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FRACTURING

T
he purpose of doing fracture stimulation on a well is to increase the 
production rate  of  that well. This is done by improving the natural 
drainage conditions around the well bore. This is called improving natural 
conductivity.

Fracture stimulation involves pumping a fluid down the well at high pressure.
This causes the rock in the vicinity of the well bottom to fracture (to split or 
crack). Vertical fractures can be induced ranging in length from a few 
metres to some hundreds of meters.

Fracture widths are in the range of a few millimetres.

The fractures open up drainage channels to the well bore. This increases
the exposed face of the reservoir rock.

Fluids pumped are selected according to the type of rock to be fractured.
Formations with solubilities less than 50% are more often fracture stimulated
with water or oil‑based fracturing fluids. These are normally sandstone
type formations.

PROPPANTS

The function of a 'proppant' is to prevent the fractures created during
fracture stimulation from closing up again once the fracturing pressure is 
released. 
Fluid Injection
Selective graded sand is the most commonly used proppant.

The main considerations for selecting a proppant are:

  • The proppant material must resist crushing under high overburden pressure. Overburden pressure is the weight of rock above the fracture.
  • The proppant material must resist being embedded in soft reservoir rocks.

Proppant sand grains are graded by passing the grains through different sized sieves (meshes). Large grains and undersize grains are removed.

FRACTURING FLUIDS

Fracturing fluids must have the following properties:

·         The ability to carry and correctly place adequate amounts of proppant.

·         Low fluid loss. (The fluid must not enter the formation rock.)

·         Low friction loss. (The fluid must be easily pumped.)

·         Compatibliity with the formation and formation fluids. The fluid must not 
          cause swelling of shales (clays) or form emulsions with formation fluids.

·         Have rapid clean‑up ability (easily flow back out of the formation).

FLUID ADDITIVES

There are many additives that can be used with fracturing fluids to obtain these 
properties.

Natural gums, cellulose gums and polymers are used to increase fluid viscosity
and to form gels. These also act as a spacer between the proppant sand grains.

The gels can be made to last for any period of time up to 100 hours.

Once the gel breaks, (returns back to low viscosity fluid), it is easy for the
fracturing fluid to flow back during well clean up.

Most of these gelling additives also act as friction reducers.

Low Pressure Ground Mixer Assembly

MIXING OF FRACTURING FLUIDS

Fracturing fluids may be batch mixed or continuously mixed.

In batch Mixing all the fluid to be used an the job is pre‑mixed before the job is
started. This requires large storage volumes and careful consideration of gel life
and gel strength.

In continuous mixing the fluid is mixed as it is needed during the job. If
a down‑hole problem is met during fracturing, unmixed material can be used
later or on another job.

EQUIPMENT USED FOR FRACTURE STIMULATION

The basic equipment requirements are:

·            Storage tanks,

·            Proportioners,

·            Blenders,

·            Pumps.

The storage tanks contain the raw materials that will be mixed together
to make up the fracturing fluid.

The proportioners take the raw materials and feeds them to be mixed in the
correct proportions.

The blender does the mixing of the raw materials with the water or oil that
will be pumped.

The pumps inject the completed fracturing fluid down the well.

Holding tanks will be needed if the fracturing fluid is batch mixed.

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