Over 300 individual leak-off test data from six Gulf of Mexico (GOM) regions were analyzed to derive relationships for fracture gradient across GOM. Analysis of leak-off tests, as a function of the GOM location, water depth, and lithology, have provided bounds on reasonable values of leak-off pressure, the minimum stress, and the matrix stress ratio. Contrary to the widely-used Eaton model, results from this study suggest that the locally-calibrated matrix stress ratio, K0, appears to be approximately constant as a function of depth. As a guideline for deriving facture gradients in an area of poor calibration, we would recommend a minimum stress fracture gradient method (e.g., Matthews and Kelly method), using a value of K0 of 0.8 in areas not affected by salt and a value of K0 of 0.95-1.0 for areas near to salt. Recommendations for conducting leak-off tests and interpreting LOT for leak-off pressure and minimum stress are included.
The overburden stress in basins considered to be ”relaxed,” including much of the GOM, is generated by the weight of the overburden. Horizontal stresses in relaxed basins are generated by the lateral confinement of the sediments as a response to the weight of the overburden. The stress generated by the overburden is distributed between the weight of the rock matrix and the weight of the pore fluid. Minimum stress methods assume that significant mud losses will occur when the wellbore pressure equals the minimum in-situ stress. Minimum stress methods are applicable for cases where near wellbore effects are negligible, including cases when the wellbore contains a large crack and rock tensile strength is negligible. Hoop stress methods are based upon analytical solutions for stresses around a wellbore, predicting massive loss when the mud pressure causes the minimum hoop stress to equal the rock's tensile strength. Hoop stress fracture gradient methods assume that leakoff is highly sensitive to near-wellbore effects (i.e., in the case where the wellbore is intact or contains only very short cracks). The fracture mechanics approach determines the conditions under which a fracture will begin and end propagating. Direct methods relate fracture gradient to some other parameter, such as depth. For the purpose of developing an improved fracture gradient prediction method to minimize the risk of lost circulation, we will focus on minimum stress methods. While these methods may be considered to be conservative, they are applicable in the case where a large wellbore fracture exists. In such a case, the minimum stress defines the upper bound for the safe operating window. The ratio relating horizontal to vertical effective stress is called the effective stress ratio, or matrix stress ratio, K0. K0 designates the ratio of the minimum horizontal effective stress to the vertical effective stress under uniaxial strain conditions. Different minimum stress fracture gradient methods derive K0 in different ways (e.g., related to friction angle, Poisson's ratio, and depth). The Eaton minimum stress method for fracture gradient [3, 4] prediction is one of the most widely-used methods in the industry.
