Abstract
The channels may be formed in the unconsolidated sands reservoir due to formation failure during high-pressure water injection or frac-packing. Based on the continuum mechanics, a mathematical model has been established to simulate the formation process of big channels in unconsolidated sands reservoir during fluid injection. The model considers the effect of reservoir heterogeneity, solid particles erosion, and deposition. The dynamic formation process of channels around the borehole and its influencing factors are analyzed by this model. The results indicate that the seepage erosion plays a very significant role in the formation of the channels during fluid injection for the unconsolidated sands with extremely low strength. The formation of the channels is closely related to the duration of fluid injection, injection pressure, reservoir heterogeneity, formation plugging, and critical fluid velocity. The long channels are more likely to form as injection time increases. Higher injection pressure will lead to higher flow rate, thus eroding the solid particles and forming big channels. The increase of the rock strength will enhance the value of critical fluid velocity, which makes it difficult for the occurrence of erosional channelization. The near-wellbore damage of the formation will decrease the flow rate, and the preferential flow channels are less likely to be induced under the same injection pressure when compared with undamaged formation. In addition, we also found that the reservoir heterogeneity is essential to the formation of preferential flow channels. The channels are especially prone to be formed in the regions with high porosity and permeability at the initial time. The study can provide a theoretical reference for the optimal design of high-pressure water injection or frac-packing operation in the unconsolidated sands reservoir.
Highlights
There are a large number of unconsolidated sands reservoirs discovered in the Bohai Bay of China [1, 2], including Bozhong, Penglai, and Shengli oil fields
The results showed that the onset of erosion is mainly controlled by hydraulic gradient; Zhang et al [25] used a discrete element method code PFC2D to model the particles movement during fluid injection
A numerical model for simulating the channel formation process in the unconsolidated sand process during fluid injection was performed based on the principle of conservation of mass in continuum mechanics
Summary
There are a large number of unconsolidated sands reservoirs discovered in the Bohai Bay of China [1, 2], including Bozhong, Penglai, and Shengli oil fields. They proposed a continuum model that involves three phases (immobile solid phase, mobile solid phases, and fluid phase) and used the finite volume method to model the erosional channelization induced by fluid flow in a saturated porous medium In their model, the internal erosion rate of the granular medium was calculated by assuming that the grain will be dislodged when the pressure gradient exceeds a threshold. Yerro et al [23] attempted to use the material point method to model internal erosion process in bimodal internally unstable soils, which assumes that erosion rate is proportional to the velocity difference between fluid and solids They performed a numerical test consisting of a soil column subjected to a vertical water flow and observed that the fine grains can be eroded and are able to move freely through the stable skeleton of the soil. The dynamic evolution process of the channels near the wellbore and its influencing factors are analyzed, which can give a theoretical reference for the optimal design of high-pressure water injection and frac-packing operation
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