How to run casing and openhole pressure tests

Transcription

1 How to run casing and openhole pressure tests Prof. dr. Davorin Matanović

2 Leak-off test PRESSURE TESTING below the casing seat is generally performed for two reasons: 1. To test the cement job. 2. To determine the fracture gradient in the first sand below the casing shoe. If the cement job is faulty, remedial work can be carried out before further drilling is attempted. Usually, the next casing string depends on what can be contained with the one just set. So maximum permissible well-bore pressure that can be imposed on the formation is valuable information for the operator. 2

3 Leak-off test If for some reason the operator anticipates a need for a total hydrostatic pressure in excess of the test pressure, he should either retest with the drill string back in the last casing string or set a liner before drilling ahead. Testing initially to leak-off is an arbitrary decision that depends on the operator's objectives. If experience shows the hydrostatic pressure plus circulating pressures will not exceed 1920 kg m -3 (16 lb/gal.) then there is no need to increase the pressure to the leak-off point that may reach 2160 kg m -3 (18 lb/gal.) On the other hand, if the next casing string depends on what can be contained with the last casing string then maximum advantage can be obtained from a specific knowledge of the maximum permissible well-bore pressure that can be imposed on the formation. Thus, in this case the operator can profit from a knowledge of the maximum leak-off and rupture pressure. 3

4 Leak-off test TESTING the well bore for maximum pressure limits can be easy and accurate if certain specific procedures are followed. These procedures include correcting for mud gel strength, displaying pressure and volume limits on the test graph, having the proper pump rate, and running the test long enough. Post-test analysis is also a critical factor. A properly run leak-off test (maximum pressure test) provides the operator with a vital piece of information the maximum equivalent mud weight his casing shoe can stand before lost circulation occurs. 4

5 Leak-off test This must be known in daily operations such as: picking casing seats, weighting up the mud, and in critical operations such as shutting in a well when it kicks. 5

6 General procedure Well-bore pressure testing is achieved by: pulling the bit into the casing, conditioning the mud, closing the BOP, then slowly pumping mud down the drill pipe (or annulus) (0,048 to 0,08 m 3 min-1 ) until pressures reach the maximum pressure specified or the anticipated leak-off pressure for uncased holes. 6

7 General procedure The leak-off pressure is the point at which the exposed formation, or cement job, just starts to fracture as evidenced by a change in slope of the pressure-volume graph plotted. A typical leak-off pressure plot is shown in Fig. for a well which has a short section of open hole exposed. psi x 6895 = Pa bbl. x 0,159 = m 3 7

8 As shown, there is a constant pressure increase for each determined volume (m 3 ) or (bbl) of mud pumped so that the points fall on a relatively straight line. 8

9 The line should parallel, or fall slightly below, the "minimum volume" line calculated (or previously measured) for the drilling mud in the hole. 9

10 General procedure The straight-line trend continues until point A where the points bend to the right. At point A the formation starts to accept whole mud since a smaller pressure increase is seen for the same volume of mud pumped. Point A is referred to as the leak-off pressure and represents the point where the formation grains just start to fracture apart. This leak-off pressure is corrected for mud gel strength effects, then used to figure fracture pressures and equivalent mud weights. psi x 6895 = Pa bbl. x 0,159 = m 3 10

11 General procedure As shown in Fig. it is necessary to record several more points as the curve bends over to insure that the fracture limit has been reached. At point B the pump should be shut down, the instantaneous shut-in pressure recorded (point B), and the well left shut in to observe the rate of pressure decline. This pressure decline is an indication of the filtration rate which is useful information when evaluating the quality of the leak-off test. psi x 6895 = Pa bbl. x 0,159 = m 3 11

12 General procedure (1) After determining leak-off pressure, the formation fracture pressure can be calculated by first subtracting the mud gelation pressure from the leak-off pressure and adding the mud-weight pressure. P P ff ff = = P P lo lo P P g g + + P ρ h m g H Where: P ff - Well bore pressure next to the formation at fracture, Pa P lo - Leak-off pressure, Pa P g = Mud gelation pressure, Pa P h - Hydrostatic pressure of mud column ρ - Mud weight, kgm 3 g gravitational constant, m s -2 H - True vertical depth of formation, m 12

13 General procedure In terms of equivalent mud weight: Where: (2) EMW = P ff g H EMW Equivalent mud weight next to the formation when fracture occurs, kgm 3 13

14 Mud gelation effects Equation (3) used to calculate the mud gelation pressure when the leak-off test is run down the drill pipe. Pumping down the drill pipe: (3) P Where: P gdp - Gelation pressure in drill pipe, Pa L - Length of drill pipe, m d dp - Drill pipe ID, m Y - Gel strength of mud, Pa gdp = 4 L Y d dp 14

15 Mud gelation effects Equation (4) used when the test is made down the annulus. Pumping down the annulus: (4) Where: P ga - Gelation pressure in annulus, Pa L - Length of drill pipe, m Y - Gel strength of mud, Pa d h ID of hole or casing, m D dp - Drill pipe OD, m P ga = 4 d h L Y D dp 15

16 Mud gelation effects Pressure charts for Equations are given in Figs. 2 and 3. 16

17 Field gel strengths The most questionable quantity in Equations 3 and 4 is the gel strength of the mud. In normal practice the gel strength is measured at the surface in a rotational viscometer after the mud has been quiescent for 10 min. This method has been criticized because it is not performed at down-hole temperature and pressure and it does not reflect the properties of any contaminated mud which may be in the annulus. 17

18 Field gel strengths The most questionable quantity in Equations 3 and 4 is the gel strength of the mud. In normal practice the gel strength is measured at the surface in a rotational viscometer after the mud has been quiescent for 10 min. This method has been criticized because it is not performed at down-hole temperature and pressure and it does not reflect the properties of any contaminated mud which may be in the annulus. 18

19 Field gel strengths One way to take such factors into consideration is to determine gel strength using field circulation data not the field viscometer. After the leak-off test is run, wait 5-10 min and then turn on the pump with the BOP open. Measure the pres-sure necessary to break circulation (P bc ) while pumping at a rate equal to the rate used in the leak-off test. The pressure recorded is used to calculate the effective gel strength, (Y e ), of the mud using the equation: (5) Y e = 1, 083 L P ( ) bc dh Ddp ( d + d D ) dp h dp d dp 19

20 Leak-off graph Before starting the leak-off test it is important to prepare a leak-off graph which contains an "anticipated leak-off pressure" line and a "minimum volume" line. These lines are used as instant guides while the test is in progress. 20

21 Anticipated leak-off This line shows pressure expected so the test can be evaluated when the curve bends over. It has been found useful under the following conditions. 1. Testing the casing for leaks. Before drilling out, the casing should be tested for leaks. Maximum test pressure specified will vary according to which casing string is being tested. USGS requires that on offshore leases the conductor string be tested to 13, Pa (200 psi), the surface string to 68, Pa (1,000 psi) and the intermediate, liner, and production strings be tested to Pa (1,500 psi) or 0, Pa/0,3048 m (0.2 psi/ft), whichever is greater. In other areas the maximum test pressure is usually set by the operator. 21

22 Anticipated leak-off 2. Testing the open hole. The leak-off test run in the open hole should be at least as high as the predicted fracture pressure value for the area. This predicted value is obtained using data from nearby wells and equations presented by various authors. One method found most useful uses the equation: ff (6) ( ) S ' P P v p p P = K + K - Effective stress ratio S' v - Total vertical stress, Pa P p - Pore pressure, Pa P ff - Formation fracture pressure, Pa 22

23 Anticipated leak-off (S' v ) is often assumed to be 0, Pa/0,3048 m (1.0 psi/ft) of depth. This may not be accurate, especially when drilling offshore in very deep water. When possible, it should be determined using density logs. ff ( ' S ) v Po Po P = K + K - Effective stress ratio S' v - Total vertical stress, Pa P o - Pore pressure, Pa P ff - Formation fracture pressure, Pa K P ff = S ' v P P o o (6) 23

24 Anticipated leak-off Using Equation 6 and field data it is possible to determine the effective stress ratio, K, as a function of depth for a given area. Fig. 4 shows a typical plot of such data. ft x 0,3048 = m 24

25 Anticipated leak-off Effective stress ratio is in fact Poisson s ratio ν P ff = + 1 ν ( P ) ob Pp Pp P ob pressure of overburden formations, Pa 25

26 After P H is determined, the anticipated leak-off pressure line is calculated using the equation: Where: P = P P + P (7) a P a anticipated leak-off pressure, Pa ff h g 26

27 Minimum volume line This line provides a guide for determining if the pumping rate is fast enough when testing the open hole. The pumping rate should stay equal to, or a little below this line. The line represents the pressure necessary to compress the mud in the well bore until leak off occurs. It can be calculated with Equation 8. V i = C m (8) V w P i Where: V i - Volume of mud injected, m 3 C m - Compressibility of mud, Pa -1 V w - Volume of well bore (drill pipe plus annulus), m 3 P i - Injection pressure, Pa 27

28 Minimum volume line The compressibility of the mud can be calculated with the equation: Where: C w - Compressibility of water, Pa -1 C s - Compressibility of solids, Pa -1 C m = Cw s % water + C % solids (9) 28

29 Fig. 5 Curves A and B represent the volume correction which must be subtracted from each curve when drill pipe is in the hole. Curve A is for 4-in. drill pipe and curve B is for 5-in. drill pipe. Fig. 5 presents the volume of fluid (V i ) required to pressure up various size casings and open holes containing water for each 7 MPa injected pressure. 29

30 Fig. 6 A further correction must be applied if a weighted mud is in the hole. Fig. 6 shows the volume percent correction to be used as a function of mud weight. 30

31 As an example It takes 0,36 m 3 (2 ¼ bbl) of water to pressure 3048 m (10,000 ft) of 9 5/8-in. casing to Pa (1,000 psi). If 5 1/2-in. drill pipe is in the casing, 0,04 m 3 (¼ bbl) must be subtracted from the 0,36 m 3 (2 1/4 bbl), leaving 0,32 m 3 / Pa (2 bbl/1,000 psi). If 1560 kgm -3 (13 lb/gal) mud is in the hole Fig. 6. shows that it takes only 85% of this volume, or 0,32 m 3 / Pa x 0.85 = 0,272 m 3 / Pa. This value was used to construct the "minimum volume" line of Fig

32 Leak-off test procedure The following steps are general guidelines which have been useful in running the leak-off test. 1. Construct a graph similar to Fig. 1. The dashed lines indicate the "minimum volume" line and the anticipated leak-off pressure line. 2. While coming out of the hole, position the bit in the casing above the shoe. Fig. 1 32

33 Leak-off test procedure 3. If the mud is not of a known, uniform density it should be circulated until it is. Two common causes of nonuniform density are barite slugs in the drill pipe and formation cuttings in the annulus. 4. Close the ram preventer above the drilling spool. Fig. 1 33

34 Leak-off test procedure 5. Using a small pump (such as a cementing pump) begin pumping mud down the drill pipe at a constant rate of 0,04 to 0,24 m 3 min -1. The rate depends on conditions. With no open hole use 0,04 to 0,053 m 3 min -1. With sandstone formations exposed use 0,12 to 0,24 m 3 min -1 depending on the amount of open hole. Data obtained should fall very close (within 0,08 m 3 min -1. ) to the "minimum volume" line at leak-off. 6. Record on the graph the pressure after each 0,04 or 0,08 m 3 increment is pumped. Fig. 1 34

35 Leak-off test procedure 7. Continue pumping until the curve bends over, or until the anticipated leak-off pressure line is exceeded. Exceeding this line is often caused by only shale being exposed in the open hole. 8. When the pump is shut off, keep the well shut in and read an instantaneous pressure. Then read pressure values each minute for about 10 min. These should also be plotted on the graph as shown in Fig. 1. Fig. 1 35

36 Leak-off test procedure 9. Release the pressure and record the volume of testing fluid recovered in the trip tank if one is available. The volume of fluid recovered should approximate the volume of fluid pumped. Fig Compare the graph with typical plots to be sure it is a good test. After the test is run the leak-off pressure is picked off the graph as that point where the curve starts to bend over. Using this leak-off point, correct for mud gelation effects then calculate the equivalent mud weight which the casing seat can hold. 36

37 Leak-off test procedure The leak-off test should be run under the following drilling conditions. Casing pressure test. The test should be run before drill out with the bit positioned in the float collars. Pump rates should be 0,04 to 0,053 m 3 min -1 ( bbl/min) and continued until the maximum test pressure required is reached. Hold the pressure for the designated period of time. USGS requires a 30-min test in OCS waters. This volume-pressure plot can be used as the "minimum volume" line when running a leak-off test in open hole. 37

38 Leak-off test procedure Cement job test. After drilling out 2,44 to 3,048 m (8-10 ft) below the casing shoe and pulling the bit up above the shoe run the leak-off test pumping at 0,04 to 0,053 m 3 min -1 ( bbl/min). The cement job at the casing shoe should be tested to a leak-off pressure at least as high as the expected leak-off pressure for the area. Failure of the cement job to hold such pressures may require the casing shoe be squeeze cemented. 38

39 Leak-off test procedure A poor cement job will have a plot similar to Fig. 7. Note the departure from the "minimum volume" line and the low leak-off pressure which is repeatable. 39

40 Leak-off test procedure Formation pressure test. On the first bit run after setting casing, a test should be made on the first trip for a new bit after drilling a sand section. Procedures are the same as listed above. The pumping rate should be 0,08 to 0,12 m 3 min -1 ( bbl/min). The higher rate should cover the filtration loss to the formation and thereby keep the volume-pressure curve near the "mini-mum volume" line. Fig. 1 is an example of this type of test. 40

41 Leak-off test procedure As drilling continues, several potential problems may be anticipated which would suggest running the leak-off test to determine if the well bore has become weakened since the last leak-off test. Typical potential problems would be a lostcirculation zone, a transition zone, or a large mud weight increase. 41

42 Leak-off test procedure If the second leak-off test is lower than the first, one might suspect a failure of the cement job which would require squeeze cementing, or a weaker formation has been exposed which may require a liner. Various logging methods are available to help define the stratigraphy and suggest a solution. When testing in a long section of open hole the pump rate may need to be increased to as high as 0,24 m 3 min -1 (1.50 bbl/ min). Before leaving a dry hole it is useful to run a leak-off test to get additional formation fracture information which will be useful in defining fracture gradients for that area. 42

43 An example This example shows how the various steps perform the leakoff test. Assume 244,48 mm (9 5/8 in.) casing is set at 3048 m (10,000 ft), and the hole is then deepened to 3057 m (10,030 ft) after the primary cement job has been tested. Mud weight is 1560 kgm -3 (13 lb/gal), and the mud has a 10-min gel strength of 4,788 Pa (10 lb/100 sq ft). Before the test, 3048 m (10,000 ft) of 139,7 mm (5 1/2 in.) drill pipe is in the hole. A sand is exposed from 3048 m to 3054 m (10,000 ft to 10,020 ft) and it has a pore pressure, P 0, of 358,510 5 Pa (5,200 psi). 43

44 An example Mud gel pressure. In this case it was decided to pump down the drill pipe when running the leak-off test. Due to mud gel strength, it will be necessary to apply 4, Pa (70-psi) extra surface pressure to overcome gelation forces. This gel pressure is obtained by entering Fig. 2 with 4,788 Pa (10 lb/100 sqft) gel strength value then reading 4, Pa/3048 m (7 psi/ 1,000 ft) of pipe. For 3048 m (10,000 ft) of drill pipe, we need to apply 4, Pa (7 x 10 or 70 psi). Equation 3 gives a similar answer. If the contractor had decided to pump down the annulus he would have needed 6, Pa (100 psi) extra surface pressure to overcome the gelation forces. 44

45 An example In Fig. 3, a gel strength of 4,788 Pa (10 lb/100 sqft) and an annulus of 244,48 mm (9 5/8 in.) casing x 139,7 mm (5 ½ in.) drill pipe gives 0, Pa/304,8 m (10 psi/1000 ft) of annulus. For a 3048 m (10000-ft) annulus, apply 6, Pa (10 x 10 or 100 psi). Equation 4 gives similar results. Some operators pump down the annulus and drill pipe. If this was tried in this example the mud would, as always, take the path of least resistance and flow only down the drill pipe when leak-off occurred. No major benefit is achieved by pumping down the annulus and the drill pipe simultaneously. 45

46 An example Minimum volume line. This line, as plotted in Fig. 1, was determined earlier to have a slope of 0,272 m 3 /68, Pa (1,7 bbl/1000 psi) for the hole conditions given above. psi x 6895 = Pa bbl. x 0,159 = m 3 46

47 ff ( ' S ) v Po Po P = K + An example Anticipated leak off. Using fracture data for the area (such as that in Fig. 4) the effective stress ratio, K, is found to be 0.85 at 3048 m (10000 ft). The overburden stress gradient is assumed to be 0, Pa/0,3048 m (1,0 psi/ft) therefore S' V = 689, Pa (10000 psi.) The formation fracture pressure P ff calculated with Equation 6 is 639, Pa (9280 psi). P ff = 0.85 (689, , ) + 358, = 639, psi. 47

48 P = P P + a ff h P g An example The anticipated leak-off pressure is determined using Equation 7. P a = 639, ,81 x 1560 x , = 178, Pa This pressure is drawn on the leakoff graph (Fig. 1). 48

49 Leak-off data Before shutting in the well the bit was positioned in the casing just above the shoe, and the mud circulated until 1560 kgm -3 (13 lb/gal) mud was going in and coming out. When the kelly was removed the hole stood full and no "U-tube" action of the mud was observed. The BOP above the drilling spool was closed and mud was pumped down the drill pipe at a rate of 0,08 m 3 /min (0,50 bbl/min). As each 0,08 m 3 /min (0,50 bbl) was pumped, the pump pressure was plotted on the leak-off graph. 49

50 Leak-off data Fig. 1 shows the results. At 175, Pa (2540 psi), point A, the curve started bending over. This was the leak-off pressure of the sand exposed. Pumping continued until point B, then pressures were plotted each minute for 10 min (point C). At this time the fluid was bled from the well into the trip tank and 0,96 m 3 (6 bbl) of mud was recovered. The volume recovered checked with the amount injected. 50

51 Leak-off data To verify mud gel strength pressure, mud was pumped (after waiting 5 min) down the drill pipe at 0,08 m 3 /min (0,50 bbl/min) and the amount of pressure needed to break circulation was measured as 17, Pa (260 psi). This value was picked at the maximum pressure (Fig. 1, point D) measured before the pressures fell back. Using the 17, Pa (260 psi) gelation pressure and Equation 5 the average gel strength of the mud was calculated to be 7,22 Pa (15.1 lb/100 sq ft). 51

52 Y = e = 1,083 L 1,083 17, Leak-off data This value is higher than the value measured with the viscometer. P gdp is therefore 7, Pa (110 psi). P ( ) bc dh Ddp ( d + d D ) dp h dp dp = 5 10 ( 0,222 0,1397 ) ( 0, ,232 0,1397 ) d = 7,22 Pa 52

53 P ff = 175,13 10 Leak-off data Formation fracture pressure was determined using Equation 1. As equivalent mud weight, the formation fracture pressure becomes (Equation 2): = P P + ρ g H lo 5 = 633,65 10 Pa Pff EMW = = g H g 5 m 7, = + 9, = 633, , = 2119 kg m -3 53

54 Rock stress behavior In the example, drilling mud started to leak off when surface pressures reached 175, Pa (2540 psi). A better understanding of what is happening in the formation as well-bore pressures increase helps to dispel the belief that once the leak-off test is run the formation fracture strength cannot be regained. Some believe that a leak-off test can weaken the formations significantly. This belief is false. 54

55 Stress theory It has often been thought that a well-bore fractures much like a pipe bursts when too much internal pressure is added. This is not so. The fracture pressure of a pipe is determined by tensile strength of the pipe, but the fracture pressure of a formation is determined primarily by overburden and tectonic compressive loads on the rock grains. 55

56 Stress theory The tensile strength of most rocks such as sandstone is small relative to the compressive loads. In this article, the tensile strength of the formations is treated as equal to zero. A well bore fractures when the mud pressure in the well bore causes the rock grain stress to be decreased from high-compressive stresses to zero. At this point, additional pressure causes a fracture to form, and mud can flow into the fracture. 56

57 Stress theory Using the same well conditions as in the example, and assuming the original horizontal stress in the sandstone is 496, Pa (7200 psi), the rock stresses around the well bore before and during the leak-off test can be calculated. To do this, first consider stress conditions in the sand before the well was drilled. 57

58 After the well is drilled the rock stresses change to those shown in Fig. 8b. It is the horizontal stress tangential to the borehole wall, S' t, which is of primary concern when rock rupture is being considered. The symbol S' t describes the horizontal stress "tangential" to the well bore wall, and S' r is the horizontal stress radial to the well bore. Stress theory 58

59 Stress theory 358, Pa 466, Pa 496, Pa 3048 m 526, Pa 689, Pa 496, Pa 689, Pa 466, Pa In Fig. 8a a block of sandstone is shown with a vertical stress S' v of 689, Pa (10000 psi) on top and two equal horizontal stresses, S' h, of 496, Pa (7200 psi). 59

60 Rock grain stress Well-bore fracture is imminent when the rock grain stress decreases to zero. In this example, rock grain stress (S h ) is Pa (2000 psi) before the well is drilled and 168, Pa (2440 psi) (S t ) after the well is drilled. S h and S t are "grain stress" or "effective rock stress, calculated by: S h =S h -P o and S t =S t -P o 60

61 Rock grain stress Because of the presence of the well bore, the horizontal stress S' t is not constant but varies relative to the distance from the well bore. Fig. 9 is a plot of S' t for the 200,03 mm (7 7/8 in.) hole. 61

62 Note that it decreases from 466, Pa (7640 psi) at the well bore to near original conditions of 497, Pa (7218 psi) 508 mm (20 in.) from the center of the hole. Likewise, if the pore pressure is subtracted, grain stress goes from 168, Pa (2440 psi) at the well bore wall to 139, Pa (2018 psi). Rock grain stress 62

63 The stress distribution given in Fig. 9 can be calculated by the equation: S t = S h [1 + (a 2 /r 2 )] - [P w - P o ] a 2 /r 2 ) (11) and S' t = S t + P o Where: a = Radius of well bore, m r = Radius under consideration, m As well bore pressure (P w ) is increased during a leak-off test, horizontal grain stress (S t ) is decreased. Rock grain stress 63

64 Fig. 9 also shows the stress distribution when the total well-bore pressure has been increased to Pa (9200 psi) with the leakoff pressure of Pa (2540 psi) shown in Fig. 1. Note that the pressure of 2540 psi needed to initiate leak-off is much higher than the pressure required to form a long vertical fracture. Such a fracture is evidenced by a large drop in pump pressure and large volumes of mud loss. Rock grain stress 64

65 Rock grain stress If this occurs the remedy is simple. Bleed off pump pressure. This allows the fracture to close. After a mud filter cake is formed across the fracture on the borehole wall, it will have regained the stress effect of the well bore, and can again accept Pa (2540 psi) pump pressure. This is the process of "healing the formation" as practiced in regaining lost circulation. 65

66 Fig. 1 shows a leak-off test below surface casing at 914,4 m (3000 ft). In this case 3,048 m (10 ft) of hole was made below the surface casing. Thus this test is primarily a test of the cement job. In this example the leak-off occurred at 45, Pa (655 psi), the rupture pressure was 518, Pa (7520 psi), and the propagation pressure was 43, Pa (635 psi). Leak-off examples 66

67 Considering the leak-off pressure as the maximum permissible, results in a maximum mud weight of 1656 kg/m 3 (13,8 ppg) as shown below: Leak-off examples ρ max 5 45,2 10 = = ,4 9,81 kg/m 3 67

68 Leak-off examples This does not prove that all formation below the surface casing will hold 1656 kg/m 3 (13,8 lb/gal) mud. It does show that no more than 1656 kg/m 3 (13,8- lb/gal) mud should be used unless a retest shows an increase in strength. An increase in strength has been noted in many cases after several days of drilling. It is uncommon for a zone which held only 1656 kg/m 3 (13,8 lb/gal) as shown in Fig.1 to hold 1200 kg/m 3 (10 lb/gal) later. 68

69 Leak-off examples A knowledge of this increase in strength might be very helpful in many cases when drilling into a pressure-transition zone. The increase in strength, when it occurs, is probably due to plugging of pore spaces by drill solids. It should be emphasized that this strength increase may or may not occur. It is not something the operator can assume will happen. 69

70 Leak-off examples Open hole test Fig. 2 is a leakoff test where the drill string is back in the surface casing, but there is 1829 m (6000 ft) of open hole. It is noted that the leak-off occurred at about 45, Pa (655 psi). Also it is noted that 4,16 m 3 (26 bbl) of mud were required to reach this point while in Fig. 1 only 0,608 m 3 (3,8 bbl) of mud where required for the casing-seat test in Fig.1. 70

71 Leak-off examples The primary difference is the amount of open hole. In Fig. 1 only 3,048 m (10 ft) of hole had been opened below the casing seat. In Fig. 2, 1829 m (6000 ft) of hole had been opened. The additional mud was required because of filtration and loss of mud to very permeable sands. 71

72 Leak-off examples The leak-off pressure of 45, Pa (655 psi) in Fig. 2 shows the formation just below the casing seat will hold a 1656 kg/m 3 (13,8 lb/gal) mud. Again this does not ensure that all the open formations below 914,4 m (3000 ft) will hold 1656 kg/m 3 (13,8 lb/gal) mud, because 45, Pa (655 psi) imposed, say at 1829 m (6000 ft) would represent only a 228 kg/m 3 (1,9 lb/gal) increase in mud weight. 72

73 Leak-off examples Thus the 1829 m (6000 ft) formation has been tested to only 1428 kg/m 3 (11,9 lb/gal) in the test shown in Fig. 2. However, in young sediments normally associated with most offshore and coastal area formations the leak-off test results taken just below the casing shoe are generally indicative of the maximum mud weight that can be used. 73

74 Test for first sand. Fig. 3 is a leak-off test for the first sand below protective casing. It will be noted that the leak-off occurred when the surface pressure increase reached 134, Pa (1950 psi). This occurred with a 1620 kg/m 3 (13,5 lb/gal) mud in the hole, and the surface pres-sure plus mud weight represents a formation resistance equal to a mud weight of 2070 kg/m 3 (17,25 lb/gal): Leak-off examples ρ max 5 134,45 10 = = , kg/m 3 74

75 Special considerations Special considerations in running leak-off tests include: Pumping rate. Decision to test to a leak-off pres-sure. Which pressure to use if there is a difference in drill pipe and annulus pressure. Changes in line slope during the test. Frequency of testing and the effect on formation resistance. What is the maximum mud weight relative to that shown on a leak-off test. 75

76 Special considerations The pumping rate should be kept at a low value, such as 0,04 to 0,08 m 3 /min (0,25 to 0,5 bbl/ min). Tests in Figs. 1, 2, and 3 were run at 0,05 m 3 /min (0,3 bbl/min). This means the normal rig pump should generally not be used. Exceptions would be with plunger-type pumps where the suggested low volumes can be attained. A cementing unit, with pump and volume tank is generally to be preferred. 76

77 Special considerations If pumping rates are too high, the leak-off test may follow the pattern shown in Fig. 4. There is no indicated leak-off pressure; the formation suddenly ruptured, and whole mud was lost quickly. Even this type test will probably have no long-range detrimental effects. The primary problem is that the objective of determining the leakoff pressure has not been reached. 77

78 Objections to leak-off tests Some operators are repelled by the concept of increasing surface pressure until some mud is lost to the formation. Others may feel that their drilling conditions do not justify such tests. If all the drilling is to be performed in formations with a normal pore pressure, leak-off tests would not be necessary. The belief, commonly accepted in drilling, that, once the formation is tested to leak-off, it will never again hold that much pressure is an out-dated concept. 78

79 Objections to leak-off tests There may, however, be valid reasons for not testing to a leak-off pressure. At times, surface equipment may not permit the surface pressure necessary to reach the leak-off point. If the maximum leak-off pressure is desirable under these conditions a retest may be performed later, after the mud weight has been increased. Another reason for not testing to leak-off is that the operator knows based on offset well data or geologic information that future mud weights will not be high enough to justify a test to leak-off. 79

80 Fig. 5 shows what has been ob-served in some tests. The pressure leaked off at 41, Pa (600 psi), continued to increase to Pa (900 psi) where it leaked off again, and then continued to increase to the true leak-off. However, if the pressure at which this occurs is substantially below that anticipated, pumping should be continued to the rupture pressure, because in all probability some type remedial action will be necessary. Pressure differences 80