Bart Waltman, Fong-Fong (Allison) Foo and Marshal

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               Bart Waltman, Fong-Fong (Allison) Foo and Marshal Strobel,
               Halliburton, USA, explore a sand control modelling software
                that enables frac pack treatments to be conducted without
                      pumping and analysing a minifrac beforehand.




                   A
                                 fracturing treatment normally includes a minifrac and step rate
                                 test. The results from these two procedures are normally used to
                                 calibrate the frac model. The minifrac process and resulting fluid
                                 efficiency data are extremely valuable in low permeability (hard
                                 rock) formations but have not proved to always be reliable in high
                   permeability (soft rock) frac pack treatments. Field data has shown that the
                   fluid efficiency obtained during a minifrac treatment can approximate the fluid
                   efficiency obtained during the frac pack, but is not always accurate enough
                   to provide optimised frac geometry. The Optimised Fracture Geometry (OFG)
                   process replaces these steps with a new process resulting in an optimised frac
                   pack and savings in rig time and fluid costs.




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                                                                                                       l      Workstring pickle.
                                                                                                       l     Formation acidising.
                                                                                                       l     Minifrac.
                                                                                                       l     Step rate test.
                                                                                                       l     Frac pack.
                                                                                                           A completed frac pack treatment
                                                                                                       results in a highly conductive
                                                                                                       propped fracture that extends from
                                                                                                       the wellbore, through the damaged
                                                                                                       near wellbore region and into the
Figure 1. The tip screenout fracturing process for high permeability formations.                       undamaged reservoir.
A. The first stage is pad, typically a viscous, cross-linked polymer gel that creates the necessary        The five step stimulation process
geometry in which to place proppant. The fluid used in the pad possesses a certain efficiency,         requires an average of 15 - 30 hrs.
that is, the ratio of the fluid that stays in the fracture compared to total fluid pumped.             Two of the steps (minifrac and step
B. Following the pad, proppant laden slurry is pumped. The goal is for the pad to have                 rate test) are considered to be
completely leaked off into the formation just as slurry reaches the designed fracture length,          fracture model calibrators and can be
creating bridging at the tip of fracture (screening out) and halting further fracture extension5.      eliminated through the OFG process.
C. Once a tip screenout is achieved, pumping with slurry is continued. Pressure begins to
increase as the fracture increases in width and becomes packed with proppant. As the fracture          The frac pack process
volume is filled with immobile proppant the treatment will screen out, i.e. the fluid velocity          Properly stimulating an interval
becomes too low to transport proppant any longer. At this point the fracture is essentially filled      requires placing a predetermined
with proppant and the maximum conductivity is obtained.                                                 volume of proppant into a fracture with
                                                                                                        certain dimensions of propped width
                                                                                                        and half length designed to enhance
                                                                            reservoir productivity. The ratio of ‘width to length’ must be
                                                                            optimised. This ratio determines the fracture’s ‘dimensionless
                                                                            conductivity’2 and indicates the ability of the fracture to convey
                                                                            the reservoir fluid to the wellbore.
                                                                                Dimensionless conductivity (FCD) is the ratio between the
                                                                            fracture flow capacity and the fracture face flow capacity.
                                                                            The fracture flow capacity is the permeability of the proppant
                                                                            multiplied by the width of the propped fracture. The fracture
                                                                            face flow capacity is the permeability of the formation in contact
                                                                            with the fracture multiplied by the half length of the fracture. This
                                                                            relationship is shown in the following equation:


                                                                                             Fracture flow capactiy            Kf * Wf
Figure 2. Potential minimum and maximum frac geometry shown                          FCD =                                 =
for an OFG process designed treatment for a 500 mD formation                                 Fracture face flow capacity       K * Xf
with fluid efficiencies ranging between 15 - 30%.
                                                                               Where:
                                                                               FCD = dimensionless conductivity.
Background                                                                     Kf = permeability of the proppant.
Over the past two decades, deepwater reservoirs have become                    Wf = width of the fracture.
a key sector for the oil and gas industry. Last year in the Gulf of            K = permeability of the formation in contact with the
Mexico, approximately 74% of oil production and 43% of gas                          fracture half length.
production were derived from reservoirs in more than 1000 ft of                Xf = half length of the fracture.
water1. Deep water continues to be one of the leading growth
areas in the oil and gas industry.                                              Several authors have demonstrated that for moderate
    The cost to develop deepwater reservoirs today can be                   and high permeability reservoirs, the optimum dimensionless
extremely high with rig rates exceeding US$ 1 million/d.                    conductivity is approximately 1.6.3 The frac pack attains the
The large recoverable reserves combined with completion                     necessary dimensions of length and width through the tip
and workover costs require efficient, reliable completion                   screenout process (see Figure 1)4.
techniques. Globally, frac pack stimulation completions
have delivered reliable, cost-effective production from high                Minifrac and step rate test
permeability wells.                                                         The frac pack process is the last step in the traditional completion
    Stimulation treatments can be the most critical stage of a              procedure. It is preceded by the workstring pickle, the formation
completion programme since the treatments greatly enhance                   acidising treatment, the minifrac and the step rate test.
the connection between the wellbore and the reservoir, helping                  The minifrac and step rate test are considered fracture
make the well financially viable. A typical stimulation treatment           model calibrators because many variables such as interlayer
consists of five steps:                                                     rock stresses, Young’s Modulus and leak-off coefficients are


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 Table 1. Even though the treatments were designed based on a 1.6 FCD, higher numbers were achieved meaning better fracture conductivity. The higher FCD numbers in this case were
 due to formation ballooning enabling more proppant to be placed

 Wells             Perf interval     Perf interval     Minifrac          Time               Rate (bpm)        Proppant          Tip screen        Net pressure      FCD
                   - ft (MD)         - ft (TVD)        performed         from acid                            placed in         out (TSO)         gain (psi)
                                                                         shutdown to                          formation
                                                                         pad pump                             (lbs/TVD ft)
 One lower         135               106               Yes               4 hrs 30 min.      30                1622              No                33                0.98
 One upper         350               276               Yes               4 hrs 40 min.      60                1689              Yes               614               0.96
 Two lower         170               146               Yes               3 hrs              30                1063              Yes               276               6.5
 Two upper         80                68                Yes               2 hrs              18                1792              Yes               510               1.69
 Three             225               211               No                1 hr 30 min.       50                1399              Yes               380               2.84
 Four lower        230               224               No                1 hr               50                1358              Yes               500               1.99
 Four upper        350               337               No                50 min.            60                1354              Yes               300               2.53
 Five              164               142               No                3 min.             40                1127              Yes               300               3.61




estimated or provided as a range of values. Currently, in an                                        Fracture models are still an integral part of the OFG design
ideal minifrac process, the injection and leak-off profile of the                                process and are used to provide pre-job analysis and post-job
frac pack fluid are estimated and a fracture simulation model                                    pressure matches.
generated to enable placing the proppant volume required to
enhance reservoir productivity. In addition, the step rate test,                                 Visual representation of OFG concept
typically pumped with linear gel or brine, indicates a fracture                                  Figure 2 represents a potential OFG treatment scenario for a
extension pressure (an upper bound for closure pressure) that is                                 FCD of 1.6. The initial design for this 500 mD well called for a
used in interpretation of the minifrac pressure response.                                        treatment volume of 50 000 lbs of 20/40 ceramic proppant.
    The assumption is that in an ideal five step frac pack                                       Since this is a high permeability formation, the majority of the
process, minifrac derived parameters can be used to help                                         fluid will leak off while creating the geometry. The 15% fluid
enable placing the industry accepted volume of 1000+ lbs/ft                                      efficiency indicates a fracture half length of 61 ft that would be
perforations. However, the fluid efficiency during the fracture                                  achieved with a pad of 5000 gal., allowing an optimal propped
treatment is a function of a number of factors that are not                                      width of 1 in.
always accurately assessed during the minifrac, making                                                For the same pad volume, if less fluid leaked off resulting in
obvious the inherent shortcoming of this step. Both the minifrac                                 30% efficiency, a fracture half length of
and step rate test can be eliminated using the OFG process.                                      117 ft would be generated. If constrained to the original
                                                                                                 50 000 lbs design, the extra length generated would leave a
Theory of the OFG process                                                                        suboptimal propped width. A TSO may not have occurred by
For a predetermined volume of proppant, the OFG process                                          that point as well. With 30% efficiency, it would require 190 000
incorporates all ideal frac packs over a user specified                                          lbs of proppant to achieve the FCD of 1.6.
efficiency range. In order to obtain optimal frac geometry                                            If any efficiency between 15 - 30% was met and the majority
without these calibration steps of minifrac and step rate test,                                  of the 190 000 lbs of proppant was placed, the operator would
the OFG process utilises formation parameters to specify                                         gain enhanced conductivity (higher FCD) in the fracture.
two fixed fracture lengths with ‘optimal geometry’ (FCD=1.6).
The fluid volume to reach the required length is dictated by                                     OFG process integrated design program
efficiency at the time of tip screenout (TSO); therefore, the                                    A software program has been developed to enable inputting all the
OFG process efficiency range results in a range of TSO frac                                      reservoir, fluid parameters and cost data. The treatment aspects
pack geometries. The final proppant schedule total amount                                        are also entered and efficiency range is specified based on field,
is designed to provide optimal FCD for the highest                                               rock and fluid data. The program outputs the treatment schedule
efficiency input.                                                                                based on the OFG design process. Inputs include the following:

Procedure                                                                                        Reservoir information and cost factors
The OFG process consists of the following steps:                                                 l Input reservoir properties: pay interval, permeability and
l Specify an initial proppant volume based on industry                                             Young’s Modulus.
  standard 1000 lbs/ft of perforations (may vary based on                                        l Input cost parameters: rig rate, fluid and proppant cost, and
  well and reservoir conditions).                                                                  excess costs.
l Pick the fluid efficiency range to generate geometry, pad                                      l Input ‘low efficiency’ minimum proppant volume.
  and final proppant volumes using the OFG process.
l Evaluate final optimal proppant volume against constraints:                                    Frac pack placement aspects
  s The service tool maximum proppant limit.                                                     l Select treatment fluid and proppant type.
  s The economic proppant volume based on overall effect                                         l Select rate and maximum proppant concentration.
    on well value. The OFG based frac pack design need not
                                                                                                 l Select fluid efficiency probable range based on
    cost more than the conventional approach and can result
                                                                                                   field/rock/fluid data.
    in significant savings.



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System design outputs
l Pad volume: covers geometry at low efficiency.                     l Minimises the amount of fluid pumped into the formation
l Proppant volume: covers geometry at high efficiency.                 resulting in reduced formation damage.
l Proppant volume: can be limited by operational and                 l Minimises service tool movements due to there being no
  economic constraints.                                                need to reverse out displacements.
                                                                     l Provides enhanced value for single trip, multi-zone
Case history                                                           treatments7.
A GOM operator has pumped eight frac pack treatments in              l Allows for more efficient stimulation vessel scheduling.
water depths exceeding 6250 ft. Four of the treatments were              The patent pending OFG process is available from
performed without a minifrac or step rate test. This group           Halliburton as FracPac-NSM service. O T
included one well with a 350 ft MD (337 ft TVD) perforated
interval. The four treatments performed without minifracs and        References
                                                                     1.   MMS Gulf of Mexico Region. Deepwater Production Summary by year
step rate tests used the OFG process. Not only were these frac
                                                                          1985 - 2008, http://www.gomr.mms.gov/homepg/offshore/deepwatr/
pack treatments successfully placed with tip screenouts, net              summary.asp.
pressure gains and 100% frac pack coverage, the other benefits       2.   DEMARCHOS, A.S., CHOMATAS, A.S. and ECONOMIDES, M.J.:
                                                                          ‘Pushing the Limits in Hydraulic Fracture Design.’
were significant to the project.                                          Paper SPE 86843, 2004.
     Table 1 also shows the rig savings time per completion.         3.   ECONOMIDES, M.J., OLIGNEY, R.E. and VALKO, P.P.: Unified
                                                                          Fracture Design, Orsa Press, Alvin Tx. May 2002.)
The rig rates on these completions were US$ 42 000/hr. The           4.   SMITH, M.B., MILLER, W.K. and HAGA, J.: ‘Tip Screen-Out
OFG process saved the project approximately US$ 500 000                   Fracturing: A Technique for Soft, Unstable Formations,’
                                                                          Paper SPE 13273, 1984.
in rig time. In addition, approximately 100 000 gal. of gel was
                                                                     5.   W.J. EDWARDS, S. GAUTHIER, and B. CLARKSON, ‘New
not pumped into the formation, resulting in less damage due to            Frac-Pack Redesign Methodology Improves Estimation of Tip
polymer invasion, saving another US$ 500 000.                             Screenout Post-FET’ Paper SPE 98171-PA, 2007.
                                                                     6.   SMITH, J., VITTHAL, S., MCGOWEN, J., and DUSTERHOFT, R.:
                                                                          ‘How Minifracs Alter Leakoff and Ways to Counteract It,’.
Conclusion                                                                Paper SPE 58767, 2000.
                                                                     7.   Brad CLARKSON, Tommy GRIGSBY, Colby ROSS, Emile
By eliminating the minifrac and step rate test, rig time and fluid        SEVADJIAN, and Bruce TECHENTIEN, ‘Evolution of Single-Trip
savings have been realised without sacrificing job placement              Multiple-Zone Completion Technology: How State-of-the-Art New
                                                                          Developments Can Meet Today’s Ultra Deepwater Needs’
(optimal frac geometry). Rather than restricting the frac pack
                                                                          Paper SPE 116245 presented at SPE Annual Technical Conference
to a single potential geometry, the OFG process is utilised to            and Exhibition, 21 - 24 September 2008, Denver, Colorado, USA.
provide optimal results over a range of fracture lengths and
widths. The process provides several additional benefits:




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www.oilfieldtechnology.com                                                                                                   OCTOBER 2009