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Home > JOGMEC's Activities > Technology Development and Technical Support (Oil and Natural Gas) > Well Drilling & Field Development : Wellbore Stability (Well Drilling)

Wellbore Stability (Well Drilling)

Humble mud can hold the key to successful exploratory drilling and efficient production. JOGMEC has developed technologies for analyzing the effects of mud on wellbores to support proper decisions during drilling and production.

The Dynamics of Mud

After drilling, the earth's crust and fluid in geological formations put pressure on mud in wells. If we carefully choose the density of mud, the mud pressure can achieve a balance with this pressure. Insufficient mud pressure puts strain on the rock of the wellbore, yet too much mud pressure also strains the surrounding rock. "Mud density window" is the term for mud density within an appropriate range.

The Dynamics of Mud

Three natural conditions - pressure from the earth's crust, pressure from geological formations and the strength of the surrounding rock - and the bearing and trajectory of the well determine the mud density window. In addition, the particular chemistry of mud can impact and change the strength of the surrounding rock. Understanding the subterranean dynamics that determine the mud density window is therefore critical. The JOGMEC Technology and Research Center (TRC) has been emphasizing research into techniques for determining pressure from the earth's crust, including the use of cores and sending probes down into wells. However, we have primarily used extended leak-off tests (ELOT). This practical, cost-efficient method is an extended version of conventional leak-off tests applied to producing wells that analyzes existing logging data.

JOGMEC's Analysis Method

Water in mud infiltrates the crystalline structure of clay composed of shale, tuff and other materials, causing it to weaken, deteriorate and disintegrate. Methods we use to preclude this phenomenon include the use of oil mud that does not contain water, selection of proper salinity and type of salt, and the use of an additive that keeps water from permeating the surface of the wellbore.

We analyze minerals using X-ray diffraction to select the appropriate type of mud, and investigate hydration and swelling characteristics in ways such as measuring cation exchange capacity (CEC) and capillary suction time (CST). We also investigate mud pressure differentials as well as migration processes within rock due to osmotic pressure resulting from differences in chemical potential.

Investigating Mud Dynamics

TRC worked with the University of Oklahoma in the United States to develop testing equipment for quantitatively analyzing how mud changes the strength of rock, and is conducting analyses using samples from actual oil fields. This equipment is unique because it can use a comparatively small sample of 1.4" x 1.0" x 0.2" to test the impact of mud on the strength of highly anistropic shale.