or 50 ft./sec (ft/s), under atmospheric conditions. This velocity was believed to be high enough to remove dustlike particles in air drilling. Although big cuttings not removed from the vicinity of the bit by the circulating air are reground by the bit teeth, it would be uneconomical to lift large cuttings without first trying to control their initial size at the bit. It is reported in [7] that the gas flow rate values obtained from Angel's method were at least 25% below the actual field's needs. This motivated numerous investigators to develop more accurate models to determine the

discussion of aerated and foam drilling is beyond the scope of this chapter.

hole drilling because it considers particle-wall interaction [1, 2].

minimum required gas injection rate for gas drilling, for example, see [8–17].

Guo et al. performed a comparison of results from the model calculation and the field experience [18]. The comparison shows that, among those existing models, only the result given by Angel's is mostly consistent with actual needs. Guo et al. found that the assumption of Weymouth friction-model is the reason for the underestimation. The Weymouth friction-model is suitable for smooth pipe walls, but not for the borehole walls which are rather rough. Then Guo et al. introduced Nikuradse's friction factor into Angel's model so that the modified model become more reasonable and practical. However, due to the difficulty in determining the friction factor, the application of Guo's model is limited to some extent. The latter motivates us to develop a new mathematical model to determine the gas injection rate. Li et al. [19]

a question.

164 Drilling

Recent developments in gas drilling include thermal failure of rock and gas temperature prediction. Zhang et al. [21] determined the effect of fluid temperature on rock failure in borehole drilling with gas. Li et al. [22] identified the complexity of thermal effect on rock failure in gas-drilling shale gas wells. Li et al. [23] developed a closed-form mathematical model for predicting gas temperature in gas-drilling unconventional tight reservoirs. Guo et al. [24] presented an analytical thermal-model for optimization of gas-drilling in unconventional tight-sand reservoirs. Guo et al. [25] published a mathematical modeling of heat transfer in counter-current multiphase flow found in gas-drilling systems with formation fluid influx. Other recent development in gas drilling includes distribution of the sizes of rock cuttings in gas drilling [26] and gas-lift drilling [27].

Another key issue of air and gas drilling to be solved is the environmental pollution and wasting of resources caused by direct discharging or combustion of the returned gas. To address this problem, our research group developed a new gas recycling system [28–30]. Unlike the conventional gas drilling process, the returned gas is re-injected into the wellbore after treatment by separators and fine filters, rather than being discharged or burned directly. The impurity content, humidity and other parameters of the treated gas can fully meet the requirements of the gas suction standard of a compressor. Therefore, the returned gas can be recycled through compressors, and consequently, the objectives of saving resources, lowering the cost, and environmental protection are achieved.

In the current work, a modified mathematical model for predicting the minimum gas injection rate is derived, taking into account Charles' theory of particle grinding energy. The maximum required value of gas injection rate is estimated using the sonic flow criterion at a bit. The proposed model allows calculating the optimum range of gas injection rate more precisely. Also, we present our work in developing the gas recycling system, including the corresponding equipment, operating procedure, and results of a pilot test. The test results indicate a promising prospect of GRS.

The structure of the text is as follows. Section 2 presents the modified mathematical model for predicting the optimum gas injection rate and the comparison between the model prediction and the field experience. Section 3 demonstrates the newly developed gas recycling system. Section 4 concludes this chapter.
