Shale Stability with Water Base Muds

Principal Investigators: Martin E. Chenevert, Mukul M. Sharma

Background - Shales are sedimentary rocks that have distinct laminated layers and moderate to high clay content. Shales are vulnerable to phenomena such as swelling, shrinking, hydration, strength reduction and failure. Dzialowski et al. (1993) states that over 90% of the formations drilled worldwide are classified as shale formations. He further explains that about 75% of drilling operations' problems are related to Shales. Wellbore instability, hole enlargement, stuck pipe, high torque and drag, and side tracking are some of the most challenging drilling problems related to shale formations. Wellbore instability problems cost the petroleum industry an estimated $500 million a year (van Oort et al., 1996). There is a great need for water base muds that provide well bore stability.

Motivation - Most shale problems stem from the fact that shales are highly water-sensitive formations. Shale hydration leads to strength reduction and failure. Researchers have found that dehydration of the shale could lead to an increase in the shale strength and thus avoid wellbore failure. Chenevert (1970) states that stimulating the water flow out of the shale and into the wellbore can strengthen the shale. Water flow out of the shale and into the wellbore requires the existence of an effective osmotic gradient in the wellbore. In order to build an effective osmotic pressure gradient in the wellbore, a shale and drilling fluid system must produce a high osmotic pressure gradient in the wellbore and exhibit high membrane efficiency. The osmotic pressure gradient and shale membrane efficiency concepts have spurred much research and interest.

Objectives

1. To investigate and analyze various commercially available water base muds that have demonstrated improved shale inhibition. Such fluids will include:
• MI Ultradrill
• Baker Performax
• Newpark Deep Drill
• Baroid Hydroguard
• Amber Lonestar DFX System
• MI Ultradrill with low molecular weight starch
2. A follow up program will focus on evaluating the mechanism of any of the muds shown above that show promise in solving shale hydration.

Entrance of Oil-Base Muds into Shales

Oil-base muds (OBMs) have been developed to combat drilling problems often caused by shale hydration such as swelling, high pore pressures and mechanical failure. Thereby, is of extreme importance to understand the interaction of OBMs and the shale(s) in contact. Even though the oil filtrate of the OBMs does not hydrate the shale, it penetrates and flows through it. Therefore, is of primary interest to know what is the breakthrough pressure that needs to be overcome by an oil-base mud over a shale, and how the emulsifiers concentration and the water activity of the shale affects this pressure.

Objectives

1. To perform laboratory tests on various shale so as to determine entry pressures. For this purpose, five oil-based muds with different emulsifiers, oil and water concentrations will be used.
2. Analyze results so as to understand what the effects of shale water activity on entrance pressure are.