As intelligent completion technology matures, the field of application continues to expand to increasingly challenging environments, such as the poorly consol-idated, high permeability, high produc-tivity, clastic reservoirs common to the Gulf of Mexico, offshore West Africa, offshore Brazil, and the North Sea. These areas fit the modus operandi of intelligent well applications - high productivity wells, complex reservoirs, high capital invest-ment, and high intervention costs.
The challenge of applying downhole flow control to these areas is their propensity to produce significant amounts of formation solids. Sand control techniques have been applied in these environments with varying degrees of success, and it is safe to say that a properly conceived and executed sand control strategy can be very effective in reducing or eliminating solids production without unduly restricting productivity.
New techniques, such as expandable screens have been added to tried-and-true techniques such as gravel packs. But combining sand control technology with intelligent well technology can be a significant challenge, particularly when producing fluid from multiple, unconsolidated, high productivity zones. The intelligent completion industry is attacking this challenge in concert with the sand control industry to generate integrated solutions.
Sand production control
Production of load-bearing formation solids is a result of wellbore stress and dynamic fluid flow forces that exceed the strength of the formation and the forces that consolidate the sand grains. The potential for sand production is greatest in unconsolidated formations, where inter-granular cementation is poor, and is aggravated by the dissolution and chemical attack of natural cementing materials by reservoir fluids, injected pressure-maintenance fluids, or stimulation fluids.
Dynamic fluid flow forces have the greatest influence on sand production. Stresses are imposed on the sand grains that tend to move them into the wellbore, along with the produced fluids. Pressure gradients, fluid drag forces, and overburden load combine to produce stresses that can exceed the formation strength and cause sand production. High fluid velocity, fluid viscosity, multiphase flow, and dramatic flow transients exacerbate the condition.
The well is completed with a conventional two-stage gravel pack (or screens), including the dip tube design, isolating the two zones from each other with a section of blank pipe and a packer.
Sand control strategies focus on reducing wellbore stress, improving consolidation, or transferring stress to some form of mechanical retention.
The most basic strategy for sand control is to reduce the drawdown on the reservoir with the ultimate aim of reducing differential pressure, unit production rate, and hence fluid velocity. This strategy is contrary to the objective of maximizing well productivity, but it is an important strategy to consider.
One way of reducing drawdown and unit production rate while still maintaining high total production rate is to increase the amount of wellbore-to-reservoir communication through open-hole completions, under-reaming, horizontal or high deviation wells, multilateral wells, and hydraulic fracturing. Knowledge of borehole stability and formation strength is key in understanding the mechanics of sand production and establishing an operation envelope of production parameters to avoid destabilizing the formation.
Mechanical retention techniques rely on devices that exclude sand grains from entry into the flowing wellbore while letting fluids pass through. Slotted liners and screens are designed so that the majority of formation particles tend to bridge across the openings, yet offer maximum fluid flow area. Smaller formation particles are then retained behind the larger bridged particles. This method is highly dependent on size and distribution of the formation solids produced and can be subject to fines plugging, destabilization of the particle bridges, and erosive 'burn-through' from high velocities of fluids flowing from hot-spot perforations.
A second solution for controlling multiple zones with sand control includes a concentric screen/gravel pack design.
Pre-packed screens incorporate a resin-coated particle matrix in the annular area of two concentric screens to improve flow distribution and erosion resistance, but can be prone to plugging from fines. New generation screens incorporate layers of mesh, weave or sintered metal powder matrix to handle a larger range of particle size distribution, increase fluid flow area, and provide greater mechanical strength and erosion resistance.
Gravel packing is arguably the most popular and most effective method of mechanical sand-control and can be performed inside casing or in open hole. Chemical sand control methods involve treating the near-wellbore formation with sand consolidating or cementing chemicals such as epoxies, phenolics, or furans. This technique has limited success and its effectiveness is highly dependant on the placement and distribution of the materials in the formation matrix.
A third and most promising solution is the use of intelligent well equipment with expandable screen.
One of the most promising recent advances in mechanical sand control techniques is the use of expandable perforated liners and expandable screens. These devices are designed not only to exclude the movement of formation sand particles into the reservoir, but also to provide additional mechanical wellbore support to counteract dynamic fluid flow forces and overburden stress.
The challenge to the completions industry is how to effectively integrate intelligent well technologies with modern sand control strategies. The following issues must be considered when using intelligent flow control and monitoring in a sand-producing environment:
- Protection/isolation of zones: Intelligent well completions can be used to monitor and control flow from separate reservoirs, separate layers, or separate regions of a heterogeneous formation. Some or all of these zones could require some form of sand control, but critical to the effectiveness of the flow control is the hydraulic isolation of one zone from the other. Isolation can be achieved by using cemented and perforated liners with blank sections between zones. Open-hole completions with screens or gravel packs may require blank sections of liner with inflatable external casing packers and multi-stage gravel packing equipment.
- Equipment diameters/available space: Intelligent flow control equipment, transducer mandrels and flatpacks or control lines all take significantly more space than conventional completion equipment, and may need to be deployed directly inside the sand control equipment. This can create conflicts when attempting to keep casing and completion equipment sizes within conventional designs while maximizing flow areas to reduce flow velocity and maximize productivity.
- Fluid velocity, pressure drop, erosion: The bane of completion equipment in a solids producing environment is erosion, and restricted flow areas and tortuous flow paths (typical around and through flow control equipment) contribute to the effects of high velocity causing equipment erosion. When producing compressible fluids, such as gas, the flowing pressure drop associated with high velocity and restricted flow areas results not only in lower productivity, but also in higher flow velocity. If the producing environment is corrosive, erosion/corrosion mechanisms must also be considered in the material selection for the completion.
- Protection of sensors, cables, control lines: Control lines, cables, and sensors represent the nervous and circulatory system of an intelligent well completion, and damage to these elements may mean partial or total loss of the functionality of the intelligent completion.
- These element must be adequately protected from erosion (or the potential thereof from sand control failure), vibration, and thermal stresses by use of appropriately designed clamps and encapsulating blast joints. For example, WellDynamics Scramstrademark SmartWelltrademark technology uses dual redundant control line and electronic systems capable of operating on one system in the event of failure of the other.
- Mechanical interference of moving components: The solids produced with the fluids can interfere with movement and sealing of dynamic components, particular sleeves on flow control chokes, and valves. The design of these components must be sand tolerant - either excluding solids from entering cavities that may cause interference with movement, or be able to easily wipe the solids, or function despite the presence of solids. Actuators and spring returns must generate sufficient force to move the dynamic components despite buildup of solids or scale. Frequent cycling of the valves may prevent accumulation of significant amounts of solid, but may also cause more wear and tear on seals and bearing surfaces.
- Injection wells: In multi-zone reservoirs where the production wells require sand control, sand control should also be considered for the injection wells. Dissolution of the natural cementing materials in water injection wells can destabilize the formation strength. During shut-in of these wells, flow-back and cross-flow between layers at different reservoir pressure will result in significant production of solids into the wellbore, which can cause plugging and interference with flow control devices. Closing the flow control devices during shut-in to reduce cross-flow helps to alleviate the problem, but may not prevent it.
Intelligent well sand control
Use of intelligent completion elements can significantly contribute to the management and prevention of sand production while maximizing hydrocarbon productivity. By monitoring actual inflow condition, and controlling and restricting fluid flow into the wellbore, intelligent wells can maintain the flow below critical rates that would otherwise destabilize the formation matrix or gravel pack. Zones that develop a propensity for water production can be choked back or closed in, also reducing the tendency for sand production aggravated by multiphase flow and aqueous dissolution.
One of the simplest solutions for controlling two zones with sand control is the dip tube or siphon tube solution. The well is completed with a conventional two-stage gravel pack (or screens), isolating the two zones from each other with a section of blank pipe and a packer. The completion is composed, top down, of the production tubing, feed-through production packer, gauge mandrel, interval control valve (ICV), a shrouded interval control valve and dip tube with seal assembly which stings into a seal bore in the packer isolating the two zones.
Production from the lower zone flows through the dip tube, through the shroud on the lowermost ICV, and enters the production tubing through the lowermost ICV. Production from the upper zone flows in the annular area between the upper gravel pack screen, in the annular area between the lowermost ICV shroud and the production casing, and enters the production tubing through the uppermost ICV. The gauge mandrel enables pressure monitoring of both internal and annular areas.
A second solution for controlling multiple zones with sand control is where each zone is completed with (from top down) a hydraulic set, hydraulic feed-through isolation packer, a gravel slurry placement sleeve, a shrouded ICV with the shroud attached to the gravel pack screen base pipe and the ICV attached to an internal, concentric, through-bore, production conduit, which ties into the isolation packer of the next lower interval. The gravel pack slurry is placed with coiled tubing or a small work string stung into the sand placement sleeve which acts as a cross-over device for flow from the coil to the casing annular area for gravel packing, with returns back up the coiled tubing - tubing annulus. This completion can also be run with screens only, without gravel packing.
A limitation of the second solution is the limited flow area imposed by the multiple concentric strings and flow control equipment. This solution is only practical with a production casing (liner) size of 9 5/8-in. or greater. A variation on this theme has been designed and tested in a proof of concept well by Schlumberger, wherein the ICV has been integrated with the screen base pipe, the base pipe becomes the main flow conduit, and the screen has been designed with increased stand-off from the base pipe. Flow from the formation travels through the gravel pack, enters the screen and flows in the annular area between the screen and the base pipe to the ICV, through which it joins the flow in the main production conduit.
This solution provides increased production conduit flow area. In both design cases, the relative flow areas between the casing and the screen, the screen and the flow tube (or base pipe), and up the main production conduit must be thoroughly examined to balance fluid velocities.
A third and most promising solution is the use of intelligent well equipment with expandable screens. This solution maximizes flow areas in both the annulus and the production conduit.
Installation of several dip-tube type completions in the Gulf of Mexico for a number of operators has been successful. Two wells have been completed for British Borneo (now Agip) in the Allegheny field, while two other wells have been completed for Chevron/BHP in the Typhoon field. One well in the King's Peak field in the Gulf of Mexico was most recently completed for BP, with a completion integrated with a multi-zone gravel pack.
Additional similar completions are planned for King's Peak and the neighboring Aconcagua and Camden Hills fields. WellDynamics has also successfully installed five dip-tube type SmartWellRegistered completions in the Asia-Pacific region - four for BSP and one for SSB. Of the completions for BSP, one combines a two-zone flow control system with a gravel pack completion, and three are with expandable screen completions in the Champion West field. SSB's application in the South Furious field uses an internal gravel pack, with a SmartWell expandable screen application planned for 2002.
Integration of intelligent well completions with sand control equipment presents unique challenges, and experience is limited, but new systems and new ideas promise to deliver reliable, advanced completion solutions. The most effective of these solutions will address many issues specific to intelligent well applications in conjunction with sand control and will be the result of close cooperation among the operator, intelligent completion provider, and sand control system provider.