The theory of poromechanics was developed and introduced by Biot (1941). Poromechanics analyses comprises of poroelastic analysis, porothermoelastic analysis, and porochemoelastic analysis. The poromechanics analyses compute the coupled stresses and pore pressure around the wellbore. In other words, any changes in formation pore pressure (e.g. from diffusion) incur changes in stresses and vice versa. As such, the calculated safe drilling mudweight from the wellbore stresses distributions change with time (i.e. time-dependent mudweight). The poromechanics effects on wellbore are most pronounced in soft, unconsolidated, and low permeability formations. Single porosity analyses are applicable for formations without natural fractures networks and vugs.

Multi-Layer Analysis allows users to investigate the potential of wellbore instability problems at multiple time intervals and at multiple depths. These time intervals can be modified by end-users, from very short (minutes) to very long (tens of days), thus providing the flexibility in engineering analysis. For each of these time intervals, required mudweight to prevent collapse (Collapse Mudweight) and limit for mud loss (Fracturing Mudweight) are calculated for all depths and compared against important thresholds such as pore pressure gradient and minimum horizontal stress gradient.

The porochemoelastic analysis is an extension of poroelastic analysis that takes into account the chemical interactions between the drilling fluid and the formation due to the difference in solute (salt) concentration. In particular, the difference in solute concentration between the drilling fluid and the pore fluid can induce an osmotic pressure which alters the stresses and pore pressure distributions around the wellbore, which in turns change the required mudweights to stabilize the borehole. The temperature difference between the drilling fluid and the formation is assumed to be negligible in this analysis.

Dual porosity poromechanics are a special branch of poromechanics developed to analyze fractured rock formations.

Solid mechanics analyses comprise of elastic analysis, thermoelastic analysis, and chemoelastic analysis. The solid mechanics analyses compute the stresses around the wellbore while ignoring the coupled effects of pore pressure and stresses. In other words, changes in formation stresses distribution shall not incur any changes in pore pressure and vice versa. As such, the calculated safe drilling mudweight from the wellbore stresses distributions usually does not change with time. Practically, solid mechanics analyses are applicable in depleted formations, stiff and well-consolidated rocks, or rocks with high permeability (100 mD and higher).

TTD-WSS is an easy-to-use software that analyzes and simulates the time-dependent phenomena of wellbore instability while drilling in three-dimensions through various rock formations, hence the concept of 4D (3D + time).

The software assists drilling engineers in designing a wellbore drilling program for mudweight window and depth-gradient analyses. With the efficiency and accuracy of TTD-WSS, drilling engineers can perform a wide variety of computations and generate results in a very short time, which renders TTD-WSS a unique tool to simulate “real-time” stability analysis. TTD-WSS also has a graphical user interface that includes features for easy data editing, validation, copying, and graphical presentation of results.

Ideal or Nothing

The elastic analysis considers only the mechanical impacts of the drilling process on the rock matrix. Pore pressure inside the formation is assumed to stay at original formation pore pressure if the wellbore is impermeable. On the other hand, for permeable wellbore, pore pressure inside the formation is assumed to be the same as drilling mud pressure. Due to these simplified assumptions, collapse mudweight and fracturing mudweight calculated using elastic model are time-independent.

When elastic analysis is used, it is assumed that the temperature difference between the drilling fluid and the formation is negligible. The chemical interactions between the drilling fluid and the formation due to the difference in solute (salt) concentration are also ignored. Practically, the chemical interactions can be ignored when inert drilling fluids (nitrogen, oil, etc.) are used.

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