In a high pressure squeeze the formation is initially fractured (broken down) by a solids free breakdown fluid. A solids free fluid is used because a solids laiden fluid such as drilling mud will build up a filter cake and prevent injection into the formation. Solids free fluids such as water or brine are recommended. The direction of the fracture depends on the rock stresses present in the formation. The fracture will occur along a plane perpendicular to the direction of the least compressive stress. In general, the vertical stress, due to the overburden, will be greater than the horizontal stresses. A vertical fracture is therefore more likely.
In practice the fracture direction is difficult to predict since it may follow natural fractures in the formation. Since squeeze cementing is often used to isolate various horizontal zones a vertical fracture is of little use (vertical fluid movement is not
Effect of well depth and vertical-horizontal formation stresses on type of hydraulic fracture
induced by injected fluid. Horizontal fracture pressure is less than overburden pressure,
this is usually the case at depths greater than 3,000 feet.
After the formation is broken down a slurry of cement is spotted adjacent to the formation, and then pumped into the zone at a slow rate. The injection pressure should gradually build up as the cement fills up the fractured zone. After the cement has been squeezed the pressure is released to check for back flow. The disadvantages of this technique are:
• No control over the orientation of the fracture
• Large volumes of cement may be necessary to seal off the fracture
• Mud filled perforations may not be opened up by fracturing, so the cement may not seal them off effectively.