Stress-Induced Formation Damage, Sanding Tendency, Prediction, and Control
The need for sand control is determined by weakness of the formation and the maximum drawdown allowing flow without sand production (Weingarten, JPT, 1995). No measures for sand control may be necessary when drawdown allows production at desired rates, otherwise the well may be hydraulically fractured to obtain the desired rate without needing sand control (Weingarten, JPT, 1995). When the reservoir formation is highly weak, or the water influx due to reservoir pressure loss by production induces sand production, proper sand control measures should be taken (Weingarten, JPT, 1995).
There are two conventional strategies available in dealing with sand production (Geilikman and Dusseault, 1997):
1 avoiding sand production problems by controlling pressure gradient and fluid flow rate, selective perforation of zones, resin injection and injecting resin-coated sand into formation, and
2 excluding sand production using gravelpacks, screens, and slotted liners in the wellbore.
For small zones, chemical sand consolidation treatments, such as resin applications, may be convenient (Weingarten, 1995). However, gravel-packing is preferred in many cases. Gravel packing (GP), conventional gel packs (CGP), high-rate water packs (HRWP), and fracpacking (FP) are the most frequently applied sand control methods (JPT, 1995). Weingarten (JPT, 1995) states that:
One obvious measure of performance is that the well should not produce sand. This requires proper gravel sizing and effective packing to prevent voids. Another key measure of success is productivity, both short and long term. The well should have a low skin, and productivity should not decline over time owing to fines infiltration of the pack or perforation tunnels. King (JPT, 1995) states that "reservoirs with gas or water drive are better gravel-packing (GP) candidates." McLarty (JPT, 1995) explains that:
Laminated sands, severely damaged formations, and fines-migration problems should be considerations for frac-packing (FP) applications. FP's are not recommended in cases where poor cement has resulted or where nearby water or gas zones may also be stimulated. The wellbore's mechanical integrity must be able to accommodate higher treating pressures and the reservoir pressure should be sufficient to produce back the larger volume of fluids required. FP's would also be applicable in situations where a well has sanded up and it is suspected that the overburden pressure may have collapsed and pinched the reservoir off from the wellbore.
As explained by Gurley (JPT, 1995):
The primary objective of a frac-packing (FP) is to open and pack as many perforations as possible, and crate a highly conductive flow path through a damaged zone or into a low-permeability formation . . . The fracture needs to extend only through the damaged zone to be effective, but it is ecessary to penetrate 50 to 100 ft and to achieve a tip screen out so as to achieve a wide fracture.
King (JPT, 1995) states that "reservoirs with high peak demands are good candidates for FP's." Cornette (JPT, 1995) primarily recommends frac-packs (FP), especially for sand formations, which are prone to fines migration problems, or of low permeability or highly laminated types. Cornette (JPT, 1995) points out that conventional gel packs (CGP) cause significant formation and perforation damage, and therefore are not favored, although they tend to lead to productive completions in formations less than 20 feet thick. High-rate water packs (HRWP) are remedial short frac-packs, recommended for formations that are clean, homogeneous, and high permeability (K > 50 md for gas-bearing sands and K > 100 md for oil-bearing sands) (Cornette, JPT, 1995). They are less damaging than the conventional gel packs (CGP). Gurley (JPT, 1995) explains that The HRWP objective is to assure that the perforation tunnels are open and fully packed with gravel . . . Average fracture width will be very limited because the fractures do not extend very far into the reservoir.
McLarty (JPT, 1995) explains that Water packs provide good grain-to-grain contact and would probably be the recommended GP fluid in situations where the interval is particularly hot (> 280°F), excessively long (> 60 ft), or highly deviated (> 60° slope). They may not be economical or favorable from a formation-damage standpoint in situations where high-density brines must be applied.
When simultaneous water cut, fines migration, and well sanding problems are encountered, Hayatdavoudi (1999) recommends dropping the water level by a suitable completion technique, such as
a horizontal drilling,
b producing water from below the oil/water contact level and disposing it to another zone, and
c suppressing the water-coning by choke adjustment. Kanj et al. (1996) state that
In the final analysis, the decision on the sand control requirements has to be based on experience in the area with natural, perforated completions. However, adjustments must be made for differences in completion method and efficiency, hole deviation, well depth, reservoir pressure, depletion and drawdown, water production expectancy, and development plans. Frequently, sand production problems are alleviated by means of empirical rules-of-thumb, heuristic approaches, and know-how techniques (Kanj et al., 1996). Tables 20-1 and 20-2 by Kanj et al. (1996) compile the various heuristics and associated points available from some experts that can be used for predicting the sanding potential, and suitability of potential methods in sand control, respectively.
Gravel Design Criteria
In spite of the extensive reported work in the area of understanding and control of sand production, theoretical approaches and guidelines for predicting the sanding phenomena and techniques for sand control still need further research and development. As demonstrated by Bouhroum and Civan (1995), the majority of the available gravel sizing criteria, based on the gravel to sand grain size ratio do not actually agree with each other. Saucier (1969) and others developed gravel sizing criteria based on the gravel to mean size of the formation sand for similar percentile points of sieve analysis curves, solely from the geometrical point of view. Therefore, Bouhroum and Civan (1995) state:
A major problem with gravel-pack design criteria is the significant focus given by researchers to the geometrical retention criteria while neglecting the other aspects of the filtration process, such as clogging and hydrodynamics. Table 20-3 by Tiffin et al. (1998) outlines typical sand sorting ratios that can be determined by sieve analyses. In this table, the cumulative % sieve size distributions are denoted by the letter "D". For example,
D50 indicates the sieve opening above which 50% of the sand particles are retained. Tiffin et al. (1998) report that typical moderate size, well-sorted formation yields a D40/D90 value of 2.8, while a more poorly sorted sample yields a value of 10 for the D40/D90 ratio. Formations having different sizing may have the same sorting ratio. However, this does not matter because the focus of interest here is the ability of the formation sand particles to form bridges and plug screens or gravel pack
(Tiffin et al., 1998). Based on their investigations, Tiffin et al. (1998) proposed the following sorting criteria and completion methods:
1. (D10/D95<10, D40/D90<3, sub 325 mesh<2%) the lowest sorting values with low fines content may be bare screen completion candidates. (Need >1 Darcy formation permeability for cased and perforated completion, with possible use of prepacked screens).
2. (D10/D95<10, D40/D90<5, sub 325 mesh<5%) low to medium sorting ranges, or with fines just out of range may best be served by bare screen completions with new technology, woven mesh screens. (Need >1 Darcy formation permeability for cased and perforated completion).
3. (D10/D95<20, D40/D90<5, sub 325 mesh<5%) medium ratio ranges may be served by larger gravel (7x or 8x 50%), placed in high rate water pack, particularly if the formation sand size is consistent over the zone (no laminations and minimum streaks).
4. (D10/D95<20, D40/D90<5, sub 325 mesh<10%) medium ratio ranges with too many fines may use a combination of larger gravel and a fines-passing screen.
5. (D10/D95<20, D40/D90<5, sub 325 mesh<10%) the highest ratios, particularly those coupled with large amounts of fines signal a critical need for enlarging the wellbore (move the gravel/ formation sand interface away from the wellbore), through fracturing, horizontal or multi-lateral well technology underreaming, or large volume prepacking to minimize severe permeability damage at the gravel/sand interface due to flow.
Figure 20-1 by Skjserstein and Tronvoll (1997) indicate the in-situ stress conditions around a perforation cavity and replication of these
conditions in a laboratory test sample. Figure 20-2 by Burton (1998) describes the typical flow pattern involving a perforated, cased-hole gravel-packed completion and delineates the location of various types of skin effect. As can be seen, completion pressure losses involve low- and high-velocity flow regions that can be classified as Darcy and non-Darcy flow regions, respectively.
Prediction of Sanding Conditions
Hayatdavoudi (1999) developed a simplified model by modifying the Spangler and Handy (1982) model. The induced shear-stress, Ti , in the direction of flow or its equivalent pressure drop and the induced acceleration, at, of the formation particles can be related by Newton's second law as (Spangler and Handy, 1982):
where m is the mass of the sand particle. On the other hand, the maximum, critical, or threshold shear resistance of the sand, including the additional factors resulting from water invasion, is expressed as following by Hayatdavoudi (1999):
where ac is the effective cohesive strength of the formation sand (lb/ft2) and av is the effective vertical stress (lb/ft2). Pp ,Ps,Pos, and Pc denote, respectively, the pore fluid, clay swelling, osmotic, and capillary pressures. Qcyc denotes the cyclic angle of internal friction. Therefore, sand liquefaction occurs when the prevailing shear stress exceeds the threshold shear stress, that is,
Neglecting the fluid acceleration effect, Hayatdavoudi (1999) expressed the induced acceleration of particles by
where Vpv is the induced velocity of particles and fsw is the shear wave frequency. In case of the lack of information for Eq. 20-4, Hayatdavoudi (1999) recommends estimating a, by:
where g denotes the gravitational acceleration. Hayatdavoudi (1999) points out the importance of the buoyant unit weight of the in-situ particles when determining the particle mass, and estimates the in-situ particle mass by:
where z represents the depth or height measured from a reference datum (ft) and Yave is the average specific weight of the formation sand (lb/ft3). The latter is expressed as:
in which the specific weights of the sand grains in the water and oil zones are given, respectively, by:
and Yw and Yo are the specific weights of the water and oil phases, and G is the specific gravity of the sand grains. As a fluid flows over the face of a cohesionless bed of particles, such as sand or gravel, the particles can be detached and lifted-off when the fluid shear-stress exceeds the minimum, critical shear-stress. Yalin and Karahan (1979) developed a dimensionless correlation to predict the critical conditions for onset of particle obilization (or scouring) by fluid shear. Following their approach, Tremblay et al. (1998) developed:
where Vcr is the critical shear velocity, d is the mean particle diameter, and p and ɥ are the density and viscosity of the fluid flowing over the particle bed. Mcr is the critical mobility number given by
where Ys denotes the specific weight of the particles suspended in the fluid. Applying Eq. 20-11, Tremblay et al. (1998) correlated their experimental data of laminar flow of various liquids over a loose bed of sand particles linearly on a full logarithmic scale and obtained the following expression for the critical shear velocity:
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