Horizontal well multi-stage plug-n-perf fracturing has proven to be an effective method to develop unconventional reservoirs. Various studies have shown uneven fluid/proppant distributions across all perforation clusters. It is generally accepted that both fracturing fluid and proppant contribute to well performance. Achieving uniform fluid and proppant placement is an important step toward optimal stimulation. This paper discusses how to achieve such uniform placement in every stage via a CFD (Computational Fluid Dynamics) modeling approach.
A lab-scale CFD model was first built and calibrated using several sets of experimental data of sand transport in horizontal pipes. A field-scale model was then built and validated with downhole camera observations. With the model validated, CFD simulations were performed to evaluate the impact of key parameters on fluid and proppant placement in individual perforations and clusters. Some key parameters investigated in this study included casing size, cluster spacing, cluster count per stage, pumping rate, fluid properties, proppant properties, perforation parameters (size, orientation, number), and stress shadowing effects between clusters and stages.
Both lab and CFD results show that perforation orientation has a significant impact on proppant distribution, as bottom-side perforations receive more proppant than top-side perforations, due to gravitational effects. This bottom-side bias can be further aggravated with lower injection rates, smaller casing IDs, lower proppant concentrations, larger proppant sizes, and lower fluid viscosities. This study demonstrates that uniform fluid and proppant placement across all clusters in each stage is achievable with optimized perforation and treatment designs. Lab and CFD results also show that proppant distribution is heel-biased at lower rates and toe-biased at higher rates. Once the casing size, fluid and proppant properties are determined, there is an optimal rate that makes uniform fluid and proppant placement in all clusters more achievable. Cluster design is the building block for stage design. With the optimal pumping rate obtained, the number of clusters per stage, and perforation size, orientation and number can be optimized. CFD modeling plays an important role in this design optimization process.