Jeremy Bowden reports on the advanced materials and innovation now being applied to proppants used in hydraulic fracturing operations.
Over the last few months, advances in technology have helped sustain US unconventionals production despite a crash in prices. Among the key areas of development are the materials injected into the shale rock to hold fractures open, known as proppants.
Initially, the proppant used in hydraulic fracturing was simply sand, but other materials are increasingly being employed. There are resin-coated sands and ceramic-coated sands, more recently there are deformable polymeric proppants, and even a proppant composed of sintered (powdered) bauxite – tiny manufactured rocks. By using these materials to create the proppant it has been possible to vary its properties – including size, shape, strength and weight – more easily and accurately. By adjusting these properties, the optimum pathways can be maintained by the proppants for the extraction of hydrocarbons, increasing the conductivity, or the amount of flow that the proppant will allow.
Physical size typically ranges between 106 µm and 2.36 mm (or in proppant units between 8 and 140 mesh) to suit the structure of the shale, allowing hydrocarbons to be more effectively released. Proppant geometry can also be controlled, with certain shapes more effective in particular rock structures. Ceramic and resin proppants have superior roundness than sand proppants and are more uniform. When used, these give the fracture greater permeability and higher fluid retrieval rates. Resin-based proppants can also contribute to reducing the environmental burden related to fracking by cutting the volume of water or other ingredients required.
Generally the strength of proppant is directly correlated to its weight – the higher the strength, the greater the weight. But the new materials have also enabled lightweight proppants with sufficient strength to withstand higher pressures. Lighter proppants are able to travel further into the formation, as they stay suspended in fracking fluid longer than their heavier counterparts. They also allow downhole engineers to deliver the required high volume of proppants safely, without exceeding weight limits, whereas previously the ratio of proppant to fluid had to be below optimum levels.
Pressure and depth
The new materials offer huge potential for a large range of drilling applications. Weight or density can be adjusted, with deeper, higher pressure fracks benefiting from a heavier, more robust proppant with a higher crush resistance. Weaker sand proppants can disintegrate into smaller bits which then occupy the gaps between the fractures resulting in loss of hydrocarbon flow. In some environments proppants also need to be acid-resistant, requiring the right combination of materials.
As global energy demand continues to increase, deepwater/high pressure (DWHP) and acidic oil and gas reserves are projected to make up a greater share of long-term supply. Unconventional techniques are anticipated to be used more and more for enhanced oil recovery (EOR) at conventional wells – as well as for deep shale plays. Ceramic proppants are particularly effective in deeper drilling, where the fracture closure pressures are higher, owing to their greater crush resistance.
One firm, Melior Innovations, has developed what it claims is a step-change in proppants using a highly specialised material new to the oil and gas industry, designed to optimise production from DWHP wells. The material is consistently able to reach four key performance requirements – strength, weight, spherical shape and uniformity. It is also designed to resist extreme cyclic loading conditions for the life of the well.
High strength gives it the ability to operate at closure stress levels, and its light weight enables it to travel further into fractured formations, delivering increased hydrocarbon recovery. Finally, the spherical shape and uniform size ensure optimal conductivity throughout the proppant pack, delivering higher ultimate recovery levels.
CARBO Ceramics, meanwhile, has produced what it calls an ultra-high conductivity proppant technology for deep wells. The product is being used by a major operator in the US Gulf of Mexico’s Lower Tertiary trend, marking the largest and deepest job for these proprietary technologies to date. The technology provides higher production and estimated ultimate recoveries (EURs), maximising the operator’s ROI.
Some new proppants are also incorporating scale-inhibiting chemicals, which are released steadily over a set period, resulting in long-term protection against the formation of common oilfield scales over the whole production network.
Innovators are also offering easily detectable inert tracer technology for completions in both vertical and horizontal wells. Traceable proppants are detectable with a standard neutron logging tool, and can be mixed with sand or other proppants before injection. The tracer does not dissolve or wash away and is permanently identifiable, providing operators the flexibility to conduct post-fracture logging months or years after fracking.
The technique enables the detection and evaluation of near-wellbore proppant location and quantity, which provides an accurate measurement of perforation cluster efficiency and near-wellbore connectivity. This can help to maximise EURs. Understanding proppant placement also supports the optimisation of stage placement and proppant diversion. The information gathered enables operators to reduce costs and ultimately improve their completion efficiency.
Momentive Specialty Chemicals was among the first companies to offer proppants tagged with non-radioactive tracer technology to track proppant distribution, under its PropTrac fracture diagnostics service. UK-based Tracerco also produces a range of chemical tracers, and has recently been developing a new suite of proppant tracers which last even longer downhole.
Companies have also developed curable resin-coated proppants that can bond together under the correct amount and type of stress and temperature, which are designed to help protect against proppant embedment and flow back. Proppant flow back can be an issue in certain formations where, as the well produces, it forces proppant back into the wellbore, which can cause serious damage to surface equipment.
The resin-coated proppant is curable so it only bonds under fracture conditions, when there is differential pressure causing closure stress, within a range of temperatures. This ensures the bonding takes place only when the proppants are pumped into the fractures, rather than in the vertical wellbore or a horizontally drilled lateral.
Currently the proppant market is still dominated by sand, with 80% of wells using it at an average rate of 5 million pounds (2.27 million tonnes) per well, costing over US$100,000 each. The other 20% is split between resin coating and ceramics, with pioneering materials making up only a tiny proportion.
This fact that these advanced materials remain only small proportion of the overall market suggests that the innovation process has only just begun. But just as in any other young industry, we can expect to see rapid development continuing throughout the early years, and with more and more attention being given to unconventional techniques – now even applying them in conventional reservoirs, as a recent IHS study suggests – opportunities in both the advanced materials and high-spec proppant markets appear considerable.