Forward modeling and inversion of LWD induction data

The processing algorithms for high-frequency induction resistivity data are applied to logs acquired at different stages of well construction. Open-hole induction logging while reaming of vertical wells provides a priori information on geology and resistivity distribution. The resulting resistivity model can make reference in geosteering for deviated and horizontal drilling. Algorithms for inversion of high-frequency induction responses from layered media are used in a software package for processing LWD data. The software provides real-time inversion to recover resistivities and depths to layers in oil and gas reservoirs penetrated by wells of a complex trajectory. It also allows checking the inversion quality by analyzing the sensitivity of tool responses to model parameters with reference to the tool specifications.     

Characterising the present-day stress regime of the Georgina Basin

The onshore Georgina Basin in northern Australia is prospective for unconventional hydrocarbons; however, like many frontier basins, it is underexplored. A well-connected hydraulic fracture network has been shown to be essential for the extraction of resources from the tight reservoirs that categorise unconventional plays, as they allow for economic flows of fluid from the reservoir to the well. One of the fundamental scientific questions regarding hydraulic stimulation within the sub-surface of sedimentary basins is the degree to which local and regional tectonic stresses act as a primary control on fracture propagation. As such, an understanding of present-day stresses has become increasingly important to modern petroleum exploration and production, particularly when considering unconventional hydrocarbon reservoirs. This study characterises the regional stress regime in the Georgina Basin using existing well data. Wellbore geophysical logs, including electrical resistivity image logs, and well tests from 31 petroleum and stratigraphic wells have been used to derive stress magnitudes and constrain horizontal stress orientations. Borehole failure features interpreted from wellbore image and caliper logs yield a maximum horizontal stress orientation of 044°N. Integration of density log data results in a vertical stress gradient of 24.6 MPa km^–1. Leak-off and mini-fracture tests suggest that this is the minimum principal stress, as leak-off values are generally shown to be at or above the magnitude of vertical stress. The maximum horizontal stress gradient is calculated to be in the range of 31.3–53.9 MPa km^–1. As such, a compressional stress regime favouring reverse/reverse–strike-slip faulting is interpreted for the Georgina Basin.     

Fluid Property Discrimination in Dolostone Reservoirs Using Well Logs

The Ordovician Majiagou Formation is one of the main gas-producing strata in the Ordos Basin,China.The identification of hydrocarbon-bearing intervals via conventional well logs is a challenging task.This study describes the litholog of Ma 5(Member 5 of Majiagou Formation)dolostones,and then analyzes the responses of various conventional well logs to the presences of natural gas.The lithology of the gas bearing layers is dominantly of the dolomicrite to fine to medium crystalline dolomite.Natural gas can be produced from the low resistivity layers,and the dry layers are characterized by high resistivities.Neutron-density crossovers are not sensitive to the presences of natural gas.In addition,there are no significant increases in sonic transit times in natural gas bearing layers.NMR(nuclear magnetic resonance)logs,DSI(Dipole Sonic Imager)logs and borehole image logs(XRMI)are introduced to discriminate the fluid property in Majiagou dolostone reservoirs.The gas bearing intervals have broad NMR T_2(transverse relaxation time)spectrum with tail distributions as well as large T_(2gm)(T_2 logarithmic mean values)values,and the T_2 spectrum commonly display polymodal behaviors.In contrast,the dry layers and water layers have low T_(2gm) values and very narrow T_2 spectrum without tails.The gas bearing layers are characterized by low V_p/V_s ratios,low Poisson’s ratios and low P-wave impedances,therefore the fluid property can be discriminated using DSI logs,and the interpretation results show good matches with the gas test data.The apparent formation water resistivity(AFWR)spectrum can be derived from XRMI image logs by using the Archie’s formula in the flushed zone.The gas bearing layers have broad apparent formation water resistivity spectrum and tail distributions compared with the dry and water layers,and also the interpretation results from the image logs exhibit good agreement with the gas test data.The fluid property in Majiagou dolostone reservoirs can be discriminated through NMR logs,DSI logs and borehole image logs.This study helps establish a predictable model for fluid property in dolostones,and have implications in dolostone reservoirs with similar geological backgrounds worldwide.     

四川盆地磨溪地区龙王庙组储层特征、类型及成因分析

利用四川盆地磨溪地区12口井岩心、8口井成像测井资料、14口井的测井评价等资料,系统分析磨溪地区龙王庙组储层特征,划分了储层类型,探讨了储层形成主控因素,预测了优质储层展布。结果表明：1）川中磨溪地区龙王庙组储层主要岩石类型为中粗晶砂屑白云岩,储层中小洞发育,见到大量的构造缝和水平缝,整体表现为中低孔、中-高渗特征;2）龙王庙组储层类型可划分为溶蚀孔洞型、溶蚀孔隙型和基质孔隙型3种类型,其中高角度构造缝、水平缝、网状成岩缝和缝合线4种裂缝与3种储集类型形成整体连通的缝洞体系;3）优质储层主要在中细晶砂屑白云岩颗粒滩相中发育,颗粒滩后期经历3期岩溶作用,特别是表生期“顺层”溶蚀使溶蚀孔、洞型储层横向叠置连片,大面积分布,延伸5～20 km,井间连通性好,形成了孔洞发育的优质储层。     

Forward modeling and inversion of induction logs from shaly sand reservoirs using petrophysical conductivity models

Algorithms and software have been designed for petrophysical 2D forward simulation and inversion of induction logs from oil wells drilled in shaly sandstone using conductivity models for different clay types. As comparative analysis of relative amplitudes and phases of tool responses has demonstrated, it is possible to reliably and accurately estimate fluid saturation and porosity from induction logs. The applied 2D numerical inversion of both theoretical and well logs yielded quantitative estimates of petrophysical parameters of reservoirs, with clay volume fraction assumed according to the chosen conductivity model. Correction for the presence of clay improves inversion quality if clay content as well as type are taken into account.     

陕北志丹油田樊川区长61低阻油层成因分析与识别方法

陕北地区延长组发育低（特）渗储层,除发育通常的高阻油层外,还极易形成低阻油层;因此,低阻油层的发现,极大拓宽了该区石油勘探和开发的领域。根据志丹油田樊川油区长6¹的石油地质特征,结合大量实验和生产数据资料的分析对比,对该区低阻油层的成因机理及宏观控制因素进行了分析。结果表明:长6¹储层的孔隙结构复杂导致束缚水饱和度增高,高地层水矿化度、高泥质含量及放射性砂岩的存在,是低阻油层得以形成的主要原因。通过大量实际试油和测井资料的对比可见,应用计算自然电位与自然电位曲线重叠法可以快速直观显示其含油性,声波时差-深电阻率的交会图与自然伽马-深电阻率交会图相结合可有效识别低阻油层。对区内85口探井进行了测井资料的二次解释,在长6发现了初次解释所遗漏的45个低阻油层,油层数由原来的117个增加到162个,净增27.8%。     

Resistivity of reservoir sandstones and organic rich shales on the Barents Shelf:Implications for interpreting CSEM data

Marine controlled source electromagnetic(CSEM)data have been utilized in the past decade during petroleum exploration of the Barents Shelf,particularly for de-risking the highly porous sandstone reservoirs of the Upper Triassic to Middle Jurassic Realgrunnen Subgroup.In this contribution we compare the resistivity response from CSEM data to resistivity from wireline logs in both water-and hydrocarbon-bearing wells.We show that there is a very good match between these types of data,particularly when reservoirs are shallow.CSEM data,however,only provide information on the subsurface resistivity.Careful,geology-driven interpretation of CSEM data is required to maximize the impact on exploration success.This is particularly important when quantifying the relative re-sistivity contribution of high-saturation hydrocarbon-bearing sandstone and that of the overlying cap rock.In the presented case the cap rock comprises predominantly organic rich Upper Jurassic-Early Cretaceous shales of the Hekkingen Formation(i.e.a regional source rock).The resistivity response of the reservoir and its cap rock become merged in CSEM data due to the transverse resistance equivalence principle.As a result of this,it is imperative to understand both the relative contributions from reservoir and cap rock,and the geological sig-nificance of any lateral resistivity variation in each of the units.In this contribution,we quantify the resistivity of organic rich mudstone,i.e.source rock,and reservoir sandstones,using 131 exploration boreholes from the Barents Shelf.The highest resistivity(＞10,000 Ωm)is evident in the hydrocarbon-bearing Realgrunnen Subgroup which is reported from 48 boreholes,43 of which are used for this study.Pay zone resistivity is primarily controlled by reservoir quality(i.e.porosity and shale fraction)and fluid phase(i.e.gas,oil and water saturation).In the investigated wells,the shale dominated Hekkingen Formation exhibits enhanced resistivity compared to the background(i.e.the underlying and overlying stratigraphy),though rarely exceeds 20Ωm.Marine mudstones typically show good correlation between measured organic richness and resistivity/sonic velocity log signatures.We conclude that the resistivity contribution to the CSEM response from hydrocarbon-bearing sandstones out-weighs that of the organic rich cap rocks.     

低渗透储层的径向电阻率特征及含油性 ——以鄂尔多斯盆地洛河油田延长组长61为例

鄂尔多斯盆地中部洛河油田长61低渗透油层由于含油饱和度变化大、油层非均质强,再加淡水钻井液侵入,造成油层电阻率径向分布不均且变化大。通过双感应-八侧向测井与阵列感应测井响应特征对比分析,认为水层的径向电阻率均为增阻侵入,油层、油水同层的径向电阻率普遍具有减阻侵入、低阻环带、高阻环带特征或相关趋势。其中,减阻侵入有助于识别高含油饱和度、高电阻率油层和油水同层,低阻环带和高阻环带有助于识别低电阻率油水同层。另外,部分油水同层的双感应-八侧向测井径向电阻率组合具有“增阻侵入”且深感应电阻率低的特征,推测可能是受双感应-八侧向测井探测范围限制,为低阻环带靠近井眼附近遭受淡水钻井液侵入影响的结果,深感应测井反映的是侵入带电阻率而不是油水同层的电阻率,容易被误解释为水层。因此,对于双感应-八侧向测井中具有“增阻侵入”特征且深感应电阻低值的储层,其流体性质有水层、油水同层这两种可能性,需要结合深探测测井资料或油藏地质特征进一步分析,以提高油层、水层的识别率。     

应用测井资料识别煤体结构及分层

煤体结构类型与煤储层孔裂隙特征密切相关,对煤层气开采具有重要意义。以韩城矿区为例,通过对区内的参数井取心样品分析,把煤体结构分为块煤(I类)、块粉煤(II类)、粉煤(III类)。对区内3号、5号和11号煤层的深侧向电阻率测井、井径测井和自然伽玛测井曲线特征进行对比,归纳了各煤体结构的测井曲线组合特征:I和III类煤电阻率曲线和自然伽玛曲线为平滑的箱型或钟形,II类煤则为波动的箱形;II和III类煤扩径严重,且II类煤差异扩径明显。据此,提出了一种利用电阻率测井、井径测井和自然伽玛测井等测井曲线组合识别煤体结构的方法,并利用这种方法准确识别出同一煤层不同煤体结构的分层。      

青海木里三露天天然气水合物储层测井识别与划分

三露天天然气水合物钻探结果揭示,水合物主要赋存于裂隙型和孔隙型储层中。基于钻探结果,在测井岩性识别的基础上,综合利用常规测井和超声波成像测井等资料,总结两类天然气水合物储层的测井响应特征,形成了基于测井资料的水合物储层识别方法,并对该区储层进行划分。结果表明：(1)裂隙型水合物储层比纯泥岩层段的裂隙发育;声波速度较高,范围为1.5~4.5 km/s ;电阻率最高可达90 Ω·m。孔隙型水合物储层比砂岩水层的声波速度高,范围为2.0~4.0 km/s;电阻率亦为高值,范围为90~180 Ω·m。(2)研究区14个钻孔共识别出裂隙型储层12层,孔 隙型储层15层,总厚度为217.2 m。钻遇水合物的厚层均能识别,但少量钻遇的薄层未能识别,原因可能为薄层水合物含量少,导致测井响应不明显。总体上,测井方法在三露天天然气水合 物储层识别方面应用效果良好。     

The resistivity structure of the Penola Trough,Otway Basin from magnetotelluric data

We present inversion results for a 100 site, broadband magnetotelluric (MT) survey in the Penola Trough, Otway Basin, South Australia. The Penola Trough is host to several petroleum reservoirs and has more recently been a target for unconventional geothermal exploration. We present two interpretations of the MT data. A 1D anisotropic interpretation, where anisotropy is determined within the Otway Basin sequence and basement in the northeastern Penola Trough, fits the impedance tensor well. However, the anisotropy strike is inconsistent with the known orientation of electrically conductive fractures in the Penola Trough. On the other hand, a 3D interpretation, which incorporates lateral variations in resistivity, requires no anisotropy yet it matches the data equally well. Both the 1D and 3D inversions resolve several layers within the Otway Basin sequence, which correspond to stratigraphic units defined in wells and in the coincident Haselgrove–Balnaves 3D seismic survey. These include the Eumeralla and Dilwyn formations, which are poorly resolved in the seismic data. The basin architecture, defined in the 3D inversion, in particular the depth to basement, is consistent with previous interpretations based on seismic reflection data that show that the Otway Basin thins in the northeastern Penola Trough. This does not occur in the anisotropic model. We therefore conclude that the subsurface resistivity appears to be isotropic in the Penola Trough. This contrasts with the anisotropic resistivity structure determined in a previous study in the Koroit region, eastern Otway Basin. The difference in the MT responses between the two regions is supported by resistivity and permeability information from well logs and may reflect differences in the orientation of subsurface fractures, or differences in the present-day stress field, between the two regions.     

基于流动单元基础上的水淹层定量识别方法研究——以宝浪油田宝北区块为例

针对常规测井在低渗透油藏的水淹层解释中符合率低的问题,改变以往单纯用含油层段测井资料笼统计算含水饱和度来判断水淹层的方法,采用以流动单元为基础的新方法对水淹层进行了定量识别.首先从修正的Kozeny-Carman方程出发,将宝浪油田宝北区块的储层定量划分为5类流动单元;在此基础上将中子寿命测井结果与常规测井资料相结合,建立了不同流动单元下淡水水淹层和地层水水淹层的解释模型.用新井样点的含水饱和度数据进行了检验,运用模型计算出的含水饱和度与中子寿命测井获得的含水饱和度相比平均值只差3%.同时,对6口生产井的11个小层进行了生产测试验证,符合率达82%.     

低勘探程度区煤系致密气水平井地质导向技术及应用

在二维地震区、井控程度低及曲流河三角洲前缘沉积等复杂岩性储层中,实施水平井储层钻遇率较低。三维地质模型迭代更新与地质–工程一体化相结合的水平井地质导向技术应用逐渐广泛。鄂尔多斯盆地东南缘上古生界煤系发育山西组山₂ 3亚段北岔沟砂岩等多套致密砂岩储层,其中,大宁–吉县区块砂岩层厚度较薄、纵向岩性复杂、横向变化快、微幅构造发育,基本无出发井且末端井较远(大于1 500 m),水平井导向难度大。为了提高低勘探程度区水平井钻探效果,通过标志层识别与厘定、煤岩层对比、资料精准分析与补充、储层顶底特征识别等方法开展研究。首先,总结研究区煤层与炭质泥岩的快速区分方法,解决现场岩性识别困难的难题,构建研究区地层格架;而后,基于测井、地震、地质多种资料的地层精细对比,厘定1个Ⅰ级标志层及3个Ⅱ级标志层,在山₂ 3亚段进一步确定2个Ⅱ级标志层。最后,提出着陆点控制技术、水平段钻遇率提高技术与钻头出层判断方法,形成了三角洲前缘亚相储层水平井地质导向技术。将该技术应用于DJ-P37井区,累计完钻34口,平均砂岩钻遇率78.30%,取得了良好的钻探效果,可为类似区域煤系水平井钻进提供技术借鉴。      

致密低渗透砂岩储层多尺度天然裂缝表征方法

本文利用地表露头、岩芯、测井、三维地震和微观分析资料,根据多尺度裂缝的发育特征及不同资料的精度,探讨了多尺度裂缝的表征方法。致密低渗透砂岩储层发育多尺度天然裂缝,根据天然裂缝规模以及限制天然裂缝扩展延伸的岩石力学层界面,将天然裂缝分为大尺度裂缝、中尺度裂缝、小尺度裂缝和微尺度裂缝4级,不同尺度裂缝的发育特征存在较大差异。其中大尺度裂缝主要应用三维地震资料的叠后属性进行检测和表征;中小尺度裂缝在单井上可以应用岩芯、成像测井和常规测井相结合的方法进行表征,在平面上采用基于地质和测井约束的三维地震叠前各向异性分析以及储层地质力学相结合的方法进行综合预测和评价;微尺度裂缝一般应用铸体薄片、普通薄片、扫描电镜和三维CT扫描等微观分析方法进行观察描述,通过主控因素约束的方法进行预测。利用本文提出的多尺度裂缝表征方法实现了对鄂尔多斯盆地西南部致密砂岩储层多尺度裂缝表征,可为其它致密低渗透砂岩储层多尺度裂缝表征提供借鉴。     

致密低渗透砂岩储层多尺度天然裂缝表征方法

本文利用地表露头、岩芯、测井、三维地震和微观分析资料,根据多尺度裂缝的发育特征及不同资料的精度,探讨了多尺度裂缝的表征方法。致密低渗透砂岩储层发育多尺度天然裂缝,根据天然裂缝规模以及限制天然裂缝扩展延伸的岩石力学层界面,将天然裂缝分为大尺度裂缝、中尺度裂缝、小尺度裂缝和微尺度裂缝4级,不同尺度裂缝的发育特征存在较大差异。其中大尺度裂缝主要应用三维地震资料的叠后属性进行检测和表征;中小尺度裂缝在单井上可以应用岩芯、成像测井和常规测井相结合的方法进行表征,在平面上采用基于地质和测井约束的三维地震叠前各向异性分析以及储层地质力学相结合的方法进行综合预测和评价;微尺度裂缝一般应用铸体薄片、普通薄片、扫描电镜和三维CT扫描等微观分析方法进行观察描述,通过主控因素约束的方法进行预测。利用本文提出的多尺度裂缝表征方法实现了对鄂尔多斯盆地西南部致密砂岩储层多尺度裂缝表征,可为其它致密低渗透砂岩储层多尺度裂缝表征提供借鉴。     

Delineation of capture zones for municipal wells in fractured dolomite, Sturgeon Bay, Wisconsin, USA

A wellhead protection study for the city of Sturgeon Bay, Wisconsin, USA, demonstrates the necessity of combining detailed hydrostratigraphic analysis with groundwater modeling to delineate zones of contribution for municipal wells in a fractured dolomite aquifer. A numerical model (MODFLOW) was combined with a particle tracking code (MODPATH) to simulate the regional groundwater system and to delineate capture zones for municipal wells. The hydrostratigraphic model included vertical and horizontal fractures and high-permeability zones. Correlating stratigraphic interpretations with field data such as geophysical logs, packer tests, and fracture mapping resulted in the construction of a numerical model with five high-permeability zones related to bedding planes or facies changes. These zones serve as major conduits for horizontal groundwater flow. Dipping fracture zones were simulated as thin high-permeability layers. The locations of exposed bedrock and surficial karst features were used to identify areas of enhanced recharge. Model results show the vulnerability of the municipal wells to pollution. Capture zones for the wells extend several kilometers north and south from the city. Travel times from recharge areas to all wells were generally less than one year. The high seasonal variability of recharge in the study area made the use of a transient model necessary. Electronic Publication     

A new method of controlled-source frequency induction soundings

We suggest a new inversion method for frequency induction data implying the use of a new parameter, which has a simple analytical form in the case of a uniform subsurface. The new parameter is found from induction numbers measured in the field of a vertical magnetic dipole or a vertical magnetic dipole combined either with a horizontal electrical dipole or with a horizontal magnetic dipole. Compared with the classical methods, the new technique provides better resolved resistivity curves and faithful images of the subsurface at higher frequencies and smaller transmitter-receiver separations. The inversion algorithm is applied to amplitude and amplitude-phase data and provides reliable depth assignment of the detected resistivity layers in the latter case.     

哈得逊东河砂岩油藏外围水平井合理配产影响因素分析

哈得逊东河砂岩油藏外围水平井目前舍水上升较快,产油量出现明显递减.研究以有效抑制外围底水锥进和产量递减、提高原油采收率为目的,通过对比该油藏水平井生产情况与油层中夹层的发育程度及位置、水平井井轨迹和油水粘度比的关系,深入分析了影响该油藏外围水平井合理配产的因素,并利用了油藏数值模拟方法加以论证.研究认为,夹层发育较多、垂向渗透能力相对较低且夹层位于油水界面之上时,水平井配产可较高;水平井位于油层顶部,避水高度较高时,水平井配产可较高;油水粘度比较低时,水平井配产可较高.     

Estimation of petrophysical parameters using seismic inversion and neural network modeling in Upper Assam basin,India

Estimation of petrophysical parameters is an important issue of any reservoirs. Porosity, volume of shale and water saturation has been evaluated for reservoirs of Upper Assam basin, located in northeastern India from well log and seismic data. Absolute acoustic impedance (AAI) and relative acoustic impedance (RAI) are generated from model based inversion of 2-D post-stack seismic data. The top of geological formation, sand reservoirs, shale layers and discontinuities at faults are detected in RAI section under the study area. Tipam Sandstone (TS) and Barail Arenaceous Sandstone (BAS) are the main reservoirs, delineated from the logs of available wells and RAI section. Porosity section is obtained using porosity wavelet and porosity reflectivity from post-stack seismic data. Two multilayered feed forward neural network (MLFN) models are created with inputs: AAI, porosity, density and shear impedance and outputs: volume of shale and water saturation with single hidden layer. The estimated average porosity in TS and BAS reservoir varies from 30% to 36% and 18% to 30% respectively. The volume of shale and water saturation ranges from 10% to 30% and 20% to 60% in TS reservoir and 28% to 30% and 23% to 55% in BAS reservoir respectively.     

Assessment of natural gas hydrate reservoirs at Site GMGS3-W19 in the Shenhu area,South China Sea based on various well logs

To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19, extensive geophysical logging data and cores were analyzed to assess the reservoir properties. Sediment porosities were estimated from density, neutron, and nuclear magnetic resonance (NMR) logs. Both the resistivity and NMR logs were used to calculate gas hydrate saturations, the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data. The density porosity was closely in agreement with the core-derived porosity, and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates. The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity. For the gas hydrate reservoirs at GMGS3-W19, the porosity, gas hydrate saturation and free gas saturation was 52.7%, 42.7% and 10%, on average, respectively. The various logs provide different methods for the comprehensive evaluation of hydrate reservoir, which supports the selection of candidate site for gas hydrate production testing.©2022 China Geology Editorial Office.