Logging-while-drilling Logs Research Articles

A method to indicate heavy minerals in logging-while-drilling

The low-energy region of the gamma ray spectrum, less than 200 keV, often is either discarded or not analyzed in borehole logging or airborne radiometric surveys. There is much useful information about earth properties to be extracted from the low-energy region because it is sensitive to photoelectric absorption within the rock, a process that is highly sensitive to the atomic number of the elements within the rock. In addition, the rate of photoelectric absorption is proportional to the density of the rock. At higher energies, the spectrum is primarily sensitive to density variations only. The ratio of counts in the two energy regions may be used as a heavy mineral indicator (HMI). This ratio normalizes density effects and is primarily sensitive to variations in the average (or effective) atomic number ([Formula: see text]) of the rock. Previous work by other authors has focused on the application of this idea for wireline logging with an isotopic source and multiple scattering of copious primary gamma rays, which allows for good counting statistics in the recorded spectrum. Recent advances in gamma ray detectors and logging-while-drilling (LWD) techniques open the possibility of applying the HMI measurement to the natural gamma ray LWD logs. Drilling speeds are much slower than the running speeds of wireline tools, resulting in improved counting statistics. Using computer simulations, laboratory experiments, and field measurements, we find that the natural gamma ray spectrum, recorded while (diamond) drilling, provides sufficient counting statistics to be used for robustly calculating HMI. As iron is the most abundant and influential element in photoelectric gamma ray scattering processes in rock fabrics, we postulate that we can, in most cases, use HMI to indicate the iron content of the rocks. The HMI calculated under these conditions provides a good measure of changes in [Formula: see text] of rocks and correlates well with iron-rich sections. Our field and laboratory tests demonstrate that the HMI measures can be used as a proxy to separate many metalliferous ores from waste based on the differences in [Formula: see text].

Geological features, controlling factors and potential prospects of the gas hydrate occurrence in the east part of the Pearl River Mouth Basin, South China Sea

Logging-while-drilling (LWD) and wireline log (CWL) data were acquired during China's second gas hydrate drilling expedition (GMGS-2) in the east of Pearl River Mouth Basin, South China Sea. Disseminated and massive gas hydrates deposits were found at different sites. Gas hydrate-bearing lithologies identified from the sample coring included the fine-grained sediments and coarse-grained sediments. LWD logs from Site GMGS2-08 indicate significant gas hydrate in clay-bearing sediments including two layers with massive gas hydrate with a bulk density near to 1.08 g/cm3. High electrical resistivities with a range of 2.5–2000.0 Ω m and high P-wave velocities are simultaneously observed in the hydrate-bearing sediments. The average gas hydrate saturation estimated from the pore water freshing analysis ranges from 45 to 55% of the pore space. Buried carbonate layers above the massive gas hydrate deposit discovered at Sites GMGS2-08 indicate that the formations are likely to have formed initially at the surface and then were buried. Significant high amplitude seismic anomalies, discontinuous bottom simulating reflection (BSR) and blanking zone are detected in the drilling zone. The hydrate-bearing sediments predominantly consist of silty clay and limestone grains in which the gas hydrates are deposited primarily in the form of laminated, massive, veins or nodule. The gas hydrates occurrences are subjected to the sediment lithology, new tectonic activities, migration of fluid and gas and also the factors such as heat flow, salinity and time which affect the nucleation of gas hydrates. Its natural morphologies present massive, laminated, nodular, nugget and disseminated, of which the former four often formed in shallow fault, inter-layer's weak cementation zone and on the seabed. The “buried” gas hydrates with high saturation are good zones for gas hydrate exploitation.

Logging-while-drilling and wireline velocities: Site NGHP-01-10, Krishna–Godavari Basin, India

At Site NGHP-01-10, Krishna–Godavari Basin, India, the downhole wireline logging tool measured higher porosity and sonic velocities in gas hydrate-bearing sediments compared to the logging-while-drilling (LWD) acquired data in two closely spaced wells (∼10 m). Using a rock physics model that assumes random fractures in unconsolidated sediments using Hashin–Shtrikman bounds, we show that difference in physical properties could be due to intra-site difference in the distribution and pattern of hydrate-filled fractures rather than differences in the volume of gas hydrate. Our fracture-inclusive model suggests that between the two holes the porosity and hydrate saturation of the background sediments is similar while porosity and hydrate saturation of the fracture systems change considerably, resulting in the change of the sonic log responses. Relative changes in hydrate saturation between sediments and fractures at 90 m below the sea floor (mbsf) depth (at a prominent seismic horizon), suggests that fracture-filling hydrate could have partly originated as pore-filling material in the sediments and grew as fracture-filling material as a result of fluid flow. The volume fracture profile of hydrate (fracture porosity × fracture saturation) between the two holes further suggests that the higher concentration of fractures in the wireline hole may not be a natural in-situ condition but rather a manifestation of the drilling compounded with time lapse in data recording between the LWD and wireline log data. Frequently reported, worldwide, intra-site variability in hydrate saturation from LWD and wireline logs in close proximity could be more related to interpretative methods than actual geologic variability.

Characteristics and interpretation of fracture-filled gas hydrate – An example from the Ulleung Basin, East Sea of Korea

Through the use of 2-D and 3-D seismic data, a total of thirteen sites were selected and drilled in the East Sea of Korea in 2010. A suite of logging-while-drilling (LWD) logs was acquired at each site. LWD logs from the UBGH2-3A well indicate significant gas hydrate in clay-bearing sediments including several zones with massive gas hydrate with a bulk density less than 1.0 g/m3 for depths between 5 and 103 m below the sea floor. The UBGH2-3A well was drilled on a seismically identified chimney structure with a mound feature at the sea floor. Average gas hydrate saturations estimated from the isotropic analysis of ring resistivity and P-wave velocity logs are 80 ± 13% and 47 ± 16%, respectively, whereas they are 46 ± 17% and 45 ± 16%, respectively from the anisotropic analysis. Modeling indicates that the upper part of chimney (between 5 and 45 m below sea floor [mbsf]) is characterized by gas hydrate filling near horizontal fractures (7° dip) and the lower part of chimney (between 45 and 103 mbsf) is characterized by gas hydrate filling high angle fractures on the basis of ring resistivity and P-wave velocity. The anisotropic analysis using P40H resistivity (phase shift resistivity at 32 mHz with 40 inch spacing) and the P-wave velocity yields a gas hydrate saturation of 46 ± 15% and 46 ± 15% respectively, similar to those estimated using ring resistivity and P-wave velocity, but with quite different fracture dip angles. Differences in vertical resolution, depth of investigation, and a finite fracture dimension relative to the tool separation appear to contribute to this discrepancy. Forward modeling of anisotropic resistivity and velocity are essential to identify gas hydrate in fractures and to estimate accurate gas hydrate amounts.

Scale-dependent gas hydrate saturation estimates in sand reservoirs in the Ulleung Basin, East Sea of Korea

Through the use of 2-D and 3-D seismic data, several gas hydrate prospects were identified in the Ulleung Basin, East Sea of Korea and thirteen drill sites were established and logging-while-drilling (LWD) data were acquired from each site in 2010. Sites UBGH2–6 and UBGH2–10 were selected to test a series of high amplitude seismic reflections, possibly from sand reservoirs. LWD logs from the UBGH2–6 well indicate that there are three significant sand reservoirs with varying thickness. Two upper sand reservoirs are water saturated and the lower thinly bedded sand reservoir contains gas hydrate with an average saturation of 13%, as estimated from the P-wave velocity. The well logs at the UBGH2–6 well clearly demonstrated the effect of scale-dependency on gas hydrate saturation estimates. Gas hydrate saturations estimated from the high resolution LWD acquired ring resistivity (vertical resolution of about 5–8 cm) reaches about 90% with an average saturation of 28%, whereas gas hydrate saturations estimated from the low resolution A40L resistivity (vertical resolution of about 120 cm) reaches about 25% with an average saturation of 11%. However, in the UBGH2–10 well, gas hydrate occupies a 5-m thick sand reservoir near 135 mbsf with a maximum saturation of about 60%. In the UBGH2–10 well, the average and a maximum saturation estimated from various well logging tools are comparable, because the bed thickness is larger than the vertical resolution of the various logging tools. High resolution wireline log data further document the role of scale-dependency on gas hydrate calculations.

Anisotropic models to account for large borehole washouts to estimate gas hydrate saturations in the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II Alaminos Canyon 21 B well

Through the use of 3-D seismic amplitude mapping, several gas hydrate prospects were identified in the Alaminos Canyon (AC) area of the Gulf of Mexico. Two locations were drilled as part of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) in May of 2009 and a comprehensive set of logging-while-drilling (LWD) logs were acquired at each well site. LWD logs indicated that resistivity in the range of ∼2 ohm-m and P-wave velocity in the range of ∼1.9 km/s were measured in the target sand interval between 515 and 645 feet below sea floor. These values were slightly elevated relative to those measured in the sediment above and below the target sand. However, the initial well log analysis was inconclusive regarding the presence of gas hydrate in the logged sand interval, mainly because large washouts caused by drilling in the target interval degraded confidence in the well log measurements. To assess gas hydrate saturations in the sedimentary section drilled in the Alaminos Canyon 21 B (AC21-B) well, a method of compensating for the effect of washouts on the resistivity and acoustic velocities was developed. The proposed method models the washed-out portion of the borehole as a vertical layer filled with sea water (drilling fluid) and the apparent anisotropic resistivity and velocities caused by a vertical layer are used to correct the measured log values. By incorporating the conventional marine seismic data into the well log analysis, the average gas hydrate saturation in the target sand section in the AC21-B well can be constrained to the range of 8–28%, with 20% being our best estimate.

科学掘削におけるメタンハイドレートの物理検層解析：熊野堆積盆地の例

During Integrated Ocean Drilling Program (IODP) Expedition 314 at Nankai Trough, located on the sudbuction margin offshore Kii Peninsula, five sites were drilled and logged with logging-while-drilling (LWD). A 935-m-thick basin fill was penetrated and logged in Kumano Forearc Basin at Site C0002. The basin fill consists of repeated turbidite sequences intercalated with background hemipelagic mud. A gas hydrate-bearing zone was identified in the interval of 218-401 mbsf above a bottom-simulating reflector (BSR) by indirect evidences of electrical resistivities and P-wave velocities. More than sixty sharp resistivity spikes were identified and most of them are developed in sand layers. Thicker hydrate layers are localized near the BSR and frequency of hydrate occurrence is decreasing upward. Below the BSR, a number of turbidite sequences were identified and the thickest turbidite zone occurs at 482-547 mbsf as a possible gas/fluid conduit to produce hydrates. Distribution of turbidite sequence plays an essential role as potential conduits and containers for gas hydrate generation in Kumano Basin. Detailed observations of the LWD logs demonstrate a possible lithological control on gas migration and systematic distribution of gas hydrates in turbidite-dominant marginal basins.

A new look at multiphase invasion with applications to borehole resistivity interpretation

In well log interpretation, it is frequently necessary to correct logs for invasion. Invasion occurs in permeable formations when there is a radial differential pressure (RDP) between the borehole and formation. Other factors on which invasion depend include saturation, mobility, pressure (RDP) and capillary pressure, permeability and viscosity of fluids, and temperature transient effects associated with the mud filtrate injected into the formation. Thus, simulation of realistic invasion is not an easy task. This work reviews the famous Buckley–Leverett mathematical model in cylindrical coordinates appropriate for borehole geometries. The model predicts multiphase invasion in porous media when gravity, capillary pressure, and mud cake can be neglected. One application is to correct logging while drilling (LWD) and wireline resistivity logs for time-dependent invasion and formation temperature effects. This is important, for example, when there are possible large differences in formation and mud temperature. Modeling studies show these effects can be large enough to noticeably influence resistivity logs. However, after correction, difference in LWD and wireline logs arising from the time-dependent heat process are explained. Thus, the method, when coupled to a time-dependent heat flow model, and a response function formulation of resistivity, yields new insight into the influence of thermal and electrical transients in log interpretation.

Porosity‐formation factor and porosity‐velocity relationships in Barbados prism

Ocean Drilling Program Leg 171A collected logging while drilling (LWD) data to investigate the porosity and velocity structure of Barbados accretionary complex. Porosity and velocity measurements were needed for interpretation of prism dynamics. However, the LWD tool string did not measure velocity, and previous velocity data from Barbados prism are scarce and generally of low quality. Furthermore, most density‐based porosities are subject to errors associated with variations in smectite content. To determine porosities more accurately, we use smectite‐corrected, density‐based porosities to calibrate a porosity‐formation factor (FF) transform, then calculate porosities from resistivity logs. We used cation exchange capacity (CEC) measurements and LWD logs to evaluate the possibility that resistivity is affected by clay conduction. The CEC data warn of potential clay conduction, especially in smectite‐rich zones, but LWD data indicate that clay conduction is minor in these high‐porosity sediments. Instead, tortuosity is the secondary control of FF, after porosity. By comparing resistivity‐based porosities to velocities from vertical seismic profiles, we determine a porosity‐velocity transform applicable to Barbados. We then use this transform to calculate velocity logs from resistivity‐based porosity logs.