Hydraulic fracturing
Natural gas is stored under pressure in pores and open fractures of the shale rock (free gas), is dissolved in brine or adsorbed at the surfaces of organic fragments and mineral particles (associated gas).
Hydraulic fracturing is the key method used in shale gas production. It involves injections of highly pressurized water that contains chemical additives or another medium (e.g. carbon dioxide) or proppant. Hydraulic fracturing procedures are performed in vertical or horizontal sections of boreholes.
A number of pumps and huge amounts of equipment and fracturing fluids are required at the drill site to produce a pressure surge that is high enough to break the rock.
Hydraulic fracturing involves injection of fracturing fluid into a sealed section of the borehole with perforated casing. The rock breaks and the fracture begins to propagate in the gas-bearing shale as pressure increases beyond the minimum stress tangential to the wellbore wall.
The hydraulically induced fracture always propagates in a direction that is approximate to the maximum horizontal stress axis, at fracture pressure being higher than the minimum contemporary stress. Proppant is injected with fracturing fluid into the network of induced fractures so as to prevent fracture closure while pressure drops on completion of the operation.
On wellhead valve opening, a portion of fracturing fluid that has not been absorbed by the shale rock and the induced fractures is flowing back to the surface.
Micro-seismic monitoring records any subtle shocks from shale rock deformations that are triggered by the fracturing fluid. Recorded data are used to determine range of the fractured zone. It is worth of noting that normally micro-seismic events induced by fracturing operations are billion times weaker than earthquakes that are noticeable to a man standing on the ground surface. Exceptionally, a stronger seismic shock is generated, if the fracturing fluid activates a pre-existing critically stressed tectonic zone.
Alternatively, the flow of gas can be stimulated by flushing the borehole with liquid nitrogen or carbon dioxide to enhance the flow of gas under decreasing hydrostatic pressure of the fluid present in the wellbore. Unlike water, these fluids do not cause clay minerals to swell and actively displace methane gas from brine water and adsorption films. However, their use escalates the costs of the operation and of the well construction.
By establishing a network of induced fractures, hydraulic fracturing enables interconnections between small gas-filled voids and, by the same, production of some of the gas accumulated in the hydraulically fractured rock. In addition, gas flowing out of the rock decreases formation pressure so that the process of associated gas desorption (release) may be initiated. In order to hold the new fractures open, proppant (predominantly sand) is injected in final stages of the hydrofracturing operation.
In order for the fracturing procedure to be successful, the fracture network must be as dense as possible with fractures piercing many of the local microscopic gas migration paths within the shale rock, which include fissures of any size and laminae that are enriched in sand.
A majority of tests performed in reservoir formations is focused on investigation of mechanical parameters and stress of the shale rocks so as to enable careful planning of hydraulic fracturing procedures that are required for reservoir development. In an effort to optimize the procedures, various fluid formulations are prepared with different viscosity, compressibility, surface tension parameters, different additives reacting with the rock and having different proppant, i.e. slurry that prevents the fractures from collapsing, carrying capacity.
Moreover, fracturing effects can be optimized by choosing the best pressure increase rate or pressure modulation, as well as by selecting an appropriate sequence of periodic fracturing fluid changes, if multiple wells are drilled from a single pad. Considering in advance the presence of any geomechanical barriers that prevent fracture propagation beyond reservoir formations is an important aspect of fracturing procedure planning.
authors: Marek Jarosiński, Hubert Kiersnowski
