Petroleum companies nowadays tend to increase oil production from low-permeability heterogeneous reservoirs. Commercial development of such difficult objects is possible with all modern stimulation methods involved including hydrofracturing. Decrease in flow friction near the wellbore and increase in filtration area as a result of fracturing lead to multiple increase in production well rates and injection well capacities. Besides recovery factor increases because of oil production from poor-drained zones and layers.
The purpose of this work was to estimate hydraulic fracturing efficiency by estimation of after fracturing well rate increase, by comparative analysis of well logging held before and after frac operation and by analyzing the well test. The task was to investigate factors of fracturing effectiveness. Also the principle of well selection for fracturing and their order in FDP was examined.
Data on Priobskoe and Krasnoleninskoe (Em-Egovskaya area) fields located in Western Siberia were used in this work.
Hydraulic fracturing is a method of reservoir stimulation which represents fracturing of formation in the minimum stress direction under excess pressure of pumped fracturing fluid (fig.1). Fracturing fluids are the fluids which transfer the energy for fracturing from the surface to the wellbore. The fracture grows up under fracturing fluid pressure and becomes connected with system of natural fractures which are not penetrated by the well and with zones of higher permeability. Therefore well drainage area becomes larger. Fracture is kept open with the proppant which is transported with the fracturing fluid.
The results of hydrofracturing lead to multiple increase in production well rates and injection well capacities because of decrease in flow friction near the wellbore and increase in area of filtration. Recovery factor also becomes higher because of additional production from poor-drained zones and reservoirs.
Hydrofracturing has lots of technological decisions for different development objects and project purposes. Technologies differ mainly on fracturing fluid and proppant volumes and hence on sizes of created fractures. The most widely used method of wellbore region stimulation is local fracturing. Creation of fractures with 10–20 m length and tens cubic meters of fracturing fluids with unity tons of proppant is pretty enough. In that case well rate increases in 2–3 times. Technology of creation of high-conductive short fractures in high-permeability formations has been developed lately. Long fracture creation results in increase in wellbore region permeability, drainage area and oil recovery factor. Also watercut may decrease. Optimal well length in 10–50 mD formation is about 40–60 m and injection volume is between tens and hundreds cubic meters. Also selective fracturing is used which allows to involve in production and increase the productivity of low-permeability layers.
A great experience has been gathered in the world oil industry nowadays. Each frac operation gets an increasing attention and preparation. Data management and analysis of input information represents a large part of this preparation. Data required for fracturing operation planning can be divided into three groups:
geological formation properties (permeability, porosity, saturation, reservoir pressure, gas-oil contact, oil-water contact, etc);
geometry and orientation of the fracture (minimum horizontal stress, Young's modulus, Poisson's ratio, formation compressibility, etc);
fracturing fluid and proppant properties.
Principal information is derived from geological, petrophysical and geophysical investigations, laboratory core tests and field experiments (micro- and mini-fracs). Lately technology of integrated approach has been developing. This approach takes into account a lot of factors such as reservoir permeability, well patterns, fracture mechanics, fracturing fluid and proppant characteristics, technological and economical limits.
According to this the following stages of fracturing optimization can be emphasized:
1. Selection of wells for stimulation taking into account the existing or project development system which can guarantee maximum oil and gas production and minimum expences.