Microwave propagation in air drilling

This paper examines the feasibility in air drilling of transmitting down-hole signals by using microwaves. Firstly the basic theory of microwave propagation in the drill-pipe or casing was studied, including power loss, cutoff wavelength, and dust scattering. Theoretical analysis indicates that the microwave propagation distance in a cb214mm casing can easily reach 5,000 m. When the effect of dust particles is taken into account, the propagation distance decreases to 2,000 m. We conducted both laboratory experiments and field tests in casings commonly used in oil fields. The field tests show that the effective propagation distance of microwave in the casing is about 1,300 m. The experimental results do not match well with theoretical prediction, but are acceptable. In future commercial applications, by applying multiple relay amplifiers, the microwave propagation distance could be long enough for most drilling wells.

Relationship between Formation Water Rate,Equivalent Penetration Rate and Volume Flow Rate of Air in Air Drilling

Formation water invasion is the most troublesome problem associated with air drilling. However, it is not economical to apply mist drilling when only a small amount of water flows into wellbore from formation during air drilling. Formation water could be circulated out of the wellbore through increasing the gas injection rate. In this paper, the Angel model was modified by introducing Nikurade friction factor for the flow in coarse open holes and translating formation water rate into equivalent penetration rate. Thus the distribution of annular pressure and the relationship between minimum air injection rate and formation water rate were obtained. Real data verification indicated that the modified model is more accurate than the Angel model and can provide useful information for air drilling.

Critical condition study of borehole stability during air drilling

The purpose of this paper is to establish the existence of the critical condition of borehole stability during air drilling. Rock Failure Process Analysis Code 20 was used to set up a damage model of the borehole excavated in strain-softening rock. Damage evolution around the borehole was studied by tracking acoustic emission. The study indicates that excavation damaged zone （EDZ） is formed around borehole because of stress concentration after the borehole is excavated. There is a critical condition for borehole stability; the borehole will collapse when the critical damage condition is reached. The critical condition of underground excavation exists not only in elastic and ideal plastic material but in strainsoftening material as well. The research is helpful to developing an evaluation method of borehole stability during air drilling.

Analysis of effects of operating parameters on rate of penetration in drilling process with air down-the-hole hammer

Air down-the-hole(DTH)hammer drilling has long been recognized to have the potential of drilling faster than conventional rotary drill,especially in some hard rocks such as granite,sandstone,limestone,dolomite,etc.with the same weight on bit(WOB)and rotations per minute(RPM).So,it has been widely used in many drilling fields including mineral resource exploration drilling,oil and gas drilling and geothermal drilling.In order to reduce drilling cost by selecting optimal drilling parameters,rate of penetration(ROP)should be estimated accurately and the effects of different factors on ROP should be analyzed.In this research,ANN model with several multi-layer perception back propagation(BP)networks for predicting ROP of air DTH hammer drilling was developed using controllable parameters such as impact energy,impact frequency,WOB,RPM and bit operating time for the formations with a certain drillability index of rock.Several BP neural networks with the different neurons in hidden layers were developed and compared for selecting optimal architecture of ANN.The effects of the drilling parameters such as impact energy,impacting frequency,WOB,RPM and bit operating time on the ROP of air DTH hammer drilling were investigated by trained ANN.From the analyses,the optimum range of drilling parameters for providing high ROP were determined and analyzed for a formation with a certain drillability index of rock.The methodology proposed in this study can be used in many mathematical problems for optimization of drilling process with air DTH hammer.

New development of theories in gas drilling

Theories established from engineering fundamentals have been of great value in supporting the design and execution of drilling operations in gas drilling where gas is used as a drilling fluid.This work presents an overview of new theories developed in recent years for special gas drilling operations including horizontal wells.These new theories are found in the areas of gas-mixture flow hydraulics in deviated and horizontal boreholes,hole cleaning of solids accumulation,hole cleaning of formation water,flow diverging for washout control,bit orifice optimization,and depression of formation water influx.This paper provides drilling engineers with updated mathematical models and methods for optimizing design to improve gas drilling performance.