Darcy-Scale Experimental Investigation of Intermittent Particle Transport in Porous Media and its Impact on Injectivity

In the mature fields where oil and gas have been extracted for decades, a significant volume of water rises to the surface along with the hydrocarbons. This produced water is not merely a byproduct to be discarded; it is a complex mixture containing suspended particles, residual oil, and various chemicals. To protect the marine environment and maintain the pressure needed to keep the reservoir productive, oil companies often pump this water back underground. However, the water carries a hidden problem: the tiny solid particles it holds can clog the porous rock that stores the oil, much like a clogged artery restricts blood flow. This blockage, known as formation damage, reduces the ability to inject the water, threatening the efficiency of the entire operation. For years, engineers have studied how these particles settle and build up under steady, continuous flow, but the real world is rarely so constant. With the integration of renewable energy sources and changing operational needs, injection systems are increasingly subject to stop-and-start schedules, yet the behavior of these particles during such interruptions remained a mystery.

A team of researchers from NORCE Research AS and Equinor ASA set out to solve this puzzle by recreating the conditions of a real oil reservoir in the laboratory. They used cylindrical plugs of Bentheimer sandstone, a rock chosen for its uniform and open pore structure, which allows fluids to flow through it much like water through a sponge. Into these rock samples, they pumped a brine solution containing suspended quartz particles, mimicking the dirty water found in oil fields. The experiment was designed to be exceptionally long and detailed, running for up to twenty-four days and injecting a volume of fluid equivalent to fourteen thousand times the total space inside the rock sample. To see exactly what was happening inside the rock, the team did not rely on guesswork; they used a high-resolution computed tomography scanner, similar to a medical CT scan, to take three-dimensional images of the rock before and after the injection. They also placed sensors at multiple points along the length of the rock to measure pressure changes in real time, allowing them to track how the blockage moved and evolved.

The researchers tested four different scenarios. Three of the rock samples were subjected to a cycle of injection followed by a period of rest, or "shut-in," where the flow stopped completely. These cycles varied in length, with some injecting for seventy-two hours and resting for twenty-four, while others injected for only twenty-four hours before resting. The fourth sample served as a control, receiving a continuous, unbroken stream of the particle-laden water. The goal was to see if stopping the flow allowed the rock to recover its ability to accept water, or if the interruptions actually made the problem worse.

What they found was a dynamic and somewhat counterintuitive story of temporary relief followed by rapid recovery. When the flow stopped during the shut-in periods, the pressure required to push the water through the rock dropped noticeably. For a brief moment at the start of each new injection cycle, the rock seemed to breathe again, accepting water more easily than it had just before the stop. This suggested that the pause allowed the delicate structures formed by the trapped particles to loosen or rearrange, temporarily breaking the blockage. However, this improvement was fleeting. As soon as the water began to flow again, the pressure surged back to its previous high levels within a very short time. The particles, which had settled or shifted during the rest, quickly reorganized themselves into a new, tight barrier.

The experiments revealed that the damage occurred in two distinct phases. First, the particles traveled deep into the rock, getting stuck in the tiny tunnels and pores, creating an internal blockage that spread inward from the entrance. This stage was characterized by a rapidly increasing pressure. Once the internal pores became saturated, the particles could no longer enter, and they began to pile up on the very surface of the rock, forming a dense, external layer known as a filter cake. This external layer acted as a secondary wall, and its growth caused the pressure to rise in a steady, linear fashion. The researchers observed that the depth of the internal blockage was limited, penetrating only between five and twelve millimeters into the rock, while the external layer grew thicker over time.

The frequency of the stop-and-start cycles proved to be a critical factor. The samples subjected to more frequent interruptions did not stay open longer; instead, they developed a more compact and resistant external layer. The repeated cycles of stopping and starting seemed to act like a compaction process, squeezing the particles into a denser, less permeable structure. The rock that experienced the most frequent cycling ended up with the lowest ability to let water through, showing a much sharper increase in pressure compared to the rock that flowed continuously. Even the initial condition of the rock mattered; the samples with larger, more open pores allowed particles to travel deeper before the external layer formed, whereas the tighter rocks blocked up almost immediately at the surface.

These findings suggest that the common assumption that stopping injection might help clear a clogged well is only partially true. While a pause offers a momentary reprieve, the subsequent restart can lead to a more severe and tightly packed blockage than if the flow had never stopped. The study indicates that in systems where injection schedules are dictated by variable energy supplies or operational constraints, the rhythm of the flow is just as important as the volume of water being pumped. The researchers conclude that managing these cycles is essential for maintaining the long-term ability to inject produced water, as the repeated disruption and rebuilding of these particle layers can accelerate the decline of the injection system. The work provides a clear, experimental picture of how the microscopic world of particles and rock responds to the macroscopic reality of an intermittent energy future.