Determination of safe mud weight windows for drilling deviated wellbores: a case study in the North Perth Basin

Abstract

Drilling non-vertical wells, which is commonly used to enhance production, in particular in unconventional reservoirs with very low permeability, is subjected to rock mechanics related issues namely wellbore instabilities. Drilling in the same formation but with different deviations and at different directions would not result in similar response in terms of rock failures. This is due to the rotation of the induced stresses around the wellbore wall along its trajectory. The type of instabilities that the formation experiences around the wellbore is a function of formation's mechanical properties and the status of in-situ stresses. It is clear that none of these natural parameters can be controlled or changed during drilling operation. However, the density of the mud weigh which is used to drill the wellbore could be optimised in order to mitigate or stop wellbore instability issues in the form of tensile failure or fracturing in case of using high mud density or shear failure or breakouts when a low mud weigh is used. Therefore, a safe mud weight window (MWW) can be determined for optimum drilling in terms of having the least issues related to wellbore instabilities. This MWW will change as the wellbore deviation changes.In this paper, the concept of constructing the rock mechanical model (RMM) which includes the rock mechanical properties as well as in-situ stress profiles is briefly presented. Then the RMM output corresponding to vertical well Arrowsmith-1 which is a shale gas well drilled in the North Perth Basin of Australia will be used to calculate the safe MWW for drilling wellbores in different azimuth and deviation in the field. The results indicate how the MWW changes as a function of wellbore trajectory.

Relation Between Fracture Stability and Gas Leakage into Deep Aquifers in the North Perth Basin in Western Australia

“…Published estimates of stress at depths of greater than 1000 m indicate the stress is strike slip (Le and Rasouli 2012) becoming normal at increasing depths (Rasouli and Sutherland 2014) in the North Perth Basin. These estimates are not considered reliable because the horizontal stresses were based on empirically determined Poissons ratios derived from sonic logs.…”

A study on well design and integrity for deepwater exploratory drilling in Brazilian Equatorial Margin.

“…Then, the estimation of the geo-pressures such as pore, fracture and collapse pressures are necessary to establish the appropriate weights of the drilling fluid (mud) prior to setting and cementing a casing (ROCHA; AZEVEDO, 2009; ZHANG, 2011). Thus, geo-pressures define the limits of the mud weight operating envelope or window (LESAGE et al, 1991). Figure 2.4 represents an example of an offshore operating window, where can be noted mud weight and geo-pressure curves.…”

“…That is, in the case of the mud weight was lower than local pore pressure gradient in a very permeable rock, then a kick would occur; if it happens in a soft and impermeable rock, the well could collapse. However, in many cases drillers could develop an underbalanced drilling to increase rate of penetration (ROP), where the used mud weights are lower than pore pressure gradient (LESAGE et al, 1991).…”

“…defines the upper limit, which indicates the maximum allowable mud weight where the wellbore will not fracture, causing losses of drilling mud into the fracture formation (ZHANG, 2011;ZHANG;YIN, 2017). Moreover, with loss of circulation, the fracture could propagate in case of mud weight overpass the minimum horizontal earth stress (LESAGE et al, 1991). Consequently, the relationship between the rock properties and principal in-situ subsurface stresses shall be also considered when it is established the pressure limits of drilling fluid that ensure the wellbore stability to be maintained.…”

“…The indirect methods such as Hoop stress method, minimum stress method, specific correlations or rock methods and tensile failure method, are other possible ways to estimate the fracture gradient along the well. However, according to the available data, simplicity and because it is widely employed in the oil industry to determine fracture gradient in vertical wells, only will be explored the minimum stress method (LESAGE et al, 1991;ROCHA et al, 2004;ROCHA;AZEVEDO, 2009; ZHANG, 2011)…”