Abstract
Although a large volume of mudcake filtration test data is available in the literature, effects of mudcake on wellbore strengthening cannot be quantified without incorporating the data into a stress-analysis model. Traditional models for determining fracture initiation pressure (FIP) either consider a wellbore with an impermeable mudcake or with no mudcake at all. An analytical model considering permeable mudcake is proposed in this paper. The model can predict pore pressure and stress profiles around the wellbore, and consequently the FIP, for different mudcake thickness, permeability, and strength. Numerical examples are provided to illustrate the effects of these mudcake parameters. The results show that a low-permeability mudcake enhances FIP, mainly through restricting fluid seepage and pore pressure increase in the near-wellbore region, rather than by mudcake strength. Fluid loss pressure (FLP) should be distinguished from FIP when a mudcake is present on the wellbore wall. Fracture may occur behind the mudcake at FIP without mudcake rupture. The small effect of mudcake strength on FIP does not mean its effect on FLP is small too. Mudcake strength may play an important role in maintaining integrity of the wellbore once a fracture has initiated behind the mudcake.
Highlights
As a bit drills through a permeable formation in overbalance drilling, the base fluid of drilling mud permeates into the formation while solid components are filtered out, leaving a mudcake on the wellbore wall
Numerical examples have been provided to illustrate the effects of mudcake thickness, permeability and strength on wellbore stress and fracture initiation pressure (FIP)
(5) Despite very low permeability of the mudcake (e.g., 10-3 mD), neglecting the permeable nature of the mudcake, i.e., assuming the mudcake is impermeable, may lead to substantial overestimation of FIP. (6) Fluid loss pressure (FLP) is different from FIP when a mudcake exists on the wellbore wall
Summary
As a bit drills through a permeable formation in overbalance drilling, the base fluid of drilling mud permeates into the formation while solid components are filtered out, leaving a mudcake on the wellbore wall. Both laboratory tests and field practices have shown that the presence of a mudcake can effectively inhibit fracture creation on the wellbore and prevent lost circulation (Cook et al 2016; Guo et al 2014; Song and Rojas 2006). The model can be used to predict pore pressure and stress profiles around the wellbore, and the FIP, for different mudcake thickness, permeability, and strength. The difference between FIP and FLP is discussed in more detail in this paper
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
