Resistivity Log Data Research Articles

Частотная дисперсия электрофизических характеристик и электрическая анизотропия пород баженовской свиты по данным электрокаротажа

Частотная дисперсия электрофизических характеристик и электрическая анизотропия пород баженовской свиты по данным электрокаротажа

Estimate of gas hydrate saturations in the Krishna-Godavari basin, eastern continental margin of India, results of expedition NGHP-02

Abstract A total of 25 drill sites were established in the Krishna-Godavari (KG) and Mahanadi Basins along the eastern margin of India during the India National Gas Hydrate Program Expedition 02 (NGHP-02) with the goal to identify and characterize gas hydrate occurrences in sand-rich reservoir systems. The objective of this study is to characterize the gas hydrate bearing zones linked with two seismic inferred horizons (R1 and R2) and estimate gas hydrate saturations at two sites in Area B of the KG Basin: Sites NGHP-02-22 and NGHP-02-23. The upper seismic-imaged reservoir section (reflector R1) contains both pore-filling and fracture-filling gas hydrate, while the lower seismic-imaged reservoir section (reflector R2) has only pore-filling gas hydrate. Since electrical resistivity seems to be the most pronounced physical response to the presence of gas hydrate, we have analyzed the resistivity log data using Archie's empirical equation from two sites based on an isotropic reservoir model. In order to assess the gas hydrate saturation, we have also performed isotropic velocity modelling using the three-phase Biot equation (TPBE) and density log derived sediment porosities. At Site NGHP-02-22, the average gas hydrate saturation estimated using the resistivity log data and P-wave velocity modelling are 29% and 17%, respectively in the depth interval of 207–290 mbsf whereas at Site NGHP-02-23, the average gas hydrate saturation obtained from resistivity log data and P-wave velocity modelling are 25% and 60%, respectively in the depth interval of 271–288 mbsf. Estimated results are further validated with the pressure core derived gas hydrate saturations to ensure their reliability at the respective drill sites. It was found that P-wave velocity log-inferred gas hydrates show a slight mismatch with the pressure core derived hydrate saturations while, the electrical resistivity log based result depicts a major mismatch with the pressure core derived hydrate saturations. The reason for this discrepancy is likely due to isotropic resistivity analysis of the reservoir section having anisotropic properties.

Use of geophysical logs in hydrogeological studies and borehole designs: case study of Apapa coastal area, Lagos, Nigeria

The hydrogeological study of Apapa, a coastal part of Lagos, Nigeria, has been investigated using combined geophysical borehole logs, consisting of natural gamma and electrical resistivity components and ditch cutting samples. This study utilized data from 10 boreholes to delineate freshwater aquifer and proposes the best borehole design and construction for optimum groundwater developments in the study area. The electrical resistivity log data show that the depth to the freshwater interface varies across the study area occurring between 150 and 160 m and as shallow as 110 m in a borehole. The variations in depth of the freshwater interfaces are attributed to the extent of saline/brackish water contamination and distribution of rock types. Based on these differences and the inability to identify water quality from ditch cuttings, borehole geophysical logging is a vital tool in providing in situ subsurface information of the rock types and fluid contents in groundwater studies. This further demonstrates that geophysical borehole logs play an important part in borehole design for development of water wells.

Joint geophysical and geochemical evaluation of source rocks – A case study in Sayun-Masila basin, Yemen

Integration of geophysical and geochemical approaches is used for qualitative and quantitative evaluation of source rocks. The Upper Jurassic rocks in the Sayun-Masila basin are used as a case study due to its high hydrocarbon potentiality. Stratigraphically, these rocks could be differentiated from base to top into: Shuqra, Madbi and Nayfa formations. The total organic carbon (TOC) values were determined in the shale and/or carbonate intervals of these formations from four studied wells by the ΔlogR method using sonic, resistivity and gamma-ray log data. Then the discriminant analysis was applied in differentiating source from non-source rocks. Also, the effect of the burial and thermal histories on the organic material maturation and the oil and/or gas generation was studied through the application of two analytical methods, namely, the level of organic metamorphism (LOM) and the time-temperature index (TTI), depending on the corrected temperature logs integrated with the time and depth data. The reliability of the obtained results has been confirmed and combined with the results supplied from geochemical analyses. The Upper Jurassic sediments are found to be oil–prone source rock in a mature stage. The Madbi Formation is considered as the most effective source rock. The burial and thermal histories of the basin in four modeled wells showed that mature oil generation window and hydrocarbon expulsion would have been initiated in the depocenters from Upper Jurassic Madbi Formation source rock during Late Cretaceous to Middle Miocene time.

The effect of oil emplacement on quartz cementation in a deeply buried sandstone reservoir

Quartz is an important, porosity-occluding cement in sandstone reservoirs that have been subjected to elevated temperature (>80°C to 100°C) for a substantial period of time. The effect of oil emplacement on quartz cementation in reservoir sandstones is controversial; some studies have concluded that early oil emplacement can inhibit quartz cementation, leading to the preservation of porosity, whereas other studies have concluded that quartz cementation appears largely unaffected by oil emplacement. Here we have studied shallow marine, Upper Jurassic sandstones from Ula field, Norwegian North Sea, with reservoir temperatures of approximately 150°C, to determine whether oil emplacement had a significant impact on diagenesis with particular attention to quartz cementation. Following sedimentological description of cores, samples above and below the oil–water contact have been collected, adjacent to core-analysis plug points. These samples then underwent a series of studies, including petrographic point counting with a transmitted light microscope, scanning electron microscopy (SEM), backscattered electron microscopy, SEM-cathodoluminescence microscopy, and fluid-inclusion studies. These data were integrated with routine core-analysis and petrophysical log data. Density and resistivity log data have been used to determine the precise oil saturation of each sample studied. The distributions of all potential controls on porosity and permeability, such as grain size, sorting, matrix clay content, and degree of bioturbation, the presence of grain coatings and dolomite cement, and the amount of quartz cement, have been assessed. The presence of primary oil inclusions within quartz cement shows that oil ingress into the Ula reservoir commenced prior to the onset of quartz cementation. Very fine–grained, matrix-rich, bioturbated, microquartz-cemented sandstones have uniformly low quartz-cement contents irrespective of oil saturation. Medium-grained, graded, matrix-poor, microquartz-poor sandstones have quartz cement ranging from 1% to greater than 17%, associated with core porosities of approximately 22% and 7%, respectively. Higher oil saturations equate to higher porosities and permeabilities in the medium-grained, graded, matrix-poor, microquartz-poor sandstones, which cannot be explained by any control other than the amount of quartz cement as a function of pore fluid type. Oil emplacement therefore appears to have inhibited quartz cementation at high oil saturations and can be viewed as an important control on reservoir quality. The significance of this study is that the presence of oil in a sandstone reservoir at the time that quartz cement was growing can have a considerable impact on reservoir quality. Models that seek to predict quartz cement and reservoir quality in sandstones need to account for the timing of oil emplacement compared with other diagenetic processes.

Elastic parameters from constrained AVO inversion: Application to a BSR in the Mahanadi offshore, India

We have estimated P-wave velocities (VP) and thicknesses from traveltimes data, and S-wave velocities (VS) and densities (ρ) from amplitudes versus offset (AVO) data by constraining the VP for a set of seismic reflection data including a bottom simulating reflector or BSR (marker for gas hydrates) in the Mahanadi offshore. The elastic parameters (VP, VS and ρ) of the whole-layered model have been derived simultaneously using the genetic algorithm global optimization technique that converges into a global minimum. The result shows a low velocity (1506 m/s) free-gas zone below the BSR that lies at 241 m below the sea floor. The presence of underlying free-gas is further corroborated by the observation of increasing negative reflection coefficients with angles. The VP and VS for gas hydrates bearing sediments above the BSR are estimated to be 1655 and 468 m/s respectively, and match reasonably with the sonic log at site NGHP-01-08A passing through the seismic line. Gas-hydrates and free-gas have been qualitatively estimated as 7.0% and 0.9% using three-phase weighted equation of Lee and Gassmann equation respectively. The combined usage of VP and VS corresponding to a Poisson ratio of 0.4565 provides a better estimation of gas hydrates, which shows comparable result that has been estimated using the Simandoux equation to the resistivity log data at site NGHP-01-08A.

Electrical characteristics of rocks in fractured and caved reservoirs

The conductive paths formed by fractures and cave in complex reservoirs differ from those formed by pores and throats in clastic rocks. In this paper, a new formation model based on fractured and caved reservoirs is established, and the electrical characteristics of rocks are analyzed with different pore structures using resistance law to understand their effects on rock resistivity. The ratio of fracture width to cave radius (C e value) and fracture dip are employed to depict pore structure in this model. Our research shows that the electrical characteristics of rocks in fractured and caved reservoirs are strongly affected by pore structure and porous fluid distribution. Although the rock electrical properties associated with simple pore structure agree well with Archie formulae, the relationships between F and or between I and S w , in more complicated pore structures, are nonlinear in double logarithmic coordinates. The parameters in Archie formulae are not constant and they depend on porosity and fluid saturation. Our calculations suggest that the inclined fracture may lead to resistivity anisotropy in the formation. The bigger dip the inclining fracture has, the more anisotropy the formation resistivity has. All of these studies own practical sense for the evaluation of oil saturation using resistivity logging data.

Characterization and Prediction of the Gas Hydrate Reservoir at the Second Offshore Gas Production Test Site in the Eastern Nankai Trough, Japan

Following the world’s first offshore production test that was conducted from a gas hydrate reservoir by a depressurization technique in 2013, the second offshore production test has been planned in the eastern Nankai Trough. In 2016, the drilling survey was performed ahead of the production test, and logging data that covers the reservoir interval were newly obtained from three wells around the test site: one well for geological survey, and two wells for monitoring surveys, during the production test. The formation evaluation using the well log data suggested that our target reservoir has a more significant heterogeneity in the gas hydrate saturation distribution than we expected, although lateral continuity of sand layers is relatively good. To evaluate the spatial distribution of gas hydrate, the integration analysis using well and seismic data was performed. The seismic amplitude analysis supports the lateral reservoir heterogeneity that has a significant positive correlation with the resistivity log data at the well locations. The spatial distribution of the apparent low-resistivity interval within the reservoir observed from log data was investigated by the P-velocity volume derived from seismic inversion. The integrated results were utilized for the pre-drill prediction of the reservoir quality at the producing wells. These approaches will reduce the risk of future commercial production from the gas hydrate reservoir.

Shale volume and permeability of the Miocene unconsolidated turbidite sands of Bonga oil field, Niger delta, Nigeria

Bonga oil field is located 120km (75mi) southeast of the Niger Delta, Nigeria. It is a subsea type development located about 3500ft water depth and has produced over 330 mmstb of hydrocarbon till date with over 16 oil producing and water injection wells. The producing formation is the Middle to Late Miocene unconsolidated turbidite sandstones with lateral and vertical homogeneities in reservoir properties. This work, analysis the petrophysical properties of the reservoir units for the purpose of modeling the effect of shale content on permeability in the reservoir. Turbidite sandstones are identified by gamma-ray log signatures as intervals with 26-50 API, while sonic, neutron, resistivity, caliper and other log data are applied to estimate volume of shale ranging between 0.972 v/v for shale intervals and 0.0549 v/v for turbidite sands, water saturation of 0.34 v/v average in most sand intervals, porosity range from 0.010 for shale intervals to 0.49 v/v for clean sands and permeability values for the send interval 11.46 to2634mD, for intervals between 7100 to 9100 ft., Data were analyzed using the Interactive Petrophysical software that splits the whole curve into sand and shale zones and estimates among other petrophysical parameters the shale contents of the prospective zones. While Seismic data revealed reservoir thickness ranging from 25ft to over 140ft well log data within the five wells have identified sands of similar thickness and estimated average permeability of700mD. Within the sand units across the five wells, cross plots of estimated porosity, volume of shale and permeability values reveal strong dependence of permeability on shale volume and a general decrease in permeability in intervals with shale volume. It is concluded that sand units with high shale contents that are from0.500 to0.900v/v will not provide good quality reservoir in the field.

A novel method for estimation of remaining oil saturations in water-flooded layers

After long-term production by water injection, the conductivity features of oil reservoirs are changed, leading to difficulties in calculating remaining oil saturation by traditional well-logging techniques. There are some specific phenomena relating to petrophysical properties in water-flooded layers, and a deep investigation of them could help us understand resistivity changes of rocks during the water-injection process. We compared traditional mixed-fluid resistivity methods and determined their drawbacks. In addition, we revised the theoretical models and came up with a novel method to calculate mixed-fluid resistivity. The displacement process was divided into three stages: non-water-flooded stage, low and medium water-flooded stage, and high water-flooded stage. To study the relationships between rock resistivities and water saturations when injecting fresh-/saltwater, we used digital rock numerical methods. We first constructed 3D digital rock models based on the process-based method, and then we obtained oil and water distributions in the pore space by the pore morphology method. When resistivities of every component in digital rock models were assigned, rock resistivities could be calculated in different water saturations by the finite-element method. We found that in the case of freshwater injection, the relationship between water saturations and rock resistivities formed an S-shaped curve, but in the case of saltwater injection, if the injection water salinity was higher, the rock resistivity would be decreased by increasing the water saturation. In different rock heterogeneity and wettability conditions, these curves showed similar shapes and trends. Combined with resistivity logging data, this novel approach was used in the interpretation of a real oil well. Compared with the existing models in the interpretation practices, this new model better matched the real-field saturations and improved the accuracy of saturation interpretation in the studied water-flooded oil reservoir.

Feasibility of electromagnetic methods to detect and image steam-assisted gravity drainage steam chambers

We have investigated the use of electric and electromagnetic (EM) methods to monitor the growth of steam-assisted gravity drainage (SAGD) steam chambers. SAGD has proven to be a successful method for extracting bitumen from the Athabasca oil sands in Alberta, Canada. However, complexity and heterogeneity within the reservoir could impede steam chamber growth, thereby limiting oil recovery and increase production costs. Using seismic data collected over an existing SAGD project, we have generated a synthetic steam chamber and modeled it as a conductive body within the bitumen-rich McMurray Formation. Simulated data from standard crosswell electrical surveys, when inverted in three dimensions, show existence of the chamber but lack the resolution necessary to determine the shape and size. By expanding to EM surveys, our ability to recover and resolve the steam chamber is significantly enhanced. We use a simplified survey design procedure to design a variety of field surveys that include surface and borehole transmitters operating in the frequency or time domain. Each survey is inverted in three dimensions, and the results are compared. Importantly, despite the shielding effects of the highly conductive cap rock over the McMurray Formation, we have determined that it is possible to electromagnetically excite the steam chamber using a large-loop surface transmitter. This motivates a synthetic example, constructed using the geology and resistivity logging data of a future SAGD site, where we simulate data from single and multiple surface loop transmitters. We have found that even when measurements are restricted to the vertical component of the electric field in standard observation wells, if multiple transmitters are used, the inversion recovers three steam chambers and discerns an area of limited steam growth that results from a blockage in the reservoir. The effectiveness of the survey shows that this EM methodology is worthy of future investigation and field deployment.

Imaging Radial Distribution of Water Saturation and Porosity Near the Wellbore by Joint Inversion of Sonic and Resistivity Logging Data

Summary We present a work flow for joint inversion of sonic flexural-wave dispersion data and array-induction resistivity data acquired in a vertical well. The work flow estimates a pixel-based radial distribution of water saturation and porosity extending several feet into the formation at each log depth. Radial changes in saturation and porosity are caused by mud-filtrate invasion and mechanical damage, respectively. The flexural-wave and array-induction data have similar multiple investigation depths extending several feet into the formation. Furthermore, flexural-wave data are sensitive to porosity but have weak sensitivity to saturation, whereas induction data are sensitive to both porosity and saturation. Thus, integration of these data in a joint inversion can help to characterize the formation beyond the altered zone and reduce uncertainty of the interpretation. The work flow is validated on synthetic data for several scenarios of near-wellbore alteration. The work flow is then applied to field data from an offshore well drilled with oil-based mud in a gas-bearing clastic formation. The results are compared with traditional interpretation and core analysis, demonstrating an efficient and accurate inversion-based work flow that can complement traditional formation evaluation in challenging conditions.

Geostatistics-based regional characterization of groundwater chemistry in a sedimentary rock area with faulted setting

Sophisticated modeling of a hydrogeological structure and the environment are crucial for underground construction. This study characterizes hydrochemical properties of groundwater in a sedimentary rock area, clarifying their spatial distribution and correlation with geologic structure, and interpreting the groundwater source and chemical evolution. Water samples, geological columns, and well logs to 1-km depth were taken at 10 sites in Horonobe of northern Japan. Toward the objectives, a 3D model of Cl− concentration was produced in conjunction with resistivity logging data through kriging estimation and sequential Gaussian cosimulation. Variography shows that the dip of the main Omagari Fault is a control on the spatial correlation structure of Cl− concentration. The 3D model shows that this fault and its auxiliary constitute a clear boundary between high and low saline waters, and that Cl− concentrations tend to change in accord with sedimentary layer structures. The integration of stable isotope analysis suggests that deep saline water with heavy δD and δ18O originated from fossilized seawater, whereas shallow freshwater with light values is of meteoric water origin. Dilute saline water in the deep part of study area is partially attributable to dehydration of silica minerals. Vertical and lateral groundwater flows are estimated to prevail near the Omagari Fault and be general in other zones, respectively. Difference in the depth of transition zones may be caused by the dominant flow among downward, ascending, and lateral. Consequently, geostatistical techniques and data integration are useful to depict regional groundwater systems with a data set of water investigation limited by quantity and location.

A general approach for porosity estimation using artificial neural network method: a case study from Kansas gas field

This study aims to design a back-propagation artificial neural network (BP-ANN) to estimate the reliable porosity values from the well log data taken from Kansas gas field in the USA. In order to estimate the porosity, a neural network approach is applied, which uses as input sonic, density and resistivity log data, which are known to affect the porosity. This network easily sets up a relationship between the input data and the output parameters without having prior knowledge of petrophysical properties, such as porefluid type or matrix material type. The results obtained from the empirical relationship are compared with those from the neural network and a good correlation is observed. Thus, the ANN technique could be used to predict the porosity from other well log data.

시추 및 물리탐사를 이용한 석회암 공동의 분포 규모 분석

본 연구에서는 석회암 공동이 분포하는 지역에서 발생한 지반침하 현상의 원인을 파악하고 공동의 분포 위치 및 규모를 분석하기 위하여 지표지질조사, 시추조사, 전기비저항, 토모그래피 탐사 등을 실시하였다. 지표지질조사 및 시추조사 결과에 의하면 연구지역에서 석회암은 방해석의 함량에 따라 고함유대와 저함유대가 교호하며 분포한다. 시추조사 결과를 중심으로 전기비저항 및 탄성파 토모그래피 탐사 결과와 비교분석한 결과에 의하면 대부분의 공동은 지반침하가 발생한 지점을 중심으로 심도 7~14 m의 토사층과 암반의 경계부 주변에 집중되어 분포하고 있다. 또한 공동의 규모는 558~835 ㎥로 소규모이며, 침하의 형태는 사질토 지반에서 흔히 발생하는 Suffosion sinkhole에 속한다. Geological mapping, borehole drilling, electrical resistivity, and seismic tomography surveys were conducted in order to map underground cavities and better understand the mechanisms driving subsidence in a limestone region in Korea. Limestone outcrops in the study area generally alternate between calcite-rich and calcite-poor rock. The results reveal that in areas experiencing subsidence, cavities occur mainly around soil-rock boundaries at depths of 7~14 m. These results are based on comparative analyses of electrical resistivity, seismic tomography, and borehole logging data. The volumes of the cavities are relatively small in a range of 558~835 ㎥ and they have a shape typical of suffosion sinkholes, which are typically found where sandy soils overlie bedrock cavities.

Gas hydrate saturation from seismic data constrained by log data in the Krishna-Godavari Basin

A strong bottom simulating reflector (BSR) was observed along seismic line transect across site NGHP-01-05 having characteristic reverse polarity with respect to seafloor, cross-cutting of dipping strata, and high reflectivity below the BSR. The observed BSR depth along seismic profile varies from ~100 to ~125 mbsf. Gas hydrate was recovered during coring and drilling at this site during National Gas Hydrate Drilling Expedition-01 of India. Recovered pressure cores at discrete depths within gas hydrate stability zone from site NGHP-01-05 shows ~0.6–9.4 % pore volume of methane hydrate. Gas hydrate saturation was estimated along seismic profile constrained by observed velocity and density log to introduce model-based acoustic impedance inversion. It was observed that each stacked trace amplitude converted to acoustic log as a product of velocity and density. During the impedance inversion process inverted velocity log was observed at each and every trace act as an individual log. Gas hydrate saturation was estimated from inverted velocity coupled with effective medium rock physics modeling. The estimated gas hydrate saturation from the post-stack acoustic impedance inversion velocity along 2D seismic profile varies up to 15–20 % of the pore space. The gas hydrate saturation estimated from electrical resistivity log and core data shows maximum 20 % pore spaces saturated with gas hydrate from anisotropic modeling, which is comparable to gas hydrate saturation along seismic profile estimated from inverted velocity constrained by logs.

Poisson’s Ration, Deep Resistivity and Water Saturation Relationships for Shaly Sand Reservoir, SE Sirt, Murzuq and Gadames Basins, Libya (Case study)

It is important to obtain relationships between the physical quantities, overburden and reservoir composition and fluid type. This is significant in the sense that if one property, e.g., electrical resistivity, can be more easily measured than Poisson’s ratio (PR). Therefore, later parameter can be estimated and defined against resistivity log data. In fact, these relations constructed by several wells data have been taken for each studied productive reservoir from different oil fields at different sedimentary basin, in Libya. However, comparison between calculated PR, measured deep resistivity and calculated water saturation content are using to a certain extent justification of reservoir conditions (tight zone). These cross relations throw up the increase of PR range at low values of deep resistivity values and water saturation degrees, which present like a hyperbolic curves formed a two parts. The stable trend with constant PR values in hydrocarbon and clean intervals depths within a first part, while a shaly depth intervals act as a second part of the hyperbolic shape, which shows a scatter or cluster points indicates of water intervals and not a tight zone. Therefore, estimation of PR in shaly intervals in such these reservoirs are ranging above 0.3 up to 0.4.

The effect of drilling mud properties on shallow lateral resistivity logging of gas hydrate bearing sediments

Resistivity logging is one of the most important ways of identifying and estimating the saturation level of gas hydrates in permafrost and ocean regions. in practical drilling operations, resistivity loggings, especially shallow lateral resistivity logging in gas hydrate bearing sediments (ghbs), are likely to be affected by drilling mud invasions. here, we use available data from site measurements to construct a two-dimensional model for hydrate reservoirs around the borehole sh2, a hole that was drilled during the first expedition in shenhu area of south china sea to examine and drill into gas hydrates. we then use this model to investigate the characteristics of drilling mud invasions and the effect of drilling mud properties (e.g., temperature, density, and salinity) on resistivity logging using a numerical simulation. this simulation and associated calculations indicate that shallow lateral resistivity logging is significantly affected by variations in drilling mud temperature, which leads to hydrate dissociation and the formation of secondary hydrates. increasing drilling mud salinity accelerates hydrate dissociation, and has a greater effect on shallow lateral resistivity logging than the free gas produced during drilling and any potential mud density influence, which is generally dependent on the depth at which the drilling mud invasion occurred. this means that future drilling operations should focus on ensuring that the temperature, salinity, and density of drilling muds remain within a reasonable range in order to minimize the effect of mud invasions on resistivity logging data. (c) 2014 elsevier b.v. all rights reserved.

Detecting different water table levels in a shallow aquifer with combined P-, surface and SH-wave surveys: Insights from VP/VS or Poisson's ratios

When applied to hydrogeology, seismic methods are generally confined to the characterisation of aquifer geometry. The joint study of pressure- (P) and shear- (S) wave velocities (VP and VS) can provide supplementary information and improve the understanding of aquifer systems. This approach is proposed here with the estimation of VP/VS ratios in a stratified aquifer system characterised by tabular layers, well-delineated thanks to Electrical Resistivity Tomography, log and piezometer data. We carried out seismic surveys under two hydrological conditions (high and low flow regimes) to retrieve VS from both surface-wave dispersion inversion and SH-wave refraction interpretation, while VP were obtained from P-wave refraction interpretation. P-wave first arrivals provided 1D VP structures in very good agreement with the stratification and the water table level. Both VS models are similar and remain consistent with the stratification. The theoretical dispersion curves computed from both VS models present a good fit with the maxima of dispersion images, even in areas where dispersion curves could not be picked. Furthermore, VP/VS and Poisson's ratios computed with VS models obtained from both methods show a strong contrast for both flow regimes at depths consistent with the water table level, with distinct values corresponding to partially and fully saturated sediments.

Evaluation of the coal reservoir permeability using well logging data and its application in the Weibei coalbed methane field, southeast Ordos basin, China

Permeability is one of the most critical reservoir evaluation parameters in coalbed methane (CBM) exploration and development. In this study, an improved collection of sheets model was proposed for evaluation of reservoir permeability of the no. 5 coal seam in the Weibei CBM field, southeast Ordos basin, China. The evaluation model is expressed by k = 8.50 × 10− 4 nw 2 ϕ f in which the fracture width (w), fracture density (n), and porosity (φ f) can be obtained by analysis of the deep lateral resistivity logging and micro-sphere focusing logging data. The calculated permeability values are ranging from 0.005 to 2.327 mD across the Weibei coalfield. The regional distribution of permeability is mainly controlled by the factors of burial depth and tectonics stress. The permeability is high in the shallow area and the zone of normal faults, but it is low in the anticline area and the upthrust side of thrust faults. The results show that the calculation method developed in this study is reliable, providing a better understanding of the coal reservoir for CBM exploration in the study area.