New Treatment for Removal of Mud-Polymer Damage in Multilateral Wells Drilled Using Starch-Based Fluids
- K.P. O'Driscoll (BJ Services Co. Middle East) | N.M. Amin (BJ Services Co. Middle East) | I.Y. Tantawi (Zakum Development Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2000
- Document Type
- Journal Paper
- 167 - 176
- 2000. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 4.1.5 Processing Equipment, 1.6.9 Coring, Fishing, 1.6.6 Directional Drilling, 5.6.4 Drillstem/Well Testing, 3.2.4 Acidising, 5.4.10 Microbial Methods, 1.6 Drilling Operations, 2.7.1 Completion Fluids, 1.11 Drilling Fluids and Materials, 2 Well Completion, 2.5.2 Fracturing Materials (Fluids, Proppant), 1.7.7 Cuttings Transport, 1.2.3 Rock properties, 2.4.5 Gravel pack design & evaluation, 1.8 Formation Damage, 3.3.1 Production Logging, 3 Production and Well Operations, 1.10 Drilling Equipment, 1.6.1 Drilling Operation Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.1.2 Separation and Treating, 5.3.2 Multiphase Flow, 5.3.4 Integration of geomechanics in models
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Significant improvement in production from horizontal, multilateral wells has been realized through the application of specific enzyme breaker (SEB) technology for the removal of natural starch polymers used in drilling operations. This new treatment allows placement of a fluid to remove polymeric mud damage at the cessation of drilling operations for each multilateral section, which could not otherwise be re-entered for cleanup and effective stimulation during completion of the well.
The underperformance of horizontal and multilateral wells is a common problem in many areas and is largely due to near wellbore damage.1 The use of "clean" drill-in fluids has been applied to reduce the effects of mud damage particularly where openhole completions are being conducted. In theory these clean fluids lay down a thin filter cake that is easily removed by the flow of production fluids and have leak-off characteristics that prevent in depth penetration of fluid filtrate.
Commonly used "drill-in" fluids are water based, made up using seawater or brines. These fluids contain relatively high concentrations of natural polymers such as starch or cellulose which provide leak-off control and viscosity and smaller quantities of xanthan polymers for cuttings transport properties. These fluids may also contain solids used for lost circulation control and as weighting agents. The solids used are often salts or calcium carbonate and are selected for their solubility in either water or acids.
As drill in fluids leak off into higher permeability zones, significant penetration depths may be achieved while filter cake on low permeability zones is usually localized in the near wellbore region. Work carried out by Frick and Economides2 characterized the distribution of damage created by the drill-in fluids as a truncated cone with an elliptical base, with the largest penetration being near the vertical section, where permeability contrast along the well is small. This work also showed that within normal economic constraints only partial damage removal could be achieved during acidizing particularly when consideration is not given to the distribution of damage. Joshi noted that for horizontal wells a higher permeability formation would show relatively smaller damage than for a low permeability formation, which is further supported by equations developed by Sparlin and Hagen.3
To aid in the cleanup of damage from drill-in fluids, it is common practice to acidize the well during completion operations. The aim of these treatments is to remove mud filter cake, dissolve drill cuttings, soluble weighting agents (calcium carbonate), and remove or bypass in depth polymer and solids related damage. However, work carried out by Burnett has shown that the polymers can coat solid particles and prevent their reaction4 and removal with acid. Work by Hodge et al. showed that the polymers themselves do not completely degrade upon contact with the acid.5 Additionally, evidence is available that shows that acid will wormhole through the mud filter cake leaving the majority of the cake in place at the formation face6 and bypass in-depth polymeric damage.
With multilateral completions it is not always possible to re-enter a given lateral to treat the zone with acid. This means that the damage left in place during drilling will seriously impede production from these zones. Attempts at bullheading acid into a given lateral have generally produced poor results, since there is little control over where the acid will eventually enter the formation and react.
The development of wormholes in limestone and dolomite has been fairly well documented with respect to their elongation, branching and width.7,8This and other work has shown that, for limestones, highly conductive channels are formed by the acid9 at high rates, with little or no penetration at lower rates. Where channels form, these then continue to take most of the subsequent acid that is pumped under matrix conditions. It should be noted that little work has been done to establish the distribution of wormholes along the length of a wellbore nor on the effects that mud filter cake has on their formation and distribution.
Due to economic and logistic constraints, relatively small volumes of acid are used to clean and stimulate horizontal wellbores. Typically, volumes of less than 50 gal/ft of zone are used. The efficacy of these treatments to remove mud filter cake and filtrate damage is questionable, as is the ability to stimulate the formation beyond the zone of damage in the formation. These treatments are commonly placed using coiled tubing in order to ensure distribution of acid along the entire wellbore.
Because the rates achieved through coiled tubing are relatively low, it would be expected that little wormholing should result,7,8 and that effective facial cleaning of the borehole will be achieved. However, if the mud filter cake laid down on the formation face resists acid attack, then the primary means for the acid to react with the formation would be through the formation of wormholes, thus leaving a large portion of the filter cake in place.
Acid washing with coiled tubing is often supplemented with the use of rotary or indexing wash tools. It should be noted that when wash tools are deployed through completion tubulars, their relatively small diameter with respect to the openhole renders them less effective as a means of washing the filter cake from the formation face. Under downhole conditions, the jetting action of these tools is extremely limited, with an active jet being present at best 6 to 8 nozzle diameters from the tool. Additionally, when the filter cake is disrupted, fluid leakoff will occur, and redeposition of undissolved filter-cake particles will occur at the same sites, although presumably with less mass and in a less compacted form.
Acids and oxidative breakers are commonly used to reduce the viscosity of gelled fluids and to attack polymer-based filter cakes, but these substances may not necessarily reduce the amount of polymer residue that results at the end of a treatment.10-12 Acids and oxidizers work by reacting with active sights on the polymer chains and under ideal conditions will break the polymer chain at key linkages thus degrading the polymer strand to its base units of simple sugars. However, because acids and oxidizers are extremely reactive, they will attack any available active sites on a polymer strand and many other substances that they contact in the well. Thus in most cases acids and oxidizers will not react with the correct linkage site or the polymer backbone at all and will leave partially degraded and unreacted polymer strands in place.13,14
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