Study on optimization of fracturing timing based on geological-engineering integration: A case study of water injection development area in the Ordos Basin

The Ordos Basin is a typical low-pressure tight oil reservoir, which has been effectively developed through the advanced water injection + hydraulic fracturing technology model. Advanced water injection enhances formation energy and thereby improves recovery efficiency. However, from the perspective of fracturing, advanced water injection alters the in situ stress field of the reservoir, affecting hydraulic fracture propagation. This study proposes optimizing the fracturing timing to mitigate the negative impacts of advanced water injection on fracturing. By comprehensively considering reservoir physical and mechanical properties, a heterogeneous integrated geological-engineering model was established. Model calibration was performed using three methods: actual field fracturing treatment curves, microseismic monitoring results, and single-well production data to ensure model accuracy. Taking the square inverted nine-spot well pattern in the X341 well area as an example, the four-dimensional evolution patterns of pressure, stress, and saturation during the full-cycle advanced water injection–fracturing–production process were investigated. The study revealed the influence of advanced water injection on fracture morphology and identified the optimal fracturing timing for reservoirs with varying advanced water injection volumes and physical properties. Key findings include the following: (1) Advanced water injection induces changes in reservoir pore pressure, in situ stress, and water saturation, with diffusion ranges ranked as pressure field stress field saturation field. (2) Advanced water injection increases horizontal principal stresses and causes stress direction deflection (less than 3°). The maximum horizontal principal stress increases faster than the minimum horizontal principal stress along the sediment source direction, resulting in a bidirectional stress difference increase of ∼1 MPa. Conversely, perpendicular to the sediment source direction, the stress difference decreases by ∼1.6 MPa. (3) Fracturing timing significantly affects the reservoir injection-production pressure field. Excessive pre-fracturing water injection reduces fracture length and conductivity and may cause water breakthrough in new wells. Post-fracturing water injection disrupts reservoir pressure equilibrium, impairing waterflood efficiency, indicating the existence of an optimal fracturing window. (4) When permeability exceeds 1.5 mD, advanced water injection may lead to early high water cut in wells, suggesting pre-fracturing water injection should be avoided. This study provides guidance for similar advanced water injection development models.