The basic theory of massive hydraulic fracturing as it relates to rock mechanics and rock properties will be reviewed with particular emphasis on the Cotton Valley Formation. The assumptions and basic concepts in frac geometry and orientation will be discussed. The parameters required for design will be identified and their effect delineated. Although this paper will be geared for a general understanding of fracturing, recent findings in rock mechanics and key areas requiring further definition will be presented. presented
The hydraulic fracturing of reservoirs to stimulate oil and gas production revolutionized the petroleum industry. The fracturing of reservoirs greatly increases the economic lives of some wells. This is particularly true for tight gas sands, or in cases particularly true for tight gas sands, or in cases where portions of the reservoir may be depleted and will not flow adequately without stimulation.
The hydraulic fracturing of wells has developed into a highly sophisticated technique over the past thirty years. The initial treatments have evolved from small jobs using a few thousand pounds of sand to highly technical computer designed treatments that sometimes incorporate one million pounds of sand. While fracturing was growing in popularity, so was the need for more efficient frac fluids, different types of sand, special additives, sophisticated pumping equipment and the use of technically engineered pumping equipment and the use of technically engineered treatments. While techniques and equipment are constantly changing and being upgraded, there are still questions to be answered and significant improvements to be made. There is often a gap between theory and practical field application. These theories must be practical field application. These theories must be applied and tested in a usable form at the operations level.
It is the intent of this paper to review the basics of fracturing and to provide a better understanding of day to day operations.
PRINCIPLES PRINCIPLES Increased emphasis has been put on the study of rock mechanics in recent years. This study has great importance, especially in tight gas sands. It is assumed that formation rock is isotropic, homogeneous and elastic, and is generally defined by two constants, Young's Modulus and Poisson's Ratio. Although these assumptions are never completely true, their simplification is necessary to handle the complexities that arise.
Formation rock is under three principle stresses. The basic premise of rock failure is that it occurs perpendicular to the least principle stress. In perpendicular to the least principle stress. In relaxed areas with normal faulting, the principle stress is vertical with a value of approximately psi/ft. In this instance, the fracture will be psi/ft. In this instance, the fracture will be vertical with the intermediate and least principle stresses in horizontal directions, as shown in figure 1. This is normally the case in the Gulf Coast, Mid-Continent and West Texas areas, as supported by frac gradients less than one.
The relationship between the vertical maximum matrix stress (sigma v) and the horizontal matrix stresses, where the horizontal stresses are equal is given by:
(1)
where mu is Poisson's Ratio. For petroleum reservoirs, Poisson's Ratio ranges from 0.15 to 0.35. This gives a horizontal matrix stress of 18 to 55% of the vertical matrix stress.
Hydraulic fracturing is basically a process of rupturing the formation rock. In order to rupture the rock, the matrix stress, pore pressure and tensile strength of the rock must be overcome. The total stress(S) is composed of matrix stress plus pore pressure. Since the tensile strength of rock is pressure. Since the tensile strength of rock is low and highly variable, it is frequently ignored.
