Characteristics Of Reservoir Response To Drilling And Production In Gas Hydrate-bearing Sediments In The South China Sea

Natural gas hydrate with enormous reserves is a clean unconventional natural gas resource.Achieving its safe,efficient and sustainable production is of great strategic importance for alleviating the contradiction between supply and demand of conventional oil and gas resources.Therefore,hydrate exploration and exploitation have become one of the key directions of energy development in China.Based on the national special projects of the investigation and evaluation of natural gas hydrate resources and natural gas hydrate resources exploration and field trial in China seas,6 hydrate metallogenic prospective provinces,19 metallogenic zones,25 favorable blocks and 24 drilling target areas in the northern slope of the South China Sea were delineated by China Geological Survey.The natural gas reserves trapped in gas hydrate in this area could reach 185 billion tons of oil equivalent.In particular,Guangzhou Marine Geology Survey has performed hydrate drilling for 4 times in the South China Sea successively.Many high grade hydrate reservoirs with different types were found in the Shenhu,Dongsha and Qiongdongnan waters.2 hydrate deposits whose reserves are over 100 billion cubic meters were confirmed,and the field trial target area was determined based on these investigations,resulting in a series of major achievements.In May 2017,the first offshore production trial in the silty hydrate reservoir was performed,which attracted great attention from all over the world.Thus,the investigation of hydrate in China has entered the pilot stage of field trial and commercial development from the previous exploration and evaluation period.It is obvious that the development of hydrate can not be separated from drilling.In practice,exploration drilling,coring and production can alter the reservoir physical property,temperature,pore pressure and stress around the wellbore,which may induce some safety issues such as hydrate dissociation,wellbore instability,reservoir settlement,and sand production,and then the water and gas extraction can be affected.Therefore,the dynamic response characteristics of hydrate reservoir under the drilling and production conditions determine the drilling regulations,production regulations and sand control design,which are the bases for the safety risk analysis of hydrate drilling and production.Due to these reasons,the coupling thermal-hydraulic-mechanical processes during drilling and production in GHBS in the South China Sea were investigated by numerical simulation,and the corresponding response characteristics of reservoir physical property,fluid migration and stress and strain were revealed.Based on these simulation results,hydrate dissociation,wellbore stability,production potential,reservoir settlement and even sand production were fully evaluated during the whole drilling and production process.After that,a series of sensitivity analyses were performed.The above work lays a theoretical foundation for fully clarifying the reservoir response characteristics of the GHBS during drilling and production in the South China Sea,and provides some technical supports for the subsequent field trials and even the commercial development.The thesis includes six chapters and the contents of every section are as following:The first chapter,firstly the natural gas hydrates were briefly introduced,and its energy and environmental significance were emphasized.After that,the research purpose and significance were illustrated.Then,the domestic and foreign research status of GHBS during drilling and production were overviewed,and the difficulties and problems faced with during drilling and production were analyzed.Based on these,the main content and technology route were introduced.The second chapter,this chapter mainly introduced the realization process of the multi-phase and multi-component simulation method of hydrate reservoir and gave a detailed description of the actual simulation operation process.Then the basic principles of the two softwares,i.e.,TOUGH+HYDRATE and FLAC3 D,were introduced.Finally,the corresponding simulation models matching production trial in the Nankai Trough and prediction performed in the Gulf of Mexico were established using the above coupling method.The simulation results were verified with measured data and literature published before,which demonstrated the feasibility of this coupling method.The third chapter,based on the first exploration well SH2 performed in the Shenhu area of the South China Sea in 2007,reservoir response,especially the wellbore stability,at this site during drilling mud invasion was simulated and verified by the field data.Besides this analysis,the effects of both drilling mud properties(i.e.,temperature,density,and salinity)and initial deposit conditions(i.e.,porosity,permeability,and saturation)on reservoir response characteristics around the wellbore were further investigated under the condition of drilling mud invasion into the GHBS.The fourth chapter,site W19 drilled in 2015 in the Shenhu area of the South China Sea was taken as a research project in this chapter,and the production potential and reservoir geomechanical response were studied using vertical well(i.e.,casing perforation completion)under depressurization.And on this basis,the sensitivity analyses of the parameters such as bottom hole pressure,completion interval,formation permeability and porosity were systematically carried out.The fifth chapter,the production potential and geomechanical response were revealed using horizontal well design(i.e.,casing perforation completion)by means of depressurization at site W19.Similarly,the sensitivity analyses of the parameters such as bottom hole pressure,well depth layout,formation permeability and porosity were systematically conducted.The sixth chapter,the main conclusions of this thesis were summarized,and the shortcomings and future research direction were illustrated.According to the above research,the following conclusions are drawn:(1)Two main factors have been identified as the main ones behind wellbore instability problems during drilling under certain conditions: i)reduction of the effective stresses(induced by pore pressure increase during drilling),and ii)reduction of the formation strength(induced by the reduction of hydrate saturation).The decrease of the effective stress can be associated with two main factors: a)the well pressure imposed by the drilling-mud,and b)the pressure increase by any release of free gas.Increasing the temperature and salinity of the drilling-mud will imply that more free gas will be generated near the wellbore(i.e.,because of the hydrate dissociation),which will lead to a larger excess of the pore pressure,and a rapid decrease in the strength of GHBS around the borehole,which may trigger wellbore instability issues.In addition,if mud cake is poorly formed when using the high density drilling mud,the drilling mud invasion extent will increase,which can expand the hydrate dissociation area and cause the wellbore instability.(2)The effects of initial hydrate saturation(SH)and porosity on wellbore stability depend mainly on the decrease in the formation strength caused by SH reduction,rather than on the pore pressure change during drilling-mud invasion(i.e.,for the case that no free gas is released).The risk of wellbore instability decreases with an increase in the reservoir initial SH and porosity under the same conditions.It is also predicted that more permeable reservoirs(with relatively low permeability mud-cake)will control better the decrease in the effective stresses(i.e.,induced by the pressurization anticipated at the beginning of the operations)and therefore they will be safer in terms of borehole stability.(3)Because of the low formation permeability and the permeable burdens at site W19,it may be impossible to extract methane from this site economically using both vertical and horizontal well depressurization regime.Moreover,depressurization can cause effective stress concentration around the borehole,and the higher the hydrate saturation of the formation is,the more obvious the concentration of effective stress is.However,the yield failure was not predicted when the Mohr-Coulomb model was applied,but the significant seafloor settlement was observed,which can cause a great challenge for casing cementing operation and the layout of wellhead equipment.(4)Although decreasing the bottom hole pressure can promote hydrate dissociation,the increase of gas production is limited,while the water production rate can obviously increase.Furthermore,the effective stress around the borehole will increase with the reduction of well pressure,resulting in the more significant concentration phenomenon and seafloor settlement.Even though optimizing the completion interval or adjusting the depth of horizontal well can maximize gas production,the yield increasing effect is still limited.Meanwhile,the effective stress distribution around the borehole and seafloor settlement can be obviously affected.(5)Both gas and water production rates will significantly increase when improving the permeability of hydrate reservoir regardless of which well design(i.e.,vertical well or horizontal well)was applied.Even though the formation stress did not reach the yielding surface using the Mohr-Coulomb strength criterion after the increase in the permeability of GHBS,the seafloor settlement can obviously increase.As for the slight increase in the permeability of underlying free gas formation,the production of underlying free gas can be accelerated and the water production rate will not sharply increase.Although a slight increase in permeability of free gas formation will expand the area of effective stress increase in this layer,in the long term,it will reduce the the seafloor settlement to a certain extent.(6)When other parameters are the same,no matter changing hydrate reservoir or underlying free gas formation porosity,the effects on both production potential and geomechanical response are negligible.