Current well placement in unconventional shale ranges from simple geometric well placement to a gamut of patternrecognition systems and geosteering with geochemical and geomechanical analyses. The wide diversity of systems used leads to uncertainty in the effectiveness of any strategy, with confusion as to the true value or merit of a particular technique. Often, a well-placement strategy is based on what came before, with little regard as to the complexities or differences between reservoirs.This paper reviews the current common practices used in geosteering in shales, for both gas-and oil-producing reservoirs. A brief history of strategy development is outlined, with comments about its perceived effectiveness and value. Examples of successes and failures are examined to attempt to determine the viability of a particular strategy.Finally, alternative approaches and methodologies are reviewed and examined, with comments about the potential application, benefits, and value.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe efficient exploitation of hydrocarbon resources is strongly dependent on the drillers' ability to position wells accurately. This is particularly challenging for smaller targets and on long reach or horizontal wells. Conventional wireline VSP's can provide valuable assistance but such operations require either the drill string to be removed from the hole or the pumping of the geophone tool through the drill string. Both of these methods add time to the well construction. In long sections of open hole wireline operations may present risks, in the form of sticking and potential fishing operations, which prevent VSP surveys being acquired in lookahead mode. Ideally, VSP data would be acquired at several stages of the drilling process to enable corrective action to be taken to assist accurate bit placement. The costs in terms of rig time alone generally make this option prohibitive. Ongoing developments for cost effective exploitation of mature and pressure-depleted reservoirs have led to wider applications for coiled tubing drilling. The combination of a seismic tool with the coiled tubing drill string therefore provides an ideal configuration to reduce drilling costs and optimise well placement. This paper discusses the conventional methods available for geosteering of the bit, the additional benefits of incorporating a seismic tool into the drill string and describes the development of a seismic while drilling tool (SMWD) for coiled tubing drilling.
The efficient exploitation of hydrocarbon resources is strongly dependent on the drillers’ ability to position wells accurately. This is particularly challenging for smaller targets and on long reach or horizontal wells. Conventional wireline VSP's can provide valuable assistance but such operations require either the drill string to be removed from the hole or the pumping of the geophone tool through the drill string. Both of these methods add time to the well construction. In long sections of open hole wireline operations may present risks, in the form of sticking and potential fishing operations, which prevent VSP surveys being acquired in lookahead mode. Ideally, VSP data would be acquired at several stages of the drilling process to enable corrective action to be taken to assist accurate bit placement. The costs in terms of rig time alone generally make this option prohibitive. Ongoing developments for cost effective exploitation of mature and pressure-depleted reservoirs have led to wider applications for coiled tubing drilling. The combination of a seismic tool with the coiled tubing drill string therefore provides an ideal configuration to reduce drilling costs and optimise well placement. This paper discusses the conventional methods available for geosteering of the bit, the additional benefits of incorporating a seismic tool into the drill string and describes the development of a seismic while drilling tool (SMWD) for coiled tubing drilling. Introduction As many of the world's major hydrocarbon regions are reaching maturity oil companies are seeking methods to extend their productive lives. Because this will entail searching for and producing from smaller reserves such methods will need to ensure that operating costs are kept to a minimum. A major portion of the cost of well construction lies in the drilling and casing processes; a significant contribution to the economical exploitation of mature or depleted reserves may therefore be made by steering the bit to its optimum location with minimum delay and at the first attempt. The drilling location is based largely on geophysical interpretation of seismic images of the subsurface. Such images are derived from a seismic signal generated by a source at the surface. The signal travels through the earth and, after reflection from subsurface geological boundaries, is recorded by geophones that are also located at the surface. Along its travel path the signal is attenuated by absorption and scattering and is made more complex by multiple reflections between various boundaries. Such multiples can create false events of the processed seismic section and can mask the real reflectors marking the reservoir boundaries. Additionally, the horizontal position of events on the seismic section is dependent on assumptions made during the processing of the data. It is known that the velocities of seismic signals can vary according the their angle of travel relative to formation bedding planes. This effect alone, termed anisotropy, can lead to uncertainties of at least 50m in the horizontal position of seismic images. Seismic images are recorded in units of time, being the time it takes the signal to travel vertically from the surface down to each reflector and back up to the surface. For the purpose of providing drilling targets the image must be converted to units of depth. This is achieved using estimates of formation velocities. Uncertainty in these velocities clearly has an impact on the accuracy of depth information that is particularly important when drilling horizontal wells. Although the absolute depth of the bit is known its depth relative to the seismic target may not be. On many occasions the repercussion of this is an intermediate logging job costing time and money.
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