Abstract

Nowadays the use of air and gas as a circulating medium for drilling oil and gas wells is becoming an attractive practice because of its great advantage over conventional mud drilling. However, the occurrence of some troubles due to poor carrying capacity of cuttings in air/mist drilling is not uncommon. Meanwhile, the determination of air/gas injection rate desired to undertake air and gas drilling is also closely related to the actual moving regularity of cuttings in annulus. It is obvious that researches on migration rate and carrying capacity of cuttings within borehole are essential to air/mist drilling. Although there are some theoretical models for prediction of cutting migration velocity and therefore the carrying capacity of cuttings, experimental studies are also essentially important because of the complexity of actual liquid, gas and solid multiphase flow in wellbore and the uncertainty of theoretical models. In this paper, some experimental studies were performed to investigate the moving regularity and carrying capacity of cuttings in wellbore. Some comparisons between the testing results and theoretical models are also presented in the paper. It is seen that such a study is valid and effective in prediction of the migration velocity and analysis of the motion regularity of cuttings in borehole. Therefore, the required practical air injection rate to keep the necessary carrying capacity in air/mist drilling can be determined definitely.

Introduction

In air/mist drilling, the wellbore cleaning can be completed mainly through the impact energy of high-speed air/gas flow applied on cuttings or droplets in annulus. However, the study of such an issue could be extremely difficult due to both the physical and chemical effects respect to the complicated multiphase flow of liquid, gas and solid. The mass transfer between liquid and gas, the thermal exchanges between formation and the fluid in wellbore and many other factors may also directly dominate the process of wellbore clearing, which has becoming a major unsolved problem affecting the further development and practical application of air/mist drilling. Although researchers have studied on the dynamics of multiphase flow for many years and some theoretical models have been developed to describe the natures of multiphase flow behavior, the validity of each model is usually uncertain in many cases because of lacking of necessary tests and verifications. Therefore, it is important to develop a theoretical model or corresponding correlation on the basis of laboratory test.

Theoretical Model

According to the principle of the error and correlation analysis method, the major factors affecting the process of carrying capacity of cuttings can be defined and a simplified but relatively reasonable theoretical model could be established. Based on a careful study and investigation of the existing theoretical models and laboratory test results, an one-dimensional high load flowing model was to be employed to undertake the computation and analysis of carrying capacity of cuttings for air/mist drilling. See appendix A.

Test Equipments and Facilities

The test system and facilities used to simulate the real air/mist drilling process is most important to the study of carrying capacity of cuttings. Such an experimental work began in the early years in Tulsa University and University of Pennsylvania, U.S.A. and some typical and useful results have been obtained from experimental study. Based on this, a new and advanced experimental device and system has been set up in Southwest Petroleum Institute (SWPI), China for the laboratory test and researches on air/mist drilling. See fig. 1.

Air Source System.

The purpose of this system is to provide a steady airflow to be needed. It mainly consists of an air compressor, air tank and pipelines. A valve on the pipeline can control the flow rate of air from the compressor and the air tank is used to keep a constant injecting pressure and hence a steady airflow for test.

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