Slotted liners have been used for many years to provide sand control in many oil industry applications. They are commonly applied in western Canadian reservoirs that produce high-viscosity oil from horizontal wells with unconsolidated, high-permeability sands. Both primary and thermal applications are common, and the steam-assisted gravity drainage (SAGD) process1 is beginning to see widespread application in this area of the world. They are relatively inexpensive to manufacture and tolerant of installation loads, but historically they have not been able to offer the very small opening sizes of wire-wrap screens for controlling production of fine sands. However, recent advances in slot manufacturing methods provide slot openings that match and surpass the size and tolerance of wire-wrap screens. Furthermore, slotted liners offer an advantage in providing a variable slot density that can be used to optimize inflow or outflow distributions.
In the development of its south Bolney reservoir, Marathon Oil Canada and Noetic Engineering Inc. performed an analytical evaluation of inflow characteristics for a new generation of commercially available slots. Several interesting conclusions were reached, the most significant of which was that inflow resistance depends much more strongly on slot density than on open area. The inflow characterization was also used to optimize the slotdensity distribution and to promote more uniform production throughout the well. The slotting was incorporated with a deformation- management system that controlled thermally induced loads, preventing compromise of sand-control characteristics.
Sand control is a key consideration for horizontal wells used to produce from the highly permeable, unconsolidated sands typical of heavy oil reservoirs in western Canada. Typically, these wells are relatively low-rate, low-gas producers, although more prolific wells are being developed with enhanced recovery methods, such as SAGD. Regardless of the implementation, the long horizontal sections and low specific inflow rates make it impossible to transport even small grains from the toe of the well to the surface. Consequently, if inadequate sand control is provided, the wellbore will commonly sand in over time.
The economics of these wells demand the use of low-cost sandcontrol systems. Slotted liners are commonly used in many applications, and wire-wrap screens with partial coverage are often used where sand control requires small opening sizes, typically less than 0.25 mm (0.010 in.). Economic factors have precluded more expensive solutions, such as gravel packs and prepacked screens, in commercial developments.
The primary factors considered in the liner design are sand control, inflow resistance, and cost. Inflow performance is usually considered to be controlled by the open area exposed to the reservoir, with sand control governed by slot opening size. These become competing considerations in reservoirs with fine sands because slot density must be increased to maintain the open area if slot size is reduced to control sand. Furthermore, it is difficult to cut narrow slots, which previously made narrow openings unachievable for slotted liners. Recent developments in slotted-liner technology have addressed this last issue, allowing very small slot widths (less than 0.12 mm) with good antiplugging characteristics to be manufactured economically.
Although small slot openings are available, their size would demand a very high slot density to maintain the open-area targets often specified. The basis for this requirement probably stems from applying channel flow concepts to pressure loss through the slots. This basis also leads to the conclusion that fewer large slots would have less flow resistance than more small slots for the same open area. However, the basis for such conclusions ignores the most important component of slot-induced pressure loss - flow convergence in the sand that packs around the slots. In fact, the pressure loss through an open slot is negligible compared with that induced by the flow disturbance associated with the slot.
This paper presents results from a semiempirical method for evaluating pressure loss caused by the near-wellbore flow disturbance from slotted liners and gives a relationship between slot density and pressure loss. This relationship demonstrates that open area is not the best basis for characterizing flow resistance in slotted liners. It can also be used to optimize the slot density throughout the producing interval to provide a more uniform inflow distribution (or outflow for injection wells).
In the planning stages for new wells at its south Bolney project, Marathon Oil Canada Ltd. pursued a well-optimization analysis for the slotted liner. This paper describes the analysis methodology and optimization results. Several liner designs were considered, and a sensitivity study of production variations resulting from variations in the control parameters is summarized.
Marathon's initial development wells at south Bolney were completed with slotted liners of varying slot densities, generally with an open area of 2 to 2.5%. Sand production problems were ongoing, and most of the existing wells were subsequently retrofitted with wire-wrapped screens. The last phase of wells, drilled in 1999, were completed with slotted liners and sand screens simultaneously. To evaluate an alternative completion method for future development planning at Bolney, Marathon sought the services of Noetic Engineering Inc. and Regent Control Systems Ltd. for a new completion design that would mitigate sand production and provide structural integrity to the horizontal liner.
The analysis model employs common radial Darcy flow equations. Non-Darcy effects are insignificant at the specific inflow and gas rates for most heavy oil wells. In an idealized problem, the inflow rate depends on reservoir permeability, fluid viscosity, and the logarithm of the ratio between the reservoir boundary and well radii. Deviations from this idealized behavior are characterized by a skin factor, which accounts for flow impediments and enhancements. Sources of skin-factor variations can be separated into two main categories.
Permeability variations. These include initial disturbances from drilling damage and those that evolve with time because of pore-throat plugging from fines migration and solids precipitation.
Flow disturbances. These result from the flow pattern deviating from idealized radial and uniform axial distribution. All mechanical sand-control methods impose some degree of flow disturbance by blocking flow to obstruct sand influx