OVERVIEW Wellbore Stability and Sand Control Considerable effort has been spent during the past decade developing Additional constitutive models for pred by bfk20410


Wellbore Stability and Sand Control

               Considerable effort has been spent during the past decade developing             Additional
               constitutive models for predicting near-wellbore failure. While many of          Wellbore Stability
               these methods have been found to describe rock failure near the well-
               bore accurately—with particular success in categorizing hole failure in
                                                                                                and Sand Control
               the laboratory for thick-wall-cylinder and large-block specimens, for            Technical Papers
               example—the prediction or forecast of when this failure will cause a
 Goodman       specific consequence to drilling or production operations often remains          Available at the SPE eLibrary:
               a riddle that is answered by experience, one that is most often sur-             www.spe.org
mounted by use of so-called “field-calibration” techniques.
                                                                                                • SPE 86463
My opinion is that the greatest difficulty for the geomechanics practitioner today is the         The Role of the Annular Gap
application of hole-stability predictions or forecasts to drilling, completion, and produc-       in Expandable-Sand-Screen
tion operations. I suggest that the real value of geomechanics to engineering operations          Completions
is a consistent framework or reference frame against which hole-failure trends can be           [See JPT (May 2004) 44]
measured. This consistent reference frame can be adjusted/improved/calibrated as drilling
continues so that the most efficient well plan can be recognized and implemented early          • SPE 86535
in asset development.                                                                             Sand-Management Solutions
                                                                                                  for High-Rate Gas Wells:
This field-calibration process can still be linked closely with the high-end numerical-           Sawan Field, Pakistan
modeling and prediction methods available to geomechanics professionals today by use            [See JPT (September 2004) 81]
of high-fidelity measurements to monitor drilling, completion, and production opera-
tions. Examples include downhole measurement-while-drilling data to recognize hole
failure with time, sand-detection devices, and various completion-systems monitoring of
multiphase flow to picture the effect of solids production on surface facilities. The idea is
to “test” the hole failure and sanding predictions, revising the geomechanical predictions
as needed with drilling, completion, and production-systems monitoring to understand
the consequence of operations.

The challenges are (1) to know the consequences of hole failure or sand failure to the
drilling/production asset and (2) to know when this degree of instability will cause a change
in well-systems design that ensures optimized exploitation of the producing asset. JPT

Harvey E. Goodman, SPE, is Senior Staff Research Scientist at ChevronTexaco’s Energy
Technology Co. His key technical responsibilities include rock physics, acoustic rock
mechanics, and the application of geomechanics to well design by use of the common
Earth-model approach. Goodman holds BS and MS degrees in geological engineering from
the U. of Missouri–Rolla. He serves on the JPT Editorial Committee.

66                                                                                                                   OCTOBER 2004
Wellbore Stability and Sand Control

Oriented Perforations as a
Sand-Control Method

The Varg field in the central North Sea is         Data Acquisition and Analysis
in a complex compartmentalized structure           Appraisal Well Data. During the develop-             This article, written by Assistant Tech-
with different pressure regimes. Field             ment phase, a field data-acquisition cam-            nology Editor Karen Bybee, contains
stresses are complex because of a number           paign was carried out. Data from nine                highlights of paper SPE 86470, “The
of faults, and stress-field rotation has been      appraisal wells were gathered from final             Effect of Oriented Perforations as a
observed in wells near faults. A sand-pro-         drilling reports, completion reports, well-          Sand-Control Method: A Field Case
duction risk analysis was conducted                test reports, and logs. In addition, special-        Study From the Varg Field, North Sea,”
before completion of appraisal wells, and          core-test reports were examined.                     by Johan Tronvoll, SPE, SINTEF
sand-production risk was judged substan-                                                                Petroleum Research; Arne Eek, SPE,
tial. On the basis of perforation-stability        Reservoir Stress Field. Because of the com-          Pertra A/S; Idar Larsen, SPE, SINTEF
computations, selective and oriented per-          partmentalized structure of the field, differ-       Petroleum Research; and Francesco
forations were suggested as a method to            ent pressure regimes were used when esti-            Sanfilippo, SPE, Oilfield Geo-
minimize sand-production risk.                     mating intitial pore pressure before produc-         mechanics Intl., prepared for the 2004
                                                   tion and possible stress paths for the indi-         SPE International Symposium and Ex-
Introduction                                       vidual compartments. However, no precise             hibition on Formation Damage Control,
Sand production is one of the most recognized      mapping of stresses and pressures was pos-           Lafayette, Louisiana, 18–20 February.
productivity-reducing factors in weak-sand-        sible because of field complexity and the
stone formations. Different sand-control tech-     limited amout of data. Relatively consistent       rock-mechanics tests that weak sandstones
nologies to reduce or eliminate sand-produc-       density-log data from four appraisal wells         behave in a nonlinear elastoplastic manner.
tion problems have evolved over the years.         gave a general vertical-stress gradient of            A 3D finite-element (FE) model based on
Active sand control such as gravel packing         approximately 2.1 bar/10 m in the reservoir        an elastoplastic constitutive model was cali-
often has resulted in reduced productivity,        section at approximately 2900 m.                   brated by use of rock-mechanics-test data
additional cost, and operational complexity.          The minor horizontal-stress gradients           from the Varg reservoir. However, simulations
Sand management has been proposed as an            were deduced from well fracture tests. A total     based on the FE model did not improve the
alternative to gravel packing. Sand manage-        of 27 leakoff tests and formation-integrity        estimate of the horizontal-stress anisotropy.
ment involves a thorough understanding of          tests, as well as one minifracture test, were
the entire sand “life cycle,” from detachment      performed. From these, 16 tests were used in       Sand-Production Risk Analysis
and mobilization at the sandface, to trans-        interpreting the minimum horizontal-stress         An empirical analytical sand-prediction
portation into the wellbore, to settling in the    gradient. Five of these tests were discarded       model was used to analyze the conditions for
wellbore or transportation to the surface. As      because of poor quality. The observation was       sand-production onset. The benefit of such a
part of such an approach, the use of oriented      made that wells to the east of the field tend to   model is its simplicity and robustness in
perforations is a design option, for which an      show higher horizontal stresses than those to      terms of required input data. A more accurate
understanding of the basic physics of perfora-     the west that fall outside the reservoir.          3D FE analysis would have had limited value
tion-cavity failure and sand production is nec-       Maximum horizontal stress could be              because of the great uncertainty in the reser-
essary to reduce formation-failure risk. The       inferred only theoretically on the basis of        voir stress field and the limited availability of
objective is to shoot the perforations in the      fracture-initiation pressure, fracture-closure     triaxial-compression test data. The simplified
most preferable direction with respect to the      pressure (horizontal stress), and pore pres-       model was very useful in evaluating the influ-
near-wellbore stress field. To obtain maximum      sure. The major in-situ horizontal stress and      ence/effect of the uncertainty in input data.
stability, the perforation should be oriented so   stress gradient were computed. as well as the         A risk approach was selected because of the
that the stress anisotropy in a plane normal to    minor horizontal-stress gradient. The major        substantial uncertainty of the input data. Real
the cavity axis is minimized.                      horizontal stress apparently is 15 to 20%          predictions of sand-production onset condi-
   After the effects of oriented perforations      higher than the minor horizontal stress. The       tions were not made, but the sand-production
were experimentally verified in the labora-        major horizontal stress exceeds the vertical       model was used to minimize sand-production
tory, use of oriented perforations in sand-        stress, suggesting a strike-slip stress regime.    risk through a parametric sensitivity analysis in
production management was adopted in               However, such an interpretation has signifi-       which uncertainties in formation stress regime
many North Sea completions. Vertical per-          cant uncertainty because it is likely that some    and formation strength were central. This out-
forations often are shot at 180° phasing in        test data were influenced by the presence of       lined the well-completion scheme correspond-
horizontal wells in tectonically relaxed           faults, and the Kirsh solution to wellbore         ing to the minimum sand-production risk.
basins (i.e., the vertical stress is the major     stress fields tends to overestimate the major
principal stress). With today’s technology, a      horizontal stress because of the assumption        Well Perforation Strategy
perforation-orientation accuracy of ±30° or        of a linear elastic formation and no fluid         On the basis of the parametric sensitivity
better is realistic.                               leakoff to the formation. It is known from         analysis for sand-production initiation con-

OCTOBER 2004                                                                                                                                       67
                                                                        detectors are sensitive to       before production startup. Still, no correla-
                                    Sand production   Sand rate
                        6000                                      60    changes in flow conditions,      tion has been found between the weak sec-
                                                                        and proper calibration and       tions and sand production in Well A05.
      5000                                                      50
                                                                        limitation of background            The theoretical modeling before produc-

                                                                       Sand-production rate, kg/h
                                                                        noise is critical. No signifi-   tion startup clearly showed the benefit of
  Sand production, kg

      4000                                                      40
                                                                        cant sand production was         oriented perforations despite the uncertain-
      3000                                                      30      observed until Well A05          ties in stress orientations and magnitude.
                                                                        began to produce sand            For horizontal and highly deviated wells,
      2000                                                      20      sometime between October         vertical perforations or perforations parallel
                                                                        2002 and April 2003. An          to the well azimuth plane will improve per-
      1000                                                      10
                                                                        average sand rate on the         foration-cavity stability significantly. Also,
         0                                                      0       order of 5 g/s or 432 kg/d       180° phasing reduces the risk of perforating
           0      2000      4000        6000     8000       10000       was indicated. The detector      in the least-preferable perforation direction.
                                Time, h
                                                                        readings indicated that             Reservoir depletion leads to changes in the
   Fig. 1—Predicted sand-production rate and cumula-                    sand rate was variable. This     relative stress magnitude and induced signifi-
   tive sand production for Well A05.                                   may be an indication of          cant stress anisotropy. If the interpretation of a
                                                                        transient sand production        strike-slip stress regime is correct, reservoir
ditions, the decision was made to adopt a from a number of layers, or it may be caused                   depletion should reduce the difference
selectively oriented perforating strategy to by a wellbore slugging flow regime.                         between the major horizontal and vertical
minimize sand-production risk. The deci-                 Well A05 production history was analyzed        stresses. In the case of proper perforation ori-
sion was based on the following arguments. by use of an expanded pseudo-3D model that                    entation, reservoir depletion theoretically
    • The great uncertainty in the direction includes the dynamic nature of sand produc-                 should improve perforation stability and sup-
and relative magnitude (i.e., horizontal-stress tion by capturing the effects of sand mobiliza-          press sand production. Depending on initial
anisotropy) of the horizontal stresses as well tion and removal from the sandface. Well A05              stress anisotropy, this tendency may be
as the uncertainty in the vertical to maximum penetrates the reservoir at an average 45°                 reversed in the case of large depletion. The ver-
horizontal-stress ratio calls for a well archi- inclination and is perforated at 180° phasing            tical stress is likely to remain or become the
tecture designed to minimize the risk of a and 4 shots/ft (SPF) in the upper part and                    major principal stress in a depleting reservoir.
high stress concentration at the sandface.            with 90° phasing and 8 SPF in the lower,           Exceptions may be highly tectonic areas where
    • For horizontal and highly deviated more-competent sands. Because formation                         major stress anisotropy has been reported, or
wells, the risk was judged minimum for ver- strength varies throughout the pay zone, a                   wells in the immediate vicinity of faults where
tical perforations because reservoir deple- 5-Mpa unconfined compressive strength is                     the horizontal stress field may be twisted. Well
tion would lead to reductions in the hori- assumed for the weakest sands. Considering                    A05 in the northwestern part of the field is the
zontal to vertical stress ratios.                     the 30° uncertainty in perforation orientation     only well where significant sand production
    • The 180° phasing would reduce the risk of for oriented perforations, sand production as            has been reported, and it is likely that the
perforating in the most unfavorable direction. a function of time was estimated assuming a               proximity to major faults is responsible for a
    • Nonoriented, high-density perforations 20-bar production drawdown and a 180-bar                    mismatch between perforation direction and
in competent medium-permeability layers reservoir depletion. Average petrophysical                       major principal-stress direction.
in combination with oriented (i.e., 180° properties and fluid properties were assumed
phasing, low perforation density) perfora- so that realistic production rates (approxi-                  Conclusions
tions in weak, high-permeability formations mately 850 m3/d) were achieved for a 100-m                   A field data-acquisition program and a sand-
would lead to a more uniform inflow profile production interval. Fig. 1 shows the result-                production risk analysis were conducted
with lower sandface velocities in the weaker ing sand production as a function of time and               before production startup in 1998 for the Varg
and more-permeable formations.                        the cumulative sand produced with time             field. The analysis indicated the presence of
                                                      under such production conditions. A short          weak sands and a complex stress field affected
Production Data and Analysis                          spike of sand is seen initially, followed by a     by the presence of faults. The analysis showed
Production data show a clear tendency of rapid rapid decline in sand-production rate. With               that vertically oriented perforations in highly
hydrocarbon-rate decline during the first few time, substantial amounts of sand production               deviated and horizontal wells would reduce
months of production. However, some wells are predicted for the given condition. On the                  sand-production risk to a minimum even for
show improved production in limited periods basis of model computations, the steady-state                an approxiimately 100-bar reservoir depletion.
of time because of pressure maintenance or sand-production rate is less than 1 kg/h.                     Oriented perforations with 180° phasing were
temporary shut-ins followed by a natural reser-                                                          shot in the weaker parts of most production
voir-pressure buildup. Most wells experienced Discussion                                                 wells, with only sporadic sand production
water breakthrough after some months of pro- Experience from the Varga field reservoirs is               reported to date. The exception is Well A05,
duction and a water/oil ratio equal to or greater that very limited sand production was                  close to a major fault where local horizontal-
than one. The gas/oil ratio typically is on the observed during the production life of the               stress rotations are suspected.          JPT
order of 150 m3/m3, occasionally increasing to field. Because of the marginal reservoirs, the
approximately 500 m3/m3.                              field has matured fast, and even now in the
    Except for early well tests, few sand-pro- late-life production mode, no serious sanding               For a limited time, the full-length paper
duction observations were reported. Occa- problems have occurred. Later formation-                         is available free to SPE members at
sional sand-production surveys were per- strength investigations using the scratch                         www.spe.org/jpt. The paper has not
formed during 2002 and the first months of technique have revealed that parts of the pay                   been peer reviewed.
2003 by use of acoustic sand detectors. Such zone exhibit far weaker rocks than thought

68                                                                                                                                    OCTOBER 2004
Wellbore Stability and Sand Control

Novel Perforating System Results
in Improved Perforation for
Sand Management
If a completion is a candidate for sand          horizontal completion be directed to the top
management, a new gravity-force-orient-          and bottom (180° phasing) of the wellbore.         This article, written by Assistant
ed perforating strategy can eliminate tra-          After the decision is made to adopt an ori-     Technology Editor Karen Bybee, con-
ditional sand-control treatments. If the         ented-perforation strategy, the perforating        tains highlights of paper SPE 86540,
perforations are oriented in the maxi-           program must be designed to assure that the        “Novel Perforating System Used in
mum-principle-stress plane, the perfora-         perforations are placed accurately in the          North Sea Results in Improved Perfor-
tion tunnels usually will be more stable         proper stress orientation. Minor errors in         ation for Sand-Management Strategy,”
than perforations placed away from this          perforation placement can lead to problems         by Eivind Hillestad, SPE, and Paal
preferred stress plane. A new perforating        initiating hydraulic fractures, perforation-       Skillingstad, Statoil, and Kent Folse,
system was used in a North Sea field and         tunnel collapse, or sand production.               SPE, Oyvind Hjorteland, SPE, and
resulted in a 37,600-BOPD sand-free pro-         Diagnostic tools are readily available during      Janne Hauge, SPE, Halliburton Energy
duction rate. The new gun system can             the drilling phase to determine the stress         Services Inc., prepared for the 2004
be conveyed on wireline or coiled tub-           state in the near-wellbore region; however,        SPE International Symposium and Ex-
ing (CT) and also can be used in conven-         perforating hardware and accessories               hibition on Formation Damage Control,
tional tubing-conveyed perforating               required to position the perforation planes        Lafayette, Louisiana, 18–20 February.
(TCP) applications.                              have been less than reliable.

Introduction                                     Orienting Techniques                             gun-to-casing standoff is required to allow
Oriented perforating becomes especially          Each of the several existing orientation         the guns to rotate freely.
important if formation integrity is question-    methods available has certain limitations.          In the past, some completion strategies
able and sustained production without sand       For example, when perforating shorter            have called for positioning guns on depth
mobilization is desired. Normally, operators     intervals, orientation by use of gyroscopic      and running wireline gyroscopic tools to
automatically default to a conventional          tools, with guns conveyed on electric line, is   reference from a known point in the work-
sand-control treatment to ensure that pro-       quite common. These techniques generally         string to determine current position; then,
duction is not interrupted and that costly       are quite accurate; however, they normally       the entire workstring is rotated at surface
well interventions to clean out wellbore         require multiple wireline runs to confirm        and orientation is confirmed. This tech-
sand accumulations will not be required.         orientation and are limited by the gun           nique works well for shorter gun lengths
While traditional sand-control methods are       length and weight that can be conveyed           where torque on the gun assembly is negli-
capable of controlling unwanted sand pro-        safely. Additionally, wireline techniques can    gible. The ideal orienting system would
duction, significant productivity penalties      not be used in horizontal completions,           eliminate the torque and friction that induce
often result. If the completion is a candidate   where the gun system must be conveyed            unwanted rotation of the gun system.
for sand management, a gravity-force-ori-        into the wellbore on some type of pipe.
ented perforating strategy that eliminates          For CT or TCP systems, orientation has        New Gravity-Force-Orienting System
traditional sand-control treatments can be       relied on gravity-based systems to orient or     A novel orientation system has been devel-
planned. Because this system relies on grav-     rotate the guns to the proper direction. A       oped that uses the force of gravity to posi-
ity for proper orientation, the most impor-      typical TCP-oriented system consists of          tion the shaped charges. The shaped-charge
tant consideration for using this concept is     mechanical tubing swivels, explosive trans-      orientation is accomplished by moving the
that the wellbore have a 25° minimum devi-       fer swivels, and eccentered gun tandems or       swivels from the external gun tandems to
ation to orient to maximum principal stress      offset tandem fins. This type of system is       the shaped-charge tube holder inside of the
or the overburden gradient.                      designed to allow the entire gun carrier to      scalloped gun carrier. The scalloped gun
   For a vertical completion, it is advanta-     rotate so the eccentric tandem travels on the    carrier is an atmospheric chamber with
geous to define the minimum and maxi-            low side of the wellbore or the offset fins      essentially a polished bore that is free from
mum horizontal stresses and orient the per-      travel on the high side of the wellbore. The     any pipe dope, residual cement/mud, mill
forations to the maximum horizontal stress       problem inherent with this type of system is     scale, or any other types of debris that are
to improve hydraulic-fracturing treatments       that long gun assemblies experience signifi-     commonly found in the wellbore.
and increase the probability of more-stable      cant torque and drag as they are conveyed           Fig. 1 shows the orienting charge-tube
perforation tunnels for a “perforated only”      into the wellbore, leading to potential error    configuration used in the new assembly that
completion. When designing the perfora-          in perforation placement on the order of         consists of multiple charge tubes per 22-ft
tion strategy for a cased horizontal comple-     ±20° in some cases. Assuming a perfect           gun carrier that rotate independently of
tion, the dominant stress field will be the      wellbore with no torque or drag placed on        each other. Orientation is achieved by
vertical, or overburden, gradient. Therefore,    the gun assembly, the potential error band is    mounting the shaped charges relative to a
it is important that the perforations in the     still on the order of ±10° because sufficient    series of weight strips attached to the charge

OCTOBER 2004                                                                                                                                69
                                               completion to orient perforations in larger-       retrieved from the completion by examining
                                               casing profiles. Previously, this type of per-     the exterior of the gun carriers. When the
                                               forating would not have been possible              gravity-force assembly orients properly, all
                                               because of the requirement to position the         of the independent charge-tube assemblies
                                               guns with orienting fins and mechanical            in each 22-ft gun carrier should result in a
                                               swivels. Elimination of the orienting fins         straight line of gun exit holes in the bands
                                               and other associated hardware required             down the length of the gun assembly.
                                               with the old system also speeds up the
                                               makeup and breakout of the perforating             Gravity-Force-Orientation Verification
                                               assemblies, thus reducing completion time.         To validate the new internal orienting sys-
                                                  The reduction in hardware also reduces          tem, a series of tests was performed to
                                               the makeup length and weight of the orient-        ensure gun survivability and accuracy. A
                                               ing gun assemblies, allowing longer gun            fully loaded 22-ft, 33/8-in., 4-SPF gun
                                               lengths to be conveyed on wireline in vertical     assembly was perforated at surface condi-
                                               wells or in horizontal wells with tractor assis-   tions in a surface pond in a horizontal posi-
                                               tance. In addition, the new orientation sys-       tion and then at a 30° angle. The surface-
                                               tem allows installation of roller tandems to       pond test resulted in a straight line down
                                               facilitate conveyance on wireline applications     the length of the 22-ft gun carrier for both
                                               where high well angle prevents gravity alone       gun angles. The deflection in the gun exit
                                               from positioning the gun assembly on depth.        holes from the straight line resulted in an
                                               The reduction in hardware also reduces the         orientation-error band close to ±5°. The
                                               distance between each loaded gun assembly          internal orienting gun system was placed in
                                               at the gun connectors, reducing nonperforat-       a pressure-test vessel, and the banded gun
                                               ed casing sections. For instance, for a fully      assembly survived an applied pressure of
                                               loaded 112-ft interval, the maximum number         30,000 psi at 450°F  .
                                               of perforations at 4 shots/ft (SPF) oriented at       The other test criterion for the new system
                                               180° would be 382 perforations with the old        was to ensure orientation accuracy for vari-
                                               system. For the newly developed gravity-           ous wellbore dogleg severities. To accom-
                                               force system, the total shots would be 425 in      plish this test, two 22-ft gravity-force gun
                                               the same perforation scenario. Increasing the      assemblies were connected and subjected to
                                               number of perforations results in a potential      maximum bending of 13°/100 ft. Gun orien-
                                               improvement in well productivity because of        tation was determined visually by machining
                                               better flow convergence to the wellbore.           windows at various positions on the exterior
                                                  In horizontal completions, one of the           of the gun carrier. The shaped-charge orien-
                                               major drawbacks to previous orienting tech-        tation was deemed to be essentially the same
                                               niques was that the gun system had to be           as observed during the surface-pond test.
                                               positioned on the low side of the well to ori-
                                               ent the guns. When the guns are lying direct-      Case History: North Sea
                                               ly on the casing, the row of perforations at 0°    The Norne field is approximately 125 miles
                                               phase will not have any gun-to-casing clear-       from the Norwegian coast. The field was dis-
                                               ance, which is required in many cases to           covered in December 1991, and an appraisal
                                               allow the shaped-charge jet sufficient time to     well drilled the following year confirmed
                                               develop fully as it exits the gun scallop. This    that the field was the biggest oil find for the
                                               can result in a reduction in formation pene-       group in several years. The reservoir consists
                                               tration and casing-entrance-hole size. The         of a 443-ft-thick hydrocarbon-bearing sand-
                                               other major problem with the gun on the            stone. Water depth is 1,250 ft.
                                               low side of the casing is that even if the well       Well 6608/10-B-3 H is a horizontal oil
                                               is perforated with optimum underbalance            producer that penetrates two sandstone for-
 Fig. 1—New gravity-force-orienting
                                               pressure to surge the perforation debris from      mations, the Ile and Tofte. It came on stream
                                               the perforation tunnels in the formation, the      in July 1999 and experienced water break-
                                               gun on the 0°-phase row of holes prevents          through in August 2001. After water break-
tubes. The exterior of the gun carrier is      efficient surging of the perforations.             through, a rapid decline was noted in the
modified with bands cut on the circumfer-         The newly developed gun system can be           oil-production rate. The well was shut in in
ence of the the gun to replace conventional    positioned or centralized as needed to             April 2002 because of a combination of low
scallops that are required to protect the      ensure that the low-side perforations are          reservoir pressure and high water cut.
completion from gun burrs.                     perforated at optimum gun clearance and               A multidisciplinary group made the deci-
   The primary benefit offered by this new     that perforations are cleaned up effectively.      sion to carry out a well intervention to
system is that orientation accuracy is not        Most orienting methods require very             detect and shut off water-flooded zones and
dependent on well path, fluid, or debris. In   complex gyroscopic-type tools to verify that       perforate oil-bearing zones in the heel sec-
addition, increased orientation accuracy is    proper orientation has been achieved after         tion of the well. The new gravity-force-ori-
possible in completions with restrictions;     the perforation event. With this system, the       enting perforating guns were selected
this assembly can be conveyed through the      orientation is confirmed when the guns are         because of the following capabilities.

70                                                                                                                           OCTOBER 2004
   • Orient perforations in vertical plane to
minimize risk of sand production.                   Resources
   • Orient guns in wells where sand and
scale may be present in the wellbore.
   • Use high-shot-density premium deep-                         Echometer Well Analyzer
penetrating shaped charges to maximize oil
   • Run long perforating strings (110 ft) in
each wireline run.
   • Be dispatched within the required time
   With rig operating costs close to U.S.
$400,000 a day, the majority of the interven-
tion costs are related to rig time. A dynami-
cally positioned vessel was brought on loca-                    Data acquisition and analysis are per-
tion in February 2003. A production log was                     formed using a powerful notebook com-
recorded across the reservoir at shut-in con-                   puter housed with an accurate analog to
ditions. Log data showed that all current per-                  digital converter in a rugged briefcase.
forations were fully or partly water flooded.                   A cost advantage is realized by using one
The upper areas of both the Ile and Tofte for-                  electronics package with the desired sen-
mations were believed to contain oil. Tofte 3                   sors to obtain a complete well analysis.
has a very good porosity (30 to 40%) and
permeability (approximately 1 to 2 darcy)                              •   Liquid Level Instrument
                                                                       •   Dynamometer
and has been known to produce sand.                                    •   Pressure Transient Tester
   The gravity-force-orienting perforation                             •   Motor Power Sensor
tool string, consisting of five 22-ft gun sec-
                                                                         Echometer Company
tions, was deployed by use of 7/16-in. cable                                5001 Ditto Lane
combined with a well tractor to pull the guns                           Wichita Falls, TX 76302
in place. A gamma ray collar locator was used                           Phone: (940) 767-4334
to correlate the guns, and the overall tool-                             Fax: (940) 723-7507
                                                                      e-mail: info@echometer.com
string length was approximately 138 ft. After                            www.echometer.com
perforating in the Ile formation, an attempt
was made to clean up the well. Because of
low reservoir pressure (close to hydrostatic)
and water flooding, it was decided to install a
permanent bridge plug above Ile. A second
perforation run then was performed in the
upper Tofte 3 formation. The guns were set
off with a 290-psi underbalance.

Field Results
An initial sand-free oil-production rate of
37,740 BOPD was obtained. This represented
a 20% increase in the overall Norne field pro-
duction potential at the time. Pressure-
buildup analysis gave a zero skin and a
58-bbl/(psi-D) productivity index. The com-
bination of a high-permeability zone, perfo-
rating underbalanced, vertically oriented per-
forations, and long perforation intervals con-
tributed to the high initial oil production rate.
   Operating efficiency exceeded 96% when
compared to original time estimates. All
well objectives were met, and payback for
the operation was attained within approxi-
mately 6 days of production.              JPT

  For a limited time, the full-length paper
  is available free to SPE members at
  www.spe.org/jpt. The paper has not
  been peer reviewed.

OCTOBER 2004                                                                                            71
Wellbore Stability and Sand Control

Wellbore Stability in the
Compacting and Subsiding
Valhall Field
The Valhall chalk reservoir is weak and              reservoir pressure declined during primary
started to compact when oil production               depletion from 6,500 to 2,500 psi in the crest,       This article, written by Assistant
began in 1982. The reservoir compaction              the chalk matrix has compacted, resulting in          Technology Editor Karen Bybee, con-
soon was observed at the seafloor in the             seafloor subsidence. Current seafloor subsi-          tains highlights of paper SPE 87221,
form of seafloor subsidence. As reservoir            dence is approximately 16.5 ft and continues          “Drilling Wellbore Stability in the
depletion, compaction, and subsidence                at approximately 10 in./yr.                           Compacting and Subsiding Valhall
continued into the 1990s, drilling strate-                                                                 Field,” by T.G. Kristiansen, SPE, BP
gies were developed to handle the reduc-             Drilling Experience                                   plc, prepared for the 2004 IADC/SPE
tion in fracture gradient as well as high-           At Valhall, drilling was not very difficult until     Drilling Conference, Dallas, 2–4 March.
angle extended-reach-drilling (ERD) wells            the ERD wells drilled from 1995 to 2000. In
in the overburden. An important factor for           the early part of the development, there were       study was that there would be an increased
the drilling teams in their decision-making          problems with swelling clays in the shallow         risk of wellbore instability in ERD wells
process has been development of wellbore-            sections, but this was cured by changing from       drilled from the crest compared to lower-
stability models and integration of drilling         water-based mud to oil-based mud. Fairly            angle wells drilled from a location on the
engineering, rock mechanics, and subsur-             high-angle wells were drilled in the mid-           flank. Also, there was believed to be lower
face technology. The full-length paper               1980s without any significant wellbore-stabil-      risk of casing collapse in a flank location. The
details the drilling history at Valhall.             ity problems. An interesting trend is that it       report also pointed out considerable uncer-
                                                     seemed to take more time to drill specific hole     tainties in the conclusions because of the lack
Introduction                                         sections as time went on. High-angle wells          of good data from the Valhall field itself. The
The Valhall field is an overpressured, under-        could be drilled through the Eocene easily in       ERD well that confirmed the flank reserves in
saturated Upper Cretaceous chalk reservoir in        the mid-1980s with older drilling equipment         the south of the field was drilled with very
the North Sea offshore southern Norway in            and technology but became increasingly diffi-       high nonproductive time (NPT) and a num-
226 ft of water. Valhall was discovered in           cult to drill in the late 1990s.                    ber of weeks were spent on the final wiper
1975, and a three-platform complex with 24              One of the first indications of changes at       trip. The overall economics and risk picture
slots was installed in 1981. The 24 slots on the     Valhall after reservoir depletion was lost cir-     at that point favored a crestal platform loca-
original platform were expanded to 30 in             culation experienced when drilling into the         tion and therefore required ERD wells. The
1990. In 1996, a new 19-slot wellhead plat-          more-depleted Tor. This was a problem               risk of wellbore instability was not neglected,
form for infill drilling and ERD was installed       because the exposed formations in the               and the decision was made to begin work on
next to the existing central complex. Because        Miocene, Eocene, and Paleocene required             tools and data that could assist the drillers if
of severe drilling problems during ERD, a            approximately 14.6-lbm/gal mud weight. The          drilling was challenging. The decision was
third unmanned wellhead platform was                 pore pressure was difficult to estimate in this     made to assemble relevant geological, petro-
installed on the south flank in 2002 to drain        low-permeability reservoir in infill locations,     physical, geophysical, and rock-mechanics
this area of the field more cost-effectively. A      leading to increased risk when drilling into        data to provide the best possible wellbore-sta-
similar wellhead platform was installed on the       very depleted reservoir targets. A drill-in liner   bility analysis that could be used for mud-
north flank in 2003. An additional new plat-         was developed in the early 1990s to address         weight prediction as well as drilling strate-
form will be installed in 2003 to waterflood         this challenge. This drill-in liner was used to     gies. The work focused on the overburden
the crestal part of the field. The field was orig-   drill the last approximately 100 ft of caprock      section because the reservoir was well char-
inally developed to recover 250 million bbl of       and into the reservoir. This technology has         acterized in terms of rock mechanical prop-
oil but has produced approximately twice this        been developed continuously and currently is        erties and stresses and had few drilling prob-
amount and is expected to recover more than          a very effective operation. Hydraulic fractur-      lems related to well instability.
1 billion bbl of oil from the Valhall structure.     ing always has been the preferred stimulation
   The reservoir is approximately 7,875 ft sub-      technique to achieve stability and long-term        Wellbore-Stability Modeling. Wellbore sta-
sea and consists of two oil-bearing chalk for-       productivity in the highly porous Tor chalks.       bility is a very complicated process to model
mations, Tor and Hod. Approximately two-                                                                 numerically with high accuracy. The best strat-
thirds of the oil and the majority of the pro-       Wellbore Stability                                  egy is to use the simplest possible model to
ductivity is in Tor. Effective overburden stress     As part of sanctioning the Valhall ERD pro-         answer specific questions.
in the crest of the field was approximately 500      gram to be drilled from the new wellhead               Modeling Approaches. Different modeling
to 1,000 psi at discovery because of significant     platform installed in 1996, an internal study       approaches have been used at Valhall, ranging
overpressure. The chalk is relatively weak,          was performed to examine the effects of well-       from very simplistic linear-elastic/brittle mod-
with 450- to 2,000-psi unconfined compres-           bore stability and well life for wells drilled      els to very advanced numerical elastoplastic
sive stress. Chalk production has resulted in        from the crestal-platform-complex location          models. The final model for Valhall was a lin-
lost oil production and casing deformation. As       vs. a flank location. The conclusion from this      ear-elastic model that allowed some degree of

72                                                                                                                                   OCTOBER 2004

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