石油工程师协会技术年会论文spe166200.pdf

SPE 166200 Improve Casing Design for WCD in Deepwater Wells Jiang Wu Chevron Copyright 2013 Society of Petroleum Engineers This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in New Orleans Louisiana USA 30 September 2 October 2013 This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author s Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author s The material does not necessarily reflect any position of the Society of Petroleum Engineers its officers or members Electronic reproduction distribution or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited Permission to reproduce in print is restricted to an abstract of not more than 300 words illustrations may not be copied The abstract must contain conspicuous acknowledgment of SPE copyright Abstract Intermediate casings in deepwater wells are now designed for well integrity under the worst case discharge condition WCD a severe casing collapse and axial compression condition The severe collapse condition is from a reduced casing internal pressure from an un controlled hydrocarbon fluid flow to wellhead with a possibly increased external pressure by APB annulus pressure buildup and the severe axial compression condition is from a casing de ballooning effect by the severe collapse pressure and from a casing thermal laod by elevated casing temperature on the un controlled hydrocarbon fluid flow to wellhead Re investigation of casing design methodology becomes important and necessary in order to improve the casing design safety under the WCD condition in deepwater wells This paper presents triaxial stresses model analyses on casing collapse and axial compression and proposes an improved casing collapse and axial compression design method for WCD in deepwater wells 1 to use the direct differential pressure external pressure minus internal pressure rather than the prorated differential pressure currently listed in API ISO documents on casing collapse design together with an added equivalent axial tensile stress equal to the value of casing internal pressure and 2 to use the effective axial compression load calculated by hydrostatic pressure condition of Pi internal pressure rather than the actual axial compression load on casing axial compression design The proposed casing collapse design method is also supported by full size casing collapse test data from a recent API work group study The proposed casing collapse and axial compression design method has been used in several deepwater projects in the recent years with success Introduction Casing design for well integrity under WCD in deepwater wells brings up new design challenges due to severe casing collapse and axial compression condition The severe casing collapse pressure comes from a reduction of casing internal pressure due to an un controlled hydrocarbon fluid flow to wellhead and a possible external pressure increase due to annulus pressure buildup APB of trapped annulus fluid The severe casing axial compression load comes from a casing thermal compression load due to casing temperature elevation on heat transfer of the un controlled hydrocarbon fluid flow to wellhead and from a casing de ballooning effect of severe collapse pressure condition under WCD Therefore casing design for well integrity under WCD in deepwater wells is somehow difficult in selecting proper weight and grade of casing to satisfy the casing collapse and axial compression design and maintaining the required casing drift size for a whole well planning This paper is to provide a study on casing collapse and axial compression design for WCD condition in deepwater wells and seek an improvement on the casing design safety accordingly Casing Collapse Design Analysis API American Petroleum Institute 5C31 and ISO International Organization for Standardization TR14002 specify casing 2 SPE 166200 D t PPdt D io 2 1 2 o Pdt D 2 collapse pressure primarily based on statistical analysis of casing collapse test data conducted under zero internal pressure condition Fig 1 The external pressure to collapse casing under this casing collapse test with zero internal pressure is defined as the casing collapse pressure or casing collapse rating Pc Po 1 Pc is casing collapse pressure under zero internal pressure or casing collapse rating and Po is casing external pressure Po Pi Fig 1 Zero internal pressure case Fig 2 Non zero internal pressure case For some casing collapse designs where the casing internal pressure Pi is not zero Fig 2 the current API 5C3 ISO TR1400 specification defines the following collapse pressure Pci in the presence of internal pressure Eq 2 by Clinedinst s study3 Pci Pc 1 2t D Pi 2 Pc is casing collapse pressure under zero internal pressure condition or simply is the external pressure Pi is casing internal pressure t is casing wall thickness and D is casing outside diameter Therefore Eq 2 is actually a prorated differential pressure equation as Pc Po Pci Po 1 2t D Pi 3 The Eq 3 can be viewed as a result of average hoop stress av under casing collapse condition by a two dimensional stress analysis Fig 3 For the non zero internal pressure condition the average hoop stress av gives the expression of Eq 3 av 4 While for zero internal pressure condition the average hoop stress av reduces to the external pressure or the collapse pressure at zero internal pressure condition av 5 However there is a fundamental problem with Eq 2 or 3 regarding casing collapse The fundamental problem is when we run casing in extreme deep wells where the casing internal pressure and external pressure are equal but very high at the casing bottom we would have a high casing collapse pressure by Eq 2 or 3 to collapse the casing at bottom But in reality we know the casing at bottom will not be collapsed as it is under a hydrostatic pressure condition with zero triaxial Von Mises equivalent stress The two dimensional stress analysis by only the casing hoop stress from which the Eq 2 or 3 was derived is not sufficient for casing collapse modeling SPE 166200 3 Fig 3 A two dimensional stress analysis of hoop stress under casing collapse condition Now what we need is a three dimensional or triaxial stress analysis on casing collapse condition which will improve the casing collapse pressure calculation for non zero internal pressure condition By Fig 5 the casing internal pressure and external pressure condition Original is shown to be equivalent to a modeling result condition where the internal pressure is set to zero as compared to the API casing collapse test condition It is done by subtracting a surrounding pressure of Pi from the inside outside top and bottom of the casing piece This can be done without causing any change on casing stress condition in terms of Von Mises equivalent stress through a casing triaxial stresses analysis This modeling result gives a direct differential pressure as collapse pressure Eq 6 for non zero internal pressure condition with an equivalent tensile stress of value Pi internal pressure Eq 7 Pcd Po Pi 6 eq Pi 7 The equivalent axial tensile stress Eq 7 is equal to the value of casing internal pressure and will be combined with casing actual axial load to determine if casing collapse strength reduction calculation by tension is required When the combined axial load is a compressive load which is normally the case for WCD condition no casing collapse strength reduction calculation by tension is needed The direct differential collapse pressure Eq 6 is actual not new expression to many casing designer or casing design researcher as it was traditionally used in casing collapse design However by the triaxial stresses analysis we confirm that it is the correct casing collapse pressure to use for casing design with the additional equivalent tensile stress of value Pi internal pressure Eq 7 Now we resolve the fundamental problem with Eq 2 or 3 regarding casing collapse under hydrostatic pressure condition in the above extreme deep well scenario the collapse pressure is zero by Eq 6 and no casing collapse Also notice the combination of the equivalent axial tensile stress Eq 7 and casing actual axial stress at the bottom casing on the hydrostatic condition is zero 4 SPE 166200 Fig 4 A three dimensional triaxial stress analysis of casing collapse pressure The difference between the direct differential collapse pressure Eq 6 and the prorated differential collapse pressure Eq 3 is simply proportional to the internal pressure and the ratio of casing wall thickness to OD Pc Pci Pcd 2t D Pi 8 The larger the casing internal pressure the larger the difference and the smaller the casing D t ratio the larger the difference as shown in Fig 5 Fig 5 The difference of prorated and direct differential collapse pressures For an example of intermediate casing 16 15 115ppf HPQ125 D t 16 15 0 723 22 34 used in deepwater wells with 11 000 psi external pressure and 5300 psi internal pressure near the bottom of casing under WCD condition we have the direct differential collapse pressure Eq 6 as Pcd Po Pi 11000 5300 5700 psi While the prorated differential collapse pressure Eq 3 is 6174 psi or about 10 7 higher SPE 166200 5 Pci Po 1 2t D Pi 11000 1 2 22 34 5300 6174 psi Therefore with using the direct differential collapse pressure Eq 6 in casing collapse design under this example WCD condition in deepwater wells it can improve the casing collapse design safety factor by 10 7 comparing with the use of the prorated differential collapse pressure Eq 3 The equivalent axial tensile stress Eq 7 equal to the value of casing internal pressure should not cause a casing collapse strength reduction at WCD as disucussed due to a high axial compression load under WCD condition Casing Collapse Test Result Full size casing collapse test under non zero internal pressure condition was conducted recently under API work Item 2370 study4 The following five collapse tests were conducted on 7 26ppf L80 casing with 5 collapse samples in each test 1 Zero internal pressure open end OE 2 5000 psi internal pressure open end OE 3 7500 psi internal pressure open end OE 4 Zero internal pressure closed end CE 5 5000 psi internal pressure closed end CE The zero internal pressure casing collapse tests test 1 and 4 were used as the base cases to compare with the casing collapse tests with non zero internal pressure test 2 3 and 5 The test setup of open end and closed end configurations is illustrated in Fig 6 The open end setup has a sufficient end gap to allow the casing axial movement under collapse pressure condition with a compliant seal while the closed end setup has the casing welded to the end cap and does not allow casing axial movement During the collapse test the casing test sample has an axial compressive stress at value of internal pressure in open end test Test 1 2 and 3 and an axial compressive stress from the casing de ballooning effect in close end test Test 4 and 5 o P i P Compliant seal and gap Compliant seal and gap Test chamber Mandrel Casing o P i P End cap and Weld End cap and Weld Test chamber Casing Open End TestClosed End Test Fig 6 Illustration of collapse test setup for non zero internal pressure condition 6 SPE 166200 Five joints of casing were used to cut the test samples for each test with each of the five test samples on each test such as OE1 1 OE1 2 OE1 3 OE1 4 and OE1 5 on open end test 1 being cut from different joint by the following sequence to average possible variation of casing properties OD t OV EC along the casing joint length Each test sample has a length of 58 on open end test and 70 on closed end test both longer than 8 times of casing OD Fig 7 Collapse test sample preparation OE open end sample CE closed end sample The casing test sample material yield strength was tested to be from 87 300 psi to 90 100 psi as shown below The collapse test results is summarized and listed in Table 1 On the open end tests Test 1 2 and 3 we can see that the actual casing collapse occurs at about 7326 psi average external pressure on zero internal pressure condition Test 1 and the actual casing collapse occurs at about 7313 psi average direct differential pressure on Test 2 and at about 7225 psi average direct differential pressure on Test 3 for non zero internal pressure condition They are very closed to the collapse pressure at zero internal pressure case However the prorated collapse pressure Pci at the casing collapse is much higher at SPE 166200 7 about 7827 psi Test 2 and about 7992 psi Test 3 The prorated collapse pressure Pci is larger than the direct collpse pressure Pcd by about 7 to 10 on the open end tests Test 2 and 3 We can see the similar test reslt on the close end tests Test 4 and 5 with the direct collpse pressure Pcd being closed to the collapse pressure at zero internal pressure case and the prorated collapse pressure Pci being larger than the direct collpse pressure Pcd by about 7 Table 1 Summary of 7 26 L80 Casing Collapse Test Results 8 SPE 166200 A typical test pressure plot recorded during the casing collapse test with internal pressure is shown by Figure 8 on the test sample OE 2 1 in Test 2 Fig 8 Casing collapse pressure plot on test sample OE2 1 The following plots give the graphic comparison of collapse pressures Fig 9 is the comparison between Test 1 OE1 base case of zero internal pressure and Test 2 OE2 5000 psi internal pressure Fig 10 is the comparison between between Test 1 OE1 base case of zero internal pressure and Test 3 OE3 7500 psi internal pressure Fig 11 is the comparison between Test 4 CE1 base case of zero internal pressure and Test 5 CE2 5000 psi internal pressure Note that the test sample 1 in CE2 was a premature collapse due to wrong valve setting and can be excuded We can see that the direct differential collapse pressure Pcd gives much better prediction on casing collapse than the prorated differential collapse pressure Pci Fig 9 Casing collapse pressure comparison using Test 1 OE1 and Test 2 OE2 results SPE 166200 9 Fig 10 Casing collapse pressure comparison using Test 1 OE1 and Test 3 OE3 results Fig 11 Casing collapse pressure comparison using Test 4 CE1 and Test 5 CE2 results The full size casing collapse test results confirm the triaxial stress analysis on casing collapse and support the use of the direct differential collapse pressure Pcd for casing collapse design on WCD in deepwater wells where internal pressure is not zero Using the direct differential collapse pressure Pcd for casing collapse design on WCD in deepwater wells will help improve casing collapse design safety as shown by the previous example calculation Note also that the full size collapse test data show a higher collapse pressure than the regular API minimum collapse pressure on the 7 26 L80 casing API Minimum collapse pressure 5410 psi For example of the OE1 data the average collapse pressure is 7330 psi and API Minimum collapse pressure will be 6740 psi by statistical analysis This is because the test casing samples have low ovality low eccentricity and high material yield strength which help give higher caollapse strength than regular API minimum collapse pressure just like that on high collapse casing 10 SPE 166200 Casing Axial Compression Design Analysis Under the WCD condition in deepwater wells the intermediate casing string is exposed to high axial compression load resulting from casing thermal compression due to casing temperature elevation and casing de ballooning effect due to high casing collapse pressure The casing axial compression load may approach or exceed the axial compression design limit of casing connection when the casing connection is weaker than casing body in compression How to design casing and connection safely for axial compression under WCD in deepwater wells is another issue The following triaxial stresses analysis on casing and connection axial compression design is to propose an improved casing and connection axial compression design approach for WCD condition in deepwater wells We know that the casing and connection axial compression strength is basically defined under uniaxial compression load no casing pressures with casing connection material yield strength being reached However when casing is in the wellbore and surrounded by casing internal and or external pressure casing and connection material yield will then be governed not only by axial compressive stress load but also the hoop and radial stresses produced by the surrounding pressures For an example of casing hanging in deepwater wells the bottom of casing string is under very large axial compression load by the drilling mud hydrostatic pressure Ph acting on the casing bottom but this casing string is never yielded at the bottom no matter how large is the axial compression load as the triaxial equivalent stress e at this casing bottom is actually zero due to the effect of surrounding drilling mud hydrostatic pressure Po Pi Ph Using the equations listed in Fig 12 we can confirm the following triaxial stresses to the bottom of the casing in this example a Ph Ph r Ph e 0 Fig 12 Casing Triaxial Stresses and Von Mises Equivalent Stress e The general casing yield ellipse under triaxial stresses condition can be expressed in Eq 9 and plotted in Fig 13 including the effect of casing internal pressure external pressure and axial stress SPE 166200 11 2 1 2 4 3 1 2 2 2 1 y ia y ia y oi io PP D t rr PP 9 Where Pi casing internal pressure psi Po casing external pressure psi ri casing internal radius in ro casing external radius in D casing OD in t casing wall thickness in y casing material yield strength psi a casing axial stress negative on compressive stress psi Fig 13 Example of casing yield ellipse curve Von Mises ellipse curve under triaxial stresses con