The construction of geological and geotechnical models in typical Pilbara iron ore environments is vital to enable an optimized mine design for the life of the asset, while maintaining pit wall integrity and overall mine safety. Geotechnical assessments require the measurement of geomechanical properties, such as the triaxial shear, direct shear and unconfined compressive strength tests and pressure and shear wave velocities on diamond core samples. Ideally, these velocities would be measured in Reverse Circulation (RC) boreholes as their spatial density is far higher than diamond drilled holes. Unfortunately, despite its value, such data is seldom collected as a large proportion of the holes are above the water table, limiting the use of sonic-logging tools. Even if measurements are possible, damage to the borehole caused by drilling biases the resulting velocity measurements. This paper details the results of a trial using the vertical seismic profile method to directly measure in-situ seismic velocities in RC boreholes. The method was successful in determining the velocities of the formations through the entire length of the holes. The data in several boreholes was of sufficient quality for the application of more advanced processing methods, important for geological mapping and the processing and interpretation of surface seismic data.The success of this first trial has implications for future iron-ore developments in the Pilbara. The widespread acquisition of accurate seismic velocity data is likely to enable the creation of more accurate geotechnical models and could improve future development decisions.
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A vertical seismic profile (VSP) survey involves placing sensors in a borehole to record the passage of energy transmitted using a source of seismic energy placed on the surface. The sensors are usually contained within sondes that are coupled to the borehole wall using mechanical clamps. Existing VSP acquisition systems are generally unsuitable for acquisition using the wireline units typically used for minerals logging. In this paper we describe a new multis-sonde VSP acquisition system specifically designed for acquiring high-resolution VSP surveys in hard-rock environments.
In Chevron's Gulf of Thailand (GOT) operations, costs drive logging and formation evaluation. Programs for logging and evaluation are based on consideration of perceived value and the potential for comprehensive utilization. Well lifespan is short, and economics rarely provide for the use of higher technology at non-discounted prices. A recent business initiative recognized that oil vs gas fluid identification from logging measurements was a risk that should be mitigated providing a major opportunity to add value. Historical experience has shown, that the diameter of invasion can be greater than twenty inches by the time a well is logged with wireline which is beyond the limits of investigation for density and neutron tools, rendering the interpretation of fluid types ambiguous in most hydrocarbon bearing sands in this basin. To reduce this uncertainty, comprehensive wireline formation pressure programs have been run to assess hydrocarbon gradients but because sands are thin and permeabilities are low these programs have had various degrees of success. LWD (logging while drilling) neutron tools can obtain data prior to invasion. Overlaying the LWD neutron and wireline neutron reveals time-lapse differences between the logs, shows gas intervals, and confirms liquid-filled zones. The technique can also infer inflow permeability for some of the more shaley gas reservoirs. Positive fluid identifications have been confirmed by follow up wireline pressure gradients, NMR logs, and production tests. This paper will show many great examples of the evolution of a good idea. It will also demonstrate the effective use of technology, the buy-in from drilling engineers, and value of information benefits to the asset teams. Logging programs in the GOT once thought to be static, have rapidly evolved with insightful technology and the demonstration of its value. Geology Overview GOT Chevon's GOT operation is focused within the Pattani basin which is located near the geographic center of the Gulf of Thailand containing in some areas greater than 25,000 ft of almost entirely non-marine fluvial and delta plain sediments. The basin is characterized as a primarily north-south trending extensional system with virtually no evidence of subsequent inversion. Accommodation with control from basement block faulting has enabled the formation of en echelon graben systems resulting in a multitude of fault related structural and stratigraphic closures. Significant amounts of gas and liquid hydrocarbons have migrated into these horst and graben structures from localized coal and lacustrine source rocks. The fluvial depositional systems that provide the ubiquitous reservoir rocks in the basin developed on an extensive delta plain throughout the Neogene. Meander belts and associated point bars occasionally stack to form thicker reservoir units but sands are generally thin (less than 20 ft) and have relatively small accumulations (column and area) of hydrocarbon (Crossley. 1990). The key success of the drilling campaigns is to locate as many of these accumulations with one well, and comingle the reservoirs of known gas and lift the liquid hydrocarbons with single gas accumulations. To accommodate this strategy identifying the producible hydrocarbons and more importantly the hydrocarbon type within the multiple reservoirs found in a given well that may or may not be correlated to the offset wells is critical for success. Misidentifying the fluid type has led to difficulties in correlation as well as numerous missed opportunities to capture the reserves in a given well using the most effective completion methods. The technology described below has been of great assistance in Chevron? s operation to mitigate this risk.
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