Monday, July 30, 2018

FRACTURING


The 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 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 meters to some hundreds of meters.

Fracture widths are in the range of a few millimeters. 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. Formation with solubilities less than 50% are more often fracture stimulated with water or oil based fracturing fluids. These are normally sandstone type formation.

Fluid Injection

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.

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 of proppant.
    • Low fluid loss (The fluid must not enter the formation rock).
    • Low frictions loss (The fluid must be easily pumped).
    • Compatibility with the formation and formation fluids. The fluids 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).

    Fluids Additivies

    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 gels break (return back to low viscosity fluid), it is easy for the fracturing fluid to flow back during well clean up.

    Most of these gelling additivesn 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 on the job is pre-mixed before the job started. This requires large storage volumes and carefull consideraion of gel life and gel strength.

    In continuous mixing, the fluid is mixed as it is needed during the job. If a downhole problem is met during fracturing, unmixed material can be used later or onather job.

    EQUIPMENT USED FOR FRACTURE STIMULATION

    The basic equipment requirements are:
    • Storage tanks - contain the raw materials that will be mixed together to make up the fracturing fluid.
    • Proportioners - take the raw materials and feeds them to be mixed in the correct proportions.
    • Blenders - does the mixing of the raw materials with the water or oil that will be pumped.
    • Pumps - inject the completed fracturing fluid down the well.

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    FRACTURING

    The purpose of doing fracture stimulation on a well is to increase the production rate of that well. This is done by improving the natura...