~J(PTPerformance of Positive Displacement

Downhole Motors under Two-phase FlowJ. LI*, R. TUDOR

Canadian Fracmaster Ltd.G. SONEGO

Black Max Downhole Tool Ltd.B. VARCOE

Computalog Ltd.* now with BJ-Nowsco Well Service Ltd.

FIGURE 1: Assembly of positive displacement motor.

Objectives

Power Sectlon

Rotor

Cavlti89

Bit Box

Output ShaftJBearing Assembly

Transmission

Bypass Valve

Power Section

There were four primary objectives for dynamometer testing ofPDMs in this study. These were as follows:

1. Establish baseline motor performance with water. This pro-vided baseline data for the condition of the rotor/stator forquality control purposes and comparison to the publisheddata.

2. Determine relative performance characteristics of the motorunder different NiH20 ratios. This provided a guideline formotor performance expectations and operational efficienciesunder different operating conditions.

3. Determine operating range for flow rate which can bepumped through motors. Modelling anticipated back pres-

drilling success and efficiency. Pressure drop across the motormust be understood to accurately simulate underbalanced condi-tions at the bottom-hole. In general, motor suppliers providemotor performance data at the baseline condition. A baseline testconsists of testing a motor at designated flow rates with water andmotor outlet open to atmosphere. When conventional drilling flu-ids are used, output torque and bit speed are determined based ondifferential pressure and flow rate. Due to the compressible natureof gas, it is difficult to predict the gas flow and RPM/torque rela-tionship based on baseline tests. In this study, the performancecharacteristics of PDMs under two-phase flow were investigatedwith a dynamometer.

AbstractPositive displacement motors have been used extensively in

underbalanced drilling operations. Motor performance character-istics are essential to ensure drilling success and efficiency.Pressure drop across the motor must be understood to accuratelysimulate underbalanced conditions bottom-hole. In this study,five positive displacement motors were tested under a variety ofmixing ratios of nitrogen and water with different back pres-sures. Comparing with the baseline test at maximum liquid flowrate, the replacement of water with nitrogen decreased the motorperformance by as much as 95% in terms of maximum torqueoutput and maximum mechanical power output. The pressuredrop across the motor was lower with commingled fluid, and thegreater the nitrogen ratio, the lower the pressure drop across themotor. It was also found that back pressure decreased the motorperformance. This paper also discusses the testing procedures,the results, and how commingled fluid affects underbalanceddrilling operations.

Underbalanced drilling is a process where the aerated or nitri-fied fluid is pumped through the drilling string to the bottom-hole.The pressure at the bottom-hole should be less than the formationpressure and allow wellbore fluids to flow during the drilling peri-0d. The circulated fluids power the downhole motor to rotate thedrill bit to cut the formations. The cuttings are then picked up bythe jetting action of the nozzles on the bit and transported by thedrilling fluid to the surface through the annulus between thedrilling string and casing. Currently, there are two main types ofdownhole motors: positive displacement (PDM) and turbines. Ingeneral, turbine motors have a high rotational speed, whereasPDMs have a low speed and high torque output. The applicationof turbine motors has been limited to less than 1% of the totalfootage drilled in the United States. PDMs are the most widelyused in the world at the present time(l).

The PDM is a hydraulically-driven downhole motor that isbased on the Moineau principle. It consists of a bypass valve,power section, transmission assembly, and bearing assembly(Figure 1). In the power section of a PDM, a helicoidal rotor withone or more lobes is placed eccentrically inside a stator havingone more cavity than the rotor. This difference between therotor/stator lobe configuration creates cavities. Under pressure,the drilling fluid will drive the rotor in an eccentric rotation,which is then translated into concentric rotation through the trans-mission assembly and transferred to the drill bit.

Motor performance characteristics are essential to ensure

Introduction

46 Journal of Canadian Petroleum Technology

sures altered pressures and resultant flow rate throughmotors due to compressible flow.

4. Establish operating parameters in terms of anticipated oper-ating pressures and pressure drops. These parameters aidedin determining expected bottom-hole pressures and pumppressures for simulation and job planning purposes.

EquipmentThe PDM was circulated with a commingled flow of nitrogen

and water and was tested on a dynamometer test bench. The lay-out of this equipment is shown in Figure 2. A circulation systemwas designed to circulate water to cool the dynamometer. The N2unit pumped the liquid nitrogen at pressures of up to 70 MPa(10,152 psi) using a cryogenic pump, and vapourized the liquid togas with a high capacity heat exchanger at rates of up to 170 stan-dard cubic metres per minute (1,070 bbllmin). The pump can pro-vide a steady supply of gaseous nitrogen at various rates and pres-sures. A fluid pump was used to pump water at a range of 10 to600 liters per minute (2.6 to 159 gpm). A 3// adjustable chokewas installed at the return line to simulate annular back pressure.Pressure transducers were located at the inlet and outlet of thePDM to monitor the pressure in and pressure out. A RID ther-mometer was used to measure the flow temperature at the inlet ofthe PDM. A turbine flow meter was installed on the water supplyline to determine the water pumped rate. The nitrogen (gas) ratewas measured by a metering system integrated with the N2 pump-ing unit. A real time data acquisition system was used to collectthe data for pressures, temperature, torque, rotational speed, andflow rates.

FIGURE 2: Layout of dynamometer test system for PDM.

Test ParametersFive different PDMs were tested. These were PDM #1: 2 7/ g"-

5:6 3.3 stage, PDM #2: 3 1/g" 7:8 4-stage, PDM #3: 3 3/g" 4:5 5-stage, PDM #4: 3 3// 7:8 2.3-stage, and PDM #5: 3 3/g" 7:8 3-stage. Detailed test parameters for these motors are listed in Table1 and the general range of the parameters is summarized asfollows:

a) Baseline test with water at flow rates of 100%, 75%, and50% maximum flow rate with no back pressure.

May 1999, Volume 38, No.5 47

b) Baseline test with water at flow rates of 100%, 75%, and50% maximum flow rate with back pressure per flow rate ofapproximately lA, 4.1, and 6.9 MPa (203, 595, and 1,000psi).

c) Slug flow tests at water rates no greater than 50% of maxi-mum flow rate; nitrogen rate varied according to rotationalspeed and back pressure of lA, 4.1, and 6.9 MPa (203, 595,and 1,000 psi).

d) Mist flow tests at a water rate of 10 Ipm (2.6 gpm) or less;nitrogen rate varied according to RPM and back pressure of1.4,4.1, and 6.9 MPa (203, 595, and 1,000 psi).

e) Post baseline tests were conducted to determine whether themotor performance changed.

Test ProcedureThere are no standard PDM test procedures documented in the

PDM industry. Different test standards and conditions are beingused by different PDM manufacturers. However, in this project,48

when performing a baseline test with water, the motor outlet wasvented to atmospheric pressure, and was also choked back todesign pressures as listed above. Care was taken to allow stabi-lization of parameters before taking readings at progressivelyincreased loading. The motor was loaded to stall or near stall (i.e.,60 RPM or less).

For multi-phase testing, both slug flow and mist flow regimeswere studied. In both cases, a choke was implemented down-stream of the motor in order to simulate bottom-hole conditions.Temperature was recorded, as temperature changes can have asignificant effect on effective volume rates through the motor.Note that the choke required adjustment as loading was increasedto maintain a constant downstream back pressure.

Results and DiscussionThe main performance parameters of PDMs are: mechanical

power, torque, and pressure drop. The mechanical power devel-oped by the motor can be calculated from the product of torque

Journal of Canadian Petroleum Technology

25

20

["15 ~0....J

10uZ:I:uW

~

010 12 14 1642

Q~208lpm1200 600 50

Q-4161pm 451000 500

40

800 400 35i ! 30i >-w 600 300 U::J Z 25a wor:

1100

1000

900

800

~ 700!. 600..:;)

500G0:I! 400

300

200

100

00 ~ ~ m ~ ~ _ ~ 400 ~

RPM

FIGURE 7: Comparison of the performance for PDM #2 31/8" 7:84-stage between pre- and post-baseline tests.

different flow rates. Therefore, in order to operate the PDMs effi-ciently, the pressure drop across a PDM has to be controlled at theoptimum range.

Figure 5 plots the performance of PDM #2 at the water baselinetest with different back pressures. It shows that the back pressurealso affected the performance of the motor for baseline water test,especially at a low RPM range with a higher flow rate. Theincreased back pressure resulted in a lower torque output for agiven rotational speed. The water expands the cavity between therotor and stator due to the high back pressure, therefore, the per-formance of the PDM changed. A higher back pressure alsoresulted in a lower operation efficiency. This is because a higherback pressure resulted in a higher pressure at the inlet of the motorand it needed higher hydraulic power to force the motor to rotate.Figure 6 compared the performance for the five tested PDMs forthe baseline test at their maximum flow rates. The figure indicatesthat PDM # 1, 2, and 3 have low-torque output and high rotationalspeed, while PDM #4 and #5 have a high-torque output and lowrotational speed. For motors #1, 2, and 3, the mechanical poweroutput was more sensitive to the RPM, while for motors #4 and#5, the mechanical power output was more sensitive to the torque.Therefore, the motors shall be selected based on the application.

As shown in Figure 4, a higher mechanical output range corre-sponded to a higher operation efficiency range. Therefore, basedon the plot in Figure 6, the optimum operation ranges for differentPDMs can be determined. For example, for motor #3, the opti-mum operation range was between points A and B, which are thecross-points between the power line of 20 kW and the perfor-mance curve of the motor.

Figure 7 shows the performance curves of pre- and post-base-line water tests for motor #2. The difference of torque between thepre-test and post-test was only 5%. However, the torque for thepost-test was higher than that for the pre-test at a given rotationalspeed. This is because after circulated with commingled fluid, theswollen elastomer increased the friction between the rotor and sta-tor and a higher friction resulted in a higher torque.

Two-phase Flow TestThe performance of the PDM with the commingled flow was

quite different from that with a water baseline test. Figures 8 and9 plot the performance of motor #2 with different NzlHP ratios.Compared with the baseline test, the PDM with a commingledflow had a lower-torque output and a higher rotational speed,especially with a lower water flow rate. The test results in thisstudy showed that the replacement of water with nitrogendecreased the motor performance by as much as 95% in terms ofmaximum torque output and maximum mechanical power output.That is why the PDM circulated with aerated or nitrified fluidswas more often apt to run away or stall than it did when being cir-culated with drilling mud. It was also noted that the operating

50

1200

1000

E 800!.w 600:;)G0:g

400

200

00 ~ 100 1~ 200 250 300 350 400

RPM

FIGURE 8: The performance of PDM #2: 3 1/87:8 4-stage two-phase flow test with water flow rate of 0.2 m"3/min.

pressure window between the no-load condition and the stall con-dition for the commingled flow was significantly smaller (asmuch as 94%) than with the baseline test(Z.3).

The NzIHzO ratio also affected the performance of the PDM.As shown in Figures 8 and 9, increasing the nitrogen rate resultedin increasing the torque output at a given rotational speed of thePDM for a fixed liquid rate and back pressure. Comparing Figure8 with Figure 9 also indicates that for a given gas flow rate, ahigher fluid flow rate resulted in a higher torque output. Thetorque output was more sensitive to the fluid flow rate, and theRPM was more sensitive to the nitrogen rate(Z).

In Figure 9, the effective flow rate for the test case withNzIHzO = 2010.01 at Pb = 3.9 MPa was 396 lpm; however, themaximum torque output was approximately 78% lower than thatfor the baseline test with a water flow rate of 416 lpm. This indi-cates that it is difficult to predict the performance of a PDM withcommingled flow based on the equivalent volume flow rate andthe baseline test.

As shown in Figure 10, the back pressure had a significantimpact on the performance of the PDM under the comlningledflow. The higher back pressure decreased the effective flow ratethrough the PDM inlet, therefore, resulted in a lower rotary speedand the power output. For the same RPM, a higher back pressureresulted in a lower torque output.

In order to run a PDM efficiently, the optimum combinationratio of NzIHzO needs to be determined. Based on the test resultsin this study, the higher the liquid rate with a lower gas rate, thebetter the performance of the PDM in terms of the torque outputand operation efficiency. It was also noted that with a higher liq-

1200

1000

~ 800!.w 600:;)G0:I! 400

200

FIGURE 9: The performance of PDM #2: 31/87:84 stage two-phase flow test with water flow rate of 0.01 m"3/min.

Journal of Canadian Petroleum Technology

1614126 10PRESSURE DROP (MPa)

500

FIGURE 11: The maximum torque output vs. the correspondingpressure drop for different PDMs.

2000

2500.,....--------------------,

~!.

~ 1500

~~ 1000::I!

~

700

600

500

E!. 400w:lII 3000...

200

100

00 100 200 300 400 500

RPM

FIGURE 10: The effect of back pressure on the performance ofPDM #2 circulated with water rate of 0,2 m"3/min. and nitrogenrate of 10 std.m"3/min.

2. The performance of PDMs with commingled fluids was verydifferent from that with a water baseline test. With the com-mingled fluid at the same effective flow rate, the outputs oftorque and mechanical power and pressure drop across themotor were lower than those for water baseline test. It wasdifficult to predict the performance of PDMs with commin-gled fluids based only on the equivalent fluid flow rate andthe performance curve of PDMs under a baseline test.

3. The back pressure had a significant impact on the perfor-mance of a PDM for commingled fluids. For a certain com-bination of Nz/HzO, a higher back pressure resulted in alower output of torque, mechanical power and pressure dropacross the motor. The back pressure can also affect the per-formance of a PDM for a water baseline test. However, com-pared with the commingled fluids, such an effect was muchless for the baseline test.

4. The fluid flow ratio of NzIHzO affected the performance of aPDM. The higher fluid fraction with a lower gas rate result-ed in a better performance of a PDM in terms of the torqueoutput and operation efficiency.

Based on the present study, the performance of PDMs withcommingled fluids differed from that with water baseline test.However, the test data in this project were limited. In order tocomprehensively evaluate the performance of PDM with commin-gled flow, more studies need to be conducted in the future.

It was also noted that there was no standard test procedure forPDM testing documented in the PDM industry. Different test stan-dards and conditions are being used by different PDM manufac-turers. Therefore, it was difficult to evaluate the performance of

uid flow ratio, the hole-cleaning capacity was higher(4). However,the liquid fraction of the drilling fluid was limited by the reservoirpressure gradient and the degree of underbalance. A higher reser-voir pressure gradient and a low degree of underbalance permittedhigher liquid contents.

It is also important to determine what the maximum torque out-put, pressure drop and RPM are and how much mechanical horse-power can be provided for a given pumped water rate and nitro-gen rate at a certain annular back pressure. Based on the testresults in this report, the conversion factors of maximum torque,power output and the pressure drop for the tested motors weredetermined. The conversion factors were used to estimate themaximum output of torque, mechanical horsepower, and the max-imum pressure drop across the motor for a particular combinationof Nz and water rates. The correlation between the maximumtorque, mechanical power, pressure and the pumped water rate, Nzflow rate, and back pressure were developed in a previous study(Z)and were also used to predict the maximum torque, mechanicalpower and pressure drop across a PDM.

Figure 11 plots the maximum torque output versus the corre-sponding pressure drop, at which the PDM generated the maxi-mum torque for different combinations of water rate, nitrogen rateand back pressure. It is interesting to note that for each particulartype of motor, the test data can be plotted linearly. If the maxi-mum pressure drop across the motor can be defined for a particu-lar combination of NzIHzO and back pressure, the stall torque forthe motor can be determined by the curve shown in Figure 11.

Figure 12 plots the maximum mechanical power output versusthe corresponding pressure drop, at which the PDMs provide themaximum mechanical power. The figure shows that all tested datafor maximum mechanical power versus the corresponding pres-sure drop can be plotted linearly for all five tested motors withdifferent combinations of NzIHzO and back pressures. If the pres-sure drop can be defined in Figure 12, then the maximum poweroutput for any combinations of NzIHzO and back pressures and forany type of motor can be determined by a plot similar to thatshown in Figure 12. However, the test results were limited to fivedifferent types of motors. More tests need to be conducted to sup-port such a conclusion.

ConclusionsIn this study, five different PDMs were tested with nitrified

fluid. Based on the test results, the conclusions are summarized asfollowing:

1. The performances of PDMs were different with differentconfigurations. The mechanical power output was more sen-sitive to the rotation speed for the low-torque motor tested(PDM #1,2 and 3), while it was more sensitive to the torquefor the high torque motor tested (PDM #4 and 5).

2S

ot1PDM

20 "nPOM xxll3PDM xi

...i 150..

:Iii 10~

4 6 7PRESSURE DROP (MPa)

FIGURE 12: The maximum mechanical power output vs. thecorresponding pressure drop for all five tested PDMs.

10

May 1999, Volume 38, No.5 51

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