Sand. Nice on a beach, but it does have a tendency to get into the most unlikely – and unhelpful – places. That’s equally true in oil and gas operations where tiny loose grains can cause equipment damage and, ultimately, reduce production. So, what can be done?
Sand can be an unwelcome by-product for the oil and gas industry. It occurs when weak rock starts to break down as reservoir pressure reduces, creating loose grains that travel with the hydrocarbons into the well. Ultimately, this can cause erosion, damage equipment and lead to reduced production. The issue can be dealt with by restricting a well’s flow rate, but this also limits oil and gas production. Techniques exist that help stop sand entering the well at all while oil and gas continue to flow—metal screens or coarser-grained gravel that act as filters are installed in the well. But, these are expensive, complex and can sometimes fail. Finding the problematic zone can also be difficult and expensive. Reservoir sections can be hundreds of metres long, but ‘sanding’ may only occur in a small section. A significant proportion of BP’s hydrocarbon reserves are found in weakly consolidated reservoirs, in areas such as the Gulf of Mexico and Angola. So, it has set up a team of ‘sand control’ specialists, focused on developing technologies that enable an equivalent level of hydrocarbon recovery from weak reservoirs as from competent rocks. Currently, the team is looking at three ways of enhancing well productivity and reliability: predictive capability—to better understand when sand will be produced; monitoring—to better pinpoint where it is entering the well; and remediation—to stop it entering at all.
Predicting the problem
Sand prediction capability gathers information by analysing reservoir rock, studying well case histories, and using sophisticated numerical models to determine the conditions that result in rock disaggregation and sand production. This allows the team to help select the most appropriate completion method, as well as advising on a well’s optimal operating limits to minimize sand production. “Our specialists will study immediate production expectations and look at how reservoir conditions might change over many years,” says Dana Aytkhozhina, drilling and completions fluids specialist. Combining this with prediction skills allows BP to optimize completion performance for the life of the well.
The ability to identify where sand is entering a well could change the way the issue is managed. By using a system known as distributed acoustic sensing, based on a fibre-optic cable placed inside the well at the sand face (the point where the well crosses the reservoir interval), the team can ‘hear and sense’ noise and vibrations, helping to pinpoint the exact location of sand entry. “The fibre provides a continuous array of ‘microphones’ along the entire well, back to surface,” says Paul Beaumont, completions technology manager. “When sand flows into the wellbore, it collides with the metal completion hardware and generates noise. The optical fibre picks it up, detects the particular spectral signature that corresponds to sand inflow and identifies its location.”
He adds: “We already have a certain amount of information available through cores, logs and our subsurface understanding, but to be able to track where and when sand production takes place, in real-time, is invaluable in optimizing production and managing sand.”
Against the grains
Once the source of sanding is identified, action can be taken. Mechanical tools can be run and installed in the well to isolate the sand-producing intervals. BP is also investigating new types of chemical ‘glue’ that bind sand grains together just strongly enough to prevent them leaving the reservoir, while letting the oil and gas flow. “We’ve done several successful onshore field trials and are now developing experience with this technology in some of our offshore wells,” says Aytkhozhina. The goal is to integrate all three areas so that a well can produce at its optimum level for as long as possible. Beaumont says: “If we can combine predictive capability with data from the distributed acoustic sensing, then we can prevent more sand from entering the well in the first place. Our acoustic sensing also provides an early warning, so that we can identify the problem zone and take action before the sanding rate becomes too high.”