Abstract:Large accumulations of methane hydrates are known to exist in the deepwater Nankai Trough located off the southern coast of Japan. Due to its enormous potential as an energy source, there is growing interest in the production of methane gas from these deposits. An offshore production test was scheduled to test the technological viability to produce the methane hydrate by using a depressurization method. However, methane hydrate production may cause large amounts of compaction and subsidence, which can damage w… Show more
Borehole collapse will pose a threat to the safety of equipment and personnel during drilling operation. In this paper, a finite element multi-field coupling model for investigating borehole collapse in hydrate reservoir was developed. In this model, fluid seepage, heat transfer, hydrate dissociation and borehole deformation are all considered. Based on which, effects of drilling fluid density on both of hydrate dissociation and borehole collapse are investigated. The investigation results show that disturbance of drilling fluid invasion to hydrate reservoir will lead to hydrate dissociation around wellbore, and dissociation range narrows obviously with the increase in drilling fluid density. When the relative fluid density is 0.98, natural gas hydrates in reservoir with a width of about 16.65 cm around wellbore dissociate completely. However, dissociation range of natural gas hydrate has decreased to 12.08 cm when the relative fluid density is 1.10. Moreover, hydrate dissociation around wellbore caused by drilling fluid invasion may lead to borehole collapse, and borehole collapse can be significantly restrained with the increase in relative fluid density. Borehole enlargement rate is 33.67% when the relative fluid density is 0.98, but nearly no collapse area displays around wellbore when the relative fluid density increases to 1.12. In addition, investigation herein can provide an idea for designing drilling fluid density in hydrate reservoir when different allowable borehole enlargement rate is considered. The minimum fluid density designed for avoiding disastrous borehole collapse increases nonlinearly when higher requirements for borehole stability are proposed.
The KS reservoir is a naturally fractured, deep, tight gas sandstone reservoir under high tectonic stress. Development wells for this reservoir are of depths in excess of 6,500 m TVD. Stimulation is required to provide production rates that sufficiently compensate for the high cost of drilling and completing wells to access this deep reservoir. Hydraulic fracture design and execution must be optimal to ensure economic production. To effectively stimulate a more than 200-m thick sandstone reservoir yielding consistently high performance, it is critical to understand the interaction between hydraulic fractures and natural fractures, as the natural fractures significantly affect the growth and geometry of hydraulic fractures.To this end, a comprehensive study was conducted involving frac pressure analysis of previously stimulated wells, microseismic data analysis, hydraulic fracturing modeling by using a fracturing simulator that honors the natural fracture system, near-wellbore 4D geomechanical simulation of mechanical response of natural fractures during hydraulic fracturing, and large block hydraulic fracturing tests. This study reveals that existing natural fractures results in complexity of hydraulic fracture systems both in the near wellbore region and in the far field region. The complexity in the far field is largely controlled by the intersection angle (defined as the angle between the natural fracture strike and the maximum horizontal stress direction) given the large differential horizontal stress in this field.Based on an understanding of the interaction mechanism, an optimization of the hydraulic fracturing strategy was implemented in KS field. Improvements were made in staging, perforation, diversion, and the pumping schedule, which increased the averaged production rate more than 50% compared with previously stimulated wells.
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