Hydraulic fracturing is a commonly used technique to improve hydrocarbon recovery from low permeability reservoirs. Practical fracturing permeability reservoirs. Practical fracturing treatments are designed to maximize the high conductivity propped fracture area in pay while minimizing the formation damage resulting from various fracturing materials within economic constraints. The effectiveness of hydraulic fractures can be greatly improved by controlling the geometry of the created fracture to maximize the fraction of the fracture area in the pay zone.
In situations where vertical fractures tend to migrate due to insufficient fracture containment, the effectiveness of a treatment may be increased by using a number of techniques. These include the proper selection of the perforation placements, controlling pumping rates and the use of placements, controlling pumping rates and the use of various fluid types and other additives. Some of these techniques are currently under investigation in a Gas Research Institute funded project in western tight sands formations.
This paper reviews general considerations regarding hydraulic fracture designs, and presents the development of a perforation placement technique and a field application to verify the technique.
Hydraulic fracture treatments in low permeability reservoirs need to be optimized for best permeability reservoirs need to be optimized for best economic returns. The fracture parameters that most strongly influence the production improvement from a well are the propped fracture length in pay, fracture conductivity and formation damage. pay, fracture conductivity and formation damage. The desired values of these fracture parameters are selected as treatment goals based on the previous production history in the area and previous production history in the area and formation properties such as porosity thickness, permeabilities, and lateral extent. Hydraulic fracture permeabilities, and lateral extent. Hydraulic fracture treatments are then designed to achieve these goals as far as possible in an economic fashion.
The results of a fracturing treatment are governed by the details of the treatment as well as properties of the formations being fractured. The formation properties of importance to the fracturing process include in-situ stress distributions, shear moduli, Poisson's ratios, leak-off coefficients and lithology. It is important to recognize that created hydraulic fractures are three-dimensional and their height, length and width are all dependent on the fracturing process. In general, a created fracture will not stay confined to the productive pay zone but tend to migrate into the surrounding strata as well. This is an undesirable situation as expensive fracturing fluids are wasted in creating non- productive fracture areas outside the pay and there productive fracture areas outside the pay and there is also a possibility of breaking into water bearing zones.
The degree of fracture migration into the surrounding strata is most strongly influenced by the in-situ stress contrast between the zones. If the minimum principal stresses in the barriers are significantly higher than that in the pay and the barriers are thick enough, most of the fracture stays confined to the pay, resulting in an optimum situation. If, however, there is a lack of strong stress barriers above or below (or both) the pay zone, the created fracture will migrate out of the pay zone. Fracturing treatment designs in such pay zone. Fracturing treatment designs in such cases need to be modified to yield economically optimum results. There are two approaches to fracture design for such cases.
A. Use a modified fracture design using viscosity contrast and diverting agents to directly influence the created fracture geometry by emplacement of a barrier.
B. Place a conventionally created fracture using modified perforation placement techniques to optimize the created fracture area in pay.
The first approach holds promise and it is general enough to provide fracture designs to encompass a wide range of situations. However, the required fracture treatments are more complex than those normally practiced.