Abstract

Hydraulic fracture azimuthal orientation depends on stress distribution in the formation and is considered to coincide with the maximal horizontal stress azimuth. The knowledge of the hydraulic fracture orientation is of critical importance in field development planning, including well spacing, pattern, water injectors location that will lead to desired line drive mechanism, optimized reservoir drainage and maximized recovery factor. That information is not less critical for infill drilling, fracturing «old» wells, re-fracturing, fracturing of sidetracks and the knowledge of hydraulic fracture orientation of the water injectors well that are fractured by the mere injection process. It is also known that re-fracturing and pore pressure re-distribution will re-orient the stress field not only in the near well bore area but also in the far field.

Theoretical modeling and world experience suggest that the hydraulic fractures do re-orient under the influence of pore pressure changes because of fluid production and water injection. Field sector formation pressure distribution makes the fracture offset from maximal stress direction towards injection wells and this effect of local stress reorientation is more likely to occur in low permeable formations with low diffusivity and low stress orientation anisotropy.

A number of complex fracture geometry orientation investigations were performed on low permeable formation of Siberia, to understand the phenomena. This included acoustic measurements and micro-seismic monitoring. Though the main purpose was to optimize fracture geometry by evaluating the measured and matching of modeled fracture parameters, the importance of fracture orientation for reservoir development was even more significant.

The results undeniable indicate that a significant deviation of the field sector fracture azimuth from maximal regional stress exists. It was discovered that the degree of deviation from the regional preferred fracture azimuth is affected by water injection and reservoir fluid production in the sector of the monitored well. Observations regarding the effect of fracture wings length asymmetry may be also explained by the disturbance of initial stress conditions. The results of this investigation will be used to further optimize hydraulic fracture design, reservoir pressure maintenance including well spacing and well pattern and water flooding strategy.

Introduction

Hydraulic propped fracturing is considered a very conventional way to effectively stimulate oil and gas wells in low to mid permeability formations. The operators in Western Siberia in general and in particular Rosneft-Yuganskneftegas use hydraulic fracturing almost without exemption in all new wells as a conventional completion method. Additionally, initial and refracturing of wells with 10–15 year of production history, as well as fracturing of sidetracks and fracturing of infill drilled wells are a common practice nowadays. The oilfields are typically developed with an inverse nine or seven spot pattern and are under intensive pressure maintenance program.

Well spacing is between 500–1000 meter in most oilfields and fracture lengths have been constantly increasing within the last few years. Moreover, it is also quite common that the ongoing water injection program is above the fracturing pressure, hence water injector wells under intensive and long injection schemes are subject to create long fractures as well. Many of the late wells are planned injector wells, or converted fractured producer wells that are accordingly fracture stimulated, therefore avoiding significant water bypass or overshooting of the water front between layers within the formation.

The uneven injectivity is aggravated because of the strong waterflood inducing also thermally fracturing in vertical sweep of injection wells. In highly variable reservoir quality, injection water tends to be injected into the best zones. This promotes cooling, and the resulting thermally induced fracturing further enhances injection into these zones. This phenomenon is well known from the literature 1 and is in detrimental in formations of highly anisotropic transmissibility.

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