TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA well completion is a critical interface between the productive formation and the wellbore. An effective completion must maintain mechanical integrity of the borehole without creating any significant restrictions in the flow capacity of the well. In this paper, we outline a process to design optimal completions for horizontal wells by applying comprehensive skin factor models that include damage and turbulence effects for all common types of completions. Slotted or perforated liner, cased and perforated completions, or gravel pack completions have been used in horizontal wells for borehole stability and sand control purposes. However, these completions may have lower productivity (as characterized by a positive skin) relative to an equivalent openhole completion because the convergent flow to perforations or slots increases fluid velocity in the near-well vicinity. In addition, any reduced permeability zones (formation damage caused by drilling, completion, or other processes) magnify the convergent flow effects, and hence, may result in severe skin factors. Compound effects of formation damage around the well completion, a crushed zone due to perforating, the plugging of slots, and turbulent flow, as well as interactions among these effects are included in the model.We first illustrate how to use the skin factor models to screen the available completion types for different applications. This screening approach considers reservoir permeability, permeability anisotropy, fluid properties, formation damage effects, and rock mechanical characteristics as the key parameters. The types of completions that yield the most productive well performance for this matrix of properties are presented.A more detailed completion design is then illustrated by showing the use of the skin factor models for selection of liner completions for viscous oil reservoirs on the North Slope of Alaska. Application of the slotted or perforated liner models to the readily available liners showed that the completion skin factor can vary by as much as 40%, depending on the detailed characteristics of the slots or perforations in the liner (slot or perforation size, density, and distribution). This showed how analyzing the performance of the completion design can greatly increase well productivity at little or no cost.
In the challenging North Slope operating environment, use of innovative production equipment has provided solutions to zonal isolation and packer integrity problems in viscous oil reservoirs. Operators have employed new tools and technology utilizing expandable rubber materials to manage annular fluid flow, control solids/shale production, and achieve zonal isolation in wells where high costs, shallow depths, and long step-outs create unique completion challenges. The new technology is allowing once bypassed zones to be added to existing developments, and making future developments more economically viable. The new design approach involves installing swelling rubber packer (SRP) technology as part of the completion. This technology is based on specially designed swelling properties of rubber in crude or mineral oil based mud (MOBM) to expand and seal the annulus. The paper describes one operator's use of as many as 17 devices in a tri-lateral horizontal undulating well to manage annular flow and minimize shale/solids production. The successful application of this technology has allowed shale interbedding to be effectively isolated behind blank pipe, thus allowing an additional zone to be added to the existing development. To date the technology has been applied to eleven wells, improving production assurance. Another major operator on the North Slope has used the technology to isolate potentially conductive fault crossings along the lateral and inadvertent zonal crossings while kicking off from the parent bore. Multiple packers have been run in single laterals to achieve the desired isolation without noticeable effects on liner running drag. Recent density caliper data shows significantly more washout than previously envisioned, increasing the desire to manage annular flow. Development and application of this SRP technology is detailed in the paper, including documentation of improved efficiencies as a result of its use. The paper will also discuss field operations, installation, and unique considerations associated with design and installation in viscous oil environments. Introduction On the North Slope of Alaska, it has been estimated that between 20–25 billion barrels OOIP of viscous oil are contained within shallow, regionally extensive sands. [1,2,3,4,5,6](Figure 1) To date, development of these viscous oil sands has been deferred in favor ofthe warmer, less viscous oil that lies below. The presence of the highly viscous oil in the shallow sands results from oil biodegradation and low reservoir temperatures due to the extreme northern latitude, the presence of 1,800 feet of permafrost, and its relatively shallow burial depth.
Summary A well completion is a critical interface between the productive formation and the wellbore. An effective completion must maintain the mechanical integrity of the borehole without creating any significant restrictions on the flow capacity of the well. In this paper, a process is outlined to design optimal completions for horizontal wells by applying comprehensive skin-factor models that include damage and turbulence effects for all common types of completions. Slotted or perforated liner, cased, perforated, or gravel-pack completions have been used in horizontal wells for borehole stability and sand-control purposes. However, these completions may have lower productivity (as characterized by a positive skin) relative to an equivalent openhole completion, because the convergent flow to perforations or slots increases fluid velocity in the near-well vicinity. In addition, any reduced permeability zones (formation damage caused by drilling, completion, or other processes) magnify the convergent flow effects and therefore may result in substantially increased skin factors. Compound effects of formation damage around the well completion, a crushed zone because of perforating, plugging of slots, and turbulent flow, as well as interactions among these effects, are included in the model. This paper illustrates how to use skin factor models to screen the available completion types for cased/perforated and slotted liner completions. This screening approach considers reservoir permeability, permeability anisotropy, fluid properties, formation damage effects, and rock mechanical characteristics as the key parameters. The types of completion that yield the most productive well performance for this matrix of properties are presented. A more detailed completion design is then illustrated by showing how the skin-factor models were used to redesign the slot configuration of liner completions for viscous oil reservoirs on the North Slope of Alaska. Application of the slotted or perforated liner models to the readily available liners showed that the completion skin factor can vary by as much as 40%, depending on the detailed characteristics of the slots or perforations in the liner (slot or perforation size, density, and distribution). The example showed that optimizing the performance of the completion can increase well productivity at little or no cost and with no loss in liner mechanical strength. Introduction The optimization of well completions to improve the inflow performance of horizontal wells is a complex but very practical and challenging problem. What is needed is a means for engineers to determine the causes of high skin values occurring under various conditions and to suggest techniques to minimize the problems. In particular, the interactions among formation and perforation damage effects and convergent flow to perforations and slots are critical issues in the design of optimal completions for horizontal wells. Numerous papers have reported on completion performance models of vertical and horizontal wells, which can be used to predict the productivity of the wells. These models can be categorized into two groups; numerical models (Dogulu 1998; Ansah et al. 2002; Tang 2001) and semi-analytical models (McLeod 1983; Karakas and Tariq 1991; Golan and Whitson 1991). The numerical models require advanced computer programs to solve the complex flow problem by applying finite-difference methods, finite element methods, or the Green's function (source function) method. All the methods require the solution of a large matrix system and greater computational time compared with analytical models, although they provide accurate solutions under a variety of conditions.
In the version of this Article originally published, in Table 1, there was an error in the 'Period (equatorial J2000)' parameter value. The value '4.276057' should have been '4.296057'. Also, in the third paragraph of the main text, in the fifth sentence, the value '4.276057' should have been '4.296059'. These have now been corrected in all versions of the Article.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA well completion is a critical interface between the productive formation and the wellbore. An effective completion must maintain mechanical integrity of the borehole without creating any significant restrictions in the flow capacity of the well. In this paper, we outline a process to design optimal completions for horizontal wells by applying comprehensive skin factor models that include damage and turbulence effects for all common types of completions. Slotted or perforated liner, cased and perforated completions, or gravel pack completions have been used in horizontal wells for borehole stability and sand control purposes. However, these completions may have lower productivity (as characterized by a positive skin) relative to an equivalent openhole completion because the convergent flow to perforations or slots increases fluid velocity in the near-well vicinity. In addition, any reduced permeability zones (formation damage caused by drilling, completion, or other processes) magnify the convergent flow effects, and hence, may result in severe skin factors. Compound effects of formation damage around the well completion, a crushed zone due to perforating, the plugging of slots, and turbulent flow, as well as interactions among these effects are included in the model.We first illustrate how to use the skin factor models to screen the available completion types for different applications. This screening approach considers reservoir permeability, permeability anisotropy, fluid properties, formation damage effects, and rock mechanical characteristics as the key parameters. The types of completions that yield the most productive well performance for this matrix of properties are presented.A more detailed completion design is then illustrated by showing the use of the skin factor models for selection of liner completions for viscous oil reservoirs on the North Slope of Alaska. Application of the slotted or perforated liner models to the readily available liners showed that the completion skin factor can vary by as much as 40%, depending on the detailed characteristics of the slots or perforations in the liner (slot or perforation size, density, and distribution). This showed how analyzing the performance of the completion design can greatly increase well productivity at little or no cost.
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