Arcasolve High Performance Drilling Damage Removal
(adapted from Arcasolve Technical Document MD1)
Contents
1. Background
2. Laboratory Tests
3. Field Application
Summary
Arcasolve acidising solves several key problems inherent in conventional drilling damage clean up treatments. The treatment fluid is non-corrosive and non-reactive when placed. This ensures even distribution of fluid across long sections with minimal leak off during pumping, unlike conventional reactive acids such as HCl. Once Arcasolve fluid has been placed acid is produced in-situ during shut-in.
Independent laboratory testing with water based and oil based muds has demonstrated the effectiveness of Arcasolve for removing drilling mud damage with typical regain permeabilities of between 80% and 97%. In tests on carbonate cores not only was damaged removed but permeability increases of to 693% were attained.
The pH created by Arcasolve acidising is also optimal for polymer breaking enzyme activity. Enzyme breakers can therefore be added to Arcasolve fluid to produce a non-corrosive fluid capable of simultaneous carbonate dissolution and polymer breakdown during shut-in. This type of treatment is impossible with conventional acid systems.
1. BACKGROUND
Following drilling, damage is frequently present arising from carbonate weighting material, polymers from the drilling fluid and carbonate fines produced during drilling. High draw-down in vertical wells in high permeability formations may be sufficient to achieve adequate clean-up of damage. Effective clean-up can be very difficult to achieve in low permeability formations, however. Increasing use of long horizontal wells with very low draw-down makes efficient damage removal even more challenging.
Residual damage can severely limit well production and financial returns (see page on Economics of effective damage removal).
Conventional approaches to removing damage include the use of mineral acids to dissolve carbonate material or use of enzyme breakers to degrade polymers. Placement of corrosive mineral acid to dissolve carbonate is problematic particularly in long horizontal wells. Reactive acids prematurely penetrate weak spots in mud filter cake and fluid leaks off before the horizontal section can be filled with acid. There may be cost or operational limitations on the use of coiled tubing to aid acid placement in long directional wells. Polymer breaking enzymes are effective at degrading polymer residues present in the wellbore but will not attack carbonate or other acid soluble materials so may leave damage behind.
Furthermore, enzyme breaker fluids generally operate under sub-optimal conditions of pH; most polymer degrading enzymes prefer slightly acidic conditions. Ideally, the filter cake will be completely solubilised but enzyme polymer breakers alone cannot achieve this as they do not dissolve carbonate and they cannot be added to conventional acidising fluids because the very low pH rapidly inactivates enzymes.
Arcasolve™ Mud Damage Removal System
Cleansorb has developed its patented Arcasolve acidizing process as the basis of highly efficient treatments for removing drilling fluid damage.
The use of Arcasolve to remove damage solves a number of problems inherent in conventional damage clean up processes. Arcasolve fluid is initially non-reactive and can be placed in the wellbore via the drillstring once drilling is completed, for example Acetic acid is produced in-situ following placement of the fluid. This acid is produced homogeneously throughout the whole treatment fluid and excellent zonal coverage is achieved; the entire wellbore is effectively acidised. The acetic acid produced dissolves carbonate to produce an acetic acid-acetate buffer with a pH of about 5.5. This pH is ideal for effective operation of many polymer degrading enzymes which may be incorporated into the Arcasolve fluid.
Arcasolve alone can frequently achieve very efficient removal of filter cake due to the effective delivery of acid along the whole of the wellbore. In other cases, the incorporation of enzyme breakers into the Arcasolve formulation may be beneficial.
Benefits of using Arcasolve for Drilling Damage Removal
2. LABORATORY TEST RESULTS 2.1 Independent Testing of Water Based Mud Damage Clean Up using Arcasolve Outline of Method Tests were conducted on carbonate core slabs to evaluate damage removal over a much
larger area than is possible with conventional core plug apparatus using 2 types water
based mud: KCl/polymer mud; CAT-1 mud. Initial permeability to 2% KCl was measured before applying a mud squeeze at 500 p.s.i
for 30 minutes. The core faces were flushed with 2% KCl to remove excess liquid mud and
permeability to 2% KCl re-measured to determine the extent of mud damage. Arcasolve fluid
was injected into the core slabs at 500 p.s.i. and 1 pore volume of fluid pumped through
the core before the core slabs were heated to formation temperature and shut in for 48
hours. Following shut in core slab permeability was re-measured using KCl to determine the
extent of damage removal: Results Initial permeabilities: Cat - 1 damaged core 24 mD; KCl -Polymer damaged core 51 mD. Conclusion The independent test laboratory concluded that: " Arcasolve shows great potential as a mud damage removal treatment as indicated
by the high regained permeability measurements obtained following an Arcasolve
treatment" 2.2 Independent Testing of Oil Based Mud Clean Up using Arcasolve Method Outline Tests were conducted on slabs of carbonate core material to evaluate damage removal
over a much larger area than is possible with conventional core plug apparatus using
Enviromul oil based mud (Baroid). Permeability measurements of the initial core slab material, mud damaged core material
and Arcasolve treated core material were carried out as in example 2.1. Instead of basing
permeability measurements on flow at a single pressure, however, multiple measurements
were made at 4 different pressures. Permeability values were then determined graphically
using a statistical line fit through the data points. Results Initial permeabilities: Test 1 core 0.0065 mD; Test 2 core 0.0061 mD. Conclusion The independent test laboratory concluded that: "Under field conditions .... 15% HCl is unlikely to provide sufficient zonal
coverage and hence effective damage removal. The Arcasolve mud damage removal tests gave
equally good results in both the top and bottom cores. No wormholes were formed but
preferential etching was observed on the core surface and Arcasolve is likely to provide
effective zonal coverage" 2.3 Independent Comparison of Zonal Coverage Achieved using Arcasolve or 15% HCl. Method Outline
Uniform dissolution of carbonate in filter cakes (unlike gel breaking enzymes)
The tests were conducted on carbonate cores using 2 Hassler Sleeve linear flow cells linked in parallel. The front face of each core was damaged using KCl- polymer mud. Provided there was no rapid breakthrough of treatment fluid through the damage on the face of the first core this would ensure that a proportion of the treatment fluid was diverted to the second core holder. Rapid breakthrough into the first core would allow treatment fluid to leak off through this core preventing diversion of treatment fluid to the second core. Thus the potential for effective zonal coverage with a given treatment fluid was indicated by the relative increase in permeability of the 2 cores.
Tests were carried out using either 15% HCl or Arcasolve fluid.
Results
| Test | Initial K mD | Regain K mD | % Increase |
| Arcasolve core 1 | 6.89 | 25.07 | 264 |
| Arcasolve core 2 | 3.97 | 68.5 | 1625 |
| HCl core 1 | 4.66 | 837 | 17,800 |
| HCl core 2 | 3.37 | 4.78 | 42 |
The massive increase in permeability of HCl core 1 was found to be due to wormholing not matrix stimulation. This explains the lack of stimulation of HCl core 2 since nearly all the HCl fluid leaked off into core during pumping and very little acid reached core 2. In contrast the impact of in-situ production of acid following placement of a non-reactive fluid is evident from the Arcasolve results. There was effective diversion of non-reactive Arcasolve fluid into core 2 during pumping before acidising and stimulation of both cores occurred during shut in. This illustrates the problem of effective placement of HCl for damage removal applications and the advantages of Arcasolve for uniform damage removal.
Conclusion
The independent test laboratory concluded that:
"Arcasolve, a delayed release acid, showed greater potential for complete zonal coverage than does HCl acid. High regained permeability results obtained following an Arcasolve treatment indicated an approximate four and eighteen fold increase in permeability in cores ... after the Arcasolve treatment.
Results with HCl showed that that once HCl has broken through the damaged zone on the core material, the high permeability provided does not allow further distribution of acid to remove additional damage"
2.4 In-House Testing of Polymer Mud Cake Clean Up Combining Simultaneous Carbonate Dissolution and Polymer Degradation
Method
Tests were conducted on calcium carbonate weighted polymer mud filter cakes. Fluid flow in the injection direction at 30 p.s.i. overpressure was measured before and after incubation of the mud in breaker fluid at 70°C (158°F) for 48 hours. Flow measurements were all taken in the injection direction so that any residual mud cake material left after the breaker treatment would still impair flow.
N.B. In the field flow will occur in the production direction following mud cake clean up which should help flush away any residual filter cake material not dissolved by the breaker treatment. Consequently the following results very likely underestimate the impact of breaker treatments on flow of produced, rather than injected, fluids.
Results
Note: With the above mud system there was an advantage in incorporating enzyme polymer breakers into the Arcasolve formulation. With other muds Arcasolve alone is very effective (see 2.1 & 2.2).
Conclusions
Mixed enzyme breaker formulation 1 alone had no impact on polymer mud filter cake and flow in the injection direction was not restored.
Arcasolve only breaker treatments restored fluid flow in the injection direction.
The greatest impact on injected fluid flow was achieved using breaker containing Arcasolve plus mixed enzyme breaker preparations.
3. FIELD APPLICATION
3.1 Outline Treatment Design
The flexibility of Arcasolve acidising allows the design of treatments to be tailored to the needs of different applications and different field requirements. The following example is based on using Arcasolve acidising to clean up drilling damage in a newly drilled horizontal well.
The Arcasolve treatment is based on filling the whole horizontal interval with Arcasolve fluid by pumping the fluid through the drill string with the drill string at the toe of the well. Drilling damage will control leak-off of Arcasolve fluid to formation as it is pumped. This will allow the Arcasolve fluid to be displaced back along the wellbore from the toe to the heel of the well ultimately filling the whole interval.
Once the interval has been filled with treatment fluid it may be beneficial to apply a transient overpressure, depending upon the relative wellbore/formation pressure, to help squeeze a proportion of the Arcasolve fluid through the drilling damage and into the near wellbore formation. The interval is then shut in to allow in-situ production of acid to run to completion resulting in drilling damage clean-up. A typical shut in time is around 48 hours but this can be adjusted depending on field requirements.
3.2 Damage Removal and Near Well Bore Stimulation
The ability to place Arcasolve fluid before acid is produced in-situ also provides the opportunity for achieving near wellbore stimulation in carbonate formations. Depending upon the volume of Arcasolve fluid pumped during a damage removal treatment it is possible to squeeze Arcasolve fluid through the filter cake to a given depth into the formation. During shut-in Arcasolve fluid in the wellbore acidises and destroys filter cake at the wellbore surface whilst Arcasolve fluid displaced to formation dissolves the rock matrix and provide additional stimulation.
3.3 Volume of Treatment Fluid
The volume of Arcasolve fluid pumped will depend upon the well geometry and treatment objective. For damage removal treatments the volume would be equivalent to the wellbore volume plus some additional fluid to allow for possible leak off of fluid to formation while the interval is being filled. For a simultaneous damage removal/near wellbore stimulation treatment the volume would be increased to allow for intentional displacement of Arcasolve fluid to formation.
3.4 Arcasolve Formulation
Arcasolve fluid is prepared at the point of use by dissolving Arcasolve substrate in make up water and then adding Arcasolve enzyme. Cleansorb’s polymer breaker enzyme preparations are added at this stage if they are being used in the treatment. Cleansorb advises customers on appropriate formulations for each particular application.
3.5 Mixing and Pumping Arcasolve Chemicals
The following are general guidelines and more detailed treatment designs are prepared once specific candidate wells have been identified and specific well data is available:
3.5.1 Batch Mixing
Arcasolve liquid substrate is dissolved in make up water, e.g. by addition of substrate to water being recirculated by pump in a clean tank. Formation water, fresh water or sea water are all usually suitable sources of make up water. For example, 400 barrel batches of Arcasolve fluid for an 800 bbl treatment have been prepared using 2 frac tanks in parallel. The water was recirculated through a conventional oilfield blender unit at approximately 20 bbl/min as substrate was pumped to the blender unit. Once all the substrate had been added, enzyme was added at the blender unit and pumping continued for a period to completely mix the contents of the frac tanks.
When mixing of the Arcasolve chemicals is complete the fluid can be pumped into the well. Arcasolve fluid pumped straight after mixing has a pH only slightly below pH 7. Even if held in the mixing tank for several hours before pumping the pH of the fluid will not drop below pH 3.5 - pH 4. Consequently there is no need for corrosion inhibitors when pumping Arcasolve and acid sensitive downhole equipment can often be left in place.
3.5.2 Continuous Mixing
Both the Arcasolve substrate and enzyme preparations are liquids. It is possible to add them "on the fly" using appropriate dosing and metering equipment provided there is adequate mixing to fully dissolve the Arcasolve chemicals prior to injection.
Consequently, mixing and pumping procedures can readily be adapted to meet specific field conditions e.g. on-shore treatments versus off-shore treatments with platform load limitations etc.
3.6 Placement of Fluid
As Arcasolve fluid is non-corrosive and low hazard it can be placed using a variety of methods without damage to tubulars or equipment in the well. In the earlier example for a newly drilled well the fluid is pumped through the drill string. In treatments on existing wells Arcasolve fluid has been pumped to formation down the tubing annulus with the down-hole pump still in place, for example. Following shut in, spent treatment fluid was immediately back produced along with the normal well fluids using the down hole pump.
3.7 Compatibility of Arcasolve with Additives and Formation Water
Arcasolve is compatible with all formation waters tested to date and is known to be compatible with a number of specific additives such as silt suspenders or chelators. Where applicable, details of any additives, biocides or other chemical treatments applied to wells prior to Arcasolve treatments should be advised to Cleansorb Ltd ). Cleansorb can carry out Arcasolve/additive compatibility testing where necessary.
3.8 Disposal of Spent Fluid
Arcasolve fluid constituents and reaction products are low hazard, non-toxic and readily biodegradable and present minimal disposal problems.
© Cleansorb Limited 2006. All rights reserved.