Offshore Fracture Gradients

Abstract

The analysis described here, based on Santa Barbara Channel field experience, shows that increasing water depth reduces the overburden stress gradient, substantially influencing the apparent fracture gradient, particularly at shallow penetrations. Two methods of correlating fracture particularly at shallow penetrations. Two methods of correlating fracture gradient are presented in the analysis. Introduction The consideration of formation fracture gradients in selecting casing setting depths is fundamental to the preparation of an acceptable drilling program. When preparation of an acceptable drilling program. When Exxon Co. U.S.A. undertook the deepwater challenges of the Santa Barbara Channel, a project was initiated to develop a fracture-gradient prediction technique that would assess the impact of water depth. Development of the Technique Onshore Fracture Gradients As frequently referenced, Hubbert and Willis, established a theoretical basis for subsequent work in the development of actual fracture-gradient prediction techniques. Later theoretical studies of porous bodies have concluded that the most important factors affecting fracture initiation pressure are the overburden stress, the formation fluid pressure, and the horizontal-to-vertical stress ratio, whereas rock compressibilities and Poisson's ratio have relatively little influence. Matthews and Kelly, in a technique presented in 1967, correctly indicated that the stress ratio, F sigma, was the most significant and yet elusive variable. The following relation was presented for the fracture gradient: ..........................................(1) By assuming a total overburden gradient of 1.0 psi/ ft, we can estimate the effective vertical stress as follows: ..........................................(2) For normally pressured formations, the stress ratio, F sigma, is empirically correlated, on the basis of field data, for a given area as a function of depth. The onshore aspect of the technique that was developed for the Santa Barbara Channel area incorporates these stress-ratio concepts with a method to estimate the actual overburden gradient rather than using the assumed value of 1.0 psi/ft. Since the overburden stress at any depth should be the cumulative weight of the formations above that depth, well data that are indicative of formation density can be used to estimate overburden. Techniques have been published that are based on velocity data from seismic published that are based on velocity data from seismic surveys and sonic logs or that utilize formation bulk density logs directly. We selected the latter method and estimated overburden with Eq. 3: ..........................................(3) where 0.4335 is the constant used to convert units of gm/cc to psi/ft. In practice, of course, a simple arithmetic average of the bulk densities of the overlaying formations, pb, will suffice: ..........................................(4) The final form of the fracture-gradient prediction formula used in the Channel is given as Eq. 5: ..........................................(5) JPT P. 910