The commonly used model for fracture pressure determination makes use of the ratio of the horizontal effective stress and the vertical stress as a function of the Poisson's ratio, the latter being inferred from seismic velocities. It has been documented that fracture pressure predicted in such a way deviates systematically from what is derived from LOT data, so the predictions provide an unreliable basis for well planning. In this paper, we analyze the reasons for the deviations and present a new methodology to predict Poisson's ratio and fracture pressure using seismic, well log and laboratory data. Examples confirm the superiority of the new methodology.
As exploration for hydrocarbons is shifted to more and more difficult environment (deep water, under salt, overpressure, etc.), the drilling gets more costly, and drilling hazards are more unwanted than ever. A pre-requisite of successful drilling is careful planning of well trajectory, casing, and drilling mud weight. Essential part of this planning consists in prediction of rock properties, pore pressure and fracture pressure profiles. For such a prediction, a well established technology exists. An essential part of the technology consists in calculation of pore pressure, minimum horizontal stress and fracture pressure from seismic velocities VP and Vs and rock density ?. The simplest, yet widely used approach to conversion of these data into the sought-for parameters is the Eaton's [1] formula (Mathematical equation (1)) (available in full paper).
Here Pfrac is the fracture pressure, Pfluid is the pore fluid pressure, Plitho is the overburden stress, and v is the Poisson's ratio. An alternative source of information on fracture pressure is the leak-off test (LOT) data and the mud weight profile established experimentally at neighbouring wells, so in the areas with a dense network of wells, the common practice is either to rely on the experimental data and disregard the Eaton's formula-based predictions, or to calibrate the latter with account for the LOT data. When implementing such a calibration, the estimates derived from formula (1) occur to be deviating from the LOT data more often than not, with the deviation increasing with depth, if for the ??values in (1), the laboratory or sonic log measurements of the Poisson's ratio are taken. Indeed, in practically all kinds of wet rocks, such measurements show the decrease of v values with the differential pressure Plitho- Pfluid [2, 3, 4, 5]. And since the differential pressure is increasing with depth as long as pore pressure gradient remains less than the overburden pressure gradient, these logor laboratory-derived ??values are commonly decreasing with depth, as, e.g., in Figure 1. However, to make the formula (1)- based predictions consistent with the LOT data and respective mud weight profiles, the ??values should increase with depth. Fig. 1. Sonic log P-wave (a) and S-wave (b) velocities and Poisson's ratio curve (c) calculated from the velocities (a) and (b) (reproduced from paper [6]), solid lines; Eaton's [1] curve v= a + b loge z (dashed line). Note inconsistency between Poisson's ratio dependencies represented by solid and dashed lines. (Available in full paper)
