Drilling muds or fluids are complex aqueous or oil-based suspensions designed to fulfil a number of important functions during the oil extraction process.
Commercially available as generic products, they are often tailored to meet the demands of specific applications, for example deep shelf drilling or extraction in environmentally sensitive areas.
Formulators control mud performance by manipulating the mud composition and the properties of the constituents, and may use a range of different additives to achieve the required behaviour.
Particle size is a significant variable for the components of drilling muds, affecting the way in which they interact with the surrounding geology as well as the rheological properties of the mud system. Particle size measurement therefore plays an important role in formulating high-performance drilling muds.
Here we examine the way drilling muds function and the role of different compounds within these industrially important suspensions, focusing particularly on two commonly used materials: barite and calcium carbonate.
We look in some detail at the need for optimal particle sizing and see how laser diffraction, a long-established method for particle size measurement, is used for the routine analysis of drilling mud.
Cooling and lubricating
The primary role of drilling muds has always been to cool and lubricate the drill bit, increasing the rate of penetration during drilling. Although this remains essential, they now fulfil much broader functions. A
well-designed mud formulation does the following:
Achieving this requires careful control of the physical, thermal and rheological properties of the mud. The concentration and density of suspended particles, for example, may be manipulated to give a fluid density and viscosity that ensures effective debris removal while maintaining the hydrostatic head needed to prevent oil and gas escape during drilling.
Particle size also impacts on these aspects of functionality but more specifically it is matched to the geology of the surrounding rock in order to stabilise the well and prevent environmental damage.
Drilling muds are formulated as either water- or oil-based systems. Water-based systems are considered the more environmentally friendly and low-cost option.
These can, however, be technically challenging to formulate and relatively costly to maintain, as the presence of salts and other additives may have a significant effect on mud particle size distribution.
Oil-based muds, on the other hand, while generally easier in terms of formulation and maintenance, represent a potential pollution risk and are more expensive to produce.
Whichever continuous phase is used, the system will generally contain the following components:
In the case of water-based systems, other additives such as starches and polymers may also be used, for example to prevent fluid loss, particularly when drilling shales. For this type of geology, polymeric additives have also proven beneficial for manipulating the water activity of the drilling mud thereby preventing water transfer into the formation1.
Clays, such as Bentonites, may be used to manipulate mud rheology and many other additives are also available to address issues such as bit balling and plugging and/or the high temperatures and thermal stability issues associated with deep drilling.
The importance of particle size
Particles smaller than the pore size of the surrounding geological formation will bridge rock pores during mud circulation, leading to the formation of a filter cake that prevents egress of fluids from the well during drilling.
This is an important function of the mud as the cake protects the surrounding rock from damage while simultaneously preventing fluid loss and stabilising the well. It is particularly important when drilling through shales, which are highly prone to fluid invasion and difficult to drill without excessive fluid and associated pressure loss.
Careful determination of the optimum particle size is essential as very small particulates may themselves penetrate the surrounding rock formation, blocking pores. This can cause irreversible damage to the production zone.
Much research has been conducted in order to better understand how to tailor the particle size of mud constituents to achieve the required performance for a specific rock formation. Abrams2, for example, proposed that the median particle size (Dv50) of a bridging additive should be equal to or slightly greater than one third of the median pore size of the rock, to prevent pore blockage. Hands3 proposed that the Dv90 (the particle size below which 90 per cent of the volume of material is found) should be equal to the pore size to limit penetration into the pore structure.
More recent research has further improved understanding, showing that the invasion of either polymeric material, from the mud, and/or fine particulates, produced during drilling, causes damage in the surrounding rock formation.
The filter cake must therefore form an effective barrier for both of these substances. This requires control of particle size distribution as well as particle size, as these two variables together control the packing of particles within the cake and hence its permeability.
While in general small particles pack more closely than larger ones, poorer packing can be offset by a broad particle size distribution. In this case, smaller particles fill the voids between large ones creating a more densely packed cake.
Two examples of materials widely used in drilling muds that are closely specified in terms of particle size are barite and calcium carbonate.
Barite is a mineral commonly used in both aqueous and non-aqueous drilling muds as a weighting agent, to control the density of the mud. Particle size is important because coarse particles have a tendency to settle out, impairing equipment performance, while excessive fines are associated with inadequate weighting and formation damage.
The American Petroleum Institute (API) therefore specifies that for barite used in drilling mud applications, the fraction above 75 microns should be minimal and the per centage of material below 6 microns no higher than 30percent by weight. Fig. 1 shows particle size distribution data for barite samples produced by different suppliers in line with this API specification.
The results indicate the samples conform to the defined requirements, particularly with respect to fines content.
Calcium carbonate is used as a bridging material and also a weighting agent, often in preference to barite because it is acid-soluble and therefore easily dissolved during final cleaning of the production zone. Because calcium carbonate is used to deliver both adequate weighting and effective bridging, optimal particle size is formation-dependent.
Different grades of calcium carbonate are therefore available ranging from coarse (>100 microns Dv50) to ultra-fine (<10microns Dv50). Fig. 2 shows data for these different grades, with each one being designed to operate within a different type of geological formation.
Measuring the particle size
The particle size data for calcium carbonate and barite shown above were measured using a Mastersizer 2000 from Malvern Instruments, a laser diffraction analyser with a broad dynamic range suitable for measuring the different components of drilling muds.
Laser diffraction is a well-established technique used widely for the analysis of wet samples; however, sample dilution is a frequent requirement as measurement is viable only if light can penetrate the material.
For mud analysis, dilution and dispersion methodologies need to be carefully considered and controlled to avoid inaccuracies in the results.
Sharma et al4, for example, showed that particle size distributions reported for Bentonites can be completely different when using either water or isopropyl alcohol (IPA) as a dispersant.
This can be confirmed using the Mastersizer 2000 (Fig. 3), with a 40 micron shift in the reported Dv50 being observed when using IPA rather than water. In both cases the maximum degree of dispersion has been achieved so the question raised by these results is – which particle size is most relevant to drilling mud performance?
In order to address this issue the dispersion conditions of the mud system during use must be considered.
In general, when measuring water-based muds, the best correlations between mud performance and particle size measurements are achieved by adjusting the ionic strength of the dispersant used in analysis to that of the mud system. This ensures that the state of dispersion of the mud is maintained during dilution, and prevents the dissolution of any salt-based compounds.
For oil-based muds, on the other hand, dispersant viscosity and surface tension are key.
The importance of dispersant choice is highlighted by Fig. 4, which presents data for an oil-based mud. Initial measurements, using an IPA/xylene mixture as a dispersant, showed a relatively fine particle size but this result did not correlate well with rheological measurements carried out on the mud system. Fine particle sizes are normally associated with high viscosities whereas the actual mud viscosity was relatively low.
Further investigation of the state of dispersion of the mud showed that dilution using the IPA/Xylene mixture caused dilution shock, and a reduction in particle size.
Dilution shock describes the phenomenon whereby rapid or dramatic dilution, changes the nature of the particles away from their original state within the sample.
The particle sizing method was therefore altered to specify use of the base oil in the mud as dispersant. The resulting larger particle size correlates well with rheological observations.
Particle size is an important parameter for drilling mud formulation, which needs to be closely matched to geological formation for optimal well performance. Laser diffraction analysis, and in particular instruments such as the Mastersizer 2000 which offer a broad dynamic range, are of considerable value to the industry for routine analysis.
By employing robust analytical methodologies, formulators can use these tools to rapidly generate reproducible particle size data which in turn facilitates the development of formulations suitable for drilling in diverse geologies.
Paul Kippax is Product Manager Diffraction Systems and Stephen Ward-Smith is Product Technical Specialist in Laser Diffraction with Malvern Instruments Limited. For more information, visit www.malvern.com