PPT Principles
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 formations 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
2qr 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 Figure 35
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 obtained by project-
Figure 35
Example of Filton Curve with Invasion (from Barkman and Davidson)
Figure 35
Example of Filton Curve with Invasion (from Barkman and Davidson)
ing 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 following equation:
V2PckcA2 D Pt pv where pc = 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 pw = 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.
Sized lost circulation materials (LCM) can also reduce cake thickness and filtration. Low permeability filter cakes are the result of optimizing the distribution of sized solids in a mud. Low permeability 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. (Figures 35 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 tendencies.
Figure 36
Differential Pressure vs. PPT Fluid Loss Field Mud Example - Gulf of Mexico 2 Darcy Disks Tested at 350F
Figure 36
Differential Pressure vs. PPT Fluid Loss Field Mud Example - Gulf of Mexico 2 Darcy Disks Tested at 350F
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.
Table 16
PPT Study of LCM Sealing on Aluoxite Disk
|
Core |
Sample |
Total Leakoff, mL |
Spurt, mL |
Slope, mL/min |
Cake, 32nd |
PPT Value |
|
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 | |
|
FA0-50 |
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 |
|
FA0-50 |
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 |
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 bento-nite in the mud which will deteriorate the filter cake. Addition of a sized calcium carbonate (BARA-CARB) 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 bulk of the tests for evaluation of the seepage control materials.
The best performance of the materials tested in the base mud was the ULTRA SEAL XP. This, at 10 ppb, gave results similar to the base mud containing 15 ppb KWIK-SEAL MEDIUM, 15 ppb KWIK-SEAL FINE, and 5 ppb BORE-PLATE. GRAN SEAL and SINGLE SEAL gave similar, although inferior, performance as evidenced by their higher spurt and total leakoff values when compared to the ULTRA SEAL XP. MIX 2 failed to provide sufficient pore bridging to allow a filter cake to form.
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