There are numerous challenges involved with a well with sanding potential. These issues can also impact the sand production at various stages of production for myriad reasons. To understand why this is the case, a knowledge of sand production phenomenon is necessary. The key factor in sand production is the formation failure, which is governed by in-situ stresses in addition to the mechanical properties of the rock. Stresses around wellbore / perforations are more concentrated and weak rocks (unconfined compressive strength less than 1000 psi) are prone to deformation under these conditions. In addition, drilling and perforating contribute to damage in the near wellbore region of the formation. The fluid production and the associated drag force applied on the weakened formation induce erosion at sandface and sand grains are transported up the wellbore.
Figure 1. Sand production process
During the initial production stage, high production rate is usually desired for economic reasons. However, too high drawdown can result in erosion at the sandface. To find how the drawdown affects the sand production, a relationship between reservoir pressure and drawdown pressure can be determined based on thick-walled cylinder tests (TWC) as shown in figure 2. Exceeding the critical drawdown pressure at the initial reservoir pressure (Pi) can cause sand production. Even if the well is completed with mechanical means of sand control such as gravel pack and fracpack, formation and gravel instability caused by high fluid velocity can lead to screen plugging and erosion on the hotspots. Therefore, it is always recommended that sand control completion is used in conjunction with drawdown control. This is optimal to reap the economic benefit of highest possible production without catastrophic failure of the downhole equipment.
Figure 2. Sand production risk dependency on reservoir and bottomhole pressure
When fields become mature, pore pressure depletion and water production bring challenges in sand control. Reservoir fluid supports the in-situ stresses in the formation along with its inherent strength. For example, the formation experiences less vertical stress than the actual weight of the formation above because the formation fluid takes the load. This stress felt by the formation is called effective stress. As depletion progresses, there is not enough reservoir fluid support for the formation, which increases the effective stress. This generally means more deformation and damages around the perforation which lead to a greater risk of sandface erosion. This phenomenon is shown in fig 2 – when the reservoir pressure declines from the initial pressure (Pi) to the critical reservoir pressure, the safe drawdown becomes zero and sand production becomes inevitable if fluid production continues at this point.
To prevent the depletion, pore pressure can be maintained through water injection. However, it has a disadvantage because water production has a tendency to exacerbate the sand production problem. This can be visualized with an example of a sand castle. The key to making a strong sand castle is to “wet” the sand. When the sand is sufficiently wet, the sand castle holds its shape and can bear loads. This is analogous to the connate water saturation in the formation; even if the formation gets deformed and disaggregated, the water at the right saturation provides enough capillary forces that bind the sand grains together. Hence, the deformed area can still maintain strength without sand production. Conversely, the incoming tide can very quickly destroy the sand castle. Likewise, water saturation gradually increases with strong aquifer drive or water injection. When it exceeds a critical limit where the post-failure sand structure cannot maintain its stability, sand production suddenly occurs in a massive scale. Considering the impact of depletion and water production, a new well with little sand production today could turn out to be a major sand producer in the future.
As discussed, sand production is a complex phenomenon with many contributing factors. It is critical to understand the formation (mechanical and petrophysical properties) and what changes occur to the reservoir throughout the production lifespan. With this understanding, analytical, empirical, and numerical approaches for sanding prediction can be used to determine critical drawdown limits at certain operating conditions. This includes incorporating reservoir simulations to predict the pressure decline and water breakthrough. Finally, sanding risks and economic studies should be taken into account to choose the best completions design and proper rate control. These steps, while arduous in the beginning, can bring the best result and economics to the asset even when the field becomes mature.
[toggle title=”Learn More About Amos Kim”]
Amos S. Kim
Consultant – Halliburton Consulting
Mr. Kim is a petroleum engineer in Halliburton’s Global Consulting Group, where he focuses on mature field development. He has worked on several projects involving production optimization by applying his expertise in production technology and sand control.
Upon joining Halliburton’s Technology group in 2011, Mr. Kim led the development of sanding prediction software and managed a joint project with a major national oil company for sand control applications. He also contributed to the revitalization of the geomechanics laboratory, identifying potential projects in geomechanics and evaluating software development. Prior to joining Halliburton, he conducted research on topics ranging from wellbore stability to artificial intelligence as a graduate research assistant while pursuing degrees in petroleum and aerospace engineering.
Mr. Kim has written several published papers and is an active member of the Society of Petroleum Engineers (SPE), American Rock Mechanics Association, and Society of Core Analysts. He holds a Master of Science degree in petroleum engineering from the University of Texas at Austin and a Master of Science degree in aerospace engineering from Texas A&M University. He received his Bachelor of Science in aerospace engineering from University of Illinois at Urbana Champaign.
For more information, please feel free to contact me.
[/toggle]
The post Sand Control Mechanism and its Impact on Mature Fields appeared first on Halliburton.