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. AbstractThe placement of multiple fractures in horizontal, deviated wells is usually more difficult than placement in vertical wells. When completed with uncemented preperforated liners, such treatments become much more difficult and often are ineffective using conventional stimulation methods. When found in offshore locations, these wells present even more problems.Recently, in the shallow waters offshore Brazil, a relatively new hydrajetting technology was used with great success. However, additional complications arose when hydrajet fracturing was elected to stimulate a subsea well. This process requires dual high-pressure connections to the wellbore, including placement of a tubing string inside the full length of the well. Maintaining the hydrajet tool in position during each fracturing stage was a big issue in a floating rig. The need to pull tubing between stages of the stimulation process also poses a challenge.Because this operation occurred in a subsea well, it was the first such application ever attempted worldwide. Extremely detailed planning was essential to the ultimate success of the operation. This paper discusses the rigorous planning involved in implementing multistage hydrajet fracture acidizing in a subsea well offshore Brazil. During the operation, all challenges, mechanical or otherwise, were meticulously scrutinized, including the proper operation of the stimulation vessel and riser motion. This paper also presents posttreatment production results evaluated and compared to other stimulation attempts in offshore horizontal completions in the area.

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Summary Cased cemented completions have not been the preferred horizontal-well completion method in offshore Brazil. Lower-cost solutions such as uncemented preperforated liners were often used in completing horizontal wells offshore and are usually very effective. Often, however, low production rates mean that stimulation treatments become necessary for many wells. The use of conventional stimulation technology has generally been ineffective for these completions, which posed a challenge for the operator to find an effective solution for continuing developments in some fields. These challenges included reevaluating the more expensive cased cemented completions to allow more effective options for future stimulation, as well as trying to find newer stimulation techniques that can be effective with lower-cost completions (noncemented liners). In the attempt to find an economical yet effective stimulation solution, the operator chose to implement a unique and relatively new hydrajet stimulation technique that has a proven success rate in onshore applications. The technique can be applied in either sandstone or carbonate formations, which are commonplace in this field; therefore, stimulation plans in this area will include fracture acidizing as well as propped fracture stimulations that use a high concentration of proppants or curable resin-coated proppants (RCPs). This paper discusses the early results of this investigation. Wells that were completed and evaluated using different completion schemes are reviewed. Background As with vertical wells, underachieving wells are commonplace in horizontal completions. This situation can be caused by many things, such as unexpectedly low permeabilities in the area. However, among horizontal wells, underachieving wells are probably most often attributable to one or more of the following circumstances (McDaniel et al. 2002a; Surjaatmadja et al. 1994; Surjaatmadja et al. 2002a; Surjaatmadja et al. 2003; Abass et al. 1994, 1996):Permeability anisotropy (especially vertical permeability limitations).Skin damage or near-wellbore plugging of a natural fracture network.Ineffective stimulation techniques. The third circumstance is especially common in openhole horizontal wells, and even more so in wells that are completed using slotted or preperforated liners. Preperforated liners could even be deemed "unstimulatable" when conventional fracture-type stimulation techniques are being considered. Matrix-type stimulation or wellbore wall-cleaning techniques would probably be the only viable solution for production enhancement in this type of well. For moderate- to high-permeability reservoir applications, hydrajetting, when coupled with "squeeze" techniques (below fracturing pressures), has proved to be a very beneficial production-enhancement process (Rees et al. 2001; Surjaatmadja et al. 2002b). Unfortunately, because formation layers tend to be horizontal, horizontal wells tend to stay within one layer over a long distance, and permeability anisotropy becomes a serious problem. Another characteristic of horizontal wells is that they are often used in formations with very limited thickness; hence, their natural production performance has been artificially disadvantaged. Fracture stimulation techniques are believed to offer the best opportunity to achieve adequate stimulation under these conditions. One possible solution that has been attempted is the use of very high-rate water-frac treatments. These treatments have often produced a disappointing production response, and fracture-mapping techniques applied during some of these treatments have demonstrated the inefficiency of this treatment method when the operator attempts to create multiple fractures along the entire length of the wellbore (McDaniel and Willett 2002; Duttlinger 2001; Fisher et al. 2002; East et al. 2004). A logical continuation of this line of thought might lead to the use of sealing devices. Unfortunately, sealing devices such as inflatable packers tend to be ineffective when used in horizontal openhole completions, because fractures tend to jump over them, creating a passageway. For preperforated-liner applications, a passageway obviously has been established by the annulus region behind the liner. Based on these stipulations, the operator of an offshore Brazil field decided to implement a relatively new hydrajet fracturing process. This would be the first hydrajet fracturing stimulation performed offshore.

Cased cemented completions have not been the preferred horizontal wellcompletion method in offshore Brazil. Lower-cost solutions such as uncemented, preperforated liners are often used in completing horizontal wells offshore andare usually very effective. Often, however, low producing rates necessitatestimulation treatments for many wells. For some wells, this was the case atinitial completion, and for others would be after a year or more of production.Application of conventional stimulation technology has generally been eitherineffective or not cost-effective for these lower-producing completions. Thissituation poses a challenge for the operator to find more effective solutionsto continue developing some parts of these fields. Efforts to find a solutionhave included the re-evaluation of the more expensive cased cemented completionmethod to provide more effective options for stimulation treatments andattempts to find newer stimulation techniques that can be more effective withthe lower-cost completion method using noncemented liners. In the attempt to find an economical yet effective stimulation solution, theoperator chose to implement a newly emerging technology: thehydrajet-fracturing process. Although this stimulation technique has a provensuccess rate in onshore applications, this was the first offshore application.The technique can be applied in either sandstone or carbonate formations, whichare both commonplace in offshore Brazil fields. Therefore, stimulation plans inthis field include fracture acidizing and propped fracture stimulations withhigh concentration of proppants [ceramic and/or curable resin-coated proppants(RCPs)]. This paper discusses results from the first few wells in the investigation.Additionally, comparison wells that were completed and evaluated with differentcompletion schemes are reviewed in depth. Background Horizontal wells are often found to be underachieving - a situation alsooften experienced in vertical wells. Reasons could often be similar betweenthese two types of wells. However, in horizontal wells, additional possiblecauses include vertical permeability limitations, skin damage, or near-wellboreplugging of a natural fracture networks. Although the second and third reasonsare also found in vertical wellbore systems, the longer drilling times withexposed pay zones make these problems more commonplace and more severe forhorizontal completions.1–7 When these horizontal wells werestimulated, the conventional reasoning pointed to the inadequacy of thestimulation method used for the stimulation process. One inadequacy isgenerally caused by the fact that conventional fracturing techniques oftenresult in excessive multiple fracturing or near-wellbore fracture tortuosityproblems. Conventional fracturing stimulation methods encounter more limitations inhorizontal (or highly deviated) wells that are completed using slotted orpreperforated liners. Because of this, preperforated liners are often deemed as"stimulation unfriendly," at least with the economic limits most operators willface. Matrix-type stimulation or wellbore wall-cleaning techniques are oftenthe only affordable option for these type wells. For moderate- tohigh-permeability reservoir applications, hydrajetting coupled with "squeeze"techniques (below fracturing pressures) has proven to be a very beneficialproduction enhancement process. 8,9 Unfortunately, as formation layers tend to be horizontal beds, horizontalwells tend to stay within one layer over a long distance, creating a seriousproblem when permeability anisotropy is present. Another characteristic of manyexisting horizontal wells is that they have been used in formations with verylimited pay heights; hence, their natural production performance hasartificially been disadvantaged by limited drainage areas. Therefore, fracture-stimulation techniques are believed to be superior to wellbore damageremoval in these conditions.