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					30                 C ONDUCTIVITY E NDURANCE

Not All Resin-Coated Proppants
Are Created Equal1
Curable resin-coated proppant was introduced to the industry during the 1980s as a means to prevent
proppant flowback. For a hydraulic fracturing or frac-pack treatment to be effective, resin-coated prop-
pants should consolidate under downhole conditions into a long-lasting, high-strength permeable pack.

        oday, two general coating processes
        are used (Figure 1). The first is to
        precoat proppant with a resin (RCP)
in a manufacturing plant and then partially
cure it so the proppant can be conveniently
stored and transported to the job site with-
out consolidating. The second method
involves “on-the-fly” coating of the proppant
with an activated liquid resin system – LRS –
(Halliburton’s proprietary Expedite® service)
at the job site as it is used during a fracture
treatment. This system was first introduced
to the global stimulation market in late
1990s. As more wells are drilled into deeper
reservoirs, severe conditions – including high
temperatures, high production flow rates,
and high-closure stresses in propped frac-
tures – impose more constraints on the use
of curable resin systems.
    Nguyen et al.2 report on an extensive
study related to a new liquid resin coating                         Figure 1. Comparison of cure rates of RCP and high-temperature Expedite LRS system. Note that
                                                                    at 225ºF (107ºC) and 275ºF (135ºC), the Expedite system had a much slower cure rate.
system that is particularly, but not exclu-
sively, useful in controlling proppant flow-
back in high-temperature, high-flow rate                            properties to be achieved without any for-                        duction operations at the reservoir tempera-
wells. Additives included in the LRS elimi-                         mation closure stress.                                            ture. One of the targets of the Nguyen study
nate fracturing fluid interference and permit                           The study provides new answers as to                          was to provide a hydraulic fracture comple-
consolidation properties to be achieved                             why RCP consolidations sometimes fail in                          tion that can, in addition to being able to con-
without any formation closure stress.                               the field. Operators can use the information                      trol proppant flowback at high temperatures,
Results of this work demonstrate the diffi-                         presented to help them select the appropri-                       allow extremely aggressive flowback rates to
culty in designing fracturing treatments                            ate curable resin systems for their applica-                      remove the fracturing fluid in minimal time,
so the curing of resin does not occur too                           tions where consolidations are expected to                        so the production of hydrocarbons can begin
fast relative to fracture treatment time                            withstand production conditions of high                           as soon as the resin coating on the proppant is
and the time for formation closure to                               temperatures, high flow rates and high- or                        “cured” or set. There have been many tech-
achieve consolidation, but fast enough                              low-stress loading.                                               niques developed during the years to try to
to prevent flowback upon fluid recovery                                 An earlier study has shown that a compres-                    address these issues.
operations. The useful window for resin-                            sive strength of about 150psi is adequate to                          Failure mechanisms—The limitations and
coated proppants has been expanded by                               control proppant flowback in producing wells                      failure mechanisms of RCPs have been exten-
the incorporation of simple chemical addi-                          with moderate temperatures and production                         sively studied and well documented. Vreeburg
tives into the resin that react with fracturing                     rates.3 However, for a consolidated proppant                      et al. have identified two types of proppant
fluids, rendering them non-interfering to                           pack to be successful long term, one can infer                    flowback that occur when RCPs are used; one
the consolidation process. Additives                                that higher consolidation strength is required,                   is during the well-cleanup phase, and the
included in the LRS emiminate fracturing                            coupled with flexibility to handle repeated                       other is after a long period of proppant-free
fluid interference and permit consolidation                         stress changes that occur during normal pro-                      production.4
1. P. Nguyen, J. Weaver, M. Parker, M. McCabe, M. Hoogteijling and M. van der Horst, “A Novel       2. P. Nguyen, J. Weaver, M. Parker, M. McCabe, M. Hoogteijling and M. van der Horst, “A Novel
   Approach for Enhancing Proppant Consolidation: Laboratory Testing and Field Applications,” SPE      Approach for Enhancing Proppant Consolidation: Laboratory Testing and Field Applications,” SPE
   77748, presented at the SPE Annual Technical Conference, San Antonio, Sept. 29 – Oct. 2, 2002.      77748, presented at the SPE Annual Technical Conference, San Antonio, Sept. 29 – Oct. 2, 2002.
                                                                                                                                      C ONDUCTIVITY E NDURANCE                                  31

    The early production-type scenario is
thought to be caused by insufficient bonding
strength of the RCP. The factors affecting the
strength of the RCP pack include resin con-
centration, resin type, curing temperature,
resin/fracturing fluid interaction (under
shear and temperature) and erosion of the
resin from the proppant grains.
    The late flowback of proppant mecha-
nism was believed to be caused by damage
to the consolidated RCP resulting from
the stress cycling that the proppant under-
goes each time the well is shut in and put
back on production.
    During a fracturing treatment at nor-
mally used proppant concentrations, the
proppant grains are, in general, not in con-
tact (dispersed) while going downhole. In
addition, the fluid and proppant tempera-
ture is increasing during this time. Once the                         Figure 2. Expedite LRS coating remained on the proppant grains and provided consistent con-
proppant is placed in the fracture, it is                             solidation strengths despite extended stirring (simulated pumping) time.
believed that there is some proppant grain-
to-grain contact that is required in order to
form a consolidated pack. The loss of con-
solidation strength with curing under low
closure stress has been identified as a poten-
tial failure mechanism.4 Some RCPs have
been specially formulated to consolidate only
under high closure stress. Although this fea-
ture facilitates tubing cleanout after a prema-
ture screenout, it can lead to reduced
strength development of the RCP pack with
delayed or uneven closure of the formation.
    The confining stress acting on the prop-
pant pack during the curing process is prob-
ably not uniform because of variations of
the formation in-situ stress and the forma-
tion rock-mechanical properties. In addition,
some formations may not completely close
                                                                      Figure 3. In a comparison of widely used RCPs and Expedite® service at various closure
after treatments. Some hydraulic fractures do                         stresses, note that Expedite service provides about three times the conductivity of RCPs at
not completely close during the first 24                              4,000psi and 40% better conductivity at 10,000psi. Testing conditions: 2lb/sq ft, 300°F
hours after hydraulic-fracture stimulation                            (149ºC), water as flow medium, at least 48-hours per stress load. (*Note: RCP conductivity data
treatments, especially in the case of low per-                        is from supplier-published technical information.)
meability formations. In fact, it has been
reported that many reservoir rocks do not                             gle fractures in high-stress zones can screen                      also can be produced from a well that has
sufficiently close to prevent proppant flow-                          out during the proppant stages. Depending                          been perforated during a relatively short
back and settling during the first 90 days                            on the stage of the fracturing treatment, if                       interval if the treatment was not properly
after the fracturing operations.5                                     RCPs are not run throughout, portions of the                       designed, and the high proppant concentra-
    RCPs may not be effective when wells                              propped fractures may not contain any RCP                          tion RCP stages have been transported away
with multiple or large perforated intervals                           and may contain only uncoated proppant that                        from the near-wellbore area because of buoy-
are treated. Multiple fractures or parts of sin-                      can be produced back. Uncoated proppant                            ancy forces.6

3. L. Norman, et al.: “Applications of Curable Resin-Coated Proppants,” Production Engineering,           Slowly Closing Fractures,” SPE 37404, presented at the SPE Production Operations Symposium,
   November 1992, p. 343-349.                                                                             Oklahoma City, Okla., March 9-11, 1997.
4. R. Vreeburg, et al.: “Production Backproduction during Hydraulic Fracturing - a New Failure         6. M. Cleary and A. Fonseca: “Proppant Convection and Encapsulation in Hydraulic Fracturing:
   Mechanism for Resin-Coated Proppants,” Journal of Petroleum Technology, October 1994, p. 884-889.      Practical Implications of Computer and Laboratory Simulations,” SPE 24825, presented at the SPE
5. R. Blauer and D. Holcomb: “The Detection, Simulation, and Reservoir Performance Impact of              Annual Technical Conference and Exhibition, Washington, DC, Oct. 4-7, 1992.
32                 C ONDUCTIVITY E NDURANCE

                                                                                                                                      cure kinetics of the proppant’s resin coat on
                                                                                                                                      its ultimate consolidation.
                                                                                                                                          Cure kinetics and closure stresses—An ear-
                                                                                                                                      lier study by Nguyen et al.8 has concluded that
                                                                                                                                      most, if not all, of the currently available
                                                                                                                                      RCPs lose their ability to form consolidations
                                                                                                                                      with adequate strengths after being exposed
                                                                                                                                      to extended pump times in water-based frac-
                                                                                                                                      turing fluids and high temperatures. The cure
                                                                                                                                      kinetics of RCPs were found to have an
                                                                                                                                      impact on the resulting consolidation
                                                                                                                                      strength after curing. For an RCP to achieve
                                                                                                                                      maximum consolidation strength, the RCPs
                                                                                                                                      cure rate should be slow enough that there is
                                                                                                                                      minimal curing (or hardening) while the
Figure 4. With Expedite® service, capillary action causes flow of the liquid resin, concentrating                                     proppant is being pumped and until the for-
it between proppant grains and resulting in greater concentration of resin at contact points for
                                                                                                                                      mation has started closing.
increased durability. Above are photomicrographs of a widely used RCP (left) and proppant
coated using Expedite service (right). Both samples were handled identically and tested to failure.                                       By contrast, the systems that use a liquid
Resin grain-to-grain contact footprints (right) correlate closely to compressive strength. Note the                                   resin applied to the proppant as it is being
lack of contact footprints on the RCP (left).                                                                                         placed in the fracture have shown slower
                                                                                                                                      cure rates. These systems have demonstrated
    Even when RCPs are placed as designed,                             • increased shearing;                                          the ability to achieve consolidation strengths
failures can occur. Several factors that can                           • low and very high closure stress; and                        that are much higher than RCPs (Figures 2
affect RCP consolidation strengths and ulti-                           • increased stress cycling.                                    and 3). The high strengths are achieved even
mate performances are:3,4,6, 7                                        These factors are now well recognized in                        when there are long periods of time before
     • loss of consolidation strength has been                     the industry and usually are considered in                         fracture closure occurs and in some cases
       identified with increased fluid pH;                         job design and candidate selection. A factor                       even without closure stress applied to the
     • crosslinked carrier fluid incompatibility;                  not usually considered is the effect of the                        proppant grains.
                                                                                                                                          The later during the curing cycle the RCP
                                                                                                                                      is brought into grain-to-grain contact, the
                                                                                                                                      lower the ultimate consolidation strength
                                                                                                                                      that will be developed. Because the resin
                                                                                                                                      cure rate increases with temperature, the
                                                                                                                                      higher the temperature to which the prop-
                                                                                                                                      pant is exposed before grain-to-grain contact
                                                                                                                                      occurs, the lower the ultimate consolidation
                                                                                                                                      strength. In addition, fracturing fluid com-
                                                                                                                                      ponents can have a significant impact on the
                                                                                                                                      resin-curing rate (any test performed needs
                                                                                                                                      to be conducted in the actual fluid system
                                                                                                                                      planned for the fracture treatment). The
                                                                                                                                      combination of these effects of time before
                                                                                                                                      grain-to-grain contact, along with fluid
                                                                                                                                      effects and temperature, has a dramatic
                                                                                                                                      effect on the ultimate consolidation strength
                                                                                                                                      of the proppant pack.
                                                                                                                                          It can be expected that the resin systems
                                                                                                                                      with slower cure rates and those that require
                                                                                                                                      minimal closure stress for consolidation
Figure 5. Expedite® service can help prevent loss of fracture width due to proppant flowback.                                         would show less loss in consolidation strength
StimLab conducted tests at 250ºF (121ºC) and 6,000psi closure. Note that gas flow rate increased                                      in fracturing treatments with long pump
to about 130 MMscf/d with essentially no fracture width decrease due to proppant production.                                          times and slow formation closure rates.

7. S. Almond, G. Penny and M. Conway: “Factors Affecting Proppant Flowback with Resin-Coated           presented at the International Symposium on Formation Damage Control, Lafayette, La., Feb.
   Proppants,” SPE 30096, presented at the European Formation Damage Conference, The Netherlands,      18-19, 1998.
   May 15-16, 1995.                                                                                 9. B. Todd, et al.: “Resin Compositions and Methods of Consolidating Particulate Solids in Wells
8. P. Nguyen, et al.: “New Guidelines for Applying Curable Resin-Coated Proppants,” SPE 39582,         with or without Closure Pressure,” US Patent 6,311,773; issued Nov. 6, 2001.
                                                                                                   C ONDUCTIVITY E NDURANCE                 33

Temperature ratings for various Expedite liquid-   temperature, low-pressure reservoir with an       LRS was used, there was no decrease in pro-
resin systems.                                     objective to increase production and control      duction because the resin and frequency of
                         Bottomhole Static         proppant flowback. Three design criteria          workovers due to proppant flowback (sand
    Liquid Resin
                           Temperature             were pinpointed:                                  fill) was reduced by 75%.
                            Rating (˚F)                • perforation scheme;                              Conclusions—The Nguyen study and sub-
     Expedite 225              60 – 225                • hydraulic fracturing design with tip        sequent field applications demonstrate that:
     Expedite 350             200 – 350                   screenout (TSO); and                            • LRS permits aggressive well cleanup
                                                       • proppant flowback control (if                      procedures to be used after fracture
     Expedite 550              300 -550
                                                          required).                                        stimulation. This system significantly
                                                       After the optimum perforation scheme                 improves well production time to
    Why LRS-coated proppant outperforms            was completed, the hydraulic fracturing                  market;
RCPs—Halliburton’s proprietary Expedite            design was implemented to achieve TSO                  • LRS minimizes chemical interferences
LRS-treated system shows superior perform-         fracturing. A good TSO fracture treatment                with fracturing fluid and permits coat-
ance because:                                      was expected to significantly reduce proppant            ing of proppant just prior to blending
     • capillary action causes flow of the liq-    flowback. A live annulus was available so the            with the fracturing fluid;
       uid resin, concentrating it between         net pressure increase could be observed accu-          • the slow curing rate of the LRS allows
       proppant grains and resulting in            rately in real time. It showed a steady net              maximum consolidation strength to
       greater concentration of resin at con-      pressure increase, as reducing the pad from              be obtained, but sufficient early
       tact points (Figures 4 and 5);              14.5% to 4% quickly modified the design.                 strength to allow flowback to start
     • extrusion between proppant grains           This verified a good TSO was achieved.                   with a short shut-in time;
       increases porosity and fracture con-        Nevertheless, proppant flowed back even                • additives included in the liquid
       ductivity; and                              with optimum perforation design and TSO                  resin facilitate the removal of
     • LRS-treated proppant is tacky, which        fracturing. The LRS (Expedite service) was               crosslinked fracturing fluid from
       promotes grain-to-grain contact. In         chosen to control proppant flowback.                     the proppant grains;
       contrast, the RCPs, even when heated,           Twenty-three wells were fractured in this          • resin coating of the entire proppant
       are not as tacky and have little grain-     campaign (Figure 6). Of the 11 wells in                  stage eliminates the possibility for
       to-grain contact without closure            which the LRS was not applied, proppant                  uncoated proppant being produced
       stress. LRS-treated proppant has a          flowback occurred in eight wells, necessitat-            back; and
       slower cure rate and is not removed         ing workovers. Starting at the twelfth well,           • LRS consolidates proppant grains to
       from the proppant surface during stir-      the fracturing treatments included the LRS               the formation face. This produces a
       ring (simulated pumping) because the        additive as it was deemed to be the best                 larger “footprint,” which reduces fines
       resin system has been specially formu-      suited system for low bottomhole tempera-                creation and migration within the
       lated to preferentially coat proppants      tures and low closure stress. Results: When              proppant pack during time. ■
       in gel. RCPs, on the other hand, have
       faster cure rates, and the resin from
       some of these proppants has been
       shown to be leached off into the frac-
       turing fluids.
    RCPs are partially cured to provide for
storage and handling, and the portion of the
resin that is cured does not contribute to the
ultimate consolidation strength. Only a small
fraction of the resin in RCP is curable, while
all the resin in LRS is curable. Conversely,
LRS efficiently contributes to the final con-
solidation strength even with less resin:
     • LRS is formulated with additives that
       promote the removal of gelled fractur-
       ing fluid film that can sometimes
       impede grain-to-grain contact and
       consolidation9; and
     • LRS eliminates the problems of dam-
       age to coated proppants inherent in
       handling and storage.
    Case History: Indonesia—A fracture             Figure 6. Comparison between fracturing treatments not using LRS (Expedite service) and treat-
optimization process was initiated in a low-       ments using LRS.

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