Handling sand

Sand erosion can cost the industry tens of billions of dollars every year. NEL’s Marc Laing looks at what can be done to mitigate the issues.

Changes to oil and gas production over past decades have brought about unprecedented challenges surrounding the management of sand during hydrocarbon production. Furthermore, due to the number of declining fields, together with the push to develop from more complex fields, this trend is set to continue.

The problem

Having exploited most of the easier oil and gas pickings, and owing to technological advancements, oil and gas companies are now in the position to tackle some of the sandier fields. These are fields that generally exhibit weaker geological rock formations that shed sand easier than others.

This means it is easier to pull dislodged sand up through the wellbore and into the production stream. Controlling the amount of sand entering production is paramount, not just to the economics of the particular field, but also to the overall safety of the operation.

Unfortunately, this can be a difficult and often high-risk undertaking with disastrous consequences if not adequately managed. For this reason, there is currently a major focus on sand management in the offshore upstream oil and gas sector.

Operators have to first determine the optimum production conditions to control the flow of sand. If flow rates are too low, the sand can manifest downhole and block the free flow of hydrocarbons through the well. Conversely, if flowrates are too high, then large amounts of sand can be swept up through the well to the production pipeline and make contact with critical components along the way – with devastating results. This includes choke valves, which are the first line of defense when it comes to controlling the flow of hydrocarbons from the reservoir. Thereafter, it can wipe out major production components, such as pumps, seals, and flowlines. It can also fill the working volume of separators and reduce their overall performance.

Sand can erode flowmeters, which act as the primary measure for production control and hydrocarbon accounting. Even small amounts of sand over time can cause catastrophic damage to plant and equipment as it erodes even the toughest of materials.

At its most aggressive, sand has been known to eat through a half-inch pipe over a 12-hour shift. A worst-case scenario would be complete loss of hydrocarbon containment leading to pollution and environmental damage on a grand scale. Under certain conditions, reservoirs have been known to collapse due to the geological rock structure eroding.

There are a number of technologies in existence to monitor and reduce sand entrainment through oil and gas production systems. However, these are also affected by being in contact with sand, and can therefore be subjected to high levels of damage, which can greatly hinder their performance.

The challenges affecting sand management are further compounded as even sand-free fields can produce sporadic sand spikes over the course of their production life. These spikes can often be even more damaging and difficult to predict and manage. Furthermore, as fields mature and flow rates reduce, such as the case with many North Sea fields, there is a much greater risk of pushing sand up through the producing wells as the reservoir is worked much harder from below to extract the remaining hydrocarbons.

The key goal for oil and gas companies is therefore to accurately predict the flow of sand over the course of a field’s lifetime. This informs safe process conditions, production limits and furthermore identifies hot spots i.e. areas where sand will do the most damage. The latter allows the specification of critical production equipment and informs maintenance requirements. In particular, how robust certain equipment needs to be in order to survive specific conditions and therefore how frequently equipment needs to be replaced is a core consideration. Without an effective sand management strategy, such as this, oil and gas operators are subjected to very high levels of financial exposure. To put this into perspective, reportedly in one North Sea field gas production losses of up to 75% were reversed owing to predictive modeling and failure analysis.

However, as sand cannot be fully eliminated, the risk of sand monitors and protection systems failing, including downhole sand-screens, remains. The strategy of the industry has therefore been to put in place an over-engineered sand resistant infrastructure. But, this comes at a financial cost and must be balanced against the overall economics of producing a particular field.

Counting the cost: sand eroded choke valves. Images from NEL.

Erosive flow testing in the laboratory

Due to the potential risks and impact of poor sand management, NEL is seeing a lot more erosion testing being undertaken by the upstream offshore sector. This includes validating erosive-resistant production equipment, such as valves, pumps, seals, flowlines, flowmeters and materials, as well as established API and ISO standards or user-defined specifications.

Sand mitigation technologies are also being put to test to assess their performance and effectiveness over wide-ranging conditions. These, amongst others things, include sand-monitors, sand-screens and chemical erosion inhibitors. However, laboratory testing on its own can be prohibitive and confined to the capability of the test house, which often struggles to reproduce today’s field conditions – especially when considering the wide-ranging flowrates, pressures and temperatures associated with deep and ultra-deepwaters.

There are also so many different scenarios that would have to be tested, including different shapes and sizes of sand particles, travelling at different velocities and under different flow regimes. These factors have a bearing on the overall onset, location and magnitude of erosion. For this reason, industry is increasingly relying on flow modeling as a fast and cost-effective means of predicting erosion.

Predictive flow modeling

Computational fluid dynamics (CFD) is becoming a major tool for predicting sand erosion in the upstream sector. It allows engineers a method for validating a high number of designs and solutions in advance, covering the entirety of the production chain. It provides the ability to predict the rate and locations in which erosion will occur over the life of the field.

As well as informing the optimum production process conditions to keep erosion at bay, it can also predict failure. This is paramount, especially to support life extension of aging assets. However, CFD can also be limited and not as effective as physical testing when it comes to detecting hot spots and ill-performing equipment. This is because it is highly dependent on specialized and experienced modelers, and remains highly sensitive to different geometries, conditions and model setups. Due to the potential high level of uncertainty, it has been used cautiously by industry.

Taking a combined approach

In NEL’s experience, best practice is to use a combined approach of physical laboratory testing and erosive flow modeling. This allows the models to be validated against traceable experimental data and extrapolation to wider conditions thereafter, which cannot be achieved through physical testing. This provides a much greater level of confidence and certainty in the use of flow modelling to predict sand erosion, allowing operators to anticipate the lifetime costs of the field over time, and plan for improved financial control.

Marc Laing is the CFD Service Leader at NEL, a provider of technical consultancy, research, measurement, testing and flow measurement services to the energy and oil and gas industries, as well as government. Part of the TÜV SÜD Group, the company is a global center of excellence for flow measurement and fluid flow systems and is the custodian of the UK’s National Flow Measurement Standards.