储层和开采参数对天然气水合物开采产能的影响分析

开展储层参数和开采参数对天然气水合物开采产能影响的研究有助于其实际开采场址和开采方法的选择。以中国南海神狐海域SH7站位的地质参数为背景，采用TOUGH+HYDRATE软件系统地分析了储层压力、温度、孔隙度、水合物饱和度、渗透率、上覆层和下伏层渗透率等储层参数，以及降压幅度、降压井长度和出砂堵塞（通过改变井周网格渗透率反映出砂堵塞）等开采参数对天然气水合物降压开采产能的影响。数值模拟结果表明：(1)随着储层渗透率的增大，产气量有明显的增加；随着储层压力、孔隙度的增大以及上覆层和下伏层渗透率的减小，产气量有较大的增加；随着储层温度的增大，产气量有一定的增加；产气量随饱和度的增大先增大后减小。因此，实际开采时可优先选择渗透率大、上覆层和下伏层渗透率小、孔隙度大、温度较高、水合物饱和度适中的储层。(2)随着降压幅度的增大以及降压井长度增大，产气量有明显的增加；而随着出砂堵塞程度的加剧，产气量有非常明显的减少。因此，实际开采时可以通过增大降压幅度和降压井长度以及采取减轻出砂堵塞的措施来提高产气量。研究结果可以为我国将来天然气水合物开采区域及开采方式的选择和确定提供参考。     

基于数字岩心的含水合物石英砂微观渗流有限元分析

利用X射线计算机断层扫描系统(CT)获得不同饱和度下含水合物石英砂内部气、水、水合物各相态分布特征,通过有限元方法计算了不同水合物饱和度下石英砂液相渗透率变化,并模拟了流体在孔隙内的流动情况,获得了假定边界条件下孔隙流体的三维流速分布。研究结果表明,随着水合物饱和度的降低,渗透率逐渐增大,其中当水合物饱和度从56%下降到39%时,液相渗透率值增速最大;水合物分解末期,液相渗透率并未随着有效孔隙度的增大而快速升高,通过CT扫描图像显示,部分石英砂孔隙和喉道可见甲烷气泡滞留,由于气体的贾敏效应在一定程度上阻碍了液体的流动,从而导致液相渗透率增速降低。本研究建立了一种基于石英砂内部真实孔隙特征的液相渗透率和液体流速计算方法,可为水合物开采过程中储层微观渗流演化机理研究提供参考。     

水合物生成导致沉积物孔隙结构和渗透率变化的低场核磁共振观测

含水合物储层的宏观物性表现是由储层沉积物的微观孔隙特征所控制的。理解沉积物在水合物生成过程中微观孔隙结构特征变化对于其物性特征的预测和分析有重要意义。本文利用低场核磁共振(LFNMR)技术监测了不同砂样中氙气水合物的生成过程,利用横向弛豫时间(T2)谱对生成过程中的微观孔隙结构及水相渗透率演化规律进行了分析。研究表明,水合物优先生成于沉积物较大孔隙中,在半径较小的孔隙中水合物很难生成;生成前期水合物的生长速率较快,后期逐渐减缓;水合物的生成导致沉积物孔隙尺寸和分布的变化,表现为随着水合物的生成,沉积物水相孔隙空间的最大孔隙半径和平均孔隙半径逐渐减小,孔隙空间的分形系数逐渐增大;沉积物水相渗透率随水合物生成过程中水合物饱和度的增加,先迅速减小后缓慢减小;具有不同孔隙结构特征的样品水相渗透率变化规律存在差异;相较于SDR模型和Kozeny-Carman模型,分形方法能够更好地体现孔隙结构变化对渗透率的影响。     

压缩加载条件下含水合物沉积物蠕变特性分析

水合物开采可能诱发海底滑坡或其他工程地质灾害。实现水合物商业化开采需要中长期稳定产气，长期荷载下储层的蠕变特性是地层稳定性评价的基础力学参数。利用南海水合物储层粉黏土为试验介质在压缩加载条件下的系列固结排水蠕变测量试验结果，对粉黏土的蠕变特性进行了分析。结果表明，加载过程中，含水合物沉积物经历瞬时变形、固结变形和蠕变变形3个阶段；随着加载应力和水合物饱和度的提高，蠕变应变不断增加；修正的Singh-Mitchell蠕变模型可以较好预测不同应力水平和水合物饱和度下粉黏土的蠕变特性。     

多孔介质中不同饱和度天然气水合物的分解特性研究

开采天然气水合物对缓解世界能源危机、解决中国天然气资源短缺问题具有重要现实意义.降压法是开采天然气水合物一种较为简单、安全、有效的方法,但是目前对水合物饱和度较高条件下的降压分解特性缺乏深入认识.在多孔介质(石英砂)体系开展了天然气水合物的生成与分解实验,研究了 3种不同水合物饱和度(38.1％、42.1％、46.4％...     

瞬态压力脉冲法及其在松散含水合物沉积物中的应用

含水合物沉积物渗透率是水合物开采相关工作的基础参数之一。稳态法在应用于渗透率较低的多孔介质时存在着稳定渗流难和试验耗时长等缺点。目前,含水合物细颗粒沉积物渗透率试验数据积累明显不足。本文首先介绍了瞬态压力脉冲法的基本原理及数据处理方法,然后以模拟试验验证了瞬态压力脉冲法的适用性,最后探讨了该方法在松散含水合物沉积物渗透率测量方面的应用效果。结果表明:瞬态压力脉冲法近似解处理粉细砂沉积物试验数据效果较好,而处理黏土沉积物试验数据存在明显误差,建议采用数值模拟反演分析的方法处理瞬态压力脉冲法试验数据;瞬态压力脉冲法适用于松散沉积物渗透率测量,在含水合物沉积物渗透率试验研究方面具有潜在的应用前景。     

热水驱替开采Ⅱ类水合物藏规律研究

天然气水合物广泛分布于陆地冻土带和深海地层,资源潜力巨大,其中Ⅱ类水合物藏占有重要地位。为加强对Ⅱ类水合物藏开采规律的认识,结合实际水合物藏参数,使用数值模拟方法研究了热水驱替开采Ⅱ类水合物藏的动态规律,并与降压法的开采效果进行了对比分析。结果表明:①热水驱替开采Ⅱ类水合物藏时,产气速率和分解气速率首先快速上升,然后以较快速度下降至趋于相对稳定;累产气和累分解气上升较快;气体采出程度和水合物分解程度均处于较高水平(>60%)。②热水驱替对Ⅱ类水合藏的开采具有一定的适应性,与降压法开采相比,热水驱替方式下储层水合物的分解更彻底,气体采出程度、水合物分解程度也更优,但具有较低的累积气水比,产水量较大。     

世界热点地区高饱和度天然气水合物有利沉积储集条件

天然气水合物是全球未来能源的接替资源，高饱和度(S_h>50%)水合物储层是未来面向工业化开采的首要选择。截止到目前，高饱和度天然气水合物有利沉积相带与储层条件之间的关系仍缺乏系统研究。根据公开发表的文献资料，系统总结了墨西哥湾、日本南海海槽、韩国郁陵盆地、印度Krishna-Godavari盆地以及南海神狐海域等全球5个天然气水合物热点钻探区64口井取芯及井-震联合资料，对含水合物储层岩性、沉积环境、水合物饱和度等参数进行的详细总结分析表明：在必要的温压环境和气源条件下，深海平原区块体搬运沉积和浊流等高沉积速率的深水砂质沉积物赋存孔隙型水合物，水合物可分布在砂岩、极细砂岩、粉砂岩、粉砂质黏土和泥等粒级沉积物中，但高饱和度水合物主要赋存于粉砂-细砂岩中，储层孔隙度与饱和度具有一定的正相关性。中国南海神狐海域发现含有孔虫黏土质粉砂或粉砂质黏土这种特殊的细粒沉积物，其水合物饱和度可达到中高水平(20%～76%)。上述研究成果及认识奠定了下一步寻找优质天然气水合物储层的地质基础，也可为高饱和度水合物商业化勘探开发提供理论依据。     

利用海洋地震数据估算气体水合物与游离气资源量

位于南卡罗来纳近海的布莱克海台的海洋地震数据及测井表明：含有气体水合物的沉积层覆盖于含游离气的沉积层之上，造成了明显的地震似海底反射（BSRs)。我们将一个理论的岩石物理模型应用到二维的布莱克海台海洋地震数据，以确定气体水合物与游离气的饱和度。高孔隙度的海洋沉积物可以模拟为一个颗粒状的系统，其弹性波速度与孔隙度、有效压力、矿物成分、孔隙充填物的弹性属性以及孔隙空间中水、气体及气体水合物的饱和度相关。将此模型应用到地震数据时，我们首先通过叠加速度分析获得层速度。其次，除孔隙度，水、气和气体水合物的饱和度等参数外，通过地质信息获得岩石物理模型的其他输入参数。为了从层速度信息中估算孔隙度和饱和度，我们首先假定整个沉积物中不含气体水合物或游离气，然后，根据岩石物理模型由层速度直接计算孔隙度。在气体水合物与游离气出现的区域，这些孔隙度的数据特征出现异常（无气体水合物或游离气的沉积物中期望的标准数据特征与获得的数据特征相比较而言），低估水合物区域中的孔隙度，而高估含游离气区域的孔隙度。我们用带有异常值的孔隙度数据减去标准的孔隙度特征数据（不含气体水合物和气体）来计算剩余孔隙度。然后,我们应用岩石物理模型剔除气体水合物或气体饱和度引起的异常，最终获得理想的二维饱和度图。因此，这样得到的气体水合物最大饱和度占孔隙空间的13－18％之间（取决于所用模式的型式）。在布莱克海台钻井中（不在地震测线上）测量的饱和度大约为12％，这与计算结果一致，游离气体的饱和度在1－2％之间变化。饱和度的估算值对于输入的速度值相当敏感，因此，采用准确的速度对得到合理的储层特性相当关键。     

多孔介质中天然气水合物注热开采影响因素实验研究

利用自制的一维天然气水合物开采模拟实验系统,模拟海洋地质条件,在填砂模型中生成天然气水合物,再注入热盐水进行注热开采的物理模拟实验,分析注热参数对水合物分解动态及注热开采能量效率的影响.分别设定注热水温度为60、80、100℃,注热水速度为12、15、18 mL/min,最长注热水时间220 min.结果表明:在相同的...     

南海神狐海域天然气水合物饱和度的数值模拟分析

珠江口盆地神狐海域是天然气水合物钻探和试验开采的重点区域,大量钻探取心、测井与地震等综合分析表明不同站位水合物的饱和度、厚度与气源条件存在差异。本文利用天然气水合物调查及深水油气勘探所采集的测井和地震资料建立地质模型,利用PetroMod软件模拟地层的温度场、有机质成熟度、烃源岩生烃量、流体运移路径以及不同烃源岩影响下的水合物饱和度,结果表明:生物成因气分布在海底以下1500 m范围内的有机质未成熟地层,而热成因气分布在深度超过2300 m的成熟、过成熟地层。水合物稳定带内生烃量难以形成水合物,形成水合物气源主要来自于稳定带下方向上运移的生物与热成因气。模拟结果与测井结果对比分析表明,稳定带下部生物成因气能形成的水合物饱和度约为10%,在峡谷脊部的局部区域饱和度较高;相对高饱和度(>40%)水合物形成与文昌组、恩平组的热成因气沿断裂、气烟囱等流体运移通道幕式释放密切相关,W19井形成较高饱和度水合物的甲烷气体中热成因气占比达80%,W17井热成因气占比为73%,而SH2井主要以生物成因为主,因此,不同站位甲烷气体来源占比不同。     

In-situ gas hydrate hydrate saturation estimated from various well logs at the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

In 2006, the U.S. Geological Survey (USGS) completed detailed analysis and interpretation of available 2-D and 3-D seismic data and proposed a viable method for identifying sub-permafrost gas hydrate prospects within the gas hydrate stability zone in the Milne Point area of northern Alaska. To validate the predictions of the USGS and to acquire critical reservoir data needed to develop a long-term production testing program, a well was drilled at the Mount Elbert prospect in February, 2007. Numerous well log data and cores were acquired to estimate in-situ gas hydrate saturations and reservoir properties.Gas hydrate saturations were estimated from various well logs such as nuclear magnetic resonance (NMR), P- and S-wave velocity, and electrical resistivity logs along with pore-water salinity. Gas hydrate saturations from the NMR log agree well with those estimated from P- and S-wave velocity data. Because of the low salinity of the connate water and the low formation temperature, the resistivity of connate water is comparable to that of shale. Therefore, the effect of clay should be accounted for to accurately estimate gas hydrate saturations from the resistivity data. Two highly gas hydrate-saturated intervals are identified - an upper ∼43 ft zone with an average gas hydrate saturation of 54% and a lower ∼53 ft zone with an average gas hydrate saturation of 50%; both zones reach a maximum of about 75% saturation.     

南海北部荔湾3区块天然气水合物分布特征及目标识别

针对天然气水合物钻探与取样难以解决的水合物矿体空间展布等问题,利用白云-荔湾凹陷高密度分析重新处理的三维地震资料,首先基于模糊数学的多属性融合技术对水合物分布进行刻画;再通过高分辨率速度场对浅层开展高分辨率宽频无井反演技术,提高了水合物层分辨率;最后,利用岩石物理方法及多种模型对水合物饱和度进行定量预测,实现了对5～6m厚水合物层的有效辨别,进而形成了一套适合于孔隙充填型的水合物矿藏目标识别评方法。结果表明:应用该技术可有效对荔湾3水合物富集区第四条带水合物空间刻画,揭示出该区水合物饱和度最高可超40%,同时薄层与厚层水合物具有明显互层分布特征,在水合物矿体刻画及饱和度预测基础上,进一步对该区实施了井位优选,该方法预测的水合物层与实际钻探H1和H2站位吻合较好。这些结果说明常规三维油气地震数据在经过宽频处理后可应用于高分辨率水合物勘探,节约经济成本,同时提高了常规地震在水合物勘探中精度与实用性。     

Production potential and stability of hydrate-bearing sediments at the site GMGS3-W19 in the South China Sea: A preliminary feasibility study

According to the preliminary geological data of gas hydrate bearing-sediments (GHBS) at site GMGS3-W19 in the third Chinese expedition to drill gas hydrates in 2015, a production model using three different recovery pressures was established to assess the production feasibility from both production potential and geomechanical response. The simulation results show that for this special Class 1 deposit, it is a little hard for gas production rate to reach the commercial extraction rate because the degree of hydrate dissociation is limited due to the low reservoir permeability and the permeable burdens. However, the free gas accumulating in the lower part of the GHBS can significantly increase gas-to-water ratio. It also generates many secondary hydrates in the GHBS at the same time. Decreasing the well pressure can be beneficial to gas recovery, but the recovery increase is not obvious. In term of geomechanical response of the reservoir during the gas recovery, the permeable burdens are conducive to reduction of the sediment deformation, though they don't facilitate the gas recovery rate. In addition, significant stress concentration is observed in the upper and lower edges of GHBS around the borehole during depressurization because of high pressure gradient, and the greater the well pressure drop, the more obvious the phenomenon. Yield failures and sand production easily take place in the edges. Therefore, in order to achieve the purpose of safe, efficient and long-term gas production, a balance between the production pressure and reservoir stability should be reached at the hydrate site. The production pressure difference and sand production must be carefully controlled and the high stress concentration zones need strengthening or sand control treatment during gas production. Besides, the sensitivity analyses show that the hydrate saturation heterogeneity can affect the production potential and geomechanical response to some extent, especially the water extraction rate and the effective stress distribution and evolution. Increasing GHBS and its underlying free gas formation permeabilities can enhance the gas production potential, but it probably introduces geomechanical risks to gas recovery operations.     

Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs

During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna–Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie’s method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp’s law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie’s approach are related to the calibration of Archie coefficient (a), cementation factor (m) and saturation exponent (n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 (S_h = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 (S_h = 35–38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ∼720 km². Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ∼5.7 trillion-cubic feet (TCF) to ∼32.1 TCF.     

Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs

During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna–Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie’s method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp’s law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie’s approach are related to the calibration of Archie coefficient (a), cementation factor (m) and saturation exponent (n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 (S_h = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 (S_h = 35–38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ∼720 km². Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ∼5.7 trillion-cubic feet (TCF) to ∼32.1 TCF.     

Acoustic wave attenuation in the gas hydrate-bearing sediments of Well GC955H,Gulf of Mexico

A better understanding of wave attenuation in hydrate-bearing sediments is necessary for the improved geophysical quantification of marine gas hydrates. Here we compare the attenuation behavior of hydrate-saturated vs water-saturated sediments at site GC955H, in the Gulf of Mexico, which was surveyed during the JIP Leg II expedition. We compute the P-wave attenuation of the gas hydrate bearing sediments using the median frequency shift method on the monopole waveforms. The results show that P-wave attenuation due to low saturation (<?0.4) in hydrate-filled fractures of fine-grained sediment is comparable to that of the water-filled fracture case. On the contrary, P-wave attenuation due to high saturation (>?0.4) in the hydrate-filled pores of coarse-grained sediments can be up to as much as three times more than that of the water-saturated case. The correlation analysis shows that the P-wave attenuation increases with the increasing gas hydrate saturation for the highly saturated gas hydrate-bearing sand interval while the correlation of the P-wave attenuation and hydrate saturation is weak for low saturated gas hydrate-bearing shale interval. The results show that P-wave attenuation is more likely to be used as a geophysical proxy for gas hydrate quantification of highly concentrated coarse-grained sediment rather than for that of fine-grained sediment. To examine the P-wave behavior in sand, we use the improved LCAM model, which accounts for physical factors such as grain boundary roughness and squirt flow to explain the observed differences in P-wave attenuation between hydrate and water-saturated coarse-grained sediment. Our results provide further geophysical evidences for P-wave behavior in the gas hydrate-bearing sediments in the field.     

Alaska North Slope regional gas hydrate production modeling forecasts

A series of gas hydrate development scenarios were created to assess the range of outcomes predicted for the possible development of the “Eileen” gas hydrate accumulation, North Slope, Alaska. Production forecasts for the “reference case” were built using the 2002 Mallik production tests, mechanistic simulation, and geologic studies conducted by the US Geological Survey. Three additional scenarios were considered: A “downside-scenario” which fails to identify viable production, an “upside-scenario” describes results that are better than expected. To capture the full range of possible outcomes and balance the downside case, an “extreme upside scenario” assumes each well is exceptionally productive.Starting with a representative type-well simulation forecasts, field development timing is applied and the sum of individual well forecasts creating the field-wide production forecast. This technique is commonly used to schedule large-scale resource plays where drilling schedules are complex and production forecasts must account for many changing parameters. The complementary forecasts of rig count, capital investment, and cash flow can be used in a pre-appraisal assessment of potential commercial viability.Since no significant gas sales are currently possible on the North Slope of Alaska, typical parameters were used to create downside, reference, and upside case forecasts that predict from 0 to 71 BM³ (2.5 tcf) of gas may be produced in 20 years and nearly 283 BM³ (10 tcf) ultimate recovery after 100 years.Outlining a range of possible outcomes enables decision makers to visualize the pace and milestones that will be required to evaluate gas hydrate resource development in the Eileen accumulation. Critical values of peak production rate, time to meaningful production volumes, and investments required to rule out a downside case are provided. Upside cases identify potential if both depressurization and thermal stimulation yield positive results. An “extreme upside” case captures the full potential of unconstrained development with widely spaced wells. The results of this study indicate that recoverable gas hydrate resources may exist in the Eileen accumulation and that it represents a good opportunity for continued research.     

天然气水合物饱和度的预测方法

天然气水合物饱和度预测是天然气水合物资源评价的基础,饱和度预测研究有利于了解天然气水合物作为地质灾害和引发全球气候变化所起的作用,同时对了解天然气水合物的成藏规律和指导天然气水合物的勘探开发具有重要的指导意义,因此,准确预测天然气水合物饱和度非常重要。目前电阻率和氯离子浓度异常、氯离子剖面以及声阻抗测井数据均可预测水合物饱和度。