End User Focus: Bentonite under pressure

By Jessica Roberts
Published: Tuesday, 23 November 2010

Oil and gas drilling activity is on the rise, fuelling demand for oilfield minerals such as bentonite – but the drilling mineral is battling decreasing consumption

Put simply, oil and gas extraction could not exist without the technical drilling fluids and the myriad additives and minerals that go into them that are used to simultaneously clean, lubricate and seal boreholes.

Drilling muds, or fluids, encompass a wide range of products that each plays a pivotal role in the extraction of hydrocarbons. Of the three main fluid types water-based, oil-based, and synthetic water-based fluids consume the most minerals in their production.

Broadly speaking, this is the chief reason why numerous industrial minerals rely heavily on oil and gas markets to fuel demand; the majority of drilling that takes place at present uses water-based fluids, owing to economic reasons and, importantly, environmental concerns.

The use of synthetic or oil-based muds, meanwhile, tends to be reserved for technical wells, where they may be deviated, horizontal or high temperature.

The engineering behind a drilling fluid is vast: the fluids must be viscous enough to carry with them cuttings from the bottom of a borehole, but not be so viscous as to stop cuttings from settling out in the mud pit.

Sodium bentonite (or swelling bentonite) is one of the primary minerals that are relied upon to gel with cuttings in the borehole to ensure they are not re-circulated. Its derived products are a significant component of drilling fluids along with other minerals such as barytes.

Along with gel formation, bentonite’s role is principally to seal borehole walls and lubricate the cutting head meaning its consumption is directly linked to oil and gas exploration, and thus demand.

Bentonite’s clean sweep?

Consumption of bentonite for oilfield applications accounts for around 70-80% of the drilling grade bentonite produced, and around 20% of overall bentonite demand. The mineral has gained a stronghold in this market owing to its viscosifying characteristics and effective sealing and filtration properties.

These properties come from bentonite’s structure a colloidal clay comprising mainly montmorillonite meaning the mineral swells when wet.

Although bentonite is abundant its use in oilfield drilling means that, similarly to food or pharmaceuticals, stringent requirements are specified by industry bodies such as the American Petroleum Institute (API), which often eliminates many of the potential deposits that could be a drilling bentonite source (Tables 1-3).

Despite these specifications, the crucial role of bentonite cannot be denied in this market the mineral has no substitutes.

Yet for the last decade or so, consumption of bentonite for well drilling has steadily (albeit gently) declined. The two reasons behind this appear to be a) recovery of marginal hydrocarbons, requiring “clear fluids”; and b) a new generation of polymers that flocculate formation clays, but also adversely affect bentonite, itself a clay.

Once a reservoir has been sufficiently drilled and explored, the transition to a production platform from an exploration well usually involves replacing water-based muds with clear drilling fluids. Thus during times of lower exploration but sustained hydrocarbon production, bentonite-based drilling muds lose market demand.

However, bentonite-based fluids are still favoured in top hole sections where they control fluid loss.

Oil outlook

The latest rig counts for October 2010 show significant improvement on the counts taken 12 months earlier. Major oil and gas production hubs, such as Canada and the USA, are pushing increases of up to 60%, while regions less affected by the downturn are still showing promising rises, such as the Middle East (see rig count map).

Rig counts, such as those published by oilfield service providers Baker Hughes and M-I Swaco, are telling barometers for the drilling industry and its mineral suppliers. At present, all rig counts are pointing to increased mineral consumption Ð despite recent temporary bans arising from the Deepwater Horizon spill in the Gulf of Mexico (IM November 2010: Oil drilling faces new era of regulation).

The lack of any clear substitutes for bentonite, and the promising rig counts currently being reported by the industry, indicates a comfortable future for bentonite in this market. However, market production figures for all grades of bentonite (including drilling grade) have indicated, over the last few years, a gentle but sustained fall in demand. How these factors will play out over the longer term is unclear.

Drilling mud basics

Drilling muds, or fluids, can be divided into three primary types: water-based, oil-based and synthetic. The majority of fluids used today are water-based, as they are more economical and environmentally sounder than their counterparts. Water-based fluids also consume the most minerals in their production, such as bentonite.

In the well, the role of drilling fluid is to:

  • Remove cuttings from the borehole to be stored in a settling pit
  • Allow cuttings to drop from the fluid into the pit, to avoid re-circulation
  • Prevent cuttings from settling while drill pipe is installed
  • Create a layer on the borehole wall to mitigate against caving, and preserve adjacent formations from being eroded by the upward flowing drilling fluid
  • Reduce fluid loss by sealing the borehole wall
  • Lubricate the drill bit, bearings, mud pump and drill pipe

Drilling market (land and offshore) October 2010

Hydrocarbon drilling hotspots in October 2010, indicating the active drilling rig count (land and offshore) and percentage change from October 2009.

Source: Rig count data courtesy Baker Hughes

Table 1: API specifications - bentonite

Suspension properties Standard
Viscometer dial reading at 600 r/min minimum 30
Yield point/plastic viscosity ratio maximum 3
Filtrate volume maximum 15cm3
Residue of diameter greater than 75µm maximum mass fraction 4%

Table 2: API specifications - non-treated bentonite

Suspension properties Standard
Yield point/plastic velocity ratio maximum 1.5
Dispersed plastic velocity minimum 10m Pa.S
Dispersed filtrate volume maximum 12.5cm3

Table 3: API specifications - OCMA grade bentonite

Suspension properties Standard
Viscometer dial reading at 600 r/min minimum 30
Yield point/plastic viscosity ratio minimum 6
Filtrate volume maximum 16cm3
Residue of diameter greater than 75µm maximum mass fraction 2.5%
Moisture maximum mass fraction 13%

Source: API