A New Model For Fracture Gradient

Abstract

Abstract For the past eighteen years, fracture gradient has been thought to be some junction of overburden stress, horizontal stress und internal fluid pressure. Each author that has published in the intervening years has refined this idea. This paper derives such a model, but then uses the same data to turn the entire relationship around to derive (and support a new model with the following attributes.Fracture gradient is equal to the least stress field n the earth.Fracture graadient is independent of the fluid pressure gradient; it depends only on the earth's stress field.Fluid pressure gradient is dependent on fracture gradient. If the fluid pressure gradient equals the fracture gradient, the earth fractures and the thud is released to permeable beds.Most abnormal fluid pressure systems may be pressured to the fracture gradient at their structurally highest point. Introduction SINCE the puhlication of Hubhert and Willis(11) on hydraulic fracturing in 1957, most authors have held that fracture gradient is a function of overburden stress, horizontal stress and internal fluid pressure, although the exact relationship has remained in dispute. A recent article, however, by MacPherson and Berry(12) states that in calculating fracture gradient from sonic and density logs, they found no evidence that the formation fluid pressure gradient affects the fracture gradient. This strikes a sympathetic point with this author and the result is this paper. The data gathering and the original curve fitting for this project were done in 1967; but because a number of papers were published that year on fracture gradient, one more paper giving essentially the same results did not seem warranted. The original work is included; however, the purpose of this paper is to show that the same data that were used to relate fracture gradient to pressure gradient can be used to turn the entire equation around and relate pressure gradient to fracture gradient as the independent variable. This solution suggests that fracture gradient is dependent only on minimum regional stress. A body of theory has been derived that indicates that this solution is possible. Sampling Procedure The sample for this project included most of the lost returns encountered by Exxon while drilling in South Louisiana and the Louisiana Offshore from 1964 to mid-1967. A total of sixty-six cases were used, which is considered to be a statistically satisfactory sample. The geological section sampled ranged from Lower Miocene to Recent in age, with the bulk of the data being Upper Miocene. A basic assumption was made in gathering data that the point of fracture in each well was at the highest exposed depth and the lowest exposed formation fluid pressure below the deepest casing set in the well at the time of lost returns. As no cases were 'found in this sample where the formation fluid pressure decreased with depth, the fracture point was picked in all cases as the first exposed point below casing.