STRONG FAILURE OF SEABED INDUCED BY GAS HYDRATE DISSOCIATION

Transcription

1 Proceeding of the 7th International Conference on Ga Hydrate (ICGH 211), Edinburgh, Scotland, United Kingdom, July 17-21, 211. STRONG FAILURE OF SEABED INDUCED BY GAS HYDRATE DISSOCIATION Xiaobing Lu Xu Hui Zhang, Shu Yun Wang Intitute of Mechanic. Chinee Ancademy of Science Bei Si Huan Xi Rd. 15, Beijing, China Li Q. P. Yao H. Y Reearch Centre, China Ocean Oil Co. Dong Zhi Men Wai 6, Beijing China ABSTRACT Ga hydrate in the marine deoit may diociate once the environmental condition (temerature or reure), which will directly change the t rength of the hydrate deoit (HD). Meanwhile the roduced ga by diociation can alo increae the ore reure. Accordingly, ome eriou diater may haen uch a detruction of ocean latform, eabed, oil well, and even ga blowout. Two tye of trong failure of eabed, outburt and layered fracture, are oberved in exeriment and, the initiation mechanim i dicued. The failure i cloely determined by the trength of HD, the ore reure due to diociation of ga hydrate and the diociation ize. The criterion for thee two kind of failure are obtained. Keyword: hydrate diociation, diociation zone, layered fracture, outburt INTRODUCTION Natural ga hydrate i a crytalline olid comoed of mainly methane ga molecule and water molecule, and i tabilized in high reure and low temerature condition. In nature, ga hydrate i extenively ditributed in ediment of ocean, continental margin and dee lake and i regarded a a otential energy reource 1~4. Ga hydrate in the marine deoit may diociate during the exloitation of ga, oil, hydrate or by the change of the environmental condition, which will directly change the t rength of the hydrate deoit (HD). Meanwhile the roduced ga can alo increae the ore reure. Accordingly, ome eriou diater may haen uch a detruction of ocean latform, eabed, oil we ll, and even ga blowout [5]. Some tried to exlain the Bermuda mytery by ue of ga lume induced by ga hydrate diociation [6]. Briaud et al. [7] tudied the thermal conduction in HD. It i hown that the hydrate diociation front around a high temerature oil ie with 1m diameter can reach 2m afer15 year, and 3m after 4 year. However, the evolution of diociation and key Correonding author: Phone: Fax xblu@imech.ac.cn

2 arameter controlling diater are known little. It i reorted that the St oregga landlide on the Norwegian continental helf, the larget landlide 3 in the world with 25~32 m deoit being brought away, wa jut caued by the hydrate diociation[8,9]. T he loe at the US mid-atlantic coat under monitoring i in untable tate and will lide If the temerature of ea water goe u continuouly to caue the hydrate diociated[1]. Thi aer reort the theoretical analyi of the trong failure criterion of ocean floor due to ga hydrate diociation baed on exerimental obervation. The analyi i mainly roceed baed on the two-hae media by conidering the effective t re theorem and C-J often ued in teady deflagration. temerature( o C) D temerature-reure E temerature-reure(blue) F temerature-reure(blue) Red line are temerature curve time(min) Fig.1 reure-temerature evolution curve reure(mpa) INTRODUCTION OF EXPERIMENTS Exeriment have been done in an organic gla cylinder with a ize of inner diameter height = 1cm 3cm. In thi model, an immerion heater with a ower of 4W and with a ize of length diameter = 12cm 1cm i vertically laced 1cm below the lower boundary of the over ca. During rearing amle, ilty and with a denity of 1.6g/cm 3 i firt comacted to be the keleton and then aturated by THF liquor with a ma fraction of 19%. Two tye of over ca, 5cm ilty and layer containing 1%(ma fraction) water and 5cm hydrate deoit (HD), are et u reectively to invetigate the effect of ermeability and idewall friction. Fig.2 Layered fracture Fig. 1 how the develoment of temerature and ore reure in three exeriment named A, B and C. It i oberved that layered fracture occur in exeriment A and B, while outburt occur henomenon in exeriment C. It i hown that in the exeriment A and B, the ore reure i le than that in exeriment C

3 and layered fracture occur becaue the over ca i ermeable oil layer with low trength. Generally, there occur two layered fracture, one i located at the interface between HD and over ca, and the other i located near the uer art in HD. The maximum ore reure rior to layered fracture failure i.22mpa (Fig. 2). When the over ca i contituted of HD (exeriment C), the deoit outburt accomanied by loud whitle with the temerature riing fat to about 1 C in 4 minute after heating and the maximum ore reure riing to.3mpa. There i no weak interface in the deoit becaue the whole amle i conited of HD and o ha large trength and low ermeability in thi condition. It i meaured that the friction between the deoit and the idewall equal.18mpa. FAILURE CONDITIONS Here, the exeriment in the cylinder i taken a an examle to analyze the failure condition. The exerimental reult how that with the diociation of ga hydrate, the ore reure increae to the maximum value. If the ca i low ermeable o that it can be broken through at the maximum ore reure. The ore reure at the interface decreae to the atmohere reure. Accordingly the effective tre e become gradually to be tenile. When the tenile tre reache the tenile trength, the oil layer will become untable [11]. The t ree atify the following equation in thi cae: in which T i the tenile trength of the oil layer at and, f i the ide friction, i the Poion ratio, i the lateral tre, i the total tre. From eq. (1), the critical ore reure can be obtained: cr T f 2 (2 Auming that outburt rocee o fat that the relative movement between ore fluid and keleton can be neglected, meanwhile the volume change of grain can be neglected, the relation between the ore reure and the train E i E in which i the initial train. (3) Prior to failure, the train can be neglected, o and the critical tre D can be rewritten a D l (4) in which 12, l T f 2, l i the tenile trength conidering the effect of lateral reure. If the ca i broken through and the oil layer become untable, the critical condition for the intability i a follow by conidering the reure decreaing to the atmohere reure : a l CD (5) If the oil layer develo from intability to tenile failure, the tre and ore reure can be exreed a (6) E Drawing eq.(5) in Fig. 1, it can be een clearly that if (ore reure ) increae from a table tate to cr,, the oil layer begin to be untable. After intability, the tre tate t, E t i in the line. E cr a ) 2 (1) T f

4 In ection 2, the exerimental reult how that the ore reure i.22 MPa for layered fracture and.3mpa for outburt. Neglecting the gravity of oil D, E, E t t layer due to it mallne, the reitance mainly come from the ide friction.18mpa. The tenile trength after diociation of ga hydrate equal zero and Poion ratio i about.15. The ide, E M M tre equal zero. So the critical ore reure E for layered fracture a l can be obtained a Fig1 Sketch of the trace of the tre tate from table tate to intability It i clear that there exit a very trong failure form. In thi cae, the total ore reure difference i M. In Fig. 1, drawing a tangent from the oint cr, to the line, E the interection i the tre tate for the very trong failure (outburt) M,. Uing the C-J condition a in the cae of teady deflagration, the criterion for the very trong failure can be obtained a: D M 1 d M (5) de c Intituting eq.(2) and (3) into the above equation 2 M D (6) Then the wave eed and the critical reure can be obtained M M 1 l (7) The condition M a mut be atified in the cae of outburt, which yield M 2 a l 2 a l 4 a 2 (8 ) So the criterion for outburt i M (9) Criterion for layered fracture i D l M (1) c MPa and M 2. 9MPa. The difference between the exerimental reult and the theoretical reult come from the neglecting of the ide tre and tenile trength. DISCUSSION AND CONCLUSIONS When the hydrate diociate fat and meanwhile the ermeability of HD i low, high ore reure formed in HD lead the deoit in the diociation zone be liquefied or even gaified. A a reult, layered fracture or outburt may aear in weak oition. If liquefiable or gaified oil ediment i overlain by le ermeable layer, the overlaying layer can retrict the ore water or ga to a through. Once the ore reure i over the weight and the trength of the overlaying layer and the boundary friction, the over layer will be uh away or even detroyed, which lead to a uddenly unloading of the ore reure at the interface between the aturated or gaified ediment and the over layer. Accordingly, an unloading wave will tranmit into the ediment. At ome lace the ediment i fractured in layered form when the exce reure i not too high; otherwie the fracture failure become continuouly and the outburt failure occur.

5 There exit critical reure which are correonding to the layered fracture failure and outburt failure. The critical value i cloe with the trength of the ediment and the boundary friction. In the exeriment of thi aer, the critical ga reure i.22mpa when layered fracture aear and it i.3mpa when outburt aear. ACKNOWLEDGEMENTS Thi tudy i uorted by the National Science Foundation (No ), National High Technology Reearch and Develoment Program of China (863) and Key Program of Chinee Academy of Science KJCX2-YW-L2, Imortant National Science & Technology Secific Project 28ZX REFERENCES [1] Sloan ED. Clathrate hydrate of natural gae, Marcel Dekker Inc., New York, [2] Gille G, David G, Alekandr M. Characterization of in itu elatic roertie of ga hydrate-bearing ediment on the Blake Ridge. Journal of Geohyical Reearch, 1999, 14(B8): [3] Riedel M, Bellefleur G, Dallimore SR, taylor A, Wright JF. Amlitude and frequency anomalie in regional 3D eimic data urrounding the Mallik 5L-38 reearch ite, Mackenzie Delta, Northwet Territorie,Canada. Geohyic, 26; 71 (6): B [4]Xu W, Germanovich LN. Exce ore reure reulting from methane hydrate diociation in marine ediment: A theoretical aroach. Journal of Geohyical reearch, 26; 111, Bo114. [5] Nabil Sultan. Comment on Exce ore reure reulting from methane hydrate diociation in marine ediment: A theoretical aroach by Wenyue Xu and Leonid N. Germanovich, Journal of Geohyical reearch, (B2): NIL_78-NIL84. [6] Briaud JL, Chaouch A. Hydrate melting in oil around hot conductor. Journal of Geotechnical and Geoenvironmental Engineering. 1997; 123(7): [7] Chaouch A,Briaud JL. Pot melting behavior of ga hydrate in oft ocean ediment. Offhore Technical Conference, May 5-8, Houton, Texa, 1997, [8] Jung WY, Peter RV. Effect of bottom water warming and ea level rie on holocene hydrate diociation and ma wating along the Norwegian Barent Continental Margin. Journal of Geohyical Reearch, 24; 19: B614. [9] Dricoll NW, Weiel JK, Goff JA. Potential for large-cale ubmarine loe failure and tunami generation along the U. S. mid- Atlantic coat. Geology, 2; 28(5): [1] Milkov AV. World ditribution of ubmarine mud volcanoe and aociated ga hydrate. Marine Geology, 2; 167: [11] Yu SB., Cheng CM., Tan QM. and Ding YS. Damage of orou media containing reurized ga by unloading and the maximum damage rincile for critical outburt. Acta Mechanica Sinica, 1997; 29(6):

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