FLAC3D井壁穩(wěn)定模擬

1、基于FLAC-3D的水合物井壁穩(wěn)定性數(shù)值模擬研究天然提水合物是一種混合了水、氣等的混合物，主要組成部分為甲烷。其他比例較小的 組分有CO2、H2S，以及極少量的乙烷和高價烴類。水合物在深?；蛘哂纼鐾恋貛Ц邏旱蜏?的下狀態(tài)很穩(wěn)定。隨著化學試劑的注入，引起溫度升高壓強降低會導致水合物的分解。水合 物的分解如同冰的融化，會伴有強烈的吸熱，并釋放出氣體和水。考慮到水合物的巨大儲量， 在將來極有可能稱為一種替代能源。然而水合物分解造成的嚴重溫室效應也可能導致極大的地質災難，威脅到海洋甚至是陸地型 油氣田，因此深海水合物鉆探有著很大風險。鉆探水合物沉積物時，壓力和溫度的改變可能會導 致水合物大規(guī)模分解。通

2、常水合物沉積物在海床表層，甚至比表層套管還要淺，因此安裝防噴器 就無從說起。鉆進過程中，溫壓控制不當造成的水合物分解也極有可能造成井身結構的破壞。如果是沒有隔水板的鉆進，氣體會進入到開放的環(huán)空空間，不斷上升，并由于吸熱造成鉆桿 柱附近的鉆具結冰。如果有隔水板鉆進，氣體進入回路環(huán)空，與鉆井液混合到一起升到表層。不 管哪種情況，都會有爆炸和火災的危險，氣體中可能含有的硫化氫更是無法控制。同時，隨著大 量氣體進入，鉆井液的密度也會降低。而隨著鉆井液密度的降低導致液柱壓力降低又將導致新一 輪的水合物分解，如此惡性循環(huán)。液柱壓力降低還有另一個副作用，那就是對井壁的支撐作用削 弱，因此而導致井壁坍塌，鉆孔

3、超徑等問題。鉆井液流變性能的改變會還影響孔內(nèi)清潔，在加上 地層強度以及空隙壓力的改變，很可能就會導致縮徑從而卡鉆。更有甚者，當水合物分解時，沉積物儲層的力學和物理性質都會發(fā)生改變，空隙流體壓力也 會增大。伴隨水合物分解造成的儲層物性變化包括：滲透率上升；熱力學性質的改變；彈性模量 降低以及固井強度低；沉積物強度降低會加劇井壁的不穩(wěn)定性。隨著水合物分解造成固井強度降 低以及空隙壓力增加會造成更多的機械故障。Despite these possible risks HBS have been drilledsuccessfully in the past with few reportable i

4、ncidents(eg. Ohara et al., 2000; Collett and Dallimore, 2002 ,Minerals Management Service 2004), see /homepg/offshore/safety/ wtrflow.html.盡管有這么多風險，過去的水合物沉積物鉆進案例還是很成功的，只出現(xiàn)了極少的問題。作為將來一種可能的深海鉆進技術，對天然氣水合物鉆進的風險形成良好的認識并在問題發(fā) 生前及時預報并采取措施是十分重要的。數(shù)字模型是模擬研究水合物沉積物機理的一個有效手段，并可以由此估算鉆井參數(shù)(例如可 用的泥漿密度，組分以及溫度)，這些目前都還缺

5、少一個安全推薦值。數(shù)值模擬中需要被考慮到 的主要因素有：Numerical modelling can be an effective tool thatenables the simulation of mechanisms leading towellbore instability in HBS, and to assess drillingparameters such as mud weight, composition andtemperature that can be used, while still remainingwithin a safe operational en

6、velope. Some of the factorsthat need to be evaluated are:effect of drilling fluid on heating the formation andchanging the stresses and pore pressure;impact of the selected constitutive model;effect of heating the formation on thermodynamicstability of the hydrates and wellbore stability; and effect

7、 of reduction in the sediment strength (i.e. particularly,loss of cohesion) due to hydrate dissociation.模型建立水深1310m,海底365m處，為簡化計算只取半徑0.5m高1m圓柱，鉆桿直徑0.09m。建立網(wǎng)格如圖:Minneapolis, MN USA模型參數(shù)比奧系數(shù)1沉積層體 積模量7.0 GPa鉆井泥漿 溫度20 C (293 K)初始沉積層孔隙率0.49沉積層剪 切模量0.69 GPa固有滲透 率1x10-15m2水合物形成后沉積層孔隙率0.25泊松比0.45氣體常數(shù)8.31441J

8、K-1M0I-1導熱率1.4 Wm-1K-1原位孔隙 水壓力17 MPa水合物晶 體體積1.728x10-27m3比熱容1.9x103K-ikg-i各向同性 應力32 MPa阿伏加德 羅常數(shù)6.02205x1023水合物線性膨脹 系數(shù)7.7x10-5 K-1垂直應力21 MPa內(nèi)聚力1.8 MPa孔隙流體的線性膨脹系數(shù)30 x10-5 K-1泥漿壓力20 MPa內(nèi)摩擦角35沉積層飽和密度2.200 kgm-3原位溫度15 C(288 K)抗拉強度1.5 MPa模型一此模型只考慮力學變形，地層無滲透性。豎向應力經(jīng)過重新分布，變化較小，初始應力21MPa，重新分布后最大壓應力為20.987MPa，

9、井壁周圍應力較?。粡较虺跏紤?2MPa，經(jīng)過應力重分布，最大壓應力 為47.635MPa，分布在井壁周圍較小范圍內(nèi)。井壁周圍曾經(jīng)因剪切到達過屈服面，現(xiàn)在已處于彈性變形階段，范圍很小。FLAC3D 3.00Step 653 Model Perspective 12:35:04 Fri Jan 14 2011Center:Rotation: TOC o 1-5 h z X: 0.000e+000X:0.000Y: 5.000e-001Y:0.000Z: 0.000e+000Z:0.000Dist: 3.265e+000 Mag.: 1Ang.: 22.500Plane Origin:X: 0.0

10、00e+000Y: 5.000e-001Z: 0.000e+000Plane Normal:X: 0.000e+000Y: 1.000e+000Z: 0.000e+000Contour of SZZPlane: onMagfac = 0.000e+000 gradient Calculation -4.6695e+007 to -4.5000e+007. -4.5000e+007 to -4.0000e+007-4.0000e+007 to -3.5000e+007-3.5000e+007 to -3.0000e+007-3.0000e+007 to -2.5000e+007-2.5000e+

11、007 to -2.0000e+007-2.0000e+007 to -1.5000e+007-1.5000e+007 to -1.1912e+007Interval = 5.0e+006Historyx10A41.81.61.41.21.00.80.60.40.22.04.06.01 Max. Unbalanced ForceItasca Consulting Group, Inc.Minneapolis MN USAPlane Origin:X: 0.000e+000Y: 5.000e-001Z: 0.000e+000Contour of SXXPlane: onMagfac = 0.00

12、0e+000Interval = 5.0e+006Plane Origin:X: 0.000e+000Y: 5.000e-001Z: 0.000e+000FLAC3D 3.00Step 653 Model Perspective 12:35:32 Fri Jan 14 2011Center:Rotation: TOC o 1-5 h z X: 0.000e+000X:0.000Y: 5.000e-001Y:0.000Z: 0.000e+000Z:0.000Dist: 3.265e+000 Mag.: 1 Ang.: 22.500Plane Normal:X: 0.000e+000Y: 1.00

13、0e+000Z: 0.000e+000radient Calculation-4.7635e+007 to -4.5000e+007-4.5000e+007 to -4.0000e+007-4.0000e+007 to -3.5000e+007-3.5000e+007 to -3.0000e+007-3.0000e+007 to -2.5000e+007-2.5000e+007 to -2.0000e+007-2.0000e+007 to -1.5000e+007-1.5000e+007 to -1.1856e+007Itasca Consulting Group, Inc. Minneapo

14、lis, MN USA _FLAC3D 3.00Step 653 Model Perspective 12:35:49 Fri Jan 14 2011Center:Rotation: TOC o 1-5 h z X: 0.000e+000X:0.000Y: 5.000e-001Y:0.000Z: 0.000e+000Z:0.000Dist: 3.265e+000 Mag.: 1 Ang.: 22.500Plane Normal:X: 0.000e+000Y: 1.000e+000Z: 0.000e+000Contour of SYYPlane: onMagfac = 0.000e+000.ra

15、dient Calculation -2.0987e+007 to -2.0750e+007. -2.0750e+007 to -2.0500e+007-2.0500e+007 to -2.0250e+007-2.0250e+007 to -2.0000e+007-2.0000e+007 to -1.9750e+007-1.9750e+007 to -1.9500e+007-1.9500e+007 to -1.9250e+007-1.9250e+007 to -1.9000e+007-1.9000e+007 to -1.8809e+007Interval = 2.5e+005FLAC3D 3.

16、00Step 653 Model Perspective 12:36:06 Fri Jan 14 2011Center:X: 0.000e+000Y: 5.000e-001Z: 0.000e+000Dist: 3.265e+000Rotation:X: 0.000Y: 0.000Z: 0.000Mag.:1Ang.: 22.500Plane Origin:Plane Normal:X: 0.000e+000X: 0.000e+000Y: 5.000e-001Y: 1.000e+000Z: 0.000e+000Z: 0.000e+000PJane: on None shear-pMN USAIt

17、asca Consulting Group, Inc.模型二考慮鉆井液滲透進入水合物地層。第一步，在鉆開水合物地層，鉆井液尚未侵入時，應力重新分布，X、Y、Z向最大壓應 力分別為 38.898MPa、21.141MPa 和 41.392MPa。井壁周圍很小范圍內(nèi)應力為拉應力，在模型1中沒有出現(xiàn)這種情況，放大查看，范圍相 當小，猜測可能與固定邊界的孔隙水壓力有關，具體原因不知。鉆孔周圍較大范圍曾因剪切到達屈服面，很小范圍內(nèi)因拉伸到達過屈服面，且目前均處 于彈性變形階段。FLAC3D 3.00 x10A4Step 1276 Model Perspective 13:19:01 Fri Jan 14

18、 2011Center:Rotation:X: 0.000e+000X:0.000Y: 5.000e-001Y:0.000Z: 0.000e+000Z:0.000Dist: 3.265e+000 Mag.: 1 Ang.: 22.500Plane Origin:X: 0.000e+000Y: 5.000e-001Z: 0.000e+000Plane Normal:X: 0.000e+000Y: 1.000e+000Z: 0.000e+000Contour of SZZPlane: onMagfac = 0.000e+000radient Calculation-I -3.8898e+007 t

19、o -3.5000e+007 -3.5000e+007 to -3.0000e+007. -3.0000e+007 to -2.5000e+007-2.5000e+007 to -2.0000e+007-2.0000e+007 to -1.5000e+007-1.5000e+007 to -1.0000e+007-1.0000e+007 to -5.0000e+006-5.0000e+006 to0.0000e+000 to5.0000e+006 to1.0000e+007 toI1 1.5000e+007 to0.0000e+0005.0000e+0061.0000e+0071.5000e+

20、0071.9645e+0071.81.61.41.21.00.80.60.40.2Itasca Consulting Group, Inc.0.20.40.60.81.01.2Minneapolis, MN USAx10A3FLAC3D 3.00Center:Rotation:X: 0.000e+000X: 0.000Y: 5.000e-001Y: 0.000Z: 0.000e+000Z: 0.000Dist: 3.265e+000Mag.:1Ang.: 22.500Plane Origin:Plane Normal:X: 0.000e+000X: 0.000e+000Y: 5.000e-00

21、1Y: 1.000e+000Z: 0.000e+000Z: 0.000e+000Step 1276 Model Perspective 12:39:41 Fri Jan 14 2011Contour of SXXPlane: onMagfac = 0.000e+000radient Calculation-4.1392e+007 to -4.0000e+007-4.0000e+007 to -3.0000e+007-3.0000e+007 to -2.0000e+007-2.0000e+007 to -1.0000e+007-1.0000e+007 to 0.0000e+0000.0000e+

22、000 to 1.0000e+0071.0000e+007 to 2.0000e+0072.0000e+007 to 2.1409e+007Interval = 1.0e+007FLAC3D 3.00Step 1276 Model Perspective 12:39:52 Fri Jan 14 2011Center:Rotation:X: 0.000e+000X:0.000Y: 5.000e-001Y:0.000Z: 0.000e+000Z:0.000Dist: 3.265e+000 Mag.: 1 Ang.: 22.500Plane Origin:X: 0.000e+000Y: 5.000e

23、-001Z: 0.000e+000Plane Normal:X: 0.000e+000Y: 1.000e+000Z: 0.000e+000Contour of SYYPlane: onMagfac = 0.000e+000radient Calculation-2.1141e+007 to -2.0000e+007-2.0000e+007 to -1.5000e+007-1.5000e+007 to -1.0000e+007-1.0000e+007 to -5.0000e+006-5.0000e+006 to 0.0000e+0000.0000e+000 to 5.0000e+0065.000

24、0e+006 to 9.8822e+006Interval = 5.0e+006Itasca Consulting Group, Inc.Minneapolis, MN USA _FLAC3D 3.00Step 1276 Model Perspective12:41:33 Fri Jan 14 2011Center:Rotation:X: 0.000e+000X:0.000Y: 5.000e-001Y:0.000Z: 0.000e+000Z:0.000Dist: 3.265e+000 Mag.: 11.6 Ang.: 22.500Plane Origin:X: 0.000e+000Y: 5.0

25、00e-001Z: 0.000e+000Plane Normal:X: 0.000e+000Y: 1.000e+000Z: 0.000e+000Contour of SXXPlane: onMagfac = 0.000e+000radient Calculation-3.0175e+007 to -3.0000e+007-3.0000e+007 to -2.0000e+007-2.0000e+007 to -1.0000e+007-1.0000e+007 to 0.0000e+0000.0000e+000 to 1.0000e+0071.0000e+007 to 2.0000e+0072.00

26、00e+007 to 2.1409e+007Interval = 1.0e+007Plane Origin:X: 0.000e+000Y: 5.000e-001Z: 0.000e+000FLAC3D 3.00Step 1276 Model Perspective 12:42:06 Fri Jan 14 2011Center:Rotation: TOC o 1-5 h z X: 0.000e+000X:0.000Y: 5.000e-001Y:0.000Z: 0.000e+000Z:0.000Dist: 3.265e+000 Mag.: 1 Ang.: 22.500Plane Normal:X:

27、0.000e+000Y: 1.000e+000Z: 0.000e+000Block StatePJane: on Noneshear-pshear-p tension-pItasca Consulting Group, Inc.Minneapolis, MN USA第二步，鉆井液侵入水合物地層，進行流固耦合計算。進行的計算無法很好的收斂。圖 為耦合計算2000步得到的應力場孔隙場云圖。井壁周圍約10cm處應力過大。井壁各處的 彈塑性狀態(tài)如圖。FLAC3D 3.00Step 3276 Model Perspective 13:20:00 Fri Jan 14 2011Center:X: 0.00

28、0e+000Y: 5.000e-001Z: 0.000e+000Dist: 3.265e+000Rotation:X: 0.000Y: 0.000Z: 0.000Mag.:1Ang.: 22.500Plane Origin:X: 0.000e+000Y: 5.000e-001Z: 0.000e+000Plane Normal:X: 0.000e+000Y: 1.000e+000Z: 0.000e+000Contour of SZZPlane: onMagfac = 0.000e+000Gradient Calculation-9.7275e+007 to -8.0000e+007-8.0000

29、e+007 to -6.0000e+007-6.0000e+007 to -4.0000e+007-4.0000e+007 to -2.0000e+007-2.0000e+007 to0.0000e+000 to2.0000e+007 to4.0000e+007 to0.0000e+0002.0000e+0074.0000e+0075.4941e+007Interval = 2.0e+007History1 Max. Unbalanced Force1.02.03.0Itasca Consulting Group, Inc.Minneapolis MN USAFLAC3D 3.00Step 3

30、276 Model Perspective 12:44:43 Fri Jan 14 2011Center:Rotation:X: 0.000e+000X:0.000Y: 5.000e-001Y:0.000Z: 0.000e+000Z:0.000Dist: 3.265e+000 Mag.: 1 Ang.: 22.500Plane Origin:X: 0.000e+000Y: 5.000e-001Z: 0.000e+000Plane Normal:X: 0.000e+000Y: 1.000e+000Z: 0.000e+000Contour of SXXPlane: onMagfac = 0.000

31、e+000radient Calculation-9.2561e+007 to -8.0000e+007-8.0000e+007 to -6.0000e+007-6.0000e+007 to -4.0000e+007-4.0000e+007 to -2.0000e+007-2.0000e+007 to 0.0000e+0000.0000e+000 to 2.0000e+0072.0000e+007 to 4.0000e+0074.0000e+007 to 5.2730e+007Interval = 2.0e+007Itasca Consulting Group, Inc.Minneapolis

32、, MN USA _FLAC3D 3.00Step 3276 Model Perspective 12:44:52 Fri Jan 14 2011Center:X: 0.000e+000Y: 5.000e-001Z: 0.000e+000Dist: 3.265e+000Rotation:X: 0.000Y: 0.000Z: 0.000Mag.:1Ang.: 22.500Plane Origin:X: 0.000e+000Y: 5.000e-001Z: 0.000e+000Plane Normal:X: 0.000e+000Y: 1.000e+000Z: 0.000e+000Contour of

33、 SYYPlane: onMagfac = 0.000e+000Gradient Calculation-9.7609e+007 to -8.0000e+007-8.0000e+007 to -6.0000e+007-6.0000e+007 to -4.0000e+007-4.0000e+007 to -2.0000e+007-2.0000e+007 to0.0000e+000 to2.0000e+007 to4.0000e+007 to0.0000e+0002.0000e+0074.0000e+0075.7148e+007Interval = 2.0e+007FLAC3D 3.00Step 3276 Model Perspective 12:45:03 Fri Jan 14 2011Center:Rotation: TOC o 1-5 h z X: 0.000e+000X:0.000Y: 5.000e-001Y:0.000Z: 0.000e+000Z:0.000Dist: 3.265e+000 Mag.: 1 Ang.: 22.500Plane Origin:X: 0.000e+000Y: 5.000e-0