A cross-link is a chemical bond that joins two separate polymer chains together. When this happens, the free movement of individual polymer chains is restricted. The fluidity of the polymer chains reduces as the degree (density) of cross-linking increases, and this will affect the physical properties of the drilling fluid. In the drilling oilfield, cross-linking polymer LCM was very useful in curing severe losses.
These pills are usually a blend of polymers and lost-circulation materials that crosslink with temperature and time to form a rubbery, spongy ductile consistency that effectively stops the loss of fluid by sealing fractures and vugular formations. These pills may also be used to prevent water flow and consolidate loose gravels.
Depending on the manufacturer, the product may be a one-sack additive containing polymers and lost-circulation materials or be individually packaged. Most offer a retarder and accelerator for specific cases if needed. In most situations, the retarder will be required to delay premature crosslinking until the slurry can be placed opposite the loss zone.
How Cross Linking Polymer Affect Drilling Mud Properteies
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The viscosity of the fluid will increase as cross linking density increases, turning into a gel-like fluid with medium density cross-linking and eventually producing a rigid structure with high density cross-linking. The flexible, elastic gel structure produced with medium density cross-linking is this property that makes cross-linked polymers effective as a cure for severe Mud Losses after conventional LCM treatments have been unsuccessful.
Cross-Linking Polymers for Severe Losses
The cross-links are produced by chemical reactions that are usually initiated by heat, pressure or adjustments to pH. The cross-linking process is often slow, but the cross-links that form are permanent. However, the process can be speeded up by adding a catalyst or “activator” to initiate the chemical reaction.
The activator is usually added to the cross-linking mix just before pumping down-hole, and the cross-linking polymer pill is designed to be pumped through mud motors and down-hole assemblies (e.g. MWD and LWD tools), avoiding the need for a round trip (check also: Tripping pipe procedures). Once spotted across the loss zone, the product gradually forms a matrix in and across natural fissures and induced fractures.
Application Of LCM Cross Linking Polymer In Drilling
The use of Cross-Linked Polymers can be effective for dealing with formation losses when traditional LCM treatments have been unsuccessful for the following applications:
- Curing severe losses in weak or fractured zones
- Controlling induced losses
- Consolidating loose formations.
The Main Components Of Cross Linking Polymer System
Cross-linking polymer systems usually comprise three components:
- Cross-Linking Polymer to provide viscosity and for sealing the loss zone
- Activator to initiate the cross-linking process for gel formation
As we said before activators added to the cross-linking pill just before pumping.
- Retarder to delay the cross-linking process for high BHT
Retarders are usually available to delay the cross-linking process and avoid premature setting of the pill on surface in hot regions, or while pumping down-hole with high bottom-hole temperatures or long pumping times.
The Mixing & Pumping Procedures Of Cross Linking Polymer Pill
Cross-linking polymer products usually come with detailed mixing procedures to prevent contamination that could have a detrimental effect on setting times, but in all cases, you can follow these steps:
- Lines and tanks must be clean and flushed with fresh water before mixing.
- High Calcium levels in the mix water may also need to be treated out.
- The cross-linking polymer is then mixed at a controlled rate and sheared through the hopper for a recommended period to ensure complete dispersion and hydration.
- At this point, conventional weighting materials can be added to obtain the required displacement density.
- The cross-linking polymer lcm pill can then be stored for use when required, although biocide treatments may be required for long-term storage.
- Prior to pumping down-hole, the required concentration of activator is quickly added at the mix hopper
- The cross linking pill is then pumped quickly down the drill string.
- Cross-linking polymer products usually come with detailed pumping procedures and the spotting method will usually depend on the nature of the losses.
- When dealing with open fractures, the cross-linking polymer pill should be pumped carefully out of the drill string to prevent localized losses that might prevent coverage of the entire loss zone.
- When dealing with induced losses, the cross-linking polymer pill should be squeezed into the weak formation.
- The drill string is then usually pulled clear of the cross linking polymer lcm pill so that it can be flushed through with mud.
- The cross-linking polymer pill should then be left in place for the recommended setting time, applying pressure if required, before resuming drilling.
- In the meantime, the mix tank and associated lines should be flushed and cleaned immediately after use, before any residual cross linking polymer has had time to set.
Cross Linking polymers are usually designed to be non-damaging and can be removed by drilling, jetting or acidizing.
Weighing Cross Linking Polymers Pill
The cross-linking polymer LCM pills may be weighted if needed for well control. Depending on the manufacturer and the specific product, the materials may generally be mixed in salt water up to saturation but cannot be used with calcium-based brines.
Salt usually acts as a retarder for the crosslinking mechanism. They may be used with non-aqueous (oil-base mud) systems by incorporating spacers while the slurries are mixed in water. The manufacturers’ recommendations should be closely followed due to the differences in the crosslinking mechanisms and product limitations.
After mixing according to the individual manufacturers’ recommendations, the pill is spotted similarly to other lost-circulation pills. Pump the Although there should be no difference between water-based mud and oil-base muds in the pressure needed to initiate a hydraulic fracturing of the formation, there is a significant difference once the fractures are formed.
- Water-base fluids typically have a higher “spurt” fluid loss causing an almost instantaneous filter cake to form that aids in formation sealing in permeable formations.
- Oil-base fluids do not display the same characteristics. Once a fracture is initiated by an oil-base fluid, the pressure necessary to propagate the fracture is much smaller by comparison. This is compounded by lack of a significant pressure drop across the filter cake of most oil-base fluids.
This allows changes slurry to the drill pipe and displaces the slurry from the bit, pull up, and squeeze the pill.
Pilot testing is highly recommended to determine the correct concentration of retarder or accelerator to be used. Typically, the pills will reach consistency at 115°F (46°C) in approximately 60 min with no retarder or accelerator used. The crosslinking is faster at higher temperatures. Therefore, in most situations, a retarder will be needed. Again, consult the manufacturers’ recommendations to avoid the premature setup of the pill.
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