The application of hydrajet technique to stimulate highly-deviated andhorizontal wells has become a successful method to improve well productivityfor different field conditions in the world. In the past 2 years, an operator company has successfully implemented arelatively new hydrajet stimulation technique in shallow waters off Brazil. Indeepwater locations, additional problems had to be overcome, which provedachievable using this new technology. This paper discusses a reservoir-based methodology to determine the optimumnumber of transversal fractures for a horizontal deepwater well. The methodstarts with the review of geology and stratigraphic aspects of the field tobetter understand the relationship between fracture orientation, geologicalfaults, and regional tectonic effects. With this preliminary characterization, well-log interpretation of the pilotwell and horizontal wellbore is performed to identify porosity and permeabilityindex of the carbonate formation being drilled. A study using well testing and nodal analysis is conducted to verifyreservoir properties based on real production data. Next, a numerical simulatoris used to obtain a production forecast varying the number of transversefractures intersecting the well. Finally an economic evaluation of net presentvalue vs. number of fractures is performed to determine the optimum number oftransversal fractures. Production results are then evaluated and compared to the other stimulationattempts in offshore horizontal completions in the area. Introduction In the complex world of hydrocarbon exploration and production, expectationsfor a certain level of economic success are not easily achieved. Obviously, there have been a few attempts rewarded with unexpectedly lucrative results.Unfortunately, this situation has become exceedingly scarce because ofever-depleting supplies in the world's known hydrocarbon-bearing formations.For obvious reasons, such cases will not be discussed in this paper. During the early years of oil exploration in Brazil, wells were drilled andcompleted as vertical wells. As the conducting surface in such wells weregenerally small (defined by p × diameter × formation height), production ratewas generally limited. Since well stimulation began in the early 1930s1 andeven more so after the present style hydraulic stimulation was invented in1949,2 most vertical wells have been stimulated at least once during the lifeof the well. Stimulation results are generally acceptable, although manytreatments did not produceeconomical results. In most cases, stimulationgenerally involves pressurization of the whole wellbore; with only a fewneeding isolation around the producing formation. As formations continue to deplete, treatment methods advance equally.Completion methods continue to advance, and the industry embarks upon adifferent approach: completing wells horizontally. These horizontal wells wereinitially completed openhole, and later were often completed with cementedcasing or uncemented preperforated liners to improve well integrity. Proponentssaid these wells, especially when completed openhole, would not require theconventional fracture stimulation, and reduce cost. Unfortunately, as alsooften experienced in vertical wells, operators in Brazil found many of theirhorizontal wells were under-achieving and therefore required stimulation.
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. AbstractNon-uniform distribution of flow rate along horizontal wells results in early water/gas coning, early breakthrough of water/gas during flooding, etc. For injector wells, injection flow rate profile along the horizontal length leads to low sweep efficiency and to large oil-filled by-passed zones.In the current work, a variable distribution of flow open area along a horizontal well, with or without sand control, is proposed, to correct the flow rate profile along it. An uniform flow profile is achieved by means of unevenly spaced slots or perforations along the liner or sand control device.An analytical model was developed for flow inside and around the horizontal well with unevenly spaced slots or perforations along the liner or along the sand control device. The model couples flows in well and in the reservoir in well vicinity. The analytical model provides an explicit formula for slot/hole density along liners or screens in injection/production horizontal wells. Applications of the model to design horizontal well in cases of injection and production, with or without sand control, with inflow control device are discussed.The technology has been successfully applied in some oilfields onshore and offshore Brazil, where horizontal producer wells were equipped with specially perforated and slotted liners. The purpose is to homogenize flow along horizontal producer or injector wells, in order to delay gas and water coning and improve the sweep. The varying slot/perforation density is calculated by the analytical model, and the calculated option was implemented in some wells.
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