Application notices of downhole motors

1) Site drilling technical persons and drillers shall know well downhole motor structure principle and technical parameters, correctly apply downhole motor in accordance with requirements specified in this manual.

2) Drilling engineer shall make operation program upon the whole hole, and select the model of downhole motor and bit to be used, determine nozzle size and downhole motor combination according to formation edge structure, hole size, depth and mechanical speed. The determined drilling program shall be strictly carried out accordingly on the location.

3) Drilling Fluid Requirement: Downhole motor shall be volumetric. The key factor to downhole motor property is motor inlet flow rate, and the pressure loss functioning onto both ends. But it is not the kind of driling fluid. Physical and chemical property of drilling fluid, except the individual damage to downhole motor running life, normally does not affect its property.

This shall be considered for the need of driling process. However, the content of hard grains in mud stream must be limited since they can speed up wear of bearing and motor, may cause shortening downhole motor running life. It is suggested that solid sand content should not be over 1%(actually when sand content is 5%, running life will be reduced by 50%). Also, take care, not let drilling fluid mixed with any gas, because the fluid with gas may easily cause "Gas Erosion". lt may speed up tool damage with change of pressure. In particular, the rubber on stator is more easily damaged by gas erosion. Enough attention shall be paid to this problem.

4) Bit Selection: For bit selection, it is important to combine and match downhole motor to be used. lt relates to success of downhole motor application. lt is advised site operators shall take care of this matter.

Application factors to combine bit withdownhole motor are as follows:

(1)Drilling Plan and Program;

(2)Formation Edge Structure;

(3)Structure of Mud Flowing Channels;

(4)Determined Mechanical Drilling Speed;

(5)Estimation of Bit and Downhole motor Running Life;

(6)Design of Bit Nozzle Pressure Loss.

Please refer relevant professional information for details of bit selection on formation structure. Hereby it briefly describes bit selection for performance of drilling operation with downhole motor.

A: Except bit nozzle pressure loss, in order to avoid producing higher pressure loss while drilling fluid flowing through bit bottom, attention shall be specially paid to this matter when the nozzle pressure loss has reached the specified loss value of this kind drilling tool. This does not affect rock bit, but for the crown passage design of PDC bit. The problem shall be considered that passing fluid area may cause much extra loss, meanwhile to ensure prompt discharge of cuttings and bit cooling.

B: Rock Bit: When this kind of bit is used with downhole motor, it is more appropriate to shorter drilling period, such as directional deflection and re-entry etc.

C: PDC Bit: It is not only used for directional deflection, but also more appropriate to longer drilling period, such as vertical well drilling. During longer period of drilling operation, the more important factor is to integrate bit and downhole motor, so as to avoid unnecessary trip operation due to some problems. It is known that PDC bit can last longer time than rock bit, and it is integral in structure, having more advantages.

D: It is also an important factor to prolong running life of bit and tool. lt is helpful to extend screw life and to develop bit property by improving stability of drive shaft, e.g. stabilizer. And it shall be considered the close relationship between diamond geometric dimension, allocation, drilling weight load and the required high speed and lower drilling weight of downhole tool. In general, it is hard to obtain satisfactory effect without strictly matching bit and downhole motor, it can also cause earlier damage to the tool.

5)Requirement for Hole Bottom Temperature: It is unfavorable to the motor property, and it may intensify all disadvantageous factors if temperature goes too high. When using oil base mud, the hole bottom temperature shall be lower than 90℃, downhole motor shall be working in optimum condition. When temperature is over 150℃, stator life may be greatly shortened, even by using optimum oil base mud or using water base mud.

In order to keep downhole motor working normally at high temperature of oil base mud, the way shall be adapted to conduct trip-in by sections and intermittent recycling, to ensure the actual temperature of stator below the ultimate value by using hollow rotor with splitter hole to speed up recycling or improve heat radiation of the fluid and other property. Our company manufacture two kinds of stators. One is conventional stator, with 95℃ of rated temperature and 120℃ of maximum temperature; another is hi-temperature stator, with 105℃ of rated temperature and 150℃ of maximum temperature.

6)Requirement for Flow Rate of Drilling Fluid: One of the downhole motor features is that the output speed is proportional to the input flow rate of drilling fluid. Each downhole motor has a certain effective range of working flow rate. It is advised to choose the range according to the recommended parameters. Otherwise, it may shorten the downhole motor's working efficiency and running life.

7)Features of Drilling Fluid and Drilling Weight: When downhole motor is in idle working condition, if keeping fluid flow rate unchanged, the pressure loss caused by both bit and downhole motor shall be a constant value. This value changes with the difference of drill model and specification. As downhole motor is running, fluid recycling pressure will gradually increase with gradual rise of drilling weight. The increment of pressure is proportional to the torque increment needed by drilling weight and stripping in. When it reaches the maximum recommended value, optimum torque shall be produced. lf drilling weight goes on rising, when the pressure loss of recycling fluid at both ends of motor is over maximum design value, leakage shall occur from the downhole motor. In normal operation, the gauge pressure rises with the drilling weight up and down. lf the gauge shows sudden rise of temperature to several Mpa, when the drilling weight goes on increasing, pump pressure shall no longer increase. This indicates there occurs some leakage from downhole motor. The seal between stator and rotor may be changed, and fluid may be flowing out from bit nozzle while going through the damaged seal cavity. When it is sticking due to this problem, fluid is still recycling through downhole motor since downhole motor braking. Once downhole motor brakes, it shall be quickly lifted away from the hole bottom to reduce drilling weight, since downhole motor may be seriously damaged with fluid flowing through the idle motor for longer time.

Additionally, the pressure loss at the bottom ends of downhole motor shall be controlled within the recommended range, to ensure downhole motor in optimum working efficiency and running life.

8)Early Hydraulic Calculation: In driling operation, the pumped mud flows successively through riser, swivel, kelly into drill pipe, collar, non-magnetic collar, motor ,bit nozzle and annular space, and upwards to the surface. During recycling, the loss caused from the friction of fluid its own and the friction of fluid onto the pipe wall, hole wall and/or partial flow will consume a certain energy.

The energy loss will be shown from the pressure loss. Total pressure loss can be calculated by accumulating pressure loss in each portion. After calculation of total pressure loss according to the determined drilling depth, it can be a parameter to determine the pump pressure. lt is complicated to calculate large amount of pressure loss. Therefore, the site operator can adapt a simple method. Only by lifting the bit a little bit from the bottom, the reading on the pressure gauge of main pipe shall be the mentioned total pressure loss at the rated flow rate.