In running the PPT test, it is important to remember that the type of mud and the pore size of the disk used for the test will influence the results. The disks are meant to simulate the porous forma-tions encountered in drilling. Trends are important to monitor rather than absolute values. Results of products run in different types of mud should not be compared.
The important parameter to consider is the ratio of the size of the particles in the mud and the pore size in the rock. Generally speaking, when the ratio of the particle size to pore size is less than 1/6, whole mud will pass through the formation and bridging will not occur. A ratio of 1/2 or greater will form a filter cake and intermediate values will show invasion with bridging (spur) until the effective bridged ratio is greater than 1/2 and the filter cake forms. No leakoff control can be obtained until a filter cake is formed.
When the mud contains very fine particles relative to the pore size, i.e., the ratio is less than 1/6, SCM is added as a bridging agent to allow a filter cake to form. SCM will bridge the pores until the effective pore size is reduced and filter cake can form. The SCM may also effect the quality of the filter cake such that both loss of whole mud and normal fluid loss (filtrate) are reduced. A given SCM’s performance will be related to the core pore size that it is tested against. The performance of a SCM will also be a function of the mud type.
The deposition of a filter cake and bridging of pores is covered in filtration theory. The equations covering filtration have been known for several years. The basic form presented here for filtration with invasion comes from Barkman and Davidson:1
where
V = cumulative throughput volume
S = slope of cumulative volume vs. square root of time t = total time of filtration
VB = cumulative volume at bridging QB = linear filtration rate at time of bridging
This is the case when both normal fluid loss and seepage occur. When total loss occurs, there is no external filter cake formed and VB does not have a finite value. This equation is illustrated in Figure35 for a suspension of silica flour on Berea sandstone.
As is seen from the figure, the slope is constant once the filter cake is formed. The values of tB, the bridging time, and VB, the bridging volume, may be solved for by iteration from the general filtration equation. In actuality, with PPT experiments at high pressure, the initial part of the curve is quite steep. Due to the extremely short bridging time, normally on the order of a few seconds, sufficient quality data is not available to accurately solve for tB and VB. The value reported, the spurt, is V S t+VB S2
2QB ---–
=
obtained by projecting the linear portion of the curve to the y-axis. In the example above the spurt is approximately 290 mL.
The slope of the linear portion of the curve, S, gives valuable information concerning the quality of the filter cake. When S=0, an impermeable filter cake is formed. A large value of S indicates that a poor quality filter cake is formed. The permeability of the cake is related to the slope by the follow-ing equation:
where
rc = bulk density of filter cake kc = filter cake permeability Ac = area of filter cake
pt = total pressure differential across filter cake and filter medium m = fluid viscosity
w = weight concentration of solids in water rw = density of fluid
As can readily be seen by examination of the equation, when only small changes are made to the fluid, the slope varies as the square root of the filter cake permeability.
Figure 35
Example of Filton Curve with Invasion (from Barkman and Davidson)
S 2ρckcAc2∆pt µwρw
---=
Sized lost circulation materials (LCM) can also reduce cake thickness and filtration. Low ability filter cakes are the result of optimizing the distribution of sized solids in a mud. Low perme-ability filter cakes not only reduce filtration, but also minimize differential pressure sticking.
Commonly-used LCM’s that reduce filtration are a variety of cullulosic material and sized solids calcium carbonate and gilsonite or similar material. Cellulosies are effective because of its ability to swell and physically bridge a wide range of pore openings.
Pilot tests which use the PPT cell, rather than conventional HTHP filtration equipment can be used to optimize additives and concentrations. Both PPT and conventional fluid loss tests should be performed to identify trends and to evaluate the effectiveness of product treatments. (Figures35 and 36 illustrate how the PPT can be used to indicate improvements in the muds.) Running a PPT with every mud check can direct mud treatments and thus minimize differential sticking tenden-cies.
Table 16 contains PPT data which compare the effectiveness of various LCMs in a specific mud system.
This table includes total leakoff, spurt, the slope of the linear portion of the volume versus the square root of time curve and filter cake thickness for some runs. The last column consists of a value, the PPT Value, that is reported by some sources. The value is defined as:
PPT Value = Spurt + 2 x [Total Volume (30 minutes) -Spurt]
The base mud is a water-based mud containing 20 ppb prehydrated bentonite, 30 ppb Rev-Dust, 0.5 ppb caustic, and 0.5 ppb PAC-L.
Figure 36
Differential Pressure vs. PPT Fluid Loss Field Mud Example - Gulf of Mexico
2 Darcy Disks Tested at 350°F
If the core does not exhibit a high spurt, the material will not show its benefits as a seepage control material but rather as a filter cake additive. As can be seen very little spurt was seen for AF-6 and AF-15. Control here is provided by the filter cake which readily forms.
AF-50 did exhibit a significant spurt, but the solids in the base mud eventually bridged the pores and allow a filter cake form. Addition of 35 ppb (10%) potassium chloride (KCl) did not change the spurt appreciable but did increase the slope from 4.00 to 12.84. This is due to the flocculation of the bentonite in the mud which will deteriorate the filter cake. Addition of a sized calcium carbonate (BARACARB) significantly decreased the spurt and caused negligible effect on the quality of the filter cake. Two cellulosic materials are included for illustration. This series of tests does illustrate the changes seen in the filter cake quality and fluid loss with different type additives.
The pores in AF-80 were large enough that no filter cake was formed during the base mud test.
The entire mud sample was passed through the core in a few seconds. This core was selected to run the Permeability Plugging Test Procedure
The PPT apparatus may be used in the lab to evaluate lab formulated muds or mud systems sub-mitted by the field. The apparatus is also portable and can be used at the rig to evaluate the mud system and to test the bridging effectiveness at downhole temperature and pressure of various additives. Information from the PPT is especially useful on critical wells - drilling depleted sands or high angle holes. (A diagram of the PPT apparatus is shown in Figure37.) The test procedure is as follows:
Table 16.
PPT Study of LCM Sealing on Aluoxite Disk
Core Sample
AF-6 Base mud 23.56 1.91 3.95 45.2
AF-15 Base mud 25.71 1.71 4.38 - 49.71
AF-50 Base mud 55.45 33.55 4.00 77.34
FAO-50 Base mud 52.36 32.80 3.57 9 71.92
AF-50 Base mud - 35 ppb KCl 98.12 27.70 12.84 16 168.46
AF-50 Base mud - 10 ppb Baricarb 150 + 10 ppb Baricarb 6
35.51 14.12 3.00 9 56.90
FAO-50 Base mud 52.30 32.80 3.57 9 71.92
AF-50 Base mud + 10 ppb Ultra Seal XP 29.17 15.48 2.50 6 42.88
AF-50 Base mud + 10 ppb MIX 2 40.67 27.54 2.40 - 53.80
AF-80 Base mud Total Total No Cake No Cake
-AF-80 Base mud + 10 ppb Ultra Seal XP 72.85 30.80 2.19 - 94.85
AF-80 Base mud + 10 ppb MIX 2 Total Total No Cake No Cake
-AF-80 Base mud + 10 ppb Single Seal 100.75 34.50 2.77 - 124.91
AF-80 Base mud + 10 ppb Gran Seal 102.72 87.52 2.73 - 17.95
AF-80 Base mud + 15 ppb Kwik Seal Fine + 10 ppb Kwik Seal Medium + 5 ppb Bore-Plate
69.47 51.47 3.29 - 87.47
[Note: Several manufacturers supply the hardware for the device. These steps are not meant to be specific but rather apply to all cells. Manufacturer instructions should be consulted before any tests are performed.]
Steps
Step 1. Apply a thin coat of stopcock grease around the o-rings of the floating piston.
Step 2. Screw the floating piston onto the T-bar wrench and install the piston into the bottom of the cell. Work the piston up and down to ensure that it moves freely. (The bottom of the cell will have a shorter recess than the top.) Position the piston so that it is near the bottom edge of the cell and unscrew it from the wrench.
Figure 37
Permeability Plugging Test Apparatus
Step 3. Install the hydraulic end cap onto the bottom of the cell.
Step 4. Turn the cell upright and fill with 350 cm3 of mud.
Step 5. Acceptable: Soak Aloxite disks for at least 5 minutes in fresh water prior to testing a water-based mud. For oil muds, the Aloxite disks should be soaked in the representative base oil for at least 5 minutes prior to use.
Step 6. Preferred: Place the aloxite disk in the appropriate solution. Place this in a vacuum cell and evacuate for 1 hour.
Step 7. Install the top cap with valve onto the cell. Close the valve and install the cell into the heating jacket. Lower cell slowly until it bottoms out, then rotate clockwise until it locks in place.
Step 8. Install a thermocouple in the small hole on the top of the cell.
Step 9. Place the filtrate reservoir onto the top of the valve. Ensure that the small o-ring is in good condition. Lock reservoir in place by installing a safety key (modified cotter key).
Step 10. Install the back pressure device onto the reservoir and lock in place with safety key.
Step 11. Open the valve directly on top of the cell (green valve). Ensure that the other valves are closed on the reservoir and backpressure devices.
Step 12. Apply the appropriate amount of back pressure to the cell for the desired test temperature using the CO2 backpressure device. (Refer to Table 17 for the recommended minimum backpressure.) Once the backpressure is applied, close the valve on the cell (green valve) to trap the pressure.
Note:Aloxite disks should NEVER be reused.
Table 17.
Recommended Minimum Back Pressure
Test Temperature Vapor Pressure Minimum Back Pressure
°F °C psi kPa psi kPa
212 100 14.7 101 100 690
250 121 30 207 100 690
300 149 67 462 100 690
LIMIT of Normal Field Testing
*350 177 135 932 160 1104
*400 204 247 1704 275 1898
*450 232 422 2912 450 3105
*DO NOT exceed equipment manufacturers’ recommendations for maximum temperatures, pressures, and volumes.
Step 13. Install the quick-connect from the hydraulic pump to the hydraulic end of the cell (bottom end). Leave the black valve on the pump open.
Step 14. Heat the cell to the desired temperature. Open the valve on the cell (green valve).
Step 15. Close the valve on the pump (black valve) and apply desired pressure to the cell with the pump.
Step 16. Once the desired pressure is applied, open the valve on the reservoir and collect the mud and/or filtrate in a graduated cylinder. Continue to collect the liquid until the reservoir blows dry. This should be recorded as the spurt loss.
Step 17. Close the valve on the reservoir and maintain the desired pressure on the cell with the hydraulic pump. (Most pumps leak slightly.) Hydraulic pressure will need to be applied to the cell to maintain pressure on the cell for 30 minutes. Monitor the flow by bleeding the cell every 5 minutes and recording the volume.
Step 18. After 30 minutes, record total amount of liquid recovered (exclude the spurt loss).
Step 19. Release the pressure on the hydraulic pump and close the top valve.
Step 20. Remove the hydraulic quick-connect from the cell to the pump.
Step 21. Bleed off the back pressure. Remove back-pressure device and repeat the same for the res-ervoir. Remove the reservoir from the top of the cell and clean.
Step 22. Turn heating jacket off. Allow cell to cool by removing it from the heating jacket or cooling it in cold water.
Step 23. Open the top valve slowly to remove trapped pressure. Repeat this process several times to ensure that all the pressure is removed from the cell.
Step 24. Remove the top cap of the cell and turn the cell upside down. Remove the hydraulic end cap (bottom cap) to expose the floating piston. Screw T-bar wrench into the piston and gently push down to force mud and disk out the opposite end of the cell.
Step 25. Recover the Aloxite disk and filter cake. Wash cake very lightly with fresh water.
Step 26. Measure filter cake thickness.
Step 27. Preferred: Plot volume versus square root of time (minutes) and calculate spurt and S.
Alternative: Total fluid loss is calculated as follows:
The total fluid loss and filter cake thickness should be recorded daily on the mud sheet for trend analysis. Whole mud and filtrate recovery should be differentiated in the spurt loss and 30 minute recovery when possible.
Step 28. Completely disassemble the cell and clean apparatus.
Note:This may cause mud to splatter out if too much pressure is applied.
Note:Total Fluid Loss = (Spurt Loss, cm3) + [(2) (30 minute fluid recovery)]