fax 01-972-952-9435. AbstractThere is an increased demand for the design and application of non-damaging fluids in reservoirs that are depleted and susceptible to deep invasion of damaging mud solids. To enable a reservoir drilling fluid to function correctly, it is necessary to very rapidly deposit an external filter cake and reduce filtrate loss to a minimum. The cake must be backflowed in the production phase with minimum liftoff pressure and leave little or no permeability reduction. To achieve this, an accurate description of the pore throats in the reservoir is required. If this cannot be obtained, or the reservoir is at risk to fracture, then another approach is required. This paper illustrates the methodology of utilizing a variable calcium carbonate bridging distribution mixed with powdered graphite which overcomes the problems of drilling reservoirs with varying pore throats and fractures. The bridging materials will enable an efficient control of filter cake deposition, and also allow for easy disruption of the cake integrity, when the well is back flowed in the production phase.Results proving the effectiveness of this approach can be shown from return permeametry work in the laboratory, and successful field trials of its application in the Southern North Sea and the Middle East.The use of these non-damaging products, and the engineered size distribution, will greatly decrease the difficulty at which fractured and depleted reservoirs can be drilled and will not reduce the productivity of these wells, that have in the past experienced formation damage due to deep fluid and solid invasion.
There is an increased demand for the design and application of non-damaging fluids in reservoirs that are depleted and susceptible to deep invasion of damaging mud solids. To enable a reservoir drilling fluid to function correctly, it is necessary to very rapidly deposit an external filter cake and reduce filtrate loss to a minimum. The cake must be backflowed in the production phase with minimum liftoff pressure and leave little or no permeability reduction. To achieve this, an accurate description of the pore throats in the reservoir is required. If this cannot be obtained, or the reservoir is at risk to fracture, then another approach is required. This paper illustrates the methodology of utilizing a variable calcium carbonate bridging distribution mixed with powdered graphite which overcomes the problems of drilling reservoirs with varying pore throats and fractures. The bridging materials will enable an efficient control of filter cake deposition, and also allow for easy disruption of the cake integrity, when the well is back flowed in the production phase. Results proving the effectiveness of this approach can be shown from return permeametry work in the laboratory, and successful field trials of its application in the Southern North Sea and the Middle East. The use of these non-damaging products, and the engineered size distribution, will greatly decrease the difficulty at which fractured and depleted reservoirs can be drilled and will not reduce the productivity of these wells, that have in the past experienced formation damage due to deep fluid and solid invasion. Introduction Alternative formation pressure differences between different formation types have always presented drillers with a complex and difficult problem. A prime example of this is the stratigraphic column in the eastern province of Kingdom of Saudi Arabia, where there are successive high pressure zones followed by low pressure zones, with pressure differences ranging from 500 psi to 3600 psi (static). The operator planned to drill six bi-lateral wells through these interbedded high and low pressure zones. Five out of these six wells were unsuccessful and had to be side-tracked with only one lateral drilled at the end of each well. It was decided that a novel approach was required to overcome these field difficulties. Success had been previously achieved when depleted zones were drilled in the Brent field in the North Sea by utilising graphite (figure 1) as a bridging material and laboratory tests were initiated to utilise this approach (Davison et al1). The addition of graphite to the mud strengthened the reservoir rock, enabling it to remain stable despite an overbalance beyond the point at which physical failure had previously occurred in offset wells. The advantage of using graphite over carbonate bridging materials is its deformability. This allows it to create a much more effective pressure seal in the throats of fractures, thus preventing fracture propagation and subsequent failure. This effect is illustrated in figure 2 and was commented on by J. Adachi et al 2 in their study into depleted zone drilling. In a previous joint industry project reported by Aston et al3, the study produced results that pointed towards calcium carbonate and graphitic blends as one of the best ways to reduce mud losses into fractures. A synergy effect is created by having the deformable graphite in conjunction with the sized calcium carbonate, thus enabling a large range of fracture sizes to be bridged, and also allowing the sealing of fractures that change in dimension as downhole pressures are altered during the drilling process. It was believed that by introducing a highly efficient filtrate reducer, this effect could be further enhanced.
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