SPE 57954

Oriented Perforating for Sand Prevention

A.L. Sulbaran, SPE, PDVSA, R.S. Carbonell, SPE, Intevep-PDVSA, J.E. López-de-Cárdenas, SPE, Schlumberger

Copyright 1999, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the 1999 SPE European Formation Damage
Conference held in The Hague, The Netherlands, 31 May–1 June 1999. 50

Sand Production (lbm/kbbl)

This paper was selected for presentation by an SPE Program Committee following review of
information contained in an abstract submitted by the author(s). Contents of the paper, as
40
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Abstract
In weak formations or in competent formations with potential Figure 1 – History of Sand Production in the field
for sanding problems from high in-situ stresses, screens and
gravel packs often are not practical or economical. However,
in such formations, the perforating program can significantly In-Situ Stress Determination
affect the production of sand. With knowledge of the in-situ A detailed geomechanical study of the field was made to
stress distribution around the target well, the technique of characterize the Eocene C reservoir.2,3 To control sand
optimal phased and oriented perforating (OPOP) was applied production and to extend the well’s productive life, the critical
in the Eocene sands of the field in Lake Maracaibo, drawdown pressure (CDP) was estimated from a 3D
Venezuela, to minimize sand production. Results were simulator.4 For a horizontal tunnel in the direction of the
excellent. In this paper, the OPOP technique is described and maximum horizontal stress, the CDP was estimated to be 40%
general guidelines are given for its use. Field cases are of the initial reservoir pressure. This value was crossvalidated
presented that compare results gained using this technique to with a highly deviated pilot well.
the performance of other wells in the same field.
The geomechanical properties in general can be determined
Introduction from laboratory measurements and from shear and
Sand production has been a major problem in the Eocene C compressional well velocity logs. The critical rock mechanical
reservoir in Lake Maracaibo, Venezuela.1 The rock of these properties needed to estimate the maximum horizontal stress
reservoirs is competent and consolidated but under high in-situ (σH) were Young’s modulus (E), Poisson’s ratio (ν), Biot
stresses caused by the complex tectonic environment. A stress coefficient (α) and the critical stress intensity factor(KIC).
study of the field has shown high contrast between the Table 1 shows the average value of the critical mechanical
maximum and minimum horizontal stresses and similar properties of the reservoir.
magnitude between the vertical and minimum horizontal
stresses. Figure 1 shows the sand production history for the
field, indicating improvement in the last years. However, the
Ε ν α KIC UCS
1/2
average sand production of about 14 lbm/kbbl is still (psi) (psi*in. ) (psi)
considered high.
3x106 0.3 0.9 1250 8000
To minimize sanding, several techniques have been used—
hydraulic fracturing, high-angle drilling and, in the last year,
Table 1. Average mechanical properties of the reservoir
OPOP. The OPOP technique described in this paper has been
used in four wells with the best improvements in sand
prevention.
2 A. SULBARAN, SPE, PDVSA, R. CARBONELL, SPE, INTEVEP-PDVSA, J.E. LÓPEZ-DE-CÁRDENAS, SPE, SCHLUMBERGER SPE

The magnitude of the in-situ stresses were obtained as follows: influenced by the fault systems and the tectonic effects that
the vertical stress (σv) was obtained by integrating the rock occur in the area. Areas I and II in Fig. 2 show variation of the
stress direction, and Area III has more uniform stress
density from density logs; the minimum horizontal stress (σh)
direction. This implies that hydraulic fracturing could develop
was obtained from microfracture, minifracture, and leakoff
high tortuosity in I and II. Additionally, the close values of σv
tests; the maximum horizontal stress (σH) was estimated by
means of the theory of poroelasticity and fracture and σh suggest a possible fracture deviation during
mechanics.6,7,8 Table 2 shows a summary of the total in-situ propagation.
stress gradients of the field where Z is the vertical depth.
Perforation Tunnel Stability
The stability of a single perforation tunnel was evaluated using
an elastic-plastic model, a finite element analysis and Mohr-
σv / Z σH / Z σh / Ζ Coulomb failure criterion from laboratory testing. As the
(psi/ft) (psi/ft) (psi/ft) perforation tunnel is rotated about the axis of the borehole,
there is a critical angle measured from the direction of the
maximum stress where the tunnel is still stable. We call this
1.10 1.35–1.40 1.05–1.10 angle γ, the allowable perforation angle. This angle is used to
select the phasing and orientation of the gun. Figures 3 and 4
Table 2. In-situ stress gradients show the meshing and strain field for this analysis with a
drawdown pressure of 1000 psi. Figure 3 shows the equivalent
plastic strain contour plot of a perforated tunnel oriented in the
An interesting feature of the field is that the maximum
direction of σH.11
horizontal stress σH is the highest of the three; that is, σH >
According to the triaxial test laboratory data, failure takes
σv ≥ σh. This is the result of field tectonics.
An observation in a more recent study of the magnitude of the
stresses in the field shows that the vertical wells in the areas
with larger contrast between the maximum and minimum
horizontal stresses have more sanding.9 This finding supports
the recommendation of perforating to avoid the directions of
high contrast between horizontal and vertical stresses for this
field.

To estimate the direction of σH, a model was used at Petroleos

de Venezuela S.A. (PDVSA) Intevep.10 The model uses 3.8E-05
borehole image log information and data from laboratory core Figure 3 – Equivalent plastic strain contour plot of a
measurements such as anelastic strain recovery, shear wave perforating cavity in the direction of the maximum in-situ
amplitude anisotropy and acoustic anisotropy analysis. stress (σH)

place after the equivalent plastic strain has exceeded the value
Stress Field (σH) of 6.5 x 10-3.
Area III
Figure 4 shows the initiation of failure of the perforation
tunnel rotated 25° from the position of Fig. 3; that is, 25°

Area II

Area I

Figure 2 – Estimated direction of the maximum

horizontal stress

6.2E-03
Figure 2 shows the map of the Eocene C reservoir with the
Figure 4 – Equivalent plastic strain contour plot of a
estimated stress directions, where wide variations are basically
perforation at an azimuth of +/- 25° from the direction of σH
SPE ORIENTED PERFORATING FOR SAND PREVENTION 3

about the well’s axis.12,13 From this analysis, we concluded 1) Deep-penetrating charges produce perforations of smaller
that, for this field, if γ stays within +/-25° the perforation diameter than big-hole charges. Perforating tunnels of
tunnel is expected to be stable. smaller diameter in rocks are more stable than large-
diameter holes.14,15,16
Recommendations: Perforating for Sand Prevention 2) The thickness of the crushed zone around the perforation
Based on experiments at the Schlumberger Perforating and tunnel increases with the diameter of the perforation
Testing Center in Rosharon, Texas,14 and the geomechanical tunnel.
studies at PDVSA Intevep in Los Teques, Venezuela, the 3) Deep-penetrating charges have more chances of
following steps were followed in perforating the Venezuela perforating beyond the borehole-damaged zone and
field and are listed here as recommendations: provide a larger effective wellbore radius.

• Determine the magnitude and direction of the in-situ If the formation is not consolidated, or if a stimulation such as
stresses around the well location. hydraulic fracturing is planned, then big-hole charges are
• Define the zone around the wellbore where the generally recommended.
perforation tunnel is expected to be stable.
• Select appropriate deep-penetrating charges. Shot Density
• Use a shot density for sufficient productivity (6 to 8 Perforating with high shot density is commonly desired
shots per foot in this case). because of the expected higher productivity and lower flow
• Select a shot phasing that offers sufficient velocity in the perforations. However, increasing the shot
perforation-to-perforation distance to avoid rock density could reduce the perforation-to-perforation distance.
failure. The mechanical failure between adjacent perforations
observed in the laboratory suggested that if the distance is not
• Orient the guns to avoid shooting in the directions
sufficient a failure may occur.14 The objective was then to find
where the perforation tunnels are less stable (for this
the minimum distance to avoid rock failure and to have
field, areas having the largest contrast between the
horizontal and vertical stresses). sufficient shot density within γ for adequate productivity.
• Perforate with sufficient underbalance.
Minimum Perforation-to-Perforation Distance
Operational restrictions allowed a minimum distance of about
Figure 5 shows a top view of the plane of the perforations of
the especially loaded tubing-conveyed perforating (TCP) guns three times the estimated diameter of the crushed zone 3φ,
compared with the perforations of a standard gun. where φ is the diameter of the outer boundary of the
perforation crushed zone at the borehole face. To verify
perforation tunnel stability under this condition, the following
Conventional gun
σH
Oriented gun
analysis was conducted.
σH
To avoid possible rock failure resulting from the interaction
σh between perforations, the minimum distance δmin (see Fig. 6)
σh was validated based on two scenarios: 1) the expected stress
profile around one perforation tunnel, and 2) the superposition
of stresses from consecutive perforation tunnels. The
σv assumptions used were an elastic, homogeneous media with
σH
two parallel cylindrical tunnels under plane strain conditions.
Perfs in direction of large No perfs in areas of high
contrast between horizontal stress contrast
and vertical stress could fail

Figure 5 – Top view of perforation

Charges
Deep-penetrating charges, as opposed to big-hole charges, are
recommended for sand prevention in competent rocks for the
following reasons:
Figure 6 – Minimum perf-to-perf distance
4 A. SULBARAN, SPE, PDVSA, R. CARBONELL, SPE, INTEVEP-PDVSA, J.E. LÓPEZ-DE-CÁRDENAS, SPE, SCHLUMBERGER SPE

2
The expected stress field around one cylindrical perforation
tunnel in an homogeneous isotropic elastic media indicates 1.5 2
that the magnitude of the tangential stress around the tunnel 1
drops to about 1.1σv ≈ 1.1σh at a distance of about 1φ. 0.5

Skin
4
0
For consecutive perforation tunnels, the stress concentration in
-0.5 6
the vicinity of one tunnel was first estimated. Then, based on 9
the Mohr-Coulomb criteria, δmin between the two cylindrical -1 12

cavities at which no failure is expected was found to be less -1.5

than 3 φ, supporting that a value of δmin = 3 φ is satisfactory. 0 5 10 15
Shot Density (spf)
Therefore, the shot density and phasing for the perforating gun
must satisfy two conditions: 1) to perforate within the
allowable perforation angle γ = +/-25° measured from the
Figure 8 – SPAN Computation of Skin
direction of σH, and 2) to maintain a distance ≥ δmin. Before
deciding the final shot density, maximization of the
productivity of the well should be considered. 1) 4½-in. TCP guns with the special phasing discussed
above
Expected Productivity as Function of Shot Density 2) orienting sub, placed on top of the guns, with an internal
Figures 7 and 8 show the productivity and skin calculations as key aligned with the guns that worked as a guide for a
a function of the shot density for this field as calculated with a gyroscope
perforating and productivity numerical analysis15. For this 3) hydraulic packer to avoid rotation to set the packer.
case, shot densities below 6 shots per foot (spf) show a rapid
reduction in productivity and a rapid increase in skin, and After positioning the guns on depth, a gyroscope was run on
above 8 spf there is no significant increase in productivity but wireline through the string. With the gyroscope sitting on the
the risk of sanding may increase. Thus, to satisfy all the orienting sub, the string was rotated until the desired
requirements for a borehole diameter of 7 in., a shot density orientation was achieved. The hydraulic packer was set and
range between 6 to 8 spf was selected. the orientation of the guns was verified with the gyroscope.
Finally, after removing the gyroscope, the guns were fired
1.2
12
underbalanced.
9
6
1 4
Field Cases
Productivity Ratio

2
0.8 Four jobs have been performed using the OPOP technique for
0.6
sand control. The improvements are evident when production
is compared with the average sand and oil production of other
0.4 wells in this field. The average production for the field is 14.3
0.2 lbm/kbbl of sand and 1500 BOPD. In all four cases, the sand
production was reduced within or below 3 lbm/kbbl.
0
0 5 10 15
At the time this paper was prepared, the first job had the most
Shot Density (spf)
complete production data, which showed sand production
below 0.5 lbm/kbbl with an initial oil production of 4400
Figure 7 – SPAN Computation of the Productivity Ratio BOPD. The second well produced about 2200 BOPD (still
above average) and 3.0 lbm/1000 bbl of sand. The sand
production of the third job was reported at 3.0 lbm/kbbl with
The first three wells were perforated at 6 spf using an initial oil production of 682 BOPD. The fourth job had a
conventional guns but with shaped charges loaded in a pattern sand production of 0.4 lbm/kbbl with an initial oil production
to satisfy the above conditions. The last well was perforated of 1085 BOPD. Table 3 compares the average oil and sand
with a custom-designed gun that allows 8 spf, still satisfies the production of these four jobs with the field’s average. In all
original requirements of the minimum perforation-to- four cases the objective of sand prevention was successfully
perforation distance, and offers a more uniform distribution of achieved.
the perforations within the allowable perforation angle γ.

OPOP Operation
Three special components were used in the completion string:
SPE ORIENTED PERFORATING FOR SAND PREVENTION 5

parameter. However, a more detailed modeling would be

Field Job Job Job Job useful to determine the optimum values of the controlling
Average 1 2 3 4 parameters. Optimization of these parameters could allow
Initial Oil 1500 4427 2189 682 1085 shooting at higher shot densities and within a wider allowable
(BOPD) perforation angle. A 3D finite element analysis could be very
Average 14.3 0.5 3.0 3.0 0.4 useful in this process.
Sand
(lbm/kbbl) Conclusions
The presented methodology seems to be effective for sand
Table 3. Oil and sand production comparison prevention in competent rocks under high in-situ stress
magnitudes and contrast.
Figure 9 shows the oil production from the first job during the
first nine months, and Fig. 10 shows the corresponding sand The controlling parameters, allowable perforation angle γ and
production. The initial oil production of 4400 BOPD is about
the minimum perforation-to-perforation distance δmin, are
three times more than the average production of the wells in
characteristic of a given reservoir (i.e., in-situ conditions and
the field. Sand production of 0.5 lbm/kbbl is only 3.5% of the
geomechanical properties) and can be obtained from the
average well in the field.
borehole stability analysis of a perforation tunnel.

Oil Production - case Study #1 An optimum gun design to prevent sand production is a
function of the formation damage, reservoir properties,
Oil production (BOPD)

5000 completion scheme and constrains given by γ and δmin.

4000
3000
2000
Using deep-penetrating charges with oriented guns and
1000 shooting underbalanced have yield good well productivity
0 while reducing sanding significantly.
10/12/98

11/12/98

12/12/98
7/12/98

8/12/98

9/12/98

1/12/99

2/12/99

3/12/99

The recommendations from laboratory observations by

Behrmann14 appear to be consistent with the field results.

Figure 9 – Oil production of first OPOP job The presented methodology can also be used to identify the
preferencial fracture plane and orient the perforations
accordingly for hydraulic fracturing operations.
In all cases, the wells are producing with an approximate 1500
psi of drawdown pressure. This suggests the model used to Acknowledgments
The authors thank PDVSA and Schlumberger for their support
predict the collapse of the perforation tunnel as it is rotated
on this work.
horizontally away from the direction of maximum horizontal
stress was adequate and conservative.
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Sand Production History - Case Study #1
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20
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Sand Production (lb/kbbl)

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1996.
Future Work
The assumptions and model used for this analysis seem to
provide adequate and conservative ranges of the controlling
6 A. SULBARAN, SPE, PDVSA, R. CARBONELL, SPE, INTEVEP-PDVSA, J.E. LÓPEZ-DE-CÁRDENAS, SPE, SCHLUMBERGER SPE

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