Mexico Gas Hydrate Joint Industry Research Articles

Archie's Saturation Exponent for Natural Gas Hydrate in Coarse‐Grained Reservoirs

AbstractAccurately quantifying the amount of naturally occurring gas hydrate in marine and permafrost environments is important for assessing its resource potential and understanding the role of gas hydrate in the global carbon cycle. Electrical resistivity well logs are often used to calculate gas hydrate saturations, Sh, using Archie's equation. Archie's equation, in turn, relies on an empirical saturation parameter, n. Though n = 1.9 has been measured for ice‐bearing sands and is widely used within the hydrate community, it is highly questionable if this n value is appropriate for hydrate‐bearing sands. In this work, we calibrate n for hydrate‐bearing sands from the Canadian permafrost gas hydrate research well, Mallik 5L‐38, by establishing an independent downhole Sh profile based on compressional‐wave velocity log data. Using the independently determined Sh profile and colocated electrical resistivity and bulk density logs, Archie's saturation equation is solved for n, and uncertainty is tracked throughout the iterative process. In addition to the Mallik 5L‐38 well, we also apply this method to two marine, coarse‐grained reservoirs from the northern Gulf of Mexico Gas Hydrate Joint Industry Project: Walker Ridge 313‐H and Green Canyon 955‐H. All locations yield similar results, each suggesting n ≈ 2.5 ± 0.5. Thus, for the coarse‐grained hydrate bearing (Sh > 0.4) of greatest interest as potential energy resources, we suggest that n = 2.5 ± 0.5 should be applied in Archie's equation for either marine or permafrost gas hydrate settings if independent estimates of n are not available.

Mapping the resistivity structure of Walker Ridge 313 in the Gulf of Mexico using the marine CSEM method

A marine controlled source electromagnetic (CSEM) campaign was carried out in the Gulf of Mexico to further develop marine electromagnetic techniques in order to aid the detection and mapping of gas hydrate deposits. Marine CSEM methods are used to obtain an electrical resistivity structure of the subsurface which can indicate the type of substance filling the pore space, such as gas hydrates which are more resistive. Results from the Walker Ridge 313 study (WR 313) are presented in this paper and compared with the Gulf of Mexico Gas Hydrate Joint Industry Project II (JIP2) logging while drilling (LWD) results and available seismic data. The hydrate, known to exist within sheeted sand deposits, is mapped as a resistive region in the two dimensional (2D) CSEM inversion models. This is consistent with the JIP2 LWD resistivity results. CSEM inversions that use seismic horizons provide more realistic results compared to the unconstrained inversions by providing sharp boundaries and architectural control on the location of the resistive and conductive regions in the CSEM model. The seismic horizons include: 1) the base of the gas hydrate stability zone (BGHSZ), 2) the top of salt, and 3) the top and bottom of a fine grained marine mud interval with near vertical hydrate filled fractures, to constrain the CSEM inversion model. The top of salt provides improved location for brines, water saturated salt, and resistive salt. Inversions of the CSEM data map the occurrence of a ‘halo’ of conductive brines above salt. The use of the BGHSZ as a constraint on the inversion helps distinguish between free gas and gas hydrate as well as gas hydrate and water saturated sediments.

Permeability and porosity of hydrate-bearing sediments in the northern Gulf of Mexico

Hydrate-bearing sands are being actively explored because they contain the highest concentrations of hydrate and are the most economically recoverable hydrate resource. However, relatively little is known about the mechanisms or timescales of hydrate formation, which are related to methane supply, fluid flux, and host sediment properties such as permeability. We used logging-while-drilling data from locations in the northern Gulf of Mexico to develop an effective medium theory-based model for predicting permeability based on clay-sized sediment fraction. The model considers permeability varying between sand and clay endpoint permeabilities that are defined from laboratory data. We verified the model using permeability measurements on core samples from three boreholes, and then used the model to predict permeability in two wells drilled in Walker Ridge Block 313 during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II expedition in 2009. We found that the cleanest sands (clay-sized fraction <0.05) had intrinsic (hydrate-free) permeability contrasts of 5–6 orders of magnitude with the surrounding clays, which is sufficient to provide focused hydrate formation due to advection of methane from a deep source or diffusion of microbial methane from nearby clay layers. In sands where the clay-sized fraction exceeds 0.05, the permeability reduces significantly and focused flow is less pronounced. In these cases, diffusion of dissolved microbial methane is most likely the preferred mode of methane supply for hydrate formation. Our results provide important constraints on methane supply mechanisms in the Walker Ridge area and have global implications for evaluating rates of methane migration and hydrate formation in hydrate-bearing sands.

Review of progress in evaluating gas hydrate drilling hazards

Shallow drilling hazard assessment consists of geological/geophysical review of drill sites to enable the avoidance or effective mitigation of problems that can affect the safe drilling of the “top-hole” section of wells. Primary seafloor hazards include infrastructure, hardgrounds, chemosynthetic communities, and areas of high slope or potential seafloor instability. Shallow subsurface hazards include strata that may contain overpressured water, free gas, or gas hydrate. Among these issues, the potential hazards posed by gas hydrate have historically been the most difficult to quantify and constrain. This paper reviews the history of gas hydrate shallow hazard assessment, much of which has been framed by difficulties in remote detection and quantification of typical low-saturation, mud-hosted hydrate occurrences. Recent drilling results indicate that such accumulations can be safely drilled using existing industry protocols. However, assessment of drilling hazards associated with high saturation, sand-hosted hydrates warrant further investigation, and therefore was a key focus of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) program conducted in 2009. Pre-drill site hazard assessment for JIP Leg II utilized standard industry methods in which geohazards associated with gas hydrate and free gas were identified in a primarily-qualitative manner using seismic amplitude analysis, seismic stratigraphic interpretation, and geo-pressure analysis. While these methods can reliably detect shallow hydrocarbons, including highly-concentrated gas hydrates, they are not sufficient to determine if free gas may exist below gas hydrate, particularly where they occur in close vertical succession. Such undetected free gas occurrences can pose potential drilling hazards that can be quantitatively assessed using advanced seismic methods.

Rock physics-based seismic trace analysis of unconsolidated sediments containing gas hydrate and free gas in Green Canyon 955, Northern Gulf of Mexico

The gas hydrate petroleum system at the 2009 Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) Green Canyon 955 (GC955) site shows a complex seismic amplitude and waveform response of highly negative and positive amplitudes with continuous and discontinuous character within inferred gas-hydrate- and gas-bearing sand reservoirs. Logging-while-drilling (LWD) data obtained during JIP Leg II and conventional 3-D seismic data allowed for the identification of thick highly concentrated hydrate layers by integrating rock physics modeling, amplitude and thin layer analysis, and spectral decomposition. Rock physics modeling with constraints from three JIP LWD holes allowed for the analysis of variations in acoustic amplitude characteristics as a product of hydrate saturation, gas saturation, and reservoir thickness. Using the well log-derived acoustic models, thick highly concentrated gas hydrate with and without underlying free gas accumulations have been identified. These results suggest that thick highly concentrated gas-hydrate-bearing sand units (with thicknesses greater than half of the seismic tuning thickness and gas hydrate saturations greater than 50%) underlain by gas can be differentiated from sands containing only gas, but thin gas-hydrate-bearing sand units with low gas hydrate concentrations (with thicknesses less than half of the seismic tuning thickness and gas hydrate saturations less than 50%) are difficult to identify from post-stack seismic amplitude data alone. Within GC955, we have identified six zones with seismic amplitude anomalies interpreted as being caused by gas hydrate deposits with variable lateral extent, thickness and saturation, and in some cases overlying free-gas-bearing intervals. Synthetic seismic images produced from well-log- and model-derived velocity and density distributions mimic similar reflection characteristics in the corresponding field seismic data.

Estimating saturation of gas hydrates using conventional 3D seismic data, Gulf of Mexico Joint Industry Project Leg II

We present a methodology for generating pre-drill estimations of hydrate saturations using conventional 3D seismic data. These seismic-based estimates will be compared with well log derived saturations from the subsequently drilled wells of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) expedition.Predicting saturation of gas hydrates (Sh-seismic) combines pre-stack seismic inversion, rock physics modeling and stratigraphic interpretation. Before the wells were drilled, no nearby sonic and density logs were available to define and calibrate the elastic property trends for the shallow target interval containing the gas hydrate stability zone. Therefore, rock property trends were established by applying principles of rock physics and shallow sediment compaction, constrained by known regional geological parameters. Sh-seismic volumes were generated by inverting pre-stack data to acoustic impedance (PI) and shear impedance (SI) volumes, and then analyzing deviations from modeled impedance trends. To enhance the quality of the inversion, the signal-to-noise ratio of the offset data was maximized by conditioning the seismic prior to inversion. Seismic stratigraphic interpretation plays an important role by identifying the more promising strata and structures for the presence of gas hydrates.The pre-drill Sh-seismic results are compared with saturations calculated from log data, Sh-log, of the wells drilled in the JIP Leg II campaign. Due to weaker seismic reflections, predictions may be less accurate for low concentrations, such as saturations less than 40%, and for thin intervals below the vertical resolution of the seismic data (about 15m). However, the integrated geophysical workflow is very effective for identifying and quantifying significant hydrate concentrations, making the method a promising prospecting technique.

Electrical anisotropy of gas hydrate-bearing sand reservoirs in the Gulf of Mexico

We present new results and interpretations of the electrical anisotropy and reservoir architecture in gas hydrate-bearing sands using logging data collected during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II. We focus specifically on sand reservoirs in Hole Alaminos Canyon 21 A (AC21-A), Hole Green Canyon 955 H (GC955-H) and Hole Walker Ridge 313 H (WR313-H). Using a new logging-while-drilling directional resistivity tool and a one-dimensional inversion developed by Schlumberger, we resolve the resistivity of the current flowing parallel to the bedding, R‖ and the resistivity of the current flowing perpendicular to the bedding, R⊥. We find the sand reservoir in Hole AC21-A to be relatively isotropic, with R‖ and R⊥ values close to 2Ωm. In contrast, the gas hydrate-bearing sand reservoirs in Holes GC955-H and WR313-H are highly anisotropic. In these reservoirs, R‖ is between 2 and 30Ωm, and R⊥ is generally an order of magnitude higher.Using Schlumberger’s WebMI models, we were able to replicate multiple resistivity measurements and determine the formation resistivity the gas hydrate-bearing sand reservoir in Hole WR313-H. The results showed that gas hydrate saturations within a single reservoir unit are highly variable. For example, the sand units in Hole WR313-H contain thin layers (on the order of 10–100cm) with varying gas hydrate saturations between 15 and 95%. Our combined modeling results clearly indicate that the gas hydrate-bearing sand reservoirs in Holes GC955-H and WR313-H are highly anisotropic due to varying saturations of gas hydrate forming in thin layers within larger sand units.

Architecture of gas-hydrate-bearing sands from Walker Ridge 313, Green Canyon 955, and Alaminos Canyon 21: Northern deepwater Gulf of Mexico

Logging-while-drilling data acquired during the 2009 Gulf of Mexico (GoM) Gas Hydrate Joint Industry Project Leg II program combined with features observed in seismic data allow assessment of the depositional environment, geometry, and internal architecture of gas-hydrate-bearing sand reservoirs from three sites in the northern Gulf of Mexico (GoM): Walker Ridge 313, Alaminos Canyon 21, and Green Canyon 955. The site descriptions assist in the understanding of the geological development of gas-hydrate-bearing sands and in the assessment of their energy production potential. Three sand-rich units are described from the Walker Ridge site, including multiple ponded sand-bodies representing turbidite channel and associated levee and terminal lobe environments within the Terrebonne basin on the lower slope of the GoM. Older units display fewer but greater-reservoir-quality channel and proximal levee facies as compared to thinner, more continuous, and unconfined sheet-like sands that characterize the younger units, suggesting a decrease in depositional gradient with time in the basin. The three wells in the Green Canyon 955 site penetrated proximal levee sands within a previously recognized Late Pleistocene basin floor turbidite-channel-levee complex. Reservoirs encountered in GC955 exhibit thin-bedded internal structure and complex fault compartmentalization. Two wells drilled in the Alaminos Canyon 21 site tested a large, shallow, sand unit within the Diana mini-basin that exhibits steep lateral margins, non-sinuous elongate form, and flat base with hummocky upper surface. These features suggest deposition as a mass-transport deposit consisting of remobilized sand-rich turbidites or as a large basin-floor fan that was potentially eroded and buried by later-stage, mud-prone, mass-transport deposits.

Gulf of Mexico Gas Hydrate Joint Industry Project Leg II logging-while-drilling data acquisition and analysis

One of the objectives of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (GOM JIP Leg II) was the collection of a comprehensive suite of logging-while-drilling (LWD) data within gas-hydrate-bearing sand reservoirs in order to make accurate estimates of the concentration of gas hydrates under various geologic conditions and to understand the geologic controls on the occurrence of gas hydrate at each of the sites drilled during this expedition. The LWD sensors just above the drill bit provided important information on the nature of the sediments and the occurrence of gas hydrate. There has been significant advancements in the use of downhole well-logging tools to acquire detailed information on the occurrence of gas hydrate in nature: From using electrical resistivity and acoustic logs to identify gas hydrate occurrences in wells to where wireline and advanced logging-while-drilling tools are routinely used to examine the petrophysical nature of gas hydrate reservoirs and the distribution and concentration of gas hydrates within various complex reservoir systems. Recent integrated sediment coring and well-log studies have confirmed that electrical resistivity and acoustic velocity data can yield accurate gas hydrate saturations in sediment grain supported (isotropic) systems such as sand reservoirs, but more advanced log analysis models are required to characterize gas hydrate in fractured (anisotropic) reservoir systems. In support of the GOM JIP Leg II effort, well-log data montages have been compiled and presented in this report which includes downhole logs obtained from all seven wells drilled during this expedition with a focus on identifying and characterizing the potential gas-hydrate-bearing sedimentary section in each of the wells. Also presented and reviewed in this report are the gas-hydrate saturation and sediment porosity logs for each of the wells as calculated from available downhole well logs.

Pore- and fracture-filling gas hydrate reservoirs in the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II Green Canyon 955 H well

High-quality logging-while-drilling (LWD) downhole logs were acquired in seven wells drilled during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II in the spring of 2009. Well logs obtained in one of the wells, the Green Canyon Block 955 H well (GC955-H), indicate that a 27.4-m thick zone at the depth of 428 m below sea floor (mbsf; 1404 feet below sea floor (fbsf)) contains gas hydrate within sand with average gas hydrate saturations estimated at 60% from the compressional-wave (P-wave) velocity and 65% (locally more than 80%) from resistivity logs if the gas hydrate is assumed to be uniformly distributed in this mostly sand-rich section. Similar analysis, however, of log data from a shallow clay-rich interval between 183 and 366 mbsf (600 and 1200 fbsf) yielded average gas hydrate saturations of about 20% from the resistivity log (locally 50−60%) and negligible amounts of gas hydrate from the P-wave velocity logs. Differences in saturations estimated between resistivity and P-wave velocities within the upper clay-rich interval are caused by the nature of the gas hydrate occurrences. In the case of the shallow clay-rich interval, gas hydrate fills vertical (or high angle) fractures in rather than filling pore space in sands. In this study, isotropic and anisotropic resistivity and velocity models are used to analyze the occurrence of gas hydrate within both the clay-rich and sand dominated gas-hydrate-bearing reservoirs in the GC955-H well.

Gas hydrate resource potential in the Terrebonne Basin, Northern Gulf of Mexico

The Terrebonne Basin is a salt-withdrawal mini-basin within the northeast portion of the Walker Ridge protraction area in northern Gulf of Mexico continental slope that contains a thick sequence of upper Pliocene and Pleistocene clastic sediment. Data acquired during the 2009 Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) logging-while-drilling (LWD) program confirmed the presence of gas hydrate within a variety of sand and clay units. Integration of the Leg II LWD data with regional seismic mapping allows for the identification of various facies assemblages within the sand units and an initial estimation of the gas hydrate in-place resources throughout the Terrebonne basin. A total of ∼4.4×109m3 (1.55×1011ft3) of gas occurs within highly saturated gas hydrate accumulations within channel, proximal levee, and distal levee facies of four primary Lower Pleistocene sand reservoirs. These sand accumulations occur at the base of gas hydrate stability and locally trap additional, unquantified accumulations of free gas. A number of additional thin hydrate-bearing sand units are also observed to occur at shallower depths. Potential recoverable volumes from this accumulation compare favorably with those realized from conventional deepwater gas reservoirs in the vicinity. In addition, Leg II LWD data delineated the occurrence of a stratal-bound occurrence of gas hydrate-filled fractures at low bulk volume saturations within a thick, shallow, and predominantly fine-grained unit. This unit is estimated to contain roughly 17.0×109m3 (5.87×1011ft3) of gas. The areal gas hydrate resource density within the Terrebonne basin is calculated at 1.183×109m3 per km2 where delineated sand reservoirs are present and 0.32×109m3 per km2 where sands are thought to be absent.

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.