Deepwater reservoirs with their inherent low reservoir fracture gradient are generally restricted in the length of horizontal section that can be safely gravel packed. Historically, increasing maximum length required introducing relatively complex apparati such as intermediate valves in the washpipe or systems providing additional flow paths to bypass any bridges after they have formed.
This paper presents an annular packoff mechanism that has the capability to potentially alleviate this problem and improve overall reliability. The component is a simple "add on," that consists of a liner that bridges the rat-hole and extends down into the open hole below the rat hole since this is the primary region of concern for premature screenout during an HzGP. The potential of screenout here is due to the reduced flow velocity and irregularities in the flow regime. The added liner is run with the standard HzGP completion equipment. This is a typical issue in the deep water and heavy oil reservoirs of Brazil where the reservoirs are predominantly found at a relatively shallow below the ocean floor in relation to water depth.
Annular flow velocity through the rat hole and area of concern is critical in avoiding premature screen-out. For a given flow rate, the annular velocity in the rat hole will be lower than that in the open hole due to the larger annular area in the rat hole. The lower annular velocity in the rathole section results in a relatively high alpha wave. When slurry flows in the area open to flow above the top of the alpha wave in the rat-hole into the open-hole section, it is forced to dip down into the area open to flow in the open hole. This phenomenon is a major area of concern during scale model testing because of the turbulence that establishes a dip in the alpha wave height followed immediately by an increase in alpha wave height in this transition area. This "pinching" effect potentially induces a premature screenout situation. The first wellbore completed with this system has verified that no negative impact on circulating rates or pressures occurs during the course of an HzGP when the new system is used. The case history discusses the pressure curves and flow-rate responses during the operation and shows how the system can increase the operating envelope and reliability of the HzGP treatment. This addition of this component has been shown to more than double the length of horizontal section that can be safely gravel packed, especially under conditions where low reservoir fracture gradient is the primary limiting factor.
Reservoirs requiring sand control pose a major completion challenge because of the cost implications and risk inherent to potential remedial treatments, particularly in sub-sea completions and/or deep-water environments. As such, it is imperative that all steps are undertaken to ensure the integrity of the sand control completion. An ongoing trend in these environments is the use of open-hole, horizontal, gravel-packed completions.