SYSTEM INFORMATION & GUIDELINES
The glycol based system is a high performance, environmentally friendly alternative to traditional Invert Oil Emulsion Mud (IEOM) systems. In addition to reducing environmental concerns, this flexible system delivers superior performance and is easily customised to meet specific operational requirements. The system is primarily used to drill moderate to highly reactive shales and claystones.
The benefits realised with the Glycol System system include: -
- Stabilise troublesome shales and claystones.
- Minimise Pore Pressure Transmission and improve wellbore stability.
- Superior lubricity characteristics.
- Low surface dilution rates, reducing drilling fluid costs and environmental discharges.
- Excellent fluid hydraulics.
- Ease of displacement and clean up.
In addition, the system is ideal for use in challenging wells located in environmentally sensitive areas i.e. high angle, extended reach wells, etc.
The principal application for low molecular weight, water-soluble glycols is in drilling reactive or sensitive shales. Wellbore and cuttings stability is significantly improved with their use. Additionally, the reduced dilution rate decreases the volume of fluid discharged. Experience has clearly demonstrated that, despite the higher initial make-up costs, the low dilution rates offset the overall drilling fluid cost as well as drilling cost due to improved performance. The benefits of reduced dilution may also be realised in less reactive areas, making the fluid viable across a wide range of applications.
The GLY-DRILL™ System
Shale Stabilisation (encapsulation)
Fluid Loss Control
KCl performs two functions within the GLY-DRILL™ system. The main function is the primary source of the inhibitive K+ cation. The inhibitive property of K+ while drilling reactive shales and claystones is commonly known and understood. The K+ level, derived from the KCl, is determined from either historical data or shale characterisation and is usually between 3% to 8% KCl. The second function is to provide a salinity level where the selected Glycol “Clouds out”.
The inhibition mechanisms related to the GLY-DRILL system can be divided into three components.
• Pore pressure transmission
• Capillary effects
• Cloud point behaviour
Pore Pressure Transmission
One of the major mechanisms which can cause shale failure is a formation pressure increase in combination with swab/surge pressures. In permeable formations such as sandstones, the pressure differential between drilling fluid and pore fluid (overbalance) generates a filter cake on the borehole wall that acts as an impermeable membrane. The fluid pressure differential will be exerted on the filter cake and provide effective fluid pressure support to the borehole wall.
Shales are normally considered non-permeable, but they do have limited permeability. This permeability is in the order of 10-6 to 10-12 Darcy. In shales, no filter cake can be formed since the permeability of shales is lower than the permeability of the “normal” filter cake. Thus, the drilling fluid pressure is directly in contact with the formation and will equalize with the pore pressure around the wellbore.
With time, the drilling fluid pressure will gradually reach further into the formation. This slow fluid pressure invasion is referred to as pore pressure penetration. This leads to pressure equalization between the mud column and the near-wellbore shale fluid. This reduce effective fluid support and thus increases the rock stress level around the wellbore. Stress levels may then become so high that compressive rock failure will occur. Swab pressures temporarily lower the effective fluid support even further, bringing the shale or clay close to failure, or causing actual failure resulting
in cavings or borehole collapse. An initially stable wellbore can become unstable with time due to pore pressure penetration in combination with swab/surge pressures.
Figure 2-2 Pore Pressure Transmission with AQUA-COL TM and 10% by Weight KCL
The degree of pore pressure penetration depends upon the type of drilling fluid, type of shale (permeability), and amount of overbalance. Pore pressure penetration can be measured in the laboratory.
Neither water-base fluids nor oil-base fluids form a solid filter cake in shale. Under normal fluid pressures, shales are permeable to water-base fluids but completely impermeable to oil. The stable behaviour of shales while drilling with oil-base fluid or synthetic-base fluids is a result of capillary action. When oil enters a shale, it has to overcome a threshold capillary pressure caused by the capillary effect between oil and the pore fluid. The capillary pressure is in the order of thousands of psi and is generally too large to be overcome by the fluid pressure differential. The threshold pressure, therefore, acts as an alternative “fluid (mud) filter cake”, providing effective fluid support to the wellbore.
A consequence of the above is that shale instability with oil and synthetic drilling fluids is normally caused by lack of fluid support, i.e., too low a fluid density.
Cloud Point Behaviour
Improvement in inhibition seen over a wide range of temperatures is directly related to the cloud point of the glycol. Cloud point is the transition point or start of phase separation at which the glycol changes from being water soluble to water insoluble.
Research carried out by other investigators (SPE 28960) concluded that the glycol is adsorbed by the clay. During the adsorption process, water is displaced from the clay surface and ordered structures of polyols are formed. A weak attachment is postulated because of the observation that there is no rapid depletion of the polyol in the drilling fluid system.
For this to be the case, there must be some mechanism of association and disassociation of the polyol in the circulating system. The nature of these structures and their stability in aqueous fluids is strongly controlled by the presence of potassium cations with certain polyols.
• Under most conditions, a single polyol layer forms on the clay in the presence of potassium. The resulting complexes are stable in water.
• A complex containing two polyol layers is formed when potassium is absent. This complex is less stable in water.
• Studies on other polyols conclude an additional contribution comes from interactions between polyol molecules at the clay surface.
Polyol fluids are effective in most shale types, particularly in young or relatively uncompacted shales with high clay contents. Compared with for example a standard KCl/polymer fluid, polyol systems give improved wellbore conditions and produce firmer cuttings that do not readily disperse into the fluid. These attributes frequently combine to give faster drilling rates, fewer or less severe drilling problems, and reduced fluid volumes that translate into reduced drilling costs.
Cloud point is a phenomenon exhibited by many glycols. The solubility of these glycols in water decreases as temperature increases, with materials that are fully soluble at room temperature forming separate phases at higher temperatures. The temperature at which the glycol and water separate is known as cloud point, since the previously clear solution becomes “cloudy” upon separation.
Engineering Cloud Point
For glycols at room temperature, water solubility decreases with increasing molecular weight. Low molecular weight glycols are typically more soluble in freshwater systems than in high molecular weight glycols. Two factors control the cloud point of freshwater soluble glycols:
• water salinity
• glycol concentration
An increase in either of these factors results in a lower cloud point temperature.
When a glycol is mixed in water below the resultant solution’s cloud point, it is evenly distributed as minute droplets known as micelles. The micelles are stabilized by hydrogen bonding between the water molecules and oxygen atoms present in the glycol molecule. The stabilization process is known as hydration. As temperature increases, hydration decreases until the micelles are no longer stable in an aqueous environment. Consequently, they coalesce in large numbers and form a separate phase that is distinct from the water.
The “clouding” process is reversible. If the solution is subsequently cooled, the two phases recombine to form a clear, single-phase solution. Baker Hughes INTEQ has engineered combinations of glycol concentration and salinity to design a drilling fluid where glycols are in solution on the surface and out of solution downhole. As bottom hole temperatures change with depth, the system’s cloud point can be adjusted to maintain optimized drilling performance and shale stability.
The great variety of cloud point temperatures seen indicates that one polyol may not be suitable for all applications. Additionally, the polyol will have to be selected based upon its cloud point at a specified salinity.
Alternatively, the cloud point of the polyol can be adjusted by altering the salinity of the liquid phase. Varying the polyol type and system salinity has afforded Baker Hughes INTEQ effective control of the cloud point over a wide operating range.
Cloud point is modified at the wellsite using an software program called GLY-CAD® which models the downhole behaviour of water-soluble glycol products such as AQUA-COL™ and AQUA-COL™ D. With this program, the concentrations of salt and glycol can be engineered to match the operational needs of the drilling fluid. Depending on downhole conditions, such as formation temperature, the optimum blend of glycols and salts is determined to achieve the most effective cloud point for shale inhibition.
Shale Inhibition and Drilling Performance
Studies have been conducted to examine the benefits of cloud point behaviour on drilling performance and shale stability. The results have shown distinct performance benefits both above and below cloud point.
Above the Cloud Point
Above the cloud point, glycols are present as emulsions while the separate phases are continually intermixed by the circulating system, particularly at the bit. These emulsions block pores in the formation, preventing fluid invasion and consequent instability in water-sensitive formations.
The GLY-DRILL system can be engineered such that drill cuttings at the bottom of the hole are initially above the system’s cloud point. When this occurs, a protective glycol layer forms around the cuttings as a result of glycol “clouding out” on its surface. This prevents the cuttings from reacting with water until they have risen to a point in the wellbore where the fluid temperature drops below the cloud point, allowing the glycol to re-dissolve into the fluid. This explains both the increased inhibition present when using the GLY-DRILL system in reactive shales and the low glycol depletion rates seen in the field.
Below the Cloud Point
Glycols still provide enhanced performance below the cloud point. Field evidence has shown that glycols below the cloud point deliver improved shale stability. Also, laboratory studies have shown that shale inhibition with glycols is more effective when water-soluble, rather than water-insoluble, glycols are employed. One explanation is that the glycols adsorb onto shale surfaces via oxygen molecules present in the glycol chain. Once the water-soluble glycols enter the formation, the higher formation temperature causes the glycol/water solution to phase separate in-situ, forming an emulsion. The hydrophobic glycol droplets in the emulsion fill and block the shale pores, preventing further fluid invasion and stabilizing the shale.
Many theories have been put forward to explain the shale inhibition mechanism of glycols. Of these, the following mechanisms are believed to be the most conclusive.
• Schlumberger Cambridge Research suggests that the main function of the polyol is to compete with water molecules for adsorption sites on the clay minerals present in shales. They also conclude that when KCl is present, there is a good correlation between inhibition and adsorption of polyol. Strong adsorption still occurs from distilled water and, although polyol intercalates are formed, they have a slightly higher basal spacing and the resulting complexes are much less stable in water.
• Jachnik & Green (SPE28963) of INTEQ Fluids studied the effects of clouding polyols in aqueous solutions with varying salinities. They conclude that the formation of an aggregation of hydrophobic molecular droplets contribute to the lowering of both static and dynamic filtration, thus achieving a reduction in pore pressure penetration of invasive fluids. They also demonstrate that a relatively narrow operating band exists for optimum benefit from clouding polyols in Thermally Active Micro Emulsion (TAME) type fluids. The ideal operational environment for TAME type fluids is when the fluid temperature is below the cloud point and the formation temperature is close to or above the cloud point. In these cases, pore plugging will occur just inside the rock matrix as the material clouds out within the hotter environment, sealing the formation against further ingress. At this point, surfactant/polymer interactions will be at their greatest, as well as any complexation of surfactant with monovalent ions in solution such as potassium. Optimum benefit for long term borehole stability should occur in this scenario.
It can be concluded that shale inhibition and formation protection is achieved by:
• the polyol displacing water from adsorption sites on clay minerals present in shales, and
• blocking the formation pores from further ingress of invasive fluids by “clouding out.”
Several operators have now carried out studies to evaluate the effect of polyols in drilling and coring fluids on reservoir permeability. These studies have shown no adverse effects on the formation samples tested. Many suggest that the polyol actually protects the productive rock from impairment by the drilling fluid provided it is above the cloud point.
As discussed earlier, the theory is that the polyol emulsion protects the formation from excessive fluid invasion by pore plugging just inside the rock matrix as the material “clouds out” within the hotter environment, thus sealing the pores against further ingress. At this point, surfactant/ polymer interactions and complexation of surfactant with monovalent ions (such as potassium in solution) will occur more frequently.
These reactions provide optimum benefit for long term borehole protection by acting as a blocking layer which compliments polymer additions. An additional benefit may be the inhibition of interstitial clays.
Once part of the clouding polyol in solution becomes hydrophobic, a significant improvement in filter cake quality is seen. Water tends to bead on the surface and the cake is easier to “peel off” the filter medium.
Production after acidization and completion of wells has been above operator expectations. Unfortunately, no information on skin damage and comparisons of production rates before and after using polyols has been made available. It is thought that quick plugging of low permeability sandstones (< 20 mDarcy) by the polyol leads to minimal formation damage inside the reservoir. Later acidization could then reach beyond the invasion zone around the wellbore leading to a better clean-up and enhanced production.
In general, polyols added at low concentrations (< 5%) have little effect on the viscosity of the drilling fluid. At increased concentrations, there will be some variation in viscosity with the maximum rheology obtained in the 110° to 130°F temperature range.
The function of Partially Hydrolised Polyacrlamide - PHPA is commonly understood and is the primary source encapsulating polymer in the system. Typically the excess PHPA is run at ±1.3 – 1.5 ppb.
GLY-DRILL System Additives
Glycols have many of the properties of mineral and synthetic oils, but contribute virtually no toxicity to the fluid. Having low vapour pressure at normally ambient temperatures, glycols are not considered an occupational health hazard.
Various glycols are used in water based drilling fluids for several types of applications. The chemistry of glycol additives can be varied to meet the demands of the product application. This formulation flexibility makes the glycol based system the ideal fluid in these applications. The Glycol type and concentration is determined from the well temperature profile and fluid salinity. Typically the treatment level for optimum performance is 3% to 5% by volume.
Note: The full benefit and cost effective use of glycols can only be realised when their use is engineered correctly.
Return permeability studies obtained by various operators on glycol fluids indicate no detrimental effects in using glycols across reservoir sections. This is particularly true at higher glycol concentrations (5% to 10% by volume).
Xanthan Gum (XCD Polymer) and Polyanionic Cellulose (PAC™ R) polymers are used to provide viscosity.
Fluid Loss Control
Polyanionic Cellulose (MIL-PAC™ R & LV) polymers are used to provide fluid loss control. Modifies starches, such as MI STARCH may be used for additional fluid loss control in saline environments.
Salinity control is usually derived from the KCl content required for inhibition. However, the GLY-DRILL™ system can be formulated with a variety of salts in different concentrations dependent on the wellbore and engineering requirements. The salinity of the fluid is determined by the water activity and cloud point required for a specific wellbore application using GLY-CAD® software.
Generally the pH is run in the 8.3 to 9.5 range, preferably at the lower end of the range to minimise clay dispersion.
Glycol Software GLY-CAD®, models the behaviour of water-soluble glycol products. With this programme, the concentration of and types of salts and glycols can be engineered to match the operational needs of the drilling fluid. Depending on downhole conditions, such as formation temperature, the optimum blend of glycol and salt is determined to achieve the most effective cloud point for wellbore protection and shale stabilisation.
Colorimetric Method (GLY-KIT®) is used to determine the concentration of glycol in drilling fluid filtrate. The glycol is extracted into dichloromethane using a blue complexing agent. The resulting blue colour of dichloromethane is compared to standard solutions and the concentration determined.
Ideally, where possible, shale characterisation should be undertaken to determine the level of inhibition required from the fluid components. This will lead to a more cost effective use of the system and it’s components will be tailored to meet specific demands of the shale encountered in the wellbore. Shale samples should be collected from the first well in order to carry out this work.