Abstract

The Lucaogou formation in Junggar basin is the most potential tight oil reservoir in China. However, the disappointing production of a pilot appraisal project with 10 horizontal wells adopting multistage fracturing hold back the further development. The multiple laminated sand-shale formation with horizontal beddings severely affect the fracture height and geometry. This paper provides comprehensive large-scale-block fracturing experiments and field microseismic monitoring data to examine the effect of the highly laminated rock fabric on fracture geometry.

Hydraulic fracturing polyaxial tests were conducted using large-scale Lucaogou tight oil outcrops simulating a vertical well and horizontal well respectively. Cross-linked gel and slickwater were used to examine the effect of fluid viscosity on fracture propagation. Fracturing pressure analysis, acoustic emission monitoring, rock splitting and tracing the dyed fluid were combined to investigate the hydraulic fracture geometry and propagation mechanism. Field microseismic monitoring results along with the wellbore trajectory, stress profiles and formation characterization were also analyzed.

Large-scale-block fracturing tests show that the treatment pressure in the fracture is increasing steady, which represent the severe fracture containment or high process-zone stress, both are indication of bad stimulation potential. Rock splitting and acoustic emission mapping results proved that fracture initiated and propagated along the horizontal bedding. And the rock heterogeneity and natural fracture play a decisive role in fracture propagation. Field microseismic monitoring results also indicate that the monitoring fracture geometry is much more complicated than predicted. The microseismic mapping shows that fracture propagates downward when the stimulated interval was drilled below the target layer and propagate upward or confined in the pay zone when the interval was drilled above the target layer. Both laboratory and field results suggest that the hydraulic fracture of Luchaogou tight oil reservoir is almost determined both by the landing place in the formation and the natural fractures. Bedding-parallel stimulation would hinder fracture height and more complexity might be observed in a microseismic cloud, but it would probably not beneficial for production. Moreover, when we start flow back and the weight of the overburden is realized, the bedding planes being propped open could lose conductivity because of embedment and proppant pack collapsed.

The identification of the target layer in the centimeter-level laminated Luchaogou tight oil formation was challenging and the multiple horizontal beddings severely confined and hinder the fracture propagating vertically, even though larger fluid and proppant volume were injected at higher rate. Well trajectory, natural fracture identification and its direction and layer heterogeneity are the major concerns when stimulated highly laminated formation, which is a typical characteristic of China's tight oil reservoirs.

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