Predict Mud Weight

In the Predict Mud Weight workflow, JewelSuite Geomechanics takes all the data defined in the Setup sub-strip and calculates the critical mud pressures that correspond to wellbore collapse, initiation of tensile fractures, fracture link-up, and fracture propagation. The application then proposes an optimal program of casing setting depths. You can also manually design casing with easy-to-use graphical tools. You can consider kick tolerances during the casing design, by incorporating them into the mud window as an adjustable safety factor.

At any depth of interest, you are able to analyze in detail the required mud weight, should the drilling direction or the rock strength change. The Predict Mud Weigth algorithm calculates three-dimensional borehole stresses from formulas by Hiramatsu and Oka (1962), and then computes four critical mud pressures: borehole collapse pressure, fracture initiation pressure, fracture link-up pressure, and fracture grow or propagation pressure.

Borehole collapse pressure is the mud pressure at which borehole breakouts of a given width will form on opposite sides of the wellbore. Breakouts form because of high stress concentrations encountered while drilling the well (Zoback et al., 1985). Throughout the Wellbore Stability strip, wellbore breakouts are strictly defined as failures along shear surfaces that are inclined to the wellbore wall. The calculations assume that the rock behaves elastically up to the point of failure, which is controlled by the failure criterion selected.

Fracture initiation pressure is the mud pressure at which a tensile fracture starts to form on the wellbore wall for a given tensile strength. The tensile strength is often very close to zero, since preexisting flaws or irregularities are usually present on the wall. Both the fracture initiation pressure and orientation of the tensile fractures are sensitive to the in situ stress and to borehole orientation; fractures are not axial if the borehole axis is not in a principal stress plane. From the viewpoint of wellbore stability however, the fracture initiation pressure is usually not dangerous because fractures are still small and do not result in lost circulation.

Fracture link-up pressure is the mud pressure at which isolated, non-axial fractures begin to link up. At this stage, the fracture openings are much larger than at fracture initiation, which may lead to some lost fluid; this fluid may be recovered if the wellbore pressure drops, for example, while making connections. Such cyclic gains and losses may be the explanation for ballooning that is often reported in deepwater wells. The Predict Mud Weight calculations of the link-up pressure are based on a theory presented by Ito et al. (1999). The link-up pressure is dependent on wellbore orientation, which means that in certain cases, lost circulation problems may be avoided by changing the directional orientation of the well.

Fracture grow pressure (also called fracture propagation pressure) is the mud pressure necessary to drive fracture propagation away from the wellbore. Once tensile fractures link up, the hydraulically driven fractures tend to propagate away from the wellbore and reorient themselves to be normal to the minimum stress, S3, because borehole drilling-induced stresses diminish quickly away from the well. The Predict Mud Weight uses a formula presented by Ito et al. (1999), which gives a theoretical relationship between the fracture grow pressure, the pore pressure (Pp), the minimum stress (S3), and the so-called “mud invasion factor” that defines, as a percentage, how much the fracture is invaded by drilling mud. Lost circulation material (LCM) should be utilized to strengthen the non-invaded tip of the fracture (i.e., to lower the “mud invasion factor”) because this results in an increase of mud pressure required for fracture growth. Thus, non-invading drilling mud allows drilling in excess of the least principal stress. In effect, this results in the widening of the mud window (see below) that will maintain wellbore stability.

The Mud Window

The mud window is the difference between the lowest and highest acceptable mud weights for a given set of drilling conditions. You can define the lower bound as the pore pressure, the borehole collapse pressure, or the maximum of these two. Selecting the pore pressure as the lower bound is a common industry standard, while selecting the borehole collapse pressure as the lower bound guarantees that the width of borehole breakouts does not exceed the given limit. The default definition in the Predict Mud Weight algorithm, is the maximum of the pore and borehole collapse pressures. You can define the upper bound of the mud window as either the least principal stress or the fracture gradient. To change either bound on the mud window, select the desired option from the drop-down lists on the Input and Boundaries form. If the upper mud-window bound exceeds the lower bound over a cased section, the values listed in the table in the Analyze MW Window section on the Along Wellbore form and the area between the two bounds in the Predict Mud Weight view, will have light green backgrounds that indicate safe drilling conditions for this hole section. If the lower bound exceeds the upper bound, the shading changes to red to indicate that no safe mud window exists. You can enter the width of the critical mud window and a kick tolerance in the relevant boxes on the Along Wellbore form, and the Predict Mud Weight algorithm automatically re-displays the mud window to reflect the changes.

Casing Design

The Predict Mud Weight’s casing design selection focuses on wellbore stability, therefore it does not incorporate other drilling factors such as maximum anticipated surface pressure (MASP), cement column heights, zonal isolation, or internal casing clearance. When establishing safe geomechanical limits, other engineering factors may influence the selected criteria for the mud window. For example, improved hydraulics allows a well to be drilled with a larger volume of produced materials per unit time, which may affect the breakout width that defines the lower bound of the mud window.

References:

Hiramatsu, Y., and Y. Oka, 1962. Stress around a shaft or level excavated in ground with a three-dimensional stress state, Mem. Fac. Eng. Kyoto Univ., XXIV(I), 56–76.

Zoback, M. D., D. Moos, L. Mastin, and R. N. Anderson, 1985. Wellbore breakouts and in situ stress, J. Geophys. Res., 90, pp. 5,523–5,530.