2. Abstract
Sand production from hydrocarbon bearing formations is a major concern in the oil and
gas industry. Sand control measures are widely employed such as mechanical
intervention, limiting production rate and/or chemical sand consolidation (CSC) commonly
involving resins. Current CSC approaches are problematic for a number of reasons
including placement issues and permeability reduction. An alternative CSC approach has
been developed to overcome the limitations of current systems. The new system utilises
novel decarboxylation reactions to deposit calcium carbonate (CaCO3) from an aqueous
solution at a controlled rate over a range of conditions to achieve and improve
cementation between sand grains. The majority of these reactions involve amino acid
salts. They do not require enzymes, catalysts or oxidising agents. Significant progress has
been made in elucidating the underlying mechanisms. Inclusion of species analogous to
those implicated in biomineralisation has further improved the degree of consolidation.
The location and composition of the CaCO3 cementation has been confirmed using optical
microscopy, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy
(EDX.)
3. Introduction
Sand production commonly occurs when the subterranean formation within proximity of a wellbore experiences hydrodynamic forces exerted
by the flow of production fluids. These forces disrupt the adhesion between sand grains which are subsequently carried away in the production
direction[1]. Sand production can compromise the integrity of the wellbore leading to collapse, especially in openhole completions drilled in
poorly consolidated sandstone formations which are typically young on a geological timescale. The produced sand acts as an abrasive to erode
production equipment leading to the requirement for more frequent maintenance and/or replacement of equipment[2]. The resultant sand in
the produced fluids also has to be separated and disposed of. Sand production constitutes a significant safety concern and reduces the net
present value (NPV) of the well. Measures taken to mitigate sand production commonly involve (i) producing the well at a reduced rate to
achieve either a ‘maximum sand free rate’ (MSFR) or ‘maximum acceptable sand rate’ (MASR)[3] and (ii) mechanical intervention commonly
involving the use of sand screens and/or gravel packs which act to filter particulates from production fluids during flow[4]. These technologies
impair the rate of production, can be expensive, problematic to install and prone to blockage.
Chemical approaches for sand control exist which act to immobilise the sand grains commonly involving resins which are cured in-situ. Resin
approaches can be problematic due to the narrow range of operating conditions, placement issues (especially over long horizontal intervals),
degradation of the resin and the hazardous nature/incompatibility of the chemicals[5-6]. Of particular concern is the potential for excessive
permeability reduction meaning a compromise has to be struck between the degree of consolidation and decline in production. Less common
CSC approaches include: silicate deposition and the use of flocculating polymers or nanoparticles[7-9]. These approaches also have a number of
limitations including placement issues and permeability reduction. The only available mineral based CSC system uses a combination of Urea
and a Urease enzyme to deposit CaCO3
[10-11]. The effectiveness of this system is dependent on the activity of the enzyme which is influenced by
a multitude of factors such as temperature, salinity, pH and certain metal ions[12]. Furthermore, cost and availability limitations of the enzyme
have limited commercial use of the Urease system to date. As a result, the scope of application for current CSC technologies is restricted. A
new mineral-based approach has been developed to overcome the limitations of current CSC technologies[13] in order to: (i) achieve and
improve cementation to immobilise sand particles and increase unconfined compressive strength, (ii) deposit a minimal amount of
consolidating material to retain permeability, (iii) achieve consolidation over long horizontal intervals with uniform zonal coverage, (iv) operate
over a wide range of conditions and (v) utilise chemicals which are environmentally friendly, cost effective and readily available.
4. CaCO3 cementation, the importance of kinetics
CaCO3 is a common authigenic mineral found in the natural cementation between sand grains in sedimentary rocks[14] which therefore
represents an attractive cementing material to artificially deposit. The ability to dissolve CaCO3 with acid also makes the process reversible.
CaCO3 can be rapidly precipitated from solution via homogenous nucleation from a displacement reaction between 2 aqueous salts such as
Sodium carbonate and Calcium chloride i.e. CaCl2(aq) + Na2CO3(aq) → CaCO3(s) + 2NaCl(aq). If the rate of precipitation is significantly reduced,
CaCO3 develops a preference to undergo heterogeneous nucleation precipitating at an interface instead of the bulk solution. If existing
particles are present during this process, CaCO3 will form on their surfaces and at the contact points between them to bind them
together[15]. Two approaches to control this rate are to: (i) slowly introduce Ca2+ cations in to a solution of CO3
2- anions, or (ii) slowly
introduce CO3
2- anions in to a solution of Ca2+ cations. Based on the available chemistries, the latter option was determined to be more
feasible because it could be achieved with a decarboxylation reaction to form CO2(aq) which can be converted to CO3
2- with a suitable base.
The Urea-Urease reaction was known to achieve the desired rate of decarboxylation (d[CO2(aq)]/dt), but, the aforementioned issues with
the enzyme made it a less than ideal candidate. Comprehensive literature and laboratory investigation determined that no other known
decarboxylation reactions could achieve the desired rate. The present study has identified and evaluated novel decarboxylation reactions
which meet the aforementioned criteria.
Amino acids and CaCO3 deposition
Whilst heating a solution of Sodium Glycinate (Na-Gly) and CaCl2 at 60oC in an open test tube, a thin layer of CaCO3 was observed to form
on the tube after a couple of hours. This reaction was proven capable of consolidating glass beads (200-300µm diameter) with significant
consolidation observed between 1-2 weeks.
5. Fig. 1 - CaCO3 scaling @ 60oC after: (A) 1 day, (B) 2 days,
(C) 3 days, (D) 4 days, (E) 5 days and (F) 6 days.
Fig. 2 - Glass beads before and after consolidation at 60oC for 2 weeks.
Similar tests were performed with 16 additional amino acid salts possessing various chemical functionality which were also proven to
precipitate CaCO3 and achieve consolidation of glass beads and different natural sands to varying degrees. One possible explanation
for CaCO3 deposition was atmospheric CO2 capture by the amino acid anion (AAA) during preparation to form a carbamate which
decomposed to form CO3
2- upon heating[16]. When this reaction was repeated under a Nitrogen atmosphere using water free of CO2,
CaCO3 precipitation was still observed (although at a reduced ammount compared to initial tests) which implies that under open
conditions, CaCO3 forms via two different mechanisms simultaneously. This suggests that Na-Gly underwent decarboxylation which is
proposed to involve AAA deamination (indicated by the smell of ammonia during heating) to form an oxocarboxylate anion (OCA.) The
OCA then undergoes nucleophilic attack by another AAA to form an iminium ion which subsequently decarboxylates to form a
stabilised azomethine ylide[17-19]. Elucidating this mechanism gave rise to additional approaches which were subsequently proven in
the laboratory such as the decarboxylation of (i) AAAs with various carbonyl containing species and (ii) OCAs with various amines. This
led to the discovery of a large number of new decarboxylation reactions.
6. Amino acid and Alkylene carbonate based CaCO3 deposition:
The decarboxylation between AAAs and 1,3-dioxolan-2-ones (known as Alkylene carbonates, ACs) was determined to be the most effective at
consolidating sand at 60oC over 1-2 days. It is proposed that the mechanism involves nucleophilic attack of AC by AAA (1) to form an N-
carboxylalkyl β-hydroxy urethane (NCABHU.) [20-21] NCABHU may undergo side reactions possibly involving isocyanate and/or oxazolidinedione
intermediates but they are not outlined in this poster. 2 different products from step 1 are available depending on which AC C-O α-bond breaks
(the preference of which is dictated by the identity of the R’’ group) but, subsequent intermediates are identical so only one route is outlined
for simplicity. NCABHU undergoes base catalysed hydrolysis (2) to form an acidic N-carboxy amino acid anion (NCAAA-H.) NCAAA-H prefers to
tautomerise forming a zwitterion which rapidly undergoes decarboxylation (3) to form CO2(aq) and reform AAA. CO2(aq) can then either react
directly with AAA (4b) to reform NCAAA-H or, AAA can deprotonate water (4a) to create a hydroxide ion which attacks CO2(aq) to form
bicarbonate (HCO3
-) which is deprotonated by another AAA (5) to form CO3
2- i.e.
O O
O
R''
O
O
NH2
O
O
N
O
O
R''
OH
H
H OH
O
O
N
O
H
OH
R' R'
OH
R'
HO
R''
OH
O
O
N
O
O
R'
H H
ACAAA NCABHU
NCAAA-H
1 2
O
O
NH2
R'
CO O+
3
H OH
HO H
C
O
HO O
4a
4b
a
b5
C
O
O O
AAA-H
AAA-H
AAAAAA-H
AAA AAA-H
Fig. 3 – Proposed decarboxylation mechanism for the AAA-AC reaction.
7. Initial observations revealed that d[CO2(aq)]/dt was too fast even at room temperature, but was subsequently proven to be significantly
reduced by using an AAA:AC mole ratio of at least 2:1. It is proposed that the second AAA equivalent acts as a base to deprotonate NCAAA-H
to prevent it tautomerising. NCAAA subsequently needs to become reprotonated by deprotonating the AAA-H zwitterion in order to
decarboxylate. Because AAA has a primary amine group whereas NCAAA has a secondary amine group (with the latter possessing a higher
pKb value), the reprotonation of NCAAA is thermodynamically unfavoured which is theorised to significantly reduce d[CO2(aq)]/dt. The ability
of AAA to recycle CO2 after decarboxylation should also act to reduce the rate of CaCO3 precipitation.
CaCO3 precipitation; improved understanding and lessons from
nature
Until recently, it was understood that CaCO3 precipitated from solution only once its saturation concentration was exceeded according to
classical nucleation theory. More recent work has proven that this process is far more complex involving a number of meta-stable
intermediates (MSI’s). Upon initial formation in aqueous solution, CaCO3 exists in a fluidic form known as a ‘pre nucleation cluster’ (PNC) or a
‘dynamically ordered liquid like oxyanion polymer’ (DOLLOP.) Subsequent nucleation of the PNCs forms solid CaCO3 nanoparticles known as
‘Amorphous calcium carbonate’ (ACC) which are initially highly disordered and hydrated. ACC subsequently dehydrates and becomes more
ordered resulting in its eventual transformation to its thermodynamically favoured anhydrous crystalline polymorphs: Vaterite, Aragonite and
Calcite[22-23]. It is theorised that because CaCO3 MSI’s are small and mobile, they should be able to access smaller confined spaces between
existing particles. Therefore, if CaCO3 MSI’s can be stabilised to increase their residence time in solution prior to crystallisation, improved
placement of cementation may be possible to increase the degree of consolidation.
8. Various organisms have evolved to utilise biomineralisation to construct intricate structures composed of CaCO3 for various functions (e.g.
mollusc shell nacre, egg shells, Sea urchin spines/teeth, coral skeletons, coccolithophore exoskeletons and calcareous sponges). The
morphological control exerted during biomineralisation is theorised to be contributed to by stabilising the MSI’s with various species of
which acidic proteins have been commonly identified. The exact mechanisms via which stabilisation is achieved are still not fully
understood. Acidic proteins and other anionic polymers have proven to stabilise CaCO3 PNCs by incorporation in the ion cluster which has
been referred to as a ‘polymer induced liquid precursor’ (PILP) process[24]. Calcium phosphate has also proven to precipitate via similar
intermediates and become utilised in biomineralisation to form structures such as teeth and bone[25]. A screen of over one hundred
synthetic analogues of species implicated in biomineralisation was performed which determined that a number were able to improve the
degree of sand consolidation achieved with the AAA –AC reactions. The result of this work was the formulation of a biomimetic cocktail of
additives to improve CaCO3 cementation.
Methodology
Qualitative tests involved leaving the treatment solution after preparation to sit for 2 hours at room temperature (to replicate mixing and
pumping time). CaCl2.2H2O was used as the Ca2+ source in most cases. 20ml of the solution was then added to a 20ml bed volume of dry
sand (particle size distribution: 50-500µm) in a skirted 50ml polypropylene centrifuge tube which was sealed, shaken and heated. After
heating, the spent solution was decanted and a spatula was pushed in to the centre of the consolidated mass to determine the relative
degree of resistance which is assumed to be proportional to the degree of consolidation. Larger scale tests were run with 100ml bed
volumes of sand in cylindrical polypropylene vessels and an Ofite 175ml double ended HPHT cell with a 10µm (2 Darcy) aloxite disk and an
internal PTFE sleeve under a 100psi N2 atmosphere. SEM and EDX analysis was performed with a Hitachi S-3200N scanning electron
microscope and an Oxford instruments x-act detector. SEM samples were prepared by flushing with deionised water, Isopropanol and
acetone then dried at 40oC for 2 days after which they were set in epoxy resin under vacuum, polished and carbon coated twice.
9. Results
To provide initial confirmation of the composition and placement of the CaCO3 cementation, SEM and EDX analysis was performed on a sand
sample consolidated with an AAA reaction at 60oC for 11 days.
Spectrum Al Si Ca
1 0 37.81 0
2 0 0.45 0.21
3 0.65 10.88 3.42
Fig. 4—SEM image of the region between two separate sand grains in
consolidated sand sample with positions of EDX analysis indicated.
All results in weight%
Fig. 5—SEM image of the region between three separate sand grains in
consolidated sand sample with positions of EDX analysis indicated.
Spectrum Al Si Ca
1 0.22 6.17 1.22
2 0.74 16.01 3.94
3 1.06 24.96 3.49
4 0 0 0 All results in weight%
10. As shown in figures 4 and 5, difficulty was experienced in visualising cementation between the grains. Nevertheless, EDX point and area
analysis confirmed the presence of Calcium (which is assumed to be CaCO3) at the contact points between the sand gains but not within the
pore space to any significant extent. When an X-ray map was taken of the site in figure 5 only for Calcium, the resultant image provided a
strong indication that the surfaces of the sand grains were coated with CaCO3. This layer is very thin accounting for the difficulty experienced
in visual identification.
Fig.6—EDX scan of sample in Fig. 5 for Calcium (appears as white in image).
CaCO3 has been identified in the gaps between the sand grains which are only a few microns wide in some places. This may indicate that the
reaction has increased the residence time of the CaCO3 MSI’s in solution thereby improving placement of the cement.
11. Conclusions
New chemical processes for sand consolidation utilising amino acids salts to deposit CaCO3 cementation have been
developed. Because the process is reversible and the chemicals are environmentally friendly, cost effective, readily
available and possess good aqueous solubility, potential field application will be significantly less restricted than
current CSC technologies. In order to achieve significant consolidation over 1-2 days, a two stage approach was been
adopted involving (i) control of CaCO3 precipitation rate by utilising new decarboxylation reactions and (ii)
physiochemical control of the resultant precipitate using chemicals analogous to those implicated in biomineralisation.
Stage (ii) is believed to represent one of the first potential ‘real world’ applications of biomimetic mineralisation. Initial
SEM and EDX analysis indicates that the CaCO3 deposited from a standalone amino acid salt reaction has preferentially
deposited at the surfaces and contact points of the sand grains with no significant precipitation observed within the
pores. These first results provide a strong indication that the AAA reactions should be capable of achieving
considerable sand consolidation whilst avoiding significant permeability reduction.
12. References
1) C. Veley, US Pat. US3741308, 1973.
2) AA. H. El-Sayed, N. Musaed, M. N. Al-Awad and E. Al-Homadhi, SPE 68225,
2001, 1-9.
3) N. Fleming, E. Berge, M. Ridene, T, Østvold, L. O. Jøsang and H. C. Rohde, SPE
144047, 2011, 1-13.
4) A. van Kranenburg, J. Twycross, C. Combe and K. Hals, SPE 144089, 2011, 1-17.
5) T. W. Hamby and W. T. Strickland, US Pat. US3646999, 1972.
6) R. H. Friedman, US Pat. US4512407A, 1985.
7) F. Haavind, S. S. Bekkelund, A. Moen, H. K. Koltar, J. S. Andrews and T. Håland,
SPE 112397, 2008, 1-15.
8) D. Espin, J. C. Chavez and A. Ranson, US Pat. US6513592B2, 2003.
9) P. Chen and H. K. Koltar, Internat. Pat. WO2005124097A1, 2004.
10) R. E. Harris and I. D. McKay, SPE 50621, 1998, 1-9.
11) R. E. Harris and I. D. McKay, US Pat. US6401819B1, 2002.
12) S. M. Al-Thawadi, J. Adv. Sci. Eng. Res., 2011, 1, 98-114.
13) D. Holdsworth, Internat. Pat. WO2013064823A1, 2011.
14) W. Schlager, Carbonate Sedimentology and Sequence Stratigraphy, Soc. Sed.
Geo., Amsterdam, 2005, ch. 2, pp. 35-38.
15) J. J. De Yoreo and P. M. Dove, Min. Soc. Am. Geo. Soc. Biomineralisation,
Virginia, 2003, ch. 3, pp. 57-93.
16) P. D. Vaidya, P. Konduru and M. Vaidyanathan, Ind. Eng. Chem. Res.,
2010, 49, 11067-11072.
17) R. Grigg, S. Surendrakumar, S. Thianpatanagul and D. Vipond, J. Chem.
Soc. Perkin. Trans., 1988, 1, 2693-2701.
18) M. F. Aly and R. Grigg, J. Chem. Soc., Chem. Commun., 1985, 1523-
1524.
19) R. Grigg, D. Henderson and A. J. Hudson, Tet. Lett., 1989, 30, 2841-
2844.
20) J. H. Clements, Ind. Eng. Chem. Res., 2003, 42, 663-674.
21) A. G. Anderson, US Pat. US5977262, 1999.
22) R. Demichelis, P. Raiteri, J. D. Gale, D. Quigley and D. Gebauer, Nat.
Comm. 2011, 2, 1-8.
23) J. H. E. Cartwright, A. G. Checa, J. D. Gale, D. Gebauer and C. I. Sainz-
Dίaz, Angew. Chem. Int. Ed. 2012, 51, 2-13.
24) M. A. Bewernitz, D. Gebauer, J. Long, H. Cölfen and L. B. Gower,
Faraday Discuss. 2012, 159, 291-312.
25) M. J. Olszta, X. Cheng, S. S. Jee, R. Kumar, Y. Kim, M. J. Kaufman, E. P.
Douglas and L. P. Gower, Mat. Sci. Eng., 2007, 58, 77-116.
14. SEM images of sand consolidated with an AAA-AC reaction for 48 hours at
60oC
15. SEM images of sand consolidated with an AAA reaction for 14 days at
60oC
16. SEM image with and without EDX false colour imaging of sand
consolidated with an AAA reaction for 14 days at 60oC
Figures 7-14 courtesy of MI-SWACO
