Abstract
Gas drilling technology not only has the advantages of avoiding well leakage, shale hydration expansion,reservoir pollution and other issues, but also can greatly improve the drilling speed 4 to 8 times, it is helpful to timely discover and effectively protect reservoir and improve recovery efficiency and single well production of oil and gas reservoir, especially has important significance for hard strata and dense reservoir exploration. However, there are some problems hindered the promotion of this technology, such as gas is hard to carries cuttings and gas hole blocking. One of the key technologies to solve these problems is the determination of gas volume. Most of today’s literature use formation temperature instead of annulus temperature to calculate minimum gas volume. Owing to gas’ PVT effect, temperature is dramatically influenced by pressure flow velocity, there will be big error if we use formation temperature instead of annulus temperature. Meanwhile, most of traditional minimum gas volume methods use top of the drill collar as key point. In this study, we found that on the top of the drill collar, the carrying kinetic energy of gas is not is not necessarily the minimum, and change of the key position is strongly influenced by wellbore structure and make-up of drill tool. Minimum gas volume determined by applying temperature - pressure coupling calculation is more close to site construction, under the given conditions, error of the minimum gas volume is about 0.4%.
Highlights
In the design of gas drilling engineering, it is necessary to calculate the minimum gas flow and surface injection pressure in order to select the appropriate ground equipment and predict the bottom hole pressure accurately
The research found that the minimum of wellbore gas carrying rock kinetic energy is not necessarily at the top of the drill collar, the change of the key point is influenced by the well structure and drilling tool combination
On the basis of previous studies, using pressure temperature coupling calculation model, consider the drill pipe fluid and annulus fluid heat exchange, annulus fluid and formation near the wellbore exchange and the temperature change of the gas flowing through the bit nozzle process, establish the borehole internal fluid pressure temperature coupling calculation model and solve it
Summary
In the design of gas drilling engineering, it is necessary to calculate the minimum gas flow and surface injection pressure in order to select the appropriate ground equipment (air compressor, booster, etc.) and predict the bottom hole pressure accurately. The research found that the minimum of wellbore gas carrying rock kinetic energy is not necessarily at the top of the drill collar, the change of the key point is influenced by the well structure and drilling tool combination. Temperature, pressure, well structure, and the geometrical dimension of well bore influence each other, so the "key point" position is very important to calculate the minimum gas injection volume. On the basis of previous studies, using pressure temperature coupling calculation model, consider the drill pipe fluid and annulus fluid heat exchange, annulus fluid and formation near the wellbore exchange and the temperature change of the gas flowing through the bit nozzle process, establish the borehole internal fluid pressure temperature coupling calculation model and solve it. The minimum gas injection volume is calculated according to the modified model [5,6] of the minimum kinetic energy method
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