Staged multicluster fracturing in horizontal wells is the key technology for forming complex fractures in shale reservoirs. The existence of shale bedding plays a conspicuous role for the propagation path of hydraulic fractures, affecting the propagation of the fracture height direction prominently. A 3D finite element model containing three clusters signed as side clusters and middle cluster was established based on the cohesive zone model and the dynamic distribution mechanism of interfracture flow. And the correctness of the model was verified by literature comparison. Some factors including cluster spacing, horizontal stress difference, shale bedding strength, perforation density, injection rate, and viscosity of fracturing fluid which influenced fracture propagation behavior of bedding shale were simulated. The results indicate that the stress interference of the middle cluster by the clusters on both sides will be prominently obvious when the cluster spacing is less than 10 m. Multiclusters will penetrate across the shale bedding when the horizontal stress difference is more than 4 MP, which will conspicuously reduce the activated probability of discontinuities and the complexity of fracture geometry. In correspondence with increase of horizontal stress difference, the interference between clusters also increases prominently, which will conspicuously decrease the propagation of the middle cluster. In order to comprehensively equalize the length of multiclusters, the inhibition of intercluster stress interference on the middle cluster propagation can be counteracted by improving pressure drop in perforation. The high injection rate and viscosity of fracturing fluid will contribute to the shale bedding shear slip increasingly, which is conducive to the formation of complex fractures in areas with well-developed bedding. The study has a certain guiding significance for the operation parameter design of multicluster fracturing in bedded shale.
The failure of managed-pressure running casing in oil and gas wells may lead to complex accidents such as overflow or leakage. The technique of using multi-density gradient drilling fluids in wellbores with negative pressure windows (NPWs) is often used to deal with this situation. Therefore, it is vital to analyze the dynamic slurry column structure and calculate the wellbore pressure during casing running. For this issue, the model of transient surge pressure is established during casing running. The calculation equation of the model is proposed, and the calculations of the wellbore pressure are carried out with the exploration of Well LT-X1, located in the Xinjiang oil field. A circulation scheme is designed as follows: Circulate 125 m3 of drilling fluid with a density of 2.45 g/cm3 and 155 m3 of drilling fluid with a density of 2.35 g/cm3 at a depth of 3560 m. From there, circulate 164 m3 of drilling fluid with a density of 2.35 g/cm3 at a depth of 5900 m. Finally, at a depth of 7050 m, circulate 250 m3 of drilling fluid with a density of 2.30 g/cm3. The casing running speeds and back-pressure values were designed as follows for the respective well sections: 0–1523 m: 0.160 m/s casing speed, 0 MPa back pressure; 1523–3560 m: 0.160 m/s casing speed, 1.641 MPa back pressure; 3560–5900 m: 0.145 m/s casing speed, 2.427 MPa back pressure; 5900–6674 m: 0.137 m/s casing speed, 4.041 MPa back pressure; 6674–7050 m: 0.124 m/s casing speed, 4.457 MPa back pressure. The results show that optimizing structure of the multi-density gradient drilling fluid with different densities and applying annular back pressure in stages, with the accurate calculation of wellbore pressure, can achieve the goals of leak-proofing and pressure-stabilization. It is concluded that this result may serve as the foundation for managed-pressure running casing technology.
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