Existing literature offers numerous success stories for stimulating cased/cemented horizontal completions in low-permeability reservoirs.

Cased cemented completions have not been the preferred horizontal-wellcompletion method in offshore Brazil. Lower-cost solutions such as uncementedpreperforated liners were often used in completing horizontal wells offshoreand are usually very effective. Often, however, low producing rates meanstimulation treatments become necessary for many wells. The use of conventionalstimulation technology has generally been ineffective for these completions, which posed a challenge for the operator to find an effective solution forcontinuing developments in some fields. These challenges included re-evaluatingthe more expensive cased cemented completions to allow more effective optionsfor future stimulation, as well as trying to find newer stimulation techniquesthat can be effective with lower-cost completions (noncemented liners). In the attempt to find an economical yet effective stimulation solution, theoperator chose to implement a unique and relatively new hydrajet stimulationtechnique that has a proven success rate in onshore applications. The techniquecan be applied in either sandstone or carbonate formations, which arecommonplace in this field; therefore, stimulation plans in this area willinclude fracture acidizing as well as propped fracture stimulations that use ahigh concentration of proppants or curable resin-coated proppants (RCP). This paper discusses the early results of this investigation. Wells thatwere completed and evaluated using different completion schemes arereviewed. Background As with vertical wells, in horizontal completions underachieving wells arecommonplace. This situation can be caused by many things, such as unexpectedlylow permeabilities in the area. However, in horizontal wells, underachievingwells are probably often attributable to one or more of the followingcircumstances:1–6Permeability anisotropy (especially vertical permeability limitations)Skin damage or near-wellbore plugging of a natural fracture networkIneffective stimulation techniques The third point is especially true in openhole horizontal wells, and evenmoreso in wells that are completed using slotted or preperforated liners.Preperforated liners could even be deemed as "unstimulatable" when conventionalfracture-type stimulation techniques are being considered. Matrix-typestimulation or wellbore wall-cleaning techniques would probably be the onlyviable solution for production enhancement in these type wells. For moderate-to high-permeability reservoir applications, hydrajetting, when coupled with"squeeze" techniques (below fracturing pressures), has proven to be a verybeneficial production enhancement process.7,8 Unfortunately, becauseformation layers tend to be horizontal, horizontal wells tend to stay withinone layer over a long distance and permeability anisotropy becomes a seriousproblem. Another characteristic of horizontal wells is that they are often used informations with very limited thickness; hence, their natural productionperformance has artificially been disadvantaged. Fracture stimulationtechniques are believed to offer the best opportunity to achieve adequatestimulation in these conditions. One possible solution that has been attempted is the use of very high-ratewaterfrac treatments. These treatments often produced disappointing productionresponse, and fracture-mapping techniques applied during some of thesetreatments have demonstrated the inefficiency of this treatment method when theoperator attempts to create multiple fractures along the entire length of thewellbore.9–12 Logical progression of thought might lead to the useof sealing devices. Unfortunately, sealing devices such as inflatable packerstend to be ineffective when used in horizontal openhole completions becausefractures tend to jump over them, creating a passageway. Forpreperforated-liner applications, a passageway has obviously been establishedby the annulus region behind the liner.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractHydrajet fracturing, a relatively new stimulation technology for horizontal completions, has already proven successful in oil and gas wells across three continents. This multistage fracture stimulation method has primarily used jointed pipe to achieve hydraulic fracturing injection rates. The recent introduction of large OD coiled-tubing (CT) to the process has improved operating flexibility, reduced job time, and significantly enhanced health, safety, and environmental (HSE) performance. For some operations, it can also provide cost savings.Before 2003, most CT-conveyed applications of this process typically were limited to hydrajet-assisted acidsqueeze injections, with only a few low-rate acid-frac treatments performed. By using larger-OD coiled tubing, operators have placed propped fracture treatments to approximately 8,000 ft measured depth (MD) at treating rates up to 10 bbl/min. The use of a combined workstring consisting of jointed pipe connected to a surface string of CT can enable these multi-stage treatments to be placed in much deeper reservoirs, effectively doubling or tripling the depth capability of a CTdeployed treatment. For many applications this capability would provide the flexibility, speed, and safety inherent to coiled-tubing operations with a spool of only a few thousand feet, and jointed pipe would not be exposed to high treating pressures at the surface. This paper reviews several field applications where coiled tubing was used to deploy hydraulic jet fracture treatments. One field case is compared directly to an earlier treatment in the same reservoir, which had been pumped through jointed pipe.

Proposal A relatively new technique for applying fractures (both proppant fracs and acid fracs)1–3 in horizontal openhole and uncemented liner completions is giving operators a more effective method for stimulation of underachieving wells. The process incorporates the known technologies of hydrajetting and fracturing in a novel way to achieve individual fracture placement at several selected locations along the lateral without the need for mechanical isolation methods, usually with only one service trip to the location. Depending on the number of fracturing stages desired, most job times vary from 6 to 12 hr if there are no hole-entry, tool, or logistic problems. Use of this new stimulation process continues to expand in the USA and Canada, and during the fourth quarter of 2002 and the first half of 2003, application of this process was successfully expanded beyond these original development areas to include offshore South America and a region of northern Asia. Before the end of 2003, numerous other locations around the globe are also planning to implement this stimulation technology on underperforming horizontal completions. Although this stimulation technique can also be used for cemented liners, its unique ability to control fracture placement in openhole and uncemented liner applications will be the focus of this paper. Only a brief review the basic technology is presented here, as numerous earlier publications provide more detail. New case histories will be presented wherein the application of this process has provided the operator with a distinct improvement in well completion economics. Background What are the possible reasons for a horizontal well completion to experience lower than expected production? As with vertical wells, in some horizontal completions, a lack of data will result in a dry hole, but this should represent only a small percentage of economic failures. More realistically, the greater number of underachieving horizontal completions is probably attributable to one or more of the following circumstances:4–9Lower permeability than anticipatedPermeability anisotropy (especially vertical permeability limitations)Skin damage or near-wellbore plugging of a natural fracture networkIneffective stimulation techniques In horizontal wells, conventional waterfrac treatments have been preferred in a number of reservoirs for many years. In additional to often disappointing production response, fracture-mapping techniques applied during some of these treatments have demonstrated the inefficiency of this treatment method when attempting to create multiple fractures along the entire length of the wellbore.10–12 Most often, the simultaneous creation and extension of multiple fractures in openhole horizontal completions results in at most two dominant fractured zones commonly referred to as heel-toe fractures (Fig. 1). Even more disconcerting is the fact that there is very little control over the placement of these fractures, which could mean that many of the more prolific intervals contacted by the wellbore will remain unstimulated and possibly also with skin damage.

Long horizontal sections have been used increasingly in moderate-to low-permeability reservoirs. In these reservoirs, a successful program for applying fracture-stimulation treatments is a key component for commercial success. Historically, such wells have been stimulated with several separate fracture treatments, requiring expensive well operations between fracture stages. Successful zonal isolation for each fracture stage has been a primary reason for the success of this technique. This paper reports recent efforts by several operators to reduce completion costs by extending the application of limited-entry fracturing techniques to very long sections of highly deviated or horizontal wellbores, while ensuring effective fracture stimulation of each perforated section.Many special innovations have been introduced recently for enhancing the applicability of limited-entry fracturing in long, openhole completions and in some uncemented liner applications. Some of these methods have been published; other unpublished methods are presented in this paper.Case histories are presented for both sandstone and carbonate reservoir completions. Considerations that guided the wellbore azimuth, casing programs, perforating schemes, and fracturing program variables are presented. Known problems and unpredicted formation responses are also discussed.