• The use of clean, low-solids-content, compatible completion fluids
• The use of underbalanced perforations, such as 500 psi in oil wells and
1500 psi in gas wells
• The use of small charges, minimizing the effect of the compacted area (stress cage) around a perforation tunnel, which can reduce the original permeability as much as a factor of 10 or more
McLeod (1982) showed that the skin caused by the compacted zone could be very large.
The purpose of perforating should be defined clearly and designed appropriately. The perforation design should be evaluated based on the expected well completion and stimulation activity (Morita and McLeod, 1994). The purposes of perforating are listed below:
• Perforation as a completion method only or with the intention of matrixStimulation
• Perforation with the intention to gravel-pack
• Perforation for hydraulic fracturing (proppant or acid fracturing)
The perforation process will be discussed thoroughly in Perforating. This chapter will only discuss oriented perforations relative to the in-situ stresses.
This concept has been introduced to solve critical problems encountered during wellbore construction.
Oriented Perforations for Hydraulic Fracturing
Experimental and theoretical work indicates that perforation orientations should be designed to eliminate problems in fracturing vertical and deviated wells (Morita and McLeod, 1994; Behrmann and Elbel, 1991; Abass et al., 1994; Venditto et al., 1993). For a successful hydraulic fracturing treatment, perforation should be in phase with the anticipated fracture direction (the direction of maximum horizontal stress). This condition will
• Obtain maximum fracture width near the wellbore
• Create a single fracture
• Reduce the breakdown and propagation pressures
Figure 5-9 presents experimental results that show a complex fracture system in which the perforations are oriented within certain angles from the fracture direction (Abass et al., 1994).
Figure 5-9 Near wellbore fracture geometry as a function of perforation orientation relative to direction of in-situ stresses
The experimental study was conducted with hydrostone samples to study the effects of oriented perforations in the high and low sides of the horizontal well in the direction of the anticipated fracture. Figure 5-10 shows that for perforation angles of 0, 15, and 30°, the average breakdown pressure was 3200 psi; this pressure steadily increased for angles higher than 30°.
Figure 5-10 Fracture width performance and fracture initiation pressure as a function of perforation orientation relative to direction of in-situ stresses
Additionally, this experiment showed that fracture width is a function of perforation orientation. Figure 5-10 suggests that the optimal perforation phasing is 60° (equivalent to at most 30° deviation from the fracture direction) or less, at which the breakdown pressure is minimal. For an explanation of the negative widths in Figure 5-10, the reader is referred to Abass et al. (1994).
Clustered Perforations for Fracturing Deviated Wells
Clustered perforations can produce a transverse fracture perpendicular to the wellbore. A short perforated interval of 1 to 2 ft with 24 shots/ft can help reduce the occurrence of multiple fractures. A prefracturing stage of hydrochloric acid (HCl) in the treatment program can help establish a better communication channel between the wellbore and the main fracture.
Hydrojetting with HCl for Fracturing Horizontal Wells
Hydrojetting can ease the near-wellbore stress concentration, resulting in a successful fracturing treatment (Haigist et al., 1995). Figure 5-11 presents the sequence of operations for creating a single fracture from a horizontal well.
Figure
Figure 5-11 Hydrojetting and acid
Oriented Perforations for Sand Control
As previously explained, a circular wellbore in a rock formation creates a new stress field around the wellbore, which causes oriented failure (breakout) or total collapse (washout). Oriented perforations can be used for breakouts or unconsolidated formations.
Consolidated Formations
If breakout exists in a consolidated formation, the following steps are recommended (Figure 5-12):
• The near-wellbore area should be consolidated with a liquid resin material that is injected into the payzone.
• Because the breakout is oriented in the direction of minimum horizontal stress, a 180° phasing should be performed in the direction of maximum horizontal stress.
• A hydraulic fracture using a fracpack design should be performed.
Figure 5-12 Oriented perforation for sand control, where the
breakout region is left undisturbed
Unconsolidated Formations
Experiments showed that a wellbore should not be drilled through an unconsolidated formation because it would create a concentrated stress field around the wellbore. The hydrocarbon can be produced through hydraulic fracturing, whether in a vertical or a horizontal well. In a horizontal well, the perforation can be oriented in the lower side (Figure 5-13).
Figure 5-13 Experimental demonstration of drilling in the boundary layer and the use of oriented perforation and fracture to communicate with the poorly consolidated sandstones (sand production exclusion)
Based on fracture propagation mechanics, the fracture will have two wings in homogeneous formation even if it is forced to propagate in one direction (Figure 5-14).
Figure 5-14 Two-wing fracture propagating from one perforation
tunnel
Figure 5-13 demonstrates this concept, where a horizontal well was drilled in a homogeneous formation (hydrostone) with zero-phasing perforations in the lower side of the wellbore. A fracture was then initiated, and two fracture wings were created. However, when one wing of the fracture encountered a medium with less resistance to fracture propagation (lower fracture toughness), the upward fracture wing stopped and the lower one continued to propagate. The disturbance of energy required for fracture propagation works favorably in the technique above.
Fracpacks
From a rock mechanics perspective, the fracpack helps reduce near-wellbore pressure drawdown, which in turn reduces or prevents cohesive failure (erosion) and tensile failure. A combination fracpack/gravel pack can often effectively control sand production in many areas.
Figure 5-22 shows a hydraulic fracturing experiment in a poorly consolidated outcropping sample that has a Young's modulus of 377,000 psi and a compressive strength of 1037 psi.
Figure 5-22 Laboratory demonstration of fracturing poorly consolidated sandstone formations
This figure shows that a poorly consolidated formation can be fractured just like any conventional formation. The fracture length should be optimized to reduce the severe near-wellbore pressure drawdown (Abass et al.,1994; Fletcher et al.,1995).
Chemical Effects
A sandstone material's granular framework and type of natural cementation are inherent characteristics that help maintain stability; drilling a wellbore in the formation and introducing foreign fluids disturbs this natural stability. This section discusses the effect of drilling and completion fluids on the natural cementation material and describes a new means of restoring cementation during drilling.
Mineralogical analyses of most sand formations will reveal quartz, feldspar, carbonate (such as dolomite), and clay (such as chlorite, smectite). Cementation materials, such as quartz, dolomite, and chlorite, provide stability to a given formation and therefore should be maintained during drilling, completion, and stimulation phases.
Yale et al. (1995) studied the effects of cementation on the difference between the static and dynamic mechanical properties. The most interesting finding is the relation between the degree of nonlinearity and the static/dynamic ratio of mechanical properties. In other words, the type of cementation controls whether the material exhibits linear elastic, nonlinear elastic, or elastoplastic behavior during loading and unloading. Since a formation is exposed to many loading and unloading cycles during wellbore construction phases, studying a formation's loading and unloading characteristics is important. For exclusive sand control, the following techniques can be used individually or in combination depending on the failure mechanism:
• A horizontal wellbore in the boundary layer and a fracture to the formation
• Oriented perforations in the direction of maximum horizontal stress
• Fracpack and/or gravel pack
• Fracpack with resin-coated sand
• Consolidation during drilling
• Consolidation and fracpack
source: Petroleum well construction
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