MEAT THE MAJOR 8 SHALE INHIBITOR MECHANISMS

Shale Inhibitor & Stabilizer Chemicals Mechanisms & Product Types

The range of shale inhibitor mechanisms and typical product types used for shale inhibition are tabulated in the table below.

Inhibitor Process	Shale Inhibitor Mechanism	Typical shale inhibitor Products
Cation Exchange	Cations like non-hydrated K+ replace hydrated cations on C.E. sites.	KCl (Some organic cations are also used)
Encapsulation	High molecular weight adsorptive polymers coat the cuttings and wellbore surfaces. Most effectively used against cuttings dispersion.	Partially Hydrolyzed PolyAcrylamide (PHPA)
Reduction of Water Permeation Rate into Shale	Increased fluid viscosity results in decreased rate of flow into shale pores. Only relatively low molecular weight compounds can flow through tiny pores in shale.	Polyglycerols, Polyalkylene Glycols, Methyl-Glucoside,
Replacement of Inter-Layer Water	The water between clay sheets is replaced by more strongly-adsorbing organic molecules (This is often combined with cation exchange).	Polyalkylene Glycols, Amine-Capped PAGs
Reduction or Reversal of Osmotic Transfer of Water into Shale	The use of strong solutions of salt or polyhydroxy compounds in the mud reduces vapor pressure (Water Activity), slowing or reversing the transfer of water molecules into shale.	NaCl, CaCl2 , Potassium formate, Methylglucoside
Plugging of Shale Pores to Reduce Pore Pressure Penetration	Micro-colloids enter and plug the tiny pores in the shale. Some polyglycols may come out of solution after penetrating the shale and warming up.	Highly cross-linked polymer microgels, Cloud Point Glycols
Sealing of Shale Micro-Fractures	Some wellbores collapse when mud filtrate enters cracks and micro-fractures. Plasticized particles bridge, coalesce and seal the fractures	Asphalt or Gilsonite particles. Sulfonated Asphalt
Reactions with Clays in the Shale to Produce Inter-Particle Cement	Clays react with lime and can become hardened (the pozzolan effect). Clays react with sodium silicate to drop pH and precipitate silica cement. Silicate also reacts with calcium, magnesium, aluminium in clays to cement particles together.	High Lime muds, Sodium Silicate solutions (Potassium Silicate is sometimes used)

The cost to the industry of hole problems and lost time incidents caused by shale hydration is immense. As a result, considerable research and product development has been invested in products and systems designed to improve wellbore stability and inhibit cuttings hydration. There are many types of shale, many ways in which shale hydration and instability can occur, and many methods that can be employed to minimize the ensuing problems, as explained below:

How Cation Exchange Provide Shale Inhibition?

Cation Exchange of hydrated cations by potassium has been described earlier. Potassium Chloride as a shale inhibitor is very effective at inhibiting the hydration of shales that have a high cation exchange capacity (i.e. shales containing a substantial concentration of montmorillonite and/or illite).

The shale inhibitor used for such mechanism

Is Encapsulation Make Shale Inhibition?

Encapsulation by the shale inhibitor called partially hydrolyzed polyacrylamide (PHPA) is an effective method for dealing with highly dispersive shales such as micaceous siltstones and kaolin-rich formations. Encapsulation is probably better at reducing cuttings dispersion than inhibiting hydration of the nearwellbore shale. PHPA is more accurately described as a high molecular weight copolymer of acrylamide and acrylic acid. The amide and carboxylate functional groups adsorb very strongly onto the surface of the cuttings, coating them with a layer of viscous polymer that helps to slow the flow of filtrate into the formation pores.

What Is The Relation Between Water Permeation Rate & Shale Inhibitor?

Reduction of Water Permeation Rate into Shale by increasing the viscosity of the aqueous phase obviously slows the hydration process. According to Darcy’s Law, the permeation flow rate is inversely proportional to the viscosity, so the flow rate of a salt-saturated filtrate into a porous solid would be half the flow rate of fresh water because the viscosity of saturated sodium chloride solution is twice that of water.

Solutions of only relatively low molecular weight materials which work as shale inhibitor (such as salts, polyglycerols, methylglucoside and polyalkylene glycols) can flow through shale pores because the pore size is so small. The graph below shows that most of the pores in a typical shale are less than 10 nanometers (0.01 microns) in diameter. For a typical PHPA polymer with a molecular weight over one million, a dissolved PHPA molecule can be as long as about 1,000 nm (or 1 micron), and it therefore cannot flow through the pore network.

Pore size distribution of a typical shale

How The Shale Inhibitor Called Polyalkylene Glycols (PAGS) Work

Replacement of Inter-Layer Water using polyalkylene glycols (PAGs) as a shale inhibitor, which are typically low (around 1,000) molecular weight copolymers of ethylene oxide and propylene oxide. Ethylene oxide is sometimes used on its own, as in polyethylene glycol or an ethoxylated alcohol such as butanol ethoxylate. The ethylene oxide type glycols need to be used in conjunction with potassium for good effect.

shale inhibitor Edge view of montmorillonite sheets inhibited by polyalkylene glycol and potassium ions — Edge view of montmorillonite sheets inhibited by polyalkylene glycol and potassium ions

Potassium ions are shown as spheres occupying the cation exchange sites.
The polyalkylene glycol molecules displace water, filling the gap between the sheets.

Ethylene oxide and propylene oxide copolymers can be effective on their own, without having to use potassium chloride. The ether-links (-C—O—C-) in PAGs adsorb onto montmorillonite sheet surfaces more strongly than water, so they can infiltrate the gap, displace water, and stop further hydration. Modified PAG molecules have recently been developed that have amine “anchor groups” at each end of a linear PAG molecule, similar to the one shown below:

Besides the PAG displacing water from the gap between clay sheets, the amine groups are cationic in nature and can adsorb strongly on the cation exchange sites, providing doubly effective inhibition. It is possible for several diamine PAG molecules to bridge the gap between two clay sheets, like “nanoropes” holding the sheets together.

The Effect Of Osmotic Transfer of Water On Shale Inhibition

Reduction or Reversal of Osmotic Transfer of Water into Shale requires a Semi-Permeable Membrane that allows water molecules, but not solutes such as salts, to diffuse through the extremely small pores in the membrane. The membrane also restricts the flow of liquid water. To an extent depending on the mineralogy and the level of compaction, shale can be described as a “leaky semi-permeable membrane”. The membrane efficiency can be improved (i.e. reducing the permeability to aqueous solutions) by using pore plugging additives.

Osmosis is mainly an issue with Water Based Mud, and it is one that has increased in importance as the efficiency of commercial additives has improved over the years. The more the permeability of a shale is reduced and the more the hydration of a shale is inhibited, the more likely it is that the shale approaches the condition of a semi-permeable membrane. Under these conditions it becomes more important to use a reduced Water Activity (Aw) brine phase in the Drilling Mud because this will avoid the osmotic build up of high pore pressure in the shale, which can lead to tight hole and/or cavings.

Water Activity is related to water vapor pressure. Water vapor is composed of individual molecules of water. In liquid water, individual molecules of water move around (diffuse) in equilibrium with larger groups of water molecules that are held transiently together by hydrogen bonding. The concentration of single water molecules is a function of temperature. For example, at boiling point several individual water molecules are moving about rapidly enough to create a vapor pressure of 1 atmosphere.

Water vapor pressure is also a function of the concentration of dissolved material in the water. For instance, the vapor pressure of brine saturated in sodium chloride (about 25% by weight NaCl) is lower than that of pure water by a factor of 0.75 and this factor is called the Water Activity (Aw) coefficient. Water molecules randomly diffusing from a drilling fluid in the wellbore into the pores of a shale will be captured by the clay surfaces and the dissolved ions in the pore water in an effort to restore the level of hydration.

If the water activity in the Drilling Fluid is high then there will be a plentiful supply of water vapor molecules diffusing from the mud into the shale. This will raise the pore pressure, which together with continued diffusion will transport water of hydration further into the shale. If, however, the water activity in the drilling mud is lower than that of the shale, diffusing water molecules are more likely to be captured by the drilling fluid, leading to a net flow into the mud and reduced hydration of the shale. This diffusion and capture of water molecules is a form of osmosis.

As shales became buried over geological time they became compacted and some of the inter-particle water was gradually squeezed out. The ions that were dissolved in the water (e.g. sodium, magnesium, calcium, chloride, etc.) were not as mobile in the shale pores as the water, due to the “leaky semi-permeable membrane” effect, so as the water drained out some of the ions were left behind, resulting in an increase in the pore water salinity. This leads to reduced water activity (i.e. reduced water vapor pressure) in the shale.

In addition, the clay particles became compressed tightly together by the overburden pressure over time, and illite and montmorillonite particles became compressed as inter-layer water escaped. This puts a lot of stored energy into the compressed shale so, given the chance, the shale wants to rehydrate and expand. So the compressed shale itself also results in reduced water activity, which adds to the contribution from the concentrated salts in the pore water. As mentioned earlier, the efficiency of the semipermeable shale membrane can be improved by using pore plugging additives, described in the next paragraph.

Plugging of Shale Pores Is Considered As A Shale Inhibition Mechanism

Plugging of Shale Pores to Reduce Pore Pressure Penetration is shown in the following illustration. If permeability of the shale near the surface of the wellbore can be substantially reduced by plugging the pores in the shale with tiny organic colloids or clouded-out PAG droplets, then the wellbore pressure and hydrating water cannot penetrate very far. The hydrostatic pressure of the drilling fluid therefore acts upon the surface of the wellbore and provides support to the near-wellbore shale.

shale inhibitor Diagram showing plugging of shale pores by colloids or clouded-out polyalkylene glycol droplets. — Diagram showing plugging of shale pores by colloids or clouded-out polyalkylene glycol droplets.

Besides organic colloids and polalkylene glycols, inhibitive drilling mud based on the shale inhibitor called sodium silicate are thought to cause near-wellbore pore plugging in shale. The alkaline silicate solution reacts with the clay in the shale and the ions in the pore water. The subsequent drop in pH causes silica to precipitate, and calcium and magnesium dissolved in the pore water causes insoluble silicate salts to precipitate. Both of these colloidal precipitates are effective pore-pluggers.

How Shale Micro Fractures Sealing Play A Role As A Shale Inhibitor Solution?

Sealing of Shale Micro-Fractures is important when shales are brittle and contain many small fractures and micro-cracks. As shown in the left diagram below, the mud pressure can leak into the cracks, leading to little or no differential pressure (ΔP) supporting the near-wellbore shale. This can result in spalling, where pieces of shale break off and fall into the wellbore, and hole gauge washouts.

Diagram showing sealing of shale micro-fractures using asphalt particles

Some shale inhibitor materials, such as blown asphalt or gilsonite, are fairly brittle solids at surface temperatures, and can be ground to a size similar to that of the microcracks in shale. When added to a drilling fluid the powdered particles lodge in the entrances to fractures in the shale. They are then softened by elevated downhole temperatures, which allows them to deform and coalesce, sealing the entrances to cracks. This stops the penetration of wellbore pressure beyond the near wellbore shale, and the differential pressure is able to support the borehole wall, preventing spalling and caving of shale into the wellbore.

How Chemical Reactions Can Cause Shale Inhibition

Reactions with Clays in the Shale to Produce Inter-Particle Cement was mentioned earlier, where the alkaline filtrate from sodium silicate fluids reacts with clay in the shale. The high alkalinty can dissolve some silica from the clay silica layers, and the alumina layers can transiently dissolve to an extent to give sodium aluminate in solution. These reactions reduce the alkalinity of the invaded filtrate, leading to the precipitation of silica, while the aluminate and silicates combine to drop-out amorphous alumino-silicates.

Besides the pore-plugging effects of these precipitates noted earlier, they act as an inter-granular cement which causes the shale to harden and gain in strength. By these processes sodium silicate fluids have provided on occasions the levels of wellbore stability only obtained previously by using oil-based mud. However, some drawbacks that have been seen with silicate sytems have included cuttings accretion onto BHAs, poor lubricity, interference with MWD tools, and difficulty in obtaining low fluid loss.

It is worth mentioning that high lime fluids have been used in shale drilling. The objective here is that the lime in the filtrate reacts with clay particles in the shale in ways similar to Portland cement, or lime mortars, to produce a calcium silicate cementitious precipitate. The use of lime fluids for shale drilling has declined with the development of more efficient inhibitive fluids and additives.

MEAT THE MAJOR 8 SHALE INHIBITOR MECHANISMS

Shale Inhibitor & Stabilizer Chemicals Mechanisms & Product Types

How Cation Exchange Provide Shale Inhibition?

Is Encapsulation Make Shale Inhibition?

What Is The Relation Between Water Permeation Rate & Shale Inhibitor?

How The Shale Inhibitor Called Polyalkylene Glycols (PAGS) Work

The Effect Of Osmotic Transfer of Water On Shale Inhibition

Plugging of Shale Pores Is Considered As A Shale Inhibition Mechanism

How Shale Micro Fractures Sealing Play A Role As A Shale Inhibitor Solution?

How Chemical Reactions Can Cause Shale Inhibition

Shale Inhibitor & Stabilizer Data Sheets From Halliburton

Shale Inhibitor & Stabilizer Data Sheets From Schlumberger

MEAT THE MAJOR 8 SHALE INHIBITOR MECHANISMS

Shale Inhibitor & Stabilizer Chemicals Mechanisms & Product Types

How Cation Exchange Provide Shale Inhibition?

Is Encapsulation Make Shale Inhibition?

What Is The Relation Between Water Permeation Rate & Shale Inhibitor?

How The Shale Inhibitor Called Polyalkylene Glycols (PAGS) Work

The Effect Of Osmotic Transfer of Water On Shale Inhibition

Plugging of Shale Pores Is Considered As A Shale Inhibition Mechanism

How Shale Micro Fractures Sealing Play A Role As A Shale Inhibitor Solution?

How Chemical Reactions Can Cause Shale Inhibition

Shale Inhibitor Related Scientific Papers

Shale Inhibitor & Stabilizer Data Sheets From Halliburton

Shale Inhibitor & Stabilizer Data Sheets From Schlumberger